A common misconception in the field of light scattering is that the size of the electrical double layer can be measured with dynamic light scattering. Put another way, too often is the reason offered, that ionic strength dependent changes in the measured hydrodynamic size are a direct consequence of expansion or contraction of the double layer. The fallacy in this logic can be revealed with simple experimental observation.
The hydrodynamic size is defined as the DLS measured size of the particle in the limit of infinite dilution. According ISO 13321 recommended procedures, the hydrodynamic size can be determined by extrapolation of a size vs. concentration graph to the limit