Measurement Methods

Acoustic spectrometry: particle size measurement in concentrated dispersions

The particle size distribution of an emulsion or suspension is a key parameter to evaluate the quality of the dispersion. Thus the choice of a suitable size measurement method is essential for an efficient quality control of research on these systems. Many instruments and methods are on the market, but most of them like light scattering systems requires a special sample preparation/dilution due to their limited concentration range. Image analysis methods (REM, TEM, etc.) on the other hand can analyze only a limited sample amount – thus the results are not representative. Due to the fact that modification/dilution is always influencing the electrochemical properties and particle size distribution of concentrated dispersions, a suitable measurement method must be able to characterize the system in its original state without any modification.

The acoustic spectrometry is a technique, which fulfil exactly this task! Concentrated suspensions like ceramic slurries, cements or paints, even paste-like, can be characterized easily in order to analyze particle size distribution and the state of agglomeration. The size range extends from nano-sized to the upper micro-size range.

Instruments

DT-100/DT-110:
Particle size 5 nm – 1000 µm;
measuring in original concentration up to 50 Vol.-%

DT-1202:
Particle size 5 nm – 1000 µm; measuring in original concentration up to 50 Vol.-%; measuring zeta potential

Measurement principle

Ultrasound waves of different frequencies will be launched into the dispersion to be characterized. During transmittance of the wave through the dispersion, it will be attenuated due to different physical mechanism. The sound receiving transducer will detect the attenuated signal – the attenuation is a function of the particle size distribution. The instruments of the DT-line, DT-100 and DT-1202, are measuring the attenuation spectrum over a wide frequency-range: 1 -100 MHz. The figure is showing the ultrasound-sender and –receiver, in between the measurement chamber with the zeta potential-, temperature-, conductivity and titration-probes.

Acoustic spectrometry / particle size

Acoustic spectrometry – measuring chamber

Further the gap between sender and receiver is variable. Thus high as well as low concentrated dispersions (0,1 – 50 vol.-%) can be characterized in a wide particle size-range (5 nm – 1000 µm). However, the focus of the method will be on Nano- and submicron particles! The samples can be either pumped through or left motionless in measurement chamber if they do not sediment or cream. Thus, online-measurements can be performed such as monitoring the grinding process of mill.

The following figure is showing exemplarily the research of a grinding process of a very fine titanium oxide (Hombitec). The shift of the size distribution of a bimodal starting product (second peak are agglomerates) down to final, primary nanoparticles (mean size 35 nm after 90 milling time) can be seen observed.

Acoustic spectrometry: particle size measurement in concentrated dispersions

Acoustic spectrometry: particle size measurement in concentrated dispersions

 

References and norms

/1/ ISO 20998-1 Measurement and characterization of particles by acoustic methods.
/2/ A. Dukhin, P. Goetz: Characterization of Liquids, Nano- and Microparticulates, and Porous Bodies using Ultrasound. 2nd Edition. Oxford: Elsevier, 2010.
/3/ PARTICLE WORLD 19; p. 18 – 19; „Characterization of liquids, dispersions, emulsions and porous materials using ultrasound“.

Gas adsorption: Determination of the specific surface area (BET surface area)

The determination of specific surface areas represents a major task regarding the characterization of porous and finely-dispersed solids. Gas adsorption is the appropriate method to solve this task. If a gas gets in contact with a solid material a part of the dosed gas molecules is being adsorbed onto the surface of this material. The adsorbed amount of gas depends on the gas pressure, the temperature, the kind of gas and the size of the surface area. After choosing the measuring gas and temperature, the specific surface area of a solid material can be reliably and comparably calculated from the adsorption isotherm. Due to practical reasons the adsorption of Nitrogen at a temperature of 77 K (liquid Nitrogen) has been established as the method for the determination of specific surface areas.

Schematic measurement setup

Schematic measurement setup

Isotherm with highlighted range for calculation of specific surface area (BET)

Isotherm with highlighted range for calculation of specific surface area (BET)

Measuring method

Speaking about the BET method, actually means the analysis of isotherm data by a method developed by Brunauer, Emmett and Teller. By means of the BET equation the amount of adsorbed gas, which build up one monolayer on the surface, can be calculated from the measured isotherm. The amount of molecules in this monolayer multiplied by the required space of one molecule gives the BET surface area. Besides the adsorption of Nitrogen at 77 K, Krypton adsorption at 77 K is recommended for the determination of very small surface areas.

Mathematical principles for calculation of BET surface area

Mathematical principles for calculation of BET surface area

The choice of the measuring instrument depends on:

  • scope of the task (only determination of BET surface area or also the pore volume and pore size distribution)
  • the range of the surface area of the materials (the determination of very small surface areas by Krypton adsorption requires a special measuring technique)
  • the desired throughput (1, 2, 3 or 4 station instrument)

Literature and norms

  • ISO 9277
  • DIN 66131
  • IUPAC Technical Report “Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution”; M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F. Rodriguez Reinoso, J. Rouquerol and K.S.W Sing, Pure Appl. Chem. 87, 1051 (2015); pdf-Download
  • PARTICLE WORLD 19; p. 34 – 35; „3P INSTRUMENTS as specialist in the field of adsorption“

Breakthrough curves: dynamic sorption of gas- and vapor mixtures

Investigation experiments of practically relevant separation processes by standard gas sorption instruments for pore size analysis are limited.

Regarding the assignment of tasks such as

  • investigation of the adsorption of CO2 from dry and wet air
  • adsorption of methane of biogas
  • differentiation of practically relevant sorption behaviour of adsorbents like gas or vapor mixtures.

BET- or pore size distribution cannot predict or model separation processes.

The dynamic flow method is applied for practical investigation such as

  • dynamic gas flow adsorption and desorption
  • determination and evaluation of breakthrough curves
  • investigation of sorption kinetics
  • investigation of co-adsorption and replacement effects
  • determination of sorption selectivities
  • determination of sorption equilibria of gas mixtures
  • transfer of technical sorption processes to lab-scale
  • investigation of thermal balance of dynamic adsorption processes

Analyzers

 

mixSorb S:

Measurement of breakthrough curves; vapor option; designed for very small sample amounts


 

mixSorb L:

Measurement of breakthrough curves; vapor option; safe and easy-to-use bench-top instrument


 

mixSorb SHP:

Measurement of breakthrough curves; vapor option; designed for very small sample amounts and high system pressures (up to 68 bar)

Measuring method

The figure above shows a breakthrough curve of CO2 at activated carbon measured with the mixSorb L. The vertical, red line marks the start of the breakthrough measurement. From this point a gas mixture of 5 Vol.-% CO2 in Helium flows through the adsorbent at a pressure of 5 bar at 40 °C with a flow of 1 L min-1. The black line represents the concentration of CO2 at the end of the adsorber column. One can observe that after 15 minutes traces of CO2 are detected. Before that time CO2 is completely withheld by adsorption on the activated carbon followed by a steep increase of the CO2 concentration, which is called mass transfer zone. From the shape of the breakthrough curve in this zone different parameters concerning sorption kinetics or concentration distribution can easily be determined. After approximately 30 minutes the activated carbon reaches the maximum sorption capacity under this experimental conditions. Consequently the CO2 concentration at the end of the column approaches the initial CO2 concentration

The releasing heat of sorption results in a temperature increase of the adsorber bed. To follow this process the mixSorb L is equipped with four temperature sensors along the adsorber. Their position is sketched at the right side of the figure. The four sensors respond in accordance to the direction of the gas flow starting with T1 and ends with T4. The measured temperature curves (orange, grey, yellow and blue) are also depicted. They give information of the released heat, the speed of energy exchange processes and the dispersion of the concentration profile in the adsorber column.

The following figure illustrates how to determine the technical usable sorption capacity from a breakthrough curve under certain experimental conditions. The BET surface area or the micropore volume, which are exactly known by standard gas sorption methods, are not accessible completely in technical flow processes for certain kind of purification or separation processes. As an example: The separation of CO2 from N2 (40 °C, 2 L min-1, 5 bar, initial gas composition: 5% CO2 in N2) occurs with the breakthrough of CO2 when the sorption capacity reaches 75% of the maximum sorption capacity. The dynamic sorption analyzer mixSorb L can help to predict technical relevant sorption processes in an easy a quick way.

