The particle size distribution of the disperse phase is a key parameter for assessing the process or further processing properties of liquid dispersions. In principle, a variety of different analytical methods are available to determine this parameter.
Dynamic light scattering (DLS) or photon correlation spectroscopy (PCS) is a very common method for measuring the particle size of suspensions and emulsions, particularly in the nano and sub-micrometer range. The method is mainly used in biochemistry and technology, pharmacy and the food industry. This method is particularly suitable if only very small sample quantities are available or the particle concentrations is extremely low.
DLS-measurement principle
The principle of DLS – realized in the measuring devices of the BeNano series – is shown in the following illustration:
The particles in the dispersion are in a constant Brownian motion. Under certain assumptions (e.g. spherical particles) and with knowledge of other parameters (e.g. viscosity of the medium), this can be described using the Stokes-Einstein equation via the parameter diffusion coefficient DT:
DT = kBT/3πηDH
Where kB is the Boltzmann constant, T is the (absolute) temperature, η is the viscosity of the dispersing medium and DH is the hydrodynamic diameter of the particle.
Dynamic light scattering (DLS) is an optical method for calculating the hydrodynamic diameter DH of dispersed particles by indirectly measuring the diffusion coefficient DT using the Stokes-Einstein equation. For this purpose, the dispersion, which is located in a transparent stationary cuvette, is illuminated with a monochromatic light source (laser), the light is elastically scattered by the particles and continuously detected at a certain angle – in the BeNano series either 90° or 173° relative to the incident radiation. As the particles move continuously, the intensity of the detected scattering intensity I(t) fluctuates as a function of time due to interference of the scattered radiation caused by the permanent change of location of the scattering centers, i.e. an analysis of the function I(t) provides information about the mobility of the scattering particles.
A so-called correlation analysis is carried out for the evaluation in the BeNano series: In principle, starting from the intensity I(t) at a randomly selected time t, the intensities I(t + τ), I(t + 2τ),… are determined and autocorrelated with the “original intensity” I(t), i.e. multiplied with each other and added up, and normalized. The resulting function G(2)(τ) is called the correlation function, τ is the so-called correlation time. Due to the fluctuation in the sign of the intensity, all correlation functions fall continuously as a function of τ until they reach zero. Large particles, which move more slowly, show a slower decay behavior than small ones.
The autocorrelation function determined in this way is now fitted using a suitable model function in order to determine the diffusion coefficient of the particles in dispersion and from this the hydrodynamic diameter and its distribution using the Stokes-Einstein equation.
A simple model is the “cummulant method”: This analysis yields a mean decay rate of the autocorrelation curve and its distribution width. The mean decay rate gives the mean diffusion coefficient and thus the hydrodynamic diameter DH, and the distribution width gives the polydispersity index PI, which is a measure of the distribution width of DH.
In addition to the cummulant method, the so-called “Contin algorithm” and the “NNLS method” are implemented in the BeNano series. In contrast to the cumulant method, both models can also be used for multi-modal distributions.
DLS backscatter technology
In classic DLS, the scattered light is detected at an angle of 90°. In addition to avoiding edge effects with rectangular standard measuring cells, this setup offers good statistics by shifting the detection zone to the center of the cell due to relatively long diffusion paths of the light through the dispersion.
However, this measurement setup has various disadvantages:
The DLS backscattering technology available in the BeNano series provides a solution to these problems:
In this setup, the scattered light from the particles is detected at a scattering angle of 173°. A first advantage is the resulting significant increase in the cross-section of the detection zone, which leads to a significantly better signal-to-noise ratio and thus a higher detection limit for samples with low scattering intensity (e.g. low particle concentration). In addition, the sensitivity for the detection of small particles is increased by the backscatter detection angle compared to coarser particles.
The variable distance of the focusing lens of the incident laser beam by means of an implemented, movable mechanism allows the detection zone to be moved to different areas of the measuring cell. By positioning the zone at the edge of the cell in the direction of the laser source, significantly higher dispersion concentrations (up to approx. 40% w/v) are possible, as multiple scattering due to long light diffusion paths through the sample is avoided.
/1/ ISO 13321:1996 – Particle size analysis – Photon correlation spectroscopy
/2/ ISO 22412:2017 – Particle size analysis — Dynamic light scattering (DLS)
/3/ CIT (Chemie Ingenieur Technik) 03/2022; „Nanomaterialidentifikation entsprechend EU-Definition“ [German only]
/4/ CIT (Chemie Ingenieur Technik) 01-02/2023; „Partikelgrößen- und Zetapotentialmessung an verdünnten und original konzentrierten Nano-Oxidsuspensionen “ [German only]
/5/ Particle World 25/2024: “BeNano series light scattering in life sciences – Now with DLS microrheology and DLS flow mode option for enhanced protein analysis”
/6/ Particle World 24/2023: “BeNano series for particle analysis: New Autotitrator and DLS” Microrheology Option”
/7/ Particle World 23/2022: “Comprehensive nanoparticle characterization with optical and acoustic methods: BeNano 180 Zeta Pro vs. DT-1202”
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