Dynamic Light Scattering, also known as DLS, enables the measurement of the hydrodynamic size of particles in dispersion by quantifying their diffusive motion. At particle sizes approaching the larger size range for DLS, thermal currents, number fluctuations, and sedimentation can imply that the detected scattering no longer accurately defines purely diffusive motion, and also particle size analysis turns out to be less accurate.
This can be observed during measurements of monodispersed samples that display artifacts in the correlation function baseline, as shown in Figure 1. While this may be alleviated by adjusting the dispersant, either viscosity or density, both provide a substandard solution as they are no longer quantifying the original system of interest.
This article explains how the low-volume disposable sizing cell can enhance data quality at large particle sizes and enable the analysis of the complete size range of DLS without modifying the dispersant system.
Figure 1. Example auto-correlation data from 1.0 µm and 3.0 µm latex in water. Measurements from both a 1.0 mm capillary and a 10.0 mm cuvette are shown. Despite these samples being mono-disperse, results for these particles in the 10 mm cuvette show an additional mode of decay in the measured auto-correlation function.
With the increase in particle size, the thermal Brownian motion is no longer adequate to retain particles in suspension, and samples may sediment over the course of time, which means that the motion of the particles is no longer random.
On the other hand, sedimentation can be excluded as the main factor in the skewing of reported particle size at higher sizes by considering the time taken by the particles to transit the incoming laser beam at the time of a DLS measurement. Figure 2 shows that the timescales linked with sedimentation are considerably larger than the correlation times used to capture data in a traditional DLS measurement, even when considering variations in material density.
Figure 2. Calculated settling time as a function of particle size for polystyrene latex (ρ =1050 kg/m3) and silica (ρ = 2650 kg/m3) particles dispersed in water at 25 oC (η = 89 x10-3 Pa s).
Measurements of particles above 1 micron in size may reveal some difference in variability as a function of temperature, indicating that thermal effects may impact the artifacts observed in the measured correlation functions. Thermal modeling of a 10 mm cuvette, with temperature regulated by contact with the instrument’s cell holder, shows that the cuvette geometry supports the formation of convection currents, as shown in Figure 3. On the other hand, capillary modeling with a similarly controlled temperature demonstrates that convection currents are not supported owing to the constraints enforced by the tapered cross-section.
While this modeling does not consider the effect of particle size on the importance of these convection effects, if you consider the diffusion rate as a function of particle size, then smaller particles will diffuse more quickly and therefore the particles’ diffusive motion is the main transport property. At larger particle sizes, the dispersion is slower and the particles’ increased cross-section means that they are more readily affected by these additional currents.
Figure 3. Thermal models of a 10 mm cuvette and a 1 mm capillary, calculated using ANSYS. The thermally driven velocity gradients shown represent the steady state condition after 120s of equilibration with a Peltier device at the bottom of each image controlling the system. The red cross-hair indicates the position of the detection volume where the DLS measurements are performed.
Results and Discussion
In order to show the enhancement of measurement accuracy at larger particle sizes enabled by the capillary, a series of polystyrene latex particles of different sizes were prepared in a 10 mm NaCl solution and measured in both a standard 10 mm cuvette and the low-volume disposable sizing cell. Figure 4 shows the mean and standard deviation particle size without any modification of the dispersant, wherein the measurements executed in a cuvette were reported beyond their specified range at about 1 μm, while measurements in the capillary were accurate and consistent up to 10 μm.
Figure 4. Discrepancy in the measured particle size, derived from cumulants analysis, and associated error, compared to the specified nominal size of a range of NIST traceable polystyrene latex particles, measured in both a 10 mm cuvette and 1 mm capillary.
As an illustration of enhanced measurements at extended size range with a polydisperse sample, a random sample of UK soil was dispersed in filtered ultra-pure water and quantified both in the low-volume disposable sizing cell and a standard 10 mm cuvette. Soil characterization is crucial to gain a better understanding of the incursion of micro- and nano-particles in the environment. Conversely, the error bars for the cuvette-based measurements, shown in Figure 5, would imply that DLS is not an appropriate method in this case.
On the other hand, the same measurements carried out with sample loaded into a capillary exhibit far better resolution and enhanced repeatability as signified by the narrower error bars, although the sample is polydisperse in size from about 100 nm to over 1 μm.
Figure 5. Intensity weighted particle size distributions for a polydisperse sample of soil, measured both in a 10 mm cuvette (left), and a 1 mm capillary. The distribution represents the average of 10 measurements and corresponding standard deviations. The measurements in the capillary show better repeatability shown by the narrower error bars, and a better resolved result.
This article has demonstrated that thermal currents are the main phenomena at the higher measurable size range of dynamic light scattering. It is these thermal currents that cause artifacts in the correlation function and thus deteriorate the accuracy of measurement.
The formation of these convection currents is not supported by the geometry of the 1 mm capillary used in the low volume disposable sizing cell and so accurate measurements can be made without altering sample dispersant over the entire measurable size range for DLS, and also repeatability for polydisperse samples is enhanced over analogous measurements in a standard cuvette.
This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.
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