Literature and Norms

dynamicsorption.com – detailed presentation of features, advantages, scientific background and examples of dynamic sorption methods (flow-methods)

  • Webinar mixed gas/vapor adsorption: Working with Vapors and Low Concentrations in Breakthrough Experiments (Video on Demand)
  • PARTICLE WORLD 19; p. 20 – 25, “From the idea to the technology behind the separation process:
    mixSorb L is gearing up!”
  • Dynamic and equilibrium-based investigations of CO2-removalfrom CH4-rich gas mixtures on microporous adsorbents; A. Möller, R. Eschrich, C. Reichenbach, J. Guderian, M. Lange, J. Möllmer: Adsorption (2017) 23: 197-209; external Link to pdf-view
  • conference poster 2017 – Deutsche Zeolithtagung, “Porous solids for heat storage applications: In-depth material testing by vapor breakthrough measurements”, pdf-Download
  • conference poster 2016 – Fundamentals of Adsorption – “Dynamic and equilibrium-based investigations of CO2 removal from CH4-rich gas mixtures on zeolites”, pdf-Download
  • conference poster 2016 – Reaktionstechniktagung – “Dynamische Untersuchungen zur Adsorption an Aktivkohlen”, pdf-Download
  • conference poster 2016 – Fachgruppe Adsorption und Gasreinigung – “dynaSim – A Modeling and Evaluation Tool for Dynamic Sorption Data” pdf-Download
  • conference poster 2015 – Carbon – “Breakthrough Curves of CO2 and CH4 on Carbon Molecular Sieves”, pdf-Download

Characterization of macroscopic, physical powder properties

The determination of the macroscopic physical properties of a powder sample is an essential part of its overall characterization. It provides a fundamental understanding of the behavior of a powder in its entirety and in particular with regard to processing, packaging, transport, storage and use of the powdered material. Since the macroscopic behavior of a powder depends very much on the respective conditions – e.g. the bulk density or the angle of repose of a powder accumulator is sensitive to the drop height and the resulting compaction – the measurement conditions for the determination of such parameters are defined in detail, depending on the material type, in different standards. The most commonly and widely used standard for macroscopic powder characterization is the ASTM D6393. Whereas many norms only define the measurement of one parameter (e.g. ISO 3953 – Metallic powders – Determination of tap density, ISO 3923 – Metallic powders – Determination of apparent density), the ASTM norm D6393 specifies the criteria for determining 10 different parameters, so-called Carr indices:

  • Measurement of Carr Angle of Repose
  • Measurement of Carr Angle of Fall Carr Angle of Collapse
  • Calculation of Carr Angle of Difference
  • Measurement of Carr Angle of Spatula
  • Measurement of Carr Loose Bulk Density
  • Measurement of Carr Packed Bulk Density or Tap Density
  • Calculation of Carr Compressibility
  • Measurement of Carr Cohesion
  • Measurement of Carr Uniformity
  • Measurement of Carr Dispersibility

Depending on the application, each of these parameters can be used for an evaluation of the powder or a statement of the powder quality can be made in the subsequent process. For example, the angle of repose and the angle of collapse should be taken into account when designing a conical storage silo. For the design of packaging bags and barrels or for the production of tablets from powder raw materials, tapping and bulk densities are decisive parameters.

Innovative, state-of-the-art instruments, such as the PowderPro A1 make it possible to determine the Carr indices fully automatically. This becomes possible thanks to the angle measurements by means of a CCD camera and an image processing routine, automatic control technology and state-of-the-art software with implemented standardized operating procedures (SOPs) as well as the direct data communication with an electronic balance, so that the weighing data are transmitted automatically and directly to the measuring system.

In addition to the individual determination of the Carr indices, the PowderPro A1 also allows the calculation of the resulting parameters, which make the characterization and evaluation of a powder sample perfect. In addition to the parameters defined in ASTM norm D6393, the PowderPro A1 can be used to determine the following parameters:

  • Voidage (powder porosity)
  • Flowability index
  • Floodability index

The flowability and floodability indices are important when powdery raw materials or products, for example, are conveyed in pipelines by air flow or when dust propensity has to be assessed from an occupational safety point of view.

Instruments

Definition of parameters for the determination of macroscopic powder properties

Carr Angle of Repose

The angle of repose is the angle between a horizontal plane and the free surface of a conical powder pile (in static equilibrium) accumulated by powder falling from a defined height.

It is determined by the height (h) and the radius (r) of the conical powder pile:

Carr Angle of Repose = tan-1 (h/r)

Carr Angle of Fall or Carr Angle of Collapse

The angle of collapse corresponds to the angle of repose of a powder heap to which a defined impulse shock has been given.

Carr Angle of Difference

The angle of difference is the difference between angle of repose and the angle of collapse.

Carr Angle of Spatula or Flat-Plate-Angle

The flat-plate angle is the angle between the surface of a powder pile and a defined spatula on which it was piled up after burying the spatula under a powder bed and extracting it.

Carr Loose Bulk Density

The bulk density is determined from the volume and mass of a loosely packed powder.

Carr Packed Bulk Density or Tap Density

The tap density is determined by the volume and mass of the powder after it has been compressed, which is done through repeated dropping of a measuring cylinder from a defined height.

Carr Compressibility

The compressibility is calculated from the ratio of the difference between the bulk density (L) and tap density (P) to the determined tap density:

Carr Compressibility [%] = 100 (P – L)/P

Carr Cohesion

The cohesion is a measure for interparticular interactions and is determined by defined sieving of the powder sample (at a given sieving time).

Carr Uniformity

Uniformity is a measure for the width of the volume-based particle size distribution determined by sieving analysis.

Carr Uniformity = d60/d10

Carr Dispersibility

The dispersibility is determined by dropping 10.0 g of the powder to be characterized from a defined height into a bowl and subsequently determining the mass (m) of the powder remaining in the bowl.

Carr Dispersibility = 100 (10 g – m)/10 g

Voidage

The voidage or so called powder porosity indicates the percentage value of the interparticle volume to the total volume of the powder compacted to volume constancy.

Flowability index

The flowability index is calculated from the weighted sum of the angle of repose, flat-plate-angle, compressibility, cohesion and uniformity indices.

Floodability index

The floodability index is calculated from the weighted sum of the indices of the flowability index, the angle of collapse, the angle of difference and the dispersibility.

Literature and norms

/1/ ASTM D6393 – Standard Test Method for Bulk Solid Characterization by Carr Indices

 

Chemisorption and TPX

The chemisorption analysis is specially used to characterize catalysts. The most crucial point is to determine the chemically active part of the surface area. To determine the active surface area a measuring gas is used, which is able to strongly chemisorb at the active sites. Hydrogen gas is often used for the analysis, which creates a chemisorption bond to noble metals (e.g. platinum-alumina catalysts).

Physisorption vs Chemisorption

Physisorption vs Chemisorption

Instruments

AMI-300

dynamic chemisorption analyzer

µBenchCAT

dynamic chemisorption reactor

BenchCAT

dynamic chemisorption reactor

We are offering chemisorption analyses as contract analysis

Measuring method

Volumetric method vs Flow method

1. Static-volumetric method

A certain amount of gas is dosed onto the active material in vacuum. The determination of the amount of adsorbed gas is carried out by pressure measurement in a system with known volume. Typically two isotherms are measured during the chemisorption experiment: The first isotherm reflects the sum of physisorption and chemisorption. After that the sample is evacuated to desorb loosely bonded gas molecules (physisorbed measuring gas) from the surface. Repeating the isotherm then only shows the physisorption again due to the blockage of the active sites from the first isotherm. Subtracting the second from the first isotherm only reveals the pure chemisorption. From this isotherm the amount of active sites can be calculated.

Experimental setup volumetric method

Experimental setup volumetric method

2. Dynamic method

2.1. Isotherm (Pulse chemisorption)

An inert gas continuously flows over a solid. A thermal conductivity detector analyses the measuring signal (base line). After that pulses of the measuring gas are added successively into the flow of the inert gas and after each pulse the instrument waits until the signal reaches the base line again. Initially the active material chemisorbs the measuring gas. After a while the actives sites are getting saturated. The measurement is being continued until the thermal conductivity detector shows constant peaks and no measuring gas is chemisorbed any more. This method is called pulse- or titration method.

Chemisorption experiments are often carried out temperature-controlled, please read more at measuring method „temperature-programmed reactions“.

2.2. Temperature-programmed reactions

Non-isothermal measurements are carried out usually through linear heating of a sample and continuous recording the changes of the gas composition. Temperature-controlled reactions can involve desorption (TPD), reduction (TPR), oxidation (TPO) and other relevant reactions for the characterization of catalysts.

Experimental setup dynamic method

Experimental setup dynamic method

Before performing temperature-controlled reactions to characterize catalysts the sample is prepared in-situ. Therefore so-called macros are defined, so that the procedure of the sample preparation is done fully automatically. The further approach is then task-oriented: A TPD experiment starts with the adsorption of active gas on a sample (e.g. by pulse chemisorption) followed by the characterization of the temperature-dependent desorption process. TPR reactions are done using a reducing gas, usually H2, TPO experiments are performed with an oxidizing gas, typically O2.

The experiments are carried out in a gas flow and changes in the gas composition are recorded by the thermal conductivity detector. Before performing an experiment, it has to be considered that not only the gas composition has to change during the reaction, also the thermal conductivity has to change due to ad- and desorbing molecules. In principle gases and vapors can be divided up into two main groups regarding their thermal conductivities:

1. H2, He
2. CO, CO2, Ar, NH3, H2O, Pyridin, N2O etc.

By this classification, the experiments can be easily derived. During a chemisorption reaction a gas/vapor from both group has to be present. Examples:
1. TPR with hydrogen (group 1) needs a carrier gas from group 2, e.g. Argon.
2. TPD of NH3 (group 2) requires a carrier gas from group 1, typically Helium.
3. TPO with Oxygen (group 2) needs a carrier gas from group 1, e.g. Helium.

The advantages of temperature-controlled reactions are not only to determine the active sites of catalysts but also determining on the different strengths of chemically active sites due to the temperature dependence.

Literature and norms

  • DIN 66136

Closed cell content in foams

The determination of closed and open cell content in foams is based on the determination of the samples volume by means of gas pycnometry, which is an analytical method for density and volume analysis described separately on this homepage. When investigating foams, the most common challenge is to determine the amount of vesicular polymer cells completely closed, as these cells determine the insulation capacities of rigid foams commonly used in the thermal insulation of housing.

Analyzer

Measuring principle

The measurement is carried out as stated per DIN ISO 4590 „Determination of the volume fraction of open and closed cells in rigid foams“. Initially, a geometrically exact sample body will be cut (cube, cuboid or cylinder) and its exact dimension will be determined by means of a micrometer in order to calculate the geometric volume. Afterwards, the sample body will be analyzed in the pycnometer at a low pressure of roughly 0.25 bar. The analytical gas employed here is nitrogen, as helium will penetrate into the walls of the closed cells in the foam. This first measurement includes the volume of the sample body yielding the closed volume, into which the analysis gas cannot penetrate and from which the amount of closed cells will be derived. The so-called uncorrected method terminates here, the quotient from closed volume to geometric volume multiplied by 100% determines the percentage of closed cell content in the sample.
In the so-called corrected method relates to the fact that by cutting through the sample cells previously closed will be opened. This can be corrected by an additional measurement. For this, the sample body will be cut into smaller parts and all parts obtained after cutting will be measured in the pycnometer again. The amount and position of cuts as well as the equation for results depend on the geometry of the original sample body and can either be determined by DIN ISO 4590 or by measuring.

Literature

DIN ISO 4590

TAP density and bulk density of powders

Determination of density by means of gas pycnometry

In general terms density is defined as the quotient from mass by volume. Mass can be determined with ease by a scale. The determination of volume is more challenging, usually due to samples having irregular shapes or being powders of varying degree. Additionally, it needs to be noted that volume, and thus density, may be defined differently if pores are included (raw density) or excluded (true / absolute density) into the solid samples volume. The density is based on the solid samples volume excluding the pore volume of porous solids.

Analyzer

Measuring method

Definition of different volume terms

Definition of different volume terms

With a pycnometer (Greek, „gauged vessel”) the amount of a certain medium (liquid or Helium or other analytical gases) displaced by a solid can be determined. Examples for the use of density determinations for finely ground or bulky solids include, but are not limited to, for example the differentiation between solids, quality insurance, determination of open and closed pore volume in foams and determination of so-called vacuolar volume in the quality control of milk powders. These fields illustrate the versatility of gas pycnometry and exceed the limits of liquid pycnometry. The main advantages of the gas pycnometry are:

  • fast
  • precise
  • requires no organic liquids
  • low user expense
  • automatization
Schematic measurement setup

Schematic measurement setup

Literature and norms

  • DIN 66137

TAP density and bulk density of powders

If a powder is loosely poured into a measuring cylinder the bulk density can be determined. Bulk density considers existing pores and the interparticular voids of a loose powder bed. The tap density of powders after defined tapping steps of the powder bed can easily be calculated. Tap volume and density also consist of pores and interparticular voids, which is not based on a loose powder bed but on a bed after a defined number of tapping steps. The method for tap- and bulk density determination requires an easy measuring arrangement but leads to important material properties for storage and transport applications of powdered and bulk samples.

Analyzer

Measuring method

Different oxides were investigated regarding their bulk- and tap density. The powders were loosely filled into 250 mL measuring cylinders and tapped with the BeDensi T series. During 1250 tapping steps the powder is being compacted more and more. From the bulk- and tap volume and the weighed mass, the densities can be calculated.

Sample

mass [g]

Bulk desity
[g/cm3]

Tap volume
[cm3]

Tap desity
[g/cm3]

Compactness factor

1

68,0

0,272

192

0,354

1,30

2

56,1

0,224

180

0,312

1,39

3

116,0

0,464

198

0,586

1,26

4

125,5

0,502

191

0,657

1,31

5

130,8

0,523

181

0,723

1,38

6

111,8

0,447

186

0,601

1,34

7

97,2

0,389

199

0,488

1,26

8

70,8

0,283

181

0,391

1,38

Bulk- as well as tap density is defined as the quotient of mass and volume in g/cm³. Very low tap densities of e.g. pyrogenic oxides are often given in g/L. The tap density gives information regarding packing size and regularity of packing materials.

Measuring instruments for the determination of tap densities are called tap volumeter, such as the BeDensi T series. This is a small devices with one, two or three rotary plates. The cylinder can be used to determine the bulk volume before tapping the sample and therefore gives information how compressible the material is. The tapping number is set and automatically finished after that.
A tap starts with raising the rotary plate automatically including the filled cylinder followed by dropping it, which results in compacting of the powder bed. The volume of the sample decreases during the tapping procedure. Therefore the tap density is smaller than the bulk density.

Literature and norms

  • ISO 697 Bulk desity
  • ISO 787 Tap desity

Film formation MS-DWS

The very first commercial instrument based on MS-DWS-technology, RHEOLASER COATING enables monitoring of microstructure changes during the film formation process. It identifies the drying mechanisms and characteristic drying times on any kind of substrate and any thickness. It works on any kind of film-forming products or coatings, such as inks, paints, varnishes, resins, binders, cosmetic films…

Instruments

Measurement principle

 

Data and parameters

Fluidity Factor

Thanks to the unique and patented A.S.I.I (Adaptive Speckle Image Interferometry) processing, evolution of the “Fluidity Factor” versus time is displayed in real time providing a wide range of information such as:

  • Optical film formation analyser
  • Drying times (open time, touch-dry time, dry-hard time, …)
  • Curing times
  • Microstructure change (particle packing, particle deformation, curing…)

Trocknungsverhalten_Fluidity Factor_Abbildung 1

Characteristic times

Rheolaser COATING gives access to any characteristic time of the drying/curing process:

  • Open-time
  • Dust-free
  • Set-to-touch
  • Dry through

Trocknungsverhalten_Characteristic Times_Abbildung 2

A versatile device

RHEOLASER COATING has been designed as an open configuration to run experiments with automatic coater and/or vacuum bed. It works with any sample thickness, and on any kind of substrate: metal, glass, plastic, wood, concrete, paper, and records temperature and humidity all the time for a perfect traceability…

Abbildung 4

Main Advantages

  • Contact-free method
  • Microstructure analysis
  • Open environment: measurement on any kind of substrate, at any thickness

References and norms

/1/ A Brun, L Brunel and P Snabre, “Adaptive speckle imaging interferometry (ASII): New technology for advanced drying analysis of coatings”, Surface Coatings International Part B: Coatings Transactions Vol 89, B3, 193–268, September 2006

Particle size measurement using multiangle dynamic light scattering

The dynamic light scattering (DLS) is a technique to measure the particle size distribution of nano- and submircosized particles, which is used primarily in pharmaceutical and biochemical industry. On the one hand many application for example in the protein research or analysis of liposomes and micelles cannot be performed by means of other methods. On the other hand it is relatively easy to get an analysis result using DLS, when the hard- and software of the instrument are user-friendly designed.

Instrument

Measuring principle

In particle size measurements using dynamic light scattering (DLS), a laser beam is scattered on very small, finely dispersed particles (usually < 1 µm) in a highly diluted liquid dispersion. The scattered light of each particle will then interfere with each other. Since the particles constantly change locations due to Brownian motion, the position of the scattering centers changes with respect to each other and the interferences lead to small fluctuations in scattering intensity (this explains the name “dynamic” light scattering). The change of the scattered light intensity is measured in relation to the time at a certain fixed angle to the direction of incidence of the beam – usually 90°. This provides information about the velocity of the particles in the dispersion, the diffusion coefficient is determined by an autocorrelation of the raw data, and the particle size (hydrodynamic diameter) is determined according to the Stokes-Einstein relationship.

Measurement principle DLS

Measurement principle DLS

Literature und norms

ISO 13321 – Particle size analysis – Photon correlation spectroscopy

Confined Flow Rheology

Viscosity is an essential property to characterize fluid behaviour at flow. FLUIDICAM is designed to measure flow curves of products with various consistency (liquids, gels or semi-solid emulsions…) by combining microfluidic and imaging technologies.

Instruments

Measurement principle

Measurement principle of FLUIDICAM RHEO

A sample and a viscosity standard are pushed together through a « y-shaped » microfluidic chip at controlled flow rates. Images of the resulting laminar co-flow are acquired via an integrated optical system and the position of the interface is measured. The interface position is related to the viscosity and the flow rate ratios between the sample and the reference. Using dedicated algorithms, sample viscosity is automatically extracted as a function of shear rate and temperature.

How it works

 

Viscosity measurement is now as easy as filling a syringe. Once shear rate ranges and temperatures are selected, software will automatically control and adjust the flow rate and determine the viscosity. Not only it enables one-click experiments, but the technology is also calibration free and extremely precise.
Based on a simple intuitive principle, the interface between the fluids is detected by a video camera. Each measurement point is associated to an image accessible for control during and after the analysis. Thanks to Fluidicam, reliability reaches a new standard…

fluidicam_Detektion_Abbildung 2

Benefits and key features

  • Versatile: Wide range of shear rates (higher than 105 s-1) and wide range of viscosity: 0.1-200,000 mPas
  • Visual Control: Enhance reliability
  • Straight Forward: One click experiment, fast, automated shear rate and temperature screening
  • High precision even at very low viscosity, automatic flow control, Disposable microfluidic chip…

fluidicam_Chip_Abbildung 3

References and norms

Measurement of the electrical conductivity in organic solvents

The conventional instruments are measuring the ohmic resistance of the sample to calculate the electrical conductivity. For this method, the lower limit of this parameter is about 0.5 µS/cm (electrical conductivity of destilled water).
The innovative conductivity probe DT-700 on the other hand enables the analysis of weak polar liquids like alcohols to non-polar solvents like toluene or n-hexane.

Instrument

Measurement principle

The DT-700 probe consists of two coaxial, cylindrical electrodes. During an experiment, a sinusoidal AC voltage is applied between the electrodes and the electric current that flows between both is measured. The frequency of the applied voltage is adjusted automatically between 1 to 10 MHz in dependence of the measured conductivity. This experimental design enables the measurement of the electric current (and conductivity) over several decades. Finally the specific electrical conductivity of the sample is calculated from the ratio of current to voltage and the cell constant of the probe. This last parameter is determined by an easy calibration of the system with toluene.
The figure shows exemplarily the repeated measurements of different liquids with electrical conductivities over the whole measurement range (from 10-11 S/m (n-hexane) to 10‑4 S/m (methanol).
Leitfähigkeit Beispielmessungen

Elektroacoustic: zeta potential measurment in concentrated dispersions

The zeta potential is defined as the electrical potential difference at the shear plan of a moving particle in a liquid medium. Thus, it is a characteristic parameter for the electro-chemical properties of an emulsion droplet or rigid particle in a liquid: it gives some indication on the dispersion stability and the particle mobility in external electric fields. The isoelectric point (IEP), the zero point of the zeta potential is an important value: the particles agglomerate and the dispersion tends to flocculation.

Instruments

DT-310_transp

DT-300:    (zeta potential)
DT-310:    (zeta potential with titration unit)


DT-1202

 

DT-1202: (measurement of particle size and zeta potential)

 

 

Measurement principle

The electroacoustic method is using the measured colloidal vibration current (CVI) to calculate the zeta potential of a dispersion. The figure is showing the DT-310 with its the zeta potential probe (including titration unit, temperature-, pH- and electrical conductivity probe).

Using this method, an ultrasound pulse is directly coupled into the suspension or emulsion, which has to be analysed. Regarding the continuous phase, the colloid particles are displaced to a relative motion due to their inertia, whereby they are moved relatively to the diffuse double layer. This effect creates fluctuating dipole and an electric alternating current (CVI) is generated, which can be measured as a potential between two electrodes. The zeta potential is finally calculated from that parameter
The main advantage of the electroacoustic is the possibility to analyze the zeta potential of measure in high as well as low concentrated dispersions (0,1 – 50 vol.-%). Thus a dilution, which normally leads to a modification of the zeta potential, is not necessary. The instruments of the DT-line can be equipped with several useful options like electrical conductivity probes for aqueous and organic dispersions, special designed measurement setups for concentrated slurries and cements and more.
The following figure is showing exemplarily the zeta potential vs. pH dependence of two 40 wt.-% concentrated ceramic suspensions. Both were measured using a DT-310, the titration experiment was done fully automatic. Both slurries show a different dependence on the pH. From the graph the pH-area, were the dispersions are stable can be read as well as the location of the isoelectric point (IEP).

Zetapotential Beispielmessung

Zeta potential-vs.-pH-value of two Wertverlauf zweier high-built two-component suspensions

References and norms

/1/ ISO 13099-3 Colloidal systems – Methods for zeta potential determination.
/2/ A. Dukhin, P. Goetz: Characterization of Liquids, Nano- and Microparticulates, and Porous Bodies using Ultrasound. 2nd Edition. Oxford: Elsevier, 2010.
/3/ PARTICLE WORLD 19; p. 18 – 19; „Characterization of liquids, dispersions, emulsions and porous materials using ultrasound“

Gas adsorption: pore volume and pore size distribution

Gas adsorption for surface and pore analysis offers solutions for pore characterization between 0.3 nm and approx. 500 nm. The determination of BET surface areas and further methods to characterize pores are described on this website as separate methods. In principle the smallest pores are filled first with gas molecules. With increasing pressure successive pore filling of the larger pores takes place. Based on different evaluation models calculations are done to determine pore volumes or pore size distributions. The advantage of gas sorption lies in performing pore analytics of very small pores (micro- and mesopores).

Measuring method

1. Adsorption isotherm

Summary of isotherms- and hysteresis types of IUPAC Technical Report 2015

Summary of isotherms- and hysteresis types of IUPAC Technical Report 2015

The figure illustrates the classification of isotherm and hysteresis types according to the surface and pore structure of non-porous, micro-, meso- and macroporous materials. Besides BET surface area calculations (see corresponding method on this website) isotherms are used to determine pore volumes (micropore- and total pore volume) as well as pore size distributions. Traditional models are e.g. the BJH method for mesopore analysis, the Gurvich rule for total pore volume calculations or Dubinin equations for micropore analysis. To improve these models different groups around the world develop new calculation models. State-of-the-art models are the so-called DFT (density functional theory) models and Monte-Carlo simulations.
In contrast to mercury porosimetry gas adsorption offers different advantages such as mercury-free and easy handling with measuring cells. The measuring range already starts in the micropore range (approx. 0.3 nm) which is not accessible by mercury porosimetry.

Example
The following figure shows an isotherm of a MCM-41 material measured with Nitrogen at 77 K. The applied evaluation models depends on the type of isotherm and therefore on the kind and ratio of pores. For small mesopores the classical BJH- and modern NLDFT methods were applied. These results show that both methods differ and the real pore size of approx. 4.1 nm could not be calculated exactly by the BJH method.
Due to the large number of calculation methods for adsorption isotherms 3P INSTRUMENTS offers advanced training in terms of surface- and pore characterization analysis to exchange experiences.
During this training various topics are discussed such as sample preparation, measuring and interpretation of different kind of solids by means of concrete examples.

Isotherm of the adsorption and desorption measured by Nitrogen @ 77 K on a MCM-41-material

Isotherm of the adsorption and desorption measured by Nitrogen @ 77 K on a MCM-41-material

2. Gas adsorption at different temperatures

Pore size analysis by gas sorption is usually done in a relative pressure range between 0 and 1 by measuring the isotherm of gas at its boiling point. Due to the costs and availability of liquid Nitrogen, normally Nitrogen isotherms are measured at 77 K. In principle each gas can be used at different temperatures to investigate the sorption behaviour or to discuss the analysis of the pore structure data also in terms of practical separation processes. Following gas sorption methods have been proved:

  • Argon at 87 K for micropore determination according to the IUPAC classifications
  • Krypton at 77 K to determine small BET surface areas.
  • Krypton at 87 K to analyse small mesopores in thin, porous layers
  • CO2 at 273 K to investigate small micropores < 1.5 nm
  • H2, CH4, CO2 etc. at different temperatures to investigate gas storage applications
  • various adsorptives at different measuring temperatures to compare adsorption processes or the validation of substance-specific parameters and interpretation models for pore analysis
  • Isotherms of an adsorptive at different temperatures to calculate adsorption enthalpies (isosteric heats of adsorption)
  • Chemisorption: H2, CO, NH3, pyridine etc. to characterize active surfaces of catalysts
  • practical relevant investigation of gas- and vapor mixtures by dynamic sorption methods

Example
Isotherms of Nitrogen at 77 K, CO2 at 273 K and H2 at 77 K were measured on a zeolite 4A. In comparison to hydrogen at 77 K or CO2 at higher temperature, Nitrogen shows almost no adsorption at 77 K. This example shows that Nitrogen measurements at 77 K are not the appropriate method to analyze micropores smaller than 0.5 nm. Other adsorptives than Nitrogen and temperatures have to be used. Our LabSPA (Lab for Scientific Particle Analysis) performs test and contract analyses of different kind of gases at various ranges of temperature and pressure.

Isotherms measured by Nitrogen @ 77 K, CO2 @ 273 K and H2 @ 77 on a zeolite 4A

Isotherms measured by Nitrogen @ 77 K, CO2 @ 273 K and  H2 @ 77 on a zeolite 4A

Literature and norms

  • ISO 9277
  • IUPAC Technical Report “Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution”; M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F. Rodriguez Reinoso, J. Rouquerol and K.S.W Sing, Pure Appl. Chem. 87, 1051 (2015); pdf-Download
  • DIN 66131 (determination of BET surface areas)
  • DIN 66134 (mesopore analysis with BJH)
  • DIN 66135 (micropore characterization with different evaluation models)
  • PARTICLE WORLD 19; p. 34 – 35; „3P INSTRUMENTS as specialist in the field of adsorption“

Determination of density by means of gas pycnometry

In general terms density is defined as the quotient from mass by volume. Mass can be determined with ease by a scale. The determination of volume is more challenging, usually due to samples having irregular shapes or being powders of varying degree. Additionally, it needs to be noted that volume, and thus density, may be defined differently if pores are included (raw density) or excluded (true / absolute density) into the solid samples volume. The density is based on the solid samples volume excluding the pore volume of porous solids.

Analyzer

Measuring method

Definition of different volume terms

Definition of different volume terms

With a pycnometer (Greek, „gauged vessel”) the amount of a certain medium (liquid or Helium or other analytical gases) displaced by a solid can be determined. Examples for the use of density determinations for finely ground or bulky solids include, but are not limited to, for example the differentiation between solids, quality insurance, determination of open and closed pore volume in foams and determination of so-called vacuolar volume in the quality control of milk powders. These fields illustrate the versatility of gas pycnometry and exceed the limits of liquid pycnometry. The main advantages of the gas pycnometry are:

  • fast
  • precise
  • requires no organic liquids
  • low user expense
  • automatization
Schematic measurement setup

Schematic measurement setup

Literature and norms

  • DIN 66137

High pressure adsorption

Technological solutions for gas storage and gas separation devices of different kinds are becoming crucial for finding methods to solve future environmental problems. This includes gas storage devices for new propulsion methods (H2-storage) as well as possibilities for underground CO2-sequesterisation and machines for separating different gas and vapor mixtures. This processes are usually conducted at conditions highly different from those suitable for standard texture analysis by means of N2-physisorption at 77K. In order to determine adsorption capacities at real conditions, high pressure adsorption generates important results by itself or can be used to put adsorption analysis at lower temperatures and pressures into the correct physical and experimental context.

Analytical methods

1. Static-volumetric

The adsorption of pure gases by high pressure up to 200bar (20MPa) and a wide temperature range is useful to get technical relevant sorption equilibrium data. The following figure shows so-called excess isotherms of methane on activated carbons at different temperatures and at pressures up to 200 bar.

Excess isotherms of Methane on an active carbon for different temperatures

Excess isotherms of Methane on an active carbon for different temperatures

2. Dynamic adsorption (flow method up to 10 bar)

In order to model technical adsorption processes as well as gas mixture adsorption as best as possible, a representative aliquot of the sample and an authentic simulation of the field of flow are required within the reactor column (see figure).

With the mixSorb L a robust stainless steel construction is available to carry out investigations at temperatures up to 450°C and pressures up to 10bar. Up to four integrated mass flow controllers allow for virtually any gas flow and gas composition, including the addition of vapors. The detection of the gas composition is achieved by means of an integrated thermal conductivity detector or an additional mass spectrometer. Reversion of the flow allows substantial investigations with regards to desorption on technical adsorbents, also allowing for the analysis of regenerability and cyclic durability as well as the experimental reconstruction and simulations of complex pressure swing adsorption processes.

Analyzers

 

mixSorb S:

Measurement of breakthrough curves; vapor option; designed for very small sample amounts


 

mixSorb L:

Measurement of breakthrough curves; vapor option; safe and easy-to-use bench-top instrument


 

mixSorb SHP:

Measurement of breakthrough curves; vapor option; designed for very small sample amounts and high system pressures (up to 68 bar)

Literature and norms

dynamicsorption.com – detailed presentation of features, advantages, scientific background and examples of dynamic sorption methods (flow-methods)

  • PARTICLE WORLD 19; p. 20 – 25, “From the idea to the technology behind the separation process:
    mixSorb L is gearing up!”
  • Dynamic and equilibrium-based investigations of CO2-removalfrom CH4-rich gas mixtures on microporous adsorbents; A. Möller, R. Eschrich, C. Reichenbach, J. Guderian, M. Lange, J. Möllmer: Adsorption (2017) 23: 197-209; external link to pdf view

Particle-size and shape analysis by means of image analysis

Most particle size measurement methods are based on the assumption of spherical shaped particles. This hypothesis leads to significant errors in the analysis if the particles are flake or rod-shaped. Especially for such highly form-anisotropic particles, image analysis methods provides an excellent alternative for the determination of tailor-made size specifications.

Image analysis methods for the determination of the particle size distribution of a material offers a fundamental advantage over alternative methods such as static light scattering, sedimentation or fractionation (screening): Each particle is photographed individually! This results in several important advantages for the determination of the particle size distribution:

  • Realistic proportional values also at the edges of the size distribution, i. e. detection of oversized particles or fine particles
  • Visual assessment of the dispersing state of a sample (dispersing quality, agglomerates present)
  • Calculation of meaningful size parameters, e. g. geodetic length or Feret diameter for fibres, depending on the application
  • Selection of the appropriate distribution type (volume, number) depending on the particular task

In addition, the individual photography of the particles gives the opportunity to make statistical calculations on the particle shape, which in practice enables further differentiation of materials. For example, form anisotropy, the deviation of the particles from the ideal sphere, often plays a decisive role for their application and further processing – for example, the conveyance or compaction of powders, the influence on the rheology in dispersions or, in addition to the particle size distribution, the roughness of the particle surface plays an important role for the success of shaping or polishing.

The necessity for tailor-made particle size and shape parameters, combined with ever-increasing PC processing power, ensures that image analysis methods are becoming increasingly established on the market.

Instruments

Measurement method

The determination of the particle shape by optical image analysis includes 4 basic steps:

1. Image taking
2. Image processing
3. Object detection
4. Classification

Processing chain

The image taking is ensured by special digital cameras, if necessary in combination to a microscope, to enlarge the particles. The particles may be present neutral (e.g. on an objective) or in motion as well. The dispersing (separation) of particles is possible both in dry-mode (e.g. by simple conveying and riddling or by the usage of compressed air) but also in wet-mode in a solvent. An absolutely basic requirement to perform a successful particle shape analysis is good quality (high resolution and image sharpness, good sample dispersing resulting in individual particles, suitable enlargement etc.). Image processing by appropriate software leads to upgraded pictures: for example isolated pixels and edging particles are eliminated, variations in brightness and signal noise are retouched and connected particles are separated. The main part in object detection is image binarization, whereby every image pixel is assigned to a particle (black) or the background (white) using a threshold. The recognition of objects (particles) and feature attribution is realized by the software. In the last step, the classification, the particles are arranged in classes (e.g. size equivalent classes) on the basis of their attributed features (size and shape parameters).

Numerous size and shape parameters can be determined from the particles images by the appropriate software. Important size parameters are for example (CE) equivalent disc diameter Deq, maximum inscribed disc diameter Din, fibre length XLG (geodesic length) and fibre diameter XFD.

The equivalent disc diameter corresponds to the disc diameter of identical area to 2-D-projected particles, which is often used as size indicator for irregular shaped particles in process technology. On the contrary the maximum inscribed disc diameter of the 2-D-projected particle corresponds more or less to the sieve diameter. The geodesic length and the fibre diameter are suited very well for the characterization of fibres.

Important size parameters image analysis

Important size parameters image analysis

There are numerous and very application-specific shape parameters. The aim is to get extra morphological parameters in addition to particle size, whereby the particle characteristics can be better or basically described. Examples are “aspect ratio AD”, the ratio of length to width of the particles, “circularity ZK”, an indicator for particle deviation from the ideal circle and “concavity index C”, which reflects the ratio of area difference of convex envelope and area of the particle to convex envelope. Another important shape parameter is “perimeter”, which displays the particle coverage.

Important shape parameters image analysis

Literature and norms

/1/ ISO 13322-2: Particle size analysis – Image analysis methods – Part 2: Dynamic image analysis methods

/2/ ISO 9276-6 Representation of results of particle size analysis – Part 6: Descriptive and quantitative representation of particle shape and morphology

Particle size measurement using static light scattering

The particle size distribution as a parameter to specify a powder or dispersion plays a central role in many applications. Examples are construction material (sands, cements), lime stones, ceramics, colored pigments, fertilizers, emulsions and may more. The range of applications is increasing permanently and hence the requirements on the measurement methods regarding size range, measuring time and reproducibility. Particularly the precise and reproducible detection of particles with sizes close to the measuring range limits as well as the simultaneous determination of particle sizes of very small particles (nanometer range) as well as large particles (lower millimeter range) for the characterization of polymodally or very broadly distributed samples provides a challenge. State-of-the-art laser diffraction devices such as the Bettersizer S3 Plus solve these tasks by an innovative design of the optical bench for the detection of backscattered light of very small particles and by detecting large particles by an integrated high-speed CCD camera or the combination of static light scattering and dynamic image analysis.

Instruments

Bettersizer S3 Plus

Bettersizer S3 Plus: 
Particle size and particle shape 0.01 – 3500 µm
Bettersizer S3:
Particle size 0.01 – 3500 µm

Bettersizer 2600:
Particle size 0.02 – 2600 µm

Measuring method

In static light scattering laser light (monochromatic, coherent light) interacts with the particles which have to be characterised in terms of particle size. In dependence of the particles’ size, the light waves are scattered by the particles in a characteristic manner: the larger the particles are, the greater is the scattering in forward direction. With particles smaller about 100 nm, the scattering intensity is nearly identical in all directions.

Laser diffraction at particles with different size

Laser diffraction at particles with different size

The scattering intensity is determined by stationary detectors depending on the angle (light scattering intensity distribution). State-of-the-art laser diffraction systems such as the Bettersizer S3 Plus guarantee the determination of scattering intensities in a continuous angular range of 0.02 – 165°, i. e. in forward, side and backward direction. This is achieved by means of a so-called double lens design and oblique incidence optical system (DLOIS technology): Fourier lenses (collective lens) are positioned between the laser and particles as well as between particles and detectors. The particles will interact with the light within a parallel laser beam. This offers the advantage that the scattered light can also be detected at very large angles (in backward scattering direction) and thus even very small particles can be measured precisely. Thanks to DLOIS technology, the problems of conventional measurement setups can also be avoided. Therefore, neither the suitable lenses for the corresponding particle size measurement range have to be selected prior to the measurement (in comparison to the Fourier optics), nor do measurement inaccuracies result from different particle to detector distances, if not all particles lie in one plane (in comparison to the inverse Fourier optics).

Schematic drawing of the innovative DLOIS-technique of Bettersizer S3 PLUS and CCD-camera system (x0.5 and x10)

To calculate the particle size distribution from the measured scattering spectra, the theory of either FRAUNHOFER or MIE is applied. The FRAUNHOFER theory is based on the hypothesis of opaque and spherical particles: the scattered pattern corresponds to a thin opaque two-dimensional plate – diffraction only occurs at the edges. Therefore no additional optical input constants of the material are necessary for this calculation. However, this theory is only suitable for mean particle sizes from approx. 5 µm.

In contrast the MIE theory uses the hypothesis of virtually translucent and spherical particles, meaning that the light permeates the matter and is scattered elastically at the atoms of the particle. The knowledge of the complex refractive index of the particles and the liquid as well is necessary. This theory is applicable for particles of all sizes.

The following figure shows an example of a volume-related particle size distribution of a calcium carbonate powder – measured with a Bettersizer S3 Plus.

Laser diffraction measurement example

The cumulative throughput curve Q3 (blue) and the resulting histogram (q3, black bar) can be seen.

Literature and norms

ISO 13320 – Particle size analysis – Laser diffraction methods

Mercury porosimetry: Pore volume and pore size distribution

Pore analysis forms the base for the evaluation of many materials such as catalysts, construction materials, sediments, paper or porous plastics and ceramic green bodies. Mercury porosimetry is capable of analyzing a very broad range of pore sizes reaching from 3 nm up to 950 µm. This covers the macropore region as well as almost the complete region of mesopores as well. Pore volume is measured by recording the amount of mercury intruded into the pore system while the pore size distribution is derived from the pressure dependence of filled pore sizes.

Analytical method

Mercury (a non-wetting fluid) is intruded into the pores of a solid by pressure leading to the initial filling of larger pores while increasing pressure leads to the filling of smaller and smaller pores. This dependence of pore size and pressure is traditionally described by the Washburn equation. The so-called intrusion curves allow the calculation of pore size distributions. Further information such as surface area and raw density can also be obtained.

While porometry will only characterize through-pores and thus only generate information for filters and related materials, porosimetry will acquire all pores open to ambient in any way (with minimum pore apertures of > 3nm). The biggest asset of this method is the very large range of pore sizes.

Example

The example shown below displays the mercury intrusion curve of mixtures, e.g. silica gel (powdered SiO2) mixed with alumina oxide (granules Al2O3). Silica gel amounts are 0, 33, 50, 75 and 100 % mass percent. Pressure recorded on the x-axis rises from left to right, meaning the process of pore filling is also displayed from left to right. The analysis distinctly shows three rises in intrusion: 100 – 10 µm, 0.04 – 0.01 µm and 0.01 – 0.008 µm respectively. With regards to the particle size distribution of silica gel this data is interpreted as follows: The initial rise on the left is a result of the filling of interparticular voids. The next rise can be attributed to the filling of pores within the silica gel with the top curve corresponding to pure silica gel. The rise on the right corresponding to the smallest pores in the intrusion process is alumina oxide with the lowest curve representing pure alumina oxide.

Intrusion curves on SiO2 and Al2O3 in different mass ratio

Intrusion curves on SiO2 and Al2O3 in different mass ratio

Literature and Norms

  • DIN 66133

Adsorption of mixture: Dynamic sorption of gas and vapor mixtures

Challenges such as the adsorption of CO2 from dry and moist air, the adsorption of methane from biogas or the differentiation of the relevant physisorption behavior of adsorbents in gas and vapor mixtures are different from classical methods for texture determination. The main reason here is the fact, that a sorption of mixtures is occurring and the mixture needs to be separated accordingly in order to discover, which component is adsorbed at which magnitude.

Especially the selectivity plays a crucial role in the adsorption of mixtures, since the task requires the stronger adsorption of one component within the mixture in comparison to other residual components. In order to predict or model technical processes and extract data with practical relevance, the following investigations become more and more important:

  • dynamic adsorption and desorption from a gas flow
  • determination and evaluation of breakthrough curves
  • investigation of sorption kinetics
  • investigation of co-adsorption and displacement effects
  • determination of sorption selectivity
  • determination of sorption equilibria in gas and vapor mixtures
  • scaling of technical sorption processes
  • investigation of heat balance in dynamic adsorption processes

Analyzers

 

mixSorb S:

Measurement of breakthrough curves; vapor option; designed for very small sample amounts


 

mixSorb L:

Measurement of breakthrough curves; vapor option; safe and easy-to-use bench-top instrument


 

mixSorb SHP:

Measurement of breakthrough curves; vapor option; designed for very small sample amounts and high system pressures (up to 68 bar)

Analytical method

In order to minutely project technical adsorption processes, a representative selection of sample as well as an authentic representation of the flow field is required within the reactor bed.

The standard column of the mixSorb L with a volume of roughly 100ml and an internal diameter of 3cm has been modelled precisely towards these characteristics. The robust construction from stainless steel allows for experiments at temperatures up to 450°C and pressures of up to 10bar. Four Pt-100 thermo-couples positioned equidistantly along the reactors columns central vertical axis and allows for high resolution recording of temperature profiles within the reactor bed. Up to four internal mass-flow controllers allow for the arbitrary composition as well as streaming speed of gas mixtures. The detection and composition of gases and mixtures is carried out by an internal thermal conductivity detector or an additional mass spectrometer. Reversion of the flow direction within the column allows for in-detail studies of desorption processes in technical adsorbents. This allows for the investigation of regenerative capabilities as well as cyclic durability of technical adsorbents while at the same time allowing for the simulation and investigation of complex pressure swing adsorption processes (PSA).

Example

The following scheme exemplifies the possibilities of the mixSorb L for investigating the characteristics of practical relevance within the separation of air on a carbon-based molecular sieve. The influence of temperature on the cycling time of the adsorber as well as the breakthrough speed of oxygen was investigated. These methods require the mixSorb-software mixSorb Manager, which handles dosing, measurement and data recording and reduction fully automated.

Example of breakthrough curves on carbon molecular sieve

Furthermore, the mixSorb L comes with the simulation software 3P-Sim, which is an extraordinary tool for comparing experimental and theoretical data and calculating the experimental effort required for certain tasks by calculating gas mixture data from pure gas isotherms. The figure below showcases how diffusion parameters are merged into the linear driving force (LDF-) factor and below that, how fitting of the mass transfer coefficient kLDF describes experimental curves (both breakthrough and temperature curve) in the software simulation.

Mass Transfer coefficient kLDF
Fitting of breakthrough curves and temperature

Literature and norms

dynamicsorption.com – detailed presentation of features, advantages, scientific background and examples of dynamic sorption methods (flow-methods)

  • Webinar mixed gas/vapor adsorption: Working with Vapors and Low Concentrations in Breakthrough Experiments (Video on Demand)
  • PARTICLE WORLD 19; p. 20 – 25, “From the idea to the technology behind the separation process:
    mixSorb L is gearing up!”
  • Dynamic and equilibrium-based investigations of CO2-removalfrom CH4-rich gas mixtures on microporous adsorbents; A. Möller, R. Eschrich, C. Reichenbach, J. Guderian, M. Lange, J. Möllmer: Adsorption (2017) 23: 197-209; external link to pdf view
  • conference poster 2017 – Deutsche Zeolithtagung, “Porous solids for heat storage applications: In-depth material testing by vapor breakthrough measurements”, pdf-Download
  • conference poster 2016 – Fundamentals of Adsorption – “Dynamic and equilibrium-based investigations of CO2 removal from CH4-rich gas mixtures on zeolites”, pdf-Download
  • conference poster 2016 – Reaktionstechniktagung – “Dynamische Untersuchungen zur Adsorption an Aktivkohlen”, pdf-Download
  • conference poster 2016 – Fachgruppe Adsorption und Gasreinigung – “dynaSim – A Modeling and Evaluation Tool for Dynamic Sorption Data” pdf-Download
  • conference poster 2015 – Carbon – “Breakthrough Curves of CO2 and CH4 on Carbon Molecular Sieves”, pdf-Download

Measurement of the permittivity of pure liquids or solvent-mixtures

The relative permittivity (also dielectric constant) is a measure of resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. Thus, this parameter is important for the comprehensive electrochemical characterization of liquid dispersions. The exact calculation of the zeta potential for instance requires the relative permittivity of the medium. Especially for solvent-mixtures, this parameter cannot be calculated mathematically and must be measured.

Instrument

 

Permittivity-Probe

 

Measurement principle

A sinusoidal AC voltage (10 kHz) is applied between the cylindrical electrodes of the probe 871 in order to measure the permittivity of a liquid. The amplitude is about 7 V (measurement range 1 – 20) or 0,7 V (measurement range 1 – 200) respectively. The relative permittivity of the liquid is calculated from the measured current between the outer and inner electrode and will be shown directly on the instrument display. The instrument is calibrated easily by means of a solvent with a known permittivity.
A comparison of both methods – permittivity and measurement of the electric conductivity of organic solvents shows just a difference in the applied AC voltage (factor 1000). In case of high frequencies, the electrical current flows through shift charged particles (conductivity) while it flows due to polarization of molecules at high frequencies.

Phase transition / Crystallisation

Crystalline phase transitions of heterogeneous products can be detected with a new and innovative optical method based on Diffusing Wave Spectroscopy (DWS). This technique combines a non-destructive, non-invasive measurement with sufficiently large volumes to overcome problems of heterogeneities. This enables to measure finished products with high level of heterogeneity, such as food (chocolate, butter…) or cosmetics, and determine the transition temperatures of polymers, waxes or other fats.

Instrument

rheolaser Crystal re

 

RHEOLASER CRYSTAL

 

 

Measurement principle

Crystal_Messprinzip_Abbildung 1
RHEOLASER Crystal uses the DWS principle. Light is scattered by the particles, creating an interference pattern (Speckle Image). The variation of this image is related to the motion of the particles. By a mathematical analysis of this variation, decorrelation functions can be computed and then processed, to obtain a characteristic time τ as a function of time or temperature.

How it works – Diffusing Wave Spectroscopy (DWS)

 

Values of 1/τ or Micro-Dynamics (Hz), are then plotted against time or temperature, resulting in characteristic peaks when the product shows a microstructural evolution, such as a phase transition or any other physical event. The signal can then be integrated for an easier visualisation, obtaining the so-called Micro-Dynamics Evolution (%).

Crystal_Messprinzip_Abbildung 2

Benefits and key features

  • Monitor any physical phenomenon
    Micro-Dynamics corresponds to the speed of change at the microstructure scale in the sample.
    When there is a transition (phase transition, such as melting or crystallization, or polymorphic transition, from a crystal form to another), microstructure will move very quickly because of restructuring. This is observed by the apparition of characteristic peaks.
  • T50 & ∆T, T10 & T90
    T50 is the average transition temperature (temperature for which half the change happened). ∆T is the transition range, it represents the “polydispersity” of the microstructure. T10 & T90 are used to define the beginning and ending of the transition phenomena. They correspond to the temperatures for which 10 and 90% of the transition happened.
  • Macroscopic samples
    Thanks to the innovative measurement principle, it is possible to measure “macroscopic” samples (up to 5g), enabling to measure directly finished products, without any sample preparation or denaturation.

Crystal_Abbildung 3Crystal_Abbildung 4Crystal_Abbildung 5

 

 

 

 

 

 

 

 

References and norms

/1/ PARTICLE WORLD 19; p. 12 – 13, “Heat resistance of lipid based ointments”

Porometry: Analysis of through-pores in filters and membranes

The Capillary Flow Porometry is used for R&D and quality control in industries worldwide such as filtration, nonwovens, pharmaceutical, fuel cell, water purification and battery. Samples often tested include filter media, membranes paper, powders, ceramics, battery separators and health care products.

It measures the diameter of the most constricted part of a through pore (pore throat, largest pore diameter, Mean flow pore diameter, Pore diameter range, Pore distribution, Gas Permeability.

Analyzers

iPore Porometer

capillary flow porometer series

Analytical method

1. Capillary flow porometry

The standard capillary flow porometry is well known and very useful to characterize the pore structure of materials such as membranes, filter media, ceramics, paper, textile and similar materials.  A non-toxic liquid is allowed to spontaneously fill the pores in the sample and a non-reacting gas is allowed to displace the liquid from pores by increasing the gas pressure. First the larger pores will get emptied, as the pressure increases more and more smaller pores are progressively emptied. The pressure and flow rate of gas through the emptied pores provides the through pore distribution and the first detectable flow pressure defines the so called bubble point, which is related to the maximum pore size in a sample.

Typically curve progression of a capillary flow porometry measurement

2. Liquid-liquid porometry (LLP)

However for many materials having very small pores (< 20nm) or for materials which doesn’t withstand relatively high pressures, liquid-liquid porometry (LLP) might be the better choice. It is a valuable technique for measuring the pore structure characteristics of ultrafiltration membranes. Such membranes can act as barriers to particles, including bacteria, pollens, spores, or pesticides. Further LLP is capable of measuring pore diameters, pore size distribution and liquid flow rates of materials having very low permeability. Typical examples are reverse osmosis membranes, nanofiltration membranes, blood purification membranes or battery separators. Very low liquid permeability is measured fully automated for pore diameters down to 3 nanometers and the needed pressures are much less than those for capillary flow porometer.

Measurement principle

capillary flow porometry

Liquid-liquid porometry (LLP)

A wetting liquid spontaneously fills the pores of the material. Figure above shows the main difference between the conventional capillary flow porometry and the liquid-liquid porometry. In case of LLP, two immiscible wetting liquids are selected. Liquid 1 with lower surface tension is used to fill the pores of the sample. Liquid 2 is added to the top of the sample and is pressurized to displace the Liquid 1 from the pores and flow through the empty pores. The flow rate of Liquid 2 is also measured without wetting the sample with Liquid 1. The pore diameter is related to the surface tension of the two liquids. The flow rates yield pore distribution and liquid permeability. The working Equation for the LLP-method is:

D = 4 γ1 cos θ1 / p

with:
pore diameter → D
interfacial surface tension of liquids → γ1
contact angle of liquid  1 → θ1

A good choice for the Liquid-Liquid porometers from PMI is to take the immiscible and saturated wetting liquids such as silwick and alcohol. The pores of the material are filled with silwick as Liquid 1, and alcohol as Liquid 2 is pressurized to displace the silwick and flow through the pores. The amount of liquid flowing out is measured in balance. Alcohol flow rate and differential pressure are measured. Because surface tension of silwick and alcohol are low, contact angles are taken as zero. Mean flow pore diameter and pore distribution are computed like standard capillary flow techniques.

Literature and Norms

  • ASTM D6767
  • ASTM E128
  • ASTM F316
  • DIN ISO 4003:1990
  • DIN 58355-2:2005
  • ISO 2942
  • PARTICLE WORLD 19; p. 36 – 37; „Now Available: Liquid-Liquid-Porometry for materials with very small through-pores“

Mikrorheology MS-DWS

Microrheology enables the measurement of the evolution of viscosity and elasticity, in bulk samples, without any mechanical stress. The measurement is performed at rest, allowing to monitor sample evolution, such as gelation, rheology ageing, or samples stability. The measurement is performed in a closed glass cell, preventing any evaporation or drying, and making it safe to operate at all time.

Instrument

Measurement principle

MSDWS Prinzip Abbildung 1
The microrheology analysers of the RHEOLASER-range use MS- DWS (Multi-Speckle Diffusing Wave Spectroscopy) principle of measurement. It corresponds to Dynamic Light Scattering extended to concentrated dispersions. It measures the particles Brownian motion, which depends on the viscoelastic structure of the sample. This technique consists in sending a coherent laser beam into the sample, leading to interfering waves which create a speckle pattern captured with a video camera detector. The variations of this speckle image are directly linked to the particles Brownian motion, their speed and the distance they explore.

How it works – MS-DWS (Multi-Speckle Diffusing Wave Spectroscopy)

Benefits and key features

The Mean Square Displacement is the average distance travelled by the particles in the media (unit: nm2). This value grows linearly with time in a purely Newtonian sample, while there is a plateau in a visco-elastic fluid.
When the plateau is getting lower (shorter distance), the elasticity in the product is higher (tighter network), while if the curves get longer (longer times), the viscosity in the product is higher.
RHEOLASER MASTER enables to measure MSD curves as a function of time or temperature, allowing to monitor stability, or gelling process…

Rheolaser_MSD_Abbildung 2
Acquisition of particles MSD as a function of the gel variable enables the monitoring of any sol-gel process. A rescaling process (Time Cure Superposition) can then be applied to determine gel point and gel strength with a great accuracy.


This enables to monitor any kind of gelling process, no matter the gel variable:

  • time
  • temperature
  • pH
  • concentration of polymer, salt or additive

Solid-Liquid-Balance (SLB):

ratio between the solid-like and the liquid-like behaviour of the studied sample. Monitor properties such as: adhesion, spreadability, gel point, shape stability, physical stability, etc…

Elasticity Index:

Elasticity strength in the studied sample. Monitor properties such as: mesh/pore size, hardness, recovery, gelation, etc…

Macroscopic Viscosity Index:

quantify and compare the macroscopic viscosity at zero-shear. Monitor properties such as: effect of a thickening agent, texture, flowability, long-term stability, etc…

Rheolaser Parameter Software Abbildung 5

Rheolaser Parameter Prinzip Abbildung 4

Main Advantages

  • Measurement at rest, non-invasive and non-destructive
  • One-click experiment setup and results
  • Kinetic or aging analysis on the very same sample
  • Hazardous samples can be analysed in a closed glass cell

References and norms

/1/ Partikelwelt 16, S. 3-6 “Mikrorheologische Untersuchungen von Gelierungsprozessen nach dem Time-Cure-Superposition-Verfahren”

Sample preparation

In preparation for an analysis, it may be necessary to carry out sample selection, sample splitting and a sample preparation in order to achieve accurate and reproducible results. Procedures for sample preparation may require drying, grinding, dispersion or other procedures that will prepare the sample into a state for achieving high reproducibility of results. Reliable results with regards towards specific surface areas and pore size distributions are strongly dependent on sample preparation and creating an empty surface area and voided pore systems.

While surface and pore analysis requires several hours of sample preparation, dispersions for particle size analysis typically requires only a few minutes.

Method

1. External sample preparation station with heating and vacuum
Preparation of samples for surface area and pore analysis unifies two types of treatment into one device – temperature treatment and vacuum degassing. Some cost-efficient models can prepare different samples at the same temperature. The more expensive versions of these models also allow for different temperatures on single activation stations as well as programming individual heating ramps in order to accommodate for more complex and challenging samples.

2. External ultrasonic treatment and other dispersion techniques
Our LabSPA (Laboratory for scientific particle analysis) employs different models of shakers and dispersers. We will happily assist you with our knowledge and experience, if you direct your challenges to us.
For Bettersizer laser diffraction we recommend standard dispersion solutions on one hand and on the other hand special dispersion configurations for more challenging samples. Should a sample require higher ultrasonic powers, we offer the HD 2200 ultrasonic system. This external system can be integrated into the Bettersizer by means of a special adapted.

Stability – Turbiscan – Multiple Light Scattering (MLS)

 Stability and shelf-life are key parameters in formulation studies. TURBISCAN is the first patented technology to analyse destabilization mechanisms in concentrated media. Creaming, sedimentation, agglomeration, aggregation and coalescence are detected at a very early stage without dilution nor stress. Stability kinetics and index are provided for an efficient sample analysis.

Instruments

Measurement principle

Stabilität_MLS_Abbildung 1

Multiple light scattering (MLS) consists of sending photons (NIR light source, 880nm) into the sample. These photons, after being scattered many times by the particles (or droplets) in the dispersion emerge from the sample and are detected by the 2 detectors of the TURBISCAN™ reading head:

  • Transmission (T) for non-opaque samples (0° from light source)
  • Backscattering (BS) for opaque samples (135° from the light source)

Backscattering is directly related to the photon transport mean free path. Thus Backscattering intensity depends on particle size and concentration.

Stability – Turbiscan – Multiple Light Scattering (MLS)

 

The combination of Backscattering and Transmission sensors with a vertical scanner enables to detect physical heterogeneities (size increase or local concentration change) over the whole sample height with a vertical resolution up to 20µm. Thus, nascent destabilization phenomenon can be detected in any sample locations up to 200 times faster than visual tests.

Backscattering or Transmission profiles (i.e. signal over sample height) are recorded at different time intervals to report kinetic stability. Stability kinetics are calculated over the whole sample height for a global stability assessment or on specific zone (bottom / middle / top) depending on the stability criteria. TURBISCAN Stability Index (TSI) is calculated for easy and accurate stability reporting. Patented sample positioning enables to guaranty an optimal repeatability and reproducibility.

Benefits and key features

Migration Analysis

Local variation of light intensity corresponds to phases formation.

  • Phase thickness kinetics
  • Sedimentation rate
  • Creaming rate
  • Particles migration speed
  • Hydrodynamic diameter

Stabilität_migration_Abbildung 2Stabilität_migration_Abbildung 3Stabilität_migrations_Abbildung 4

Size variation Analysis

Global variation of light intensity corresponds to size increase.

  • Size kinetics
  • Mean diameter
  • Agglomeration rate
  • Coalescence rate
  • Dispersibility ratio

Stabilität_size_variation_Abbildung 5Stabilität_size_variation_Abbildung 6Stabilität_size_variation_Abbildung 7

Turbiscan Stability Index (TSI)

Based directly on the raw data, this unique number takes all destabilisation into account, providing you with a powerful tool to rank & compare all your formulas in just one-click. It is a key parameter for easy classification of samples regarding to their stability. Determining shelf-life of your products has never been easier!

Stabilität_tsi_Abbildung 8Stabilität_tsi_Abbildung 9

No Mechanical Stress

When it comes to shelf-life, and real-life evolution of the end-products, it makes no sense to apply a mechanical stress to a sample. Our measurements are always performed at rest, without any external stress. That way, you can be sure that the results you get are representative of what will happen in real life, only detected way earlier!

Stabilität_Abbildug 12Stabilität_Abbildung 10Stabilität_Abbildung 11

Main Advantages

  • No Sample preparation
  • 200x faster than conventional tests
  • Quantification of stability
  • No Sample dilution
  • Wide range of concentration (10-4 – 95%)
  • Wide range of size (10 nm – 1,000 µm)

References and norms

/1/ DIN ISO 18748
/2/ Partikelwelt 18, S. 16-19 “Vorhersage der Langzeitstabilität von polymerstabilisierten O/W-Emulsionen mittels statischer Mehrfachlichtstreuung”

Vapor sorption: DVS (dynamic) and SVS (static-volumetric)

The determination of water uptake and -release is highly important for a lot of products for their application, their behavior in processes or storage issues. Water uptake can be determined as a function of relative humidity. In other projects water or other vapors are used as sensors to investigate the interaction between molecules with different polarities and surfaces and compare these interactions with other molecules. Due to practical reasons the properties of porous materials under humid conditions have to be well known. Different prospects are possible to investigate the sorption behavior of gas-vapor or vapor-vapor mixtures.

Measuring method

1. Dynamic vapor sorption – DVS

In principle water vapor sorption follows the same physical rules as gas sorption. The difference to gas sorption is that condensation of vapors must not influence the results of the measurement. Because of this reason the water uptake is often measured gravimetrically in a dynamic, humid gas flow (DVS – Dynamic Vapor Sorption) – these experiments can be carried out for many materials, such as pharmaceutical products, food, packing materials, building materials etc.

Typical measurement of gravimetric water sorption experiments

Typical measurement of gravimetric water sorption experiments

2. Static-volumetric vapor sorption

Water vapor sorption can also be measured static-volumetrically in a measuring system with calibrated volume by measuring the equilibrium pressure. Corresponding analyzers exhibits a heated volume system, so that condensation in the manifold, valves or lines is avoided. The liquid is stored in a glass-vessel, which is connected to the manifold with a valve/line-system. First the glass-vessel is evacuated to remove the air then the desired vapor phase is build up above the liquid phase. Dosing of this vapor into the measuring cell removes vapor from the manifold, more liquid from the glass-vessel is evaporated until the equilibrium state between liquid and gas phase is reached again.
Determination of the water sorption isotherm (ad- and desorption) are carried out fully automatically and different possibilities are offered for the analysis of isotherms: Determination of surface areas, pore volumes or heats of adsorption.

3. Mixture adsorption of vapors

Another possibility is the determination of the sorption behavior of vapors in gas-vapor or vapor-vapor mixtures with the mixSorb S, mixSorb L and mixSorb SHP. Practical questions can be directly studied, e.g. investigation of the sorption behavior of humid adsorbents in a dry gas flow or the sorption behavior of adsorbents under a humid gas-vapor flow. There is a huge difference between these two cases and both experiments can easily be prepared. Such investigations can be done flexibly with the mixSorb series including the coupling to external analytical devices to determine e.g. trace elements.

Analyzers

 

mixSorb S:

Measurement of breakthrough curves; vapor option; designed for very small sample amounts


 

mixSorb L:

Measurement of breakthrough curves; vapor option; safe and easy-to-use bench-top instrument


 

mixSorb SHP:

Measurement of breakthrough curves; vapor option; designed for very small sample amounts and high system pressures (up to 68 bar)

Literature and norms

Contact

Contact

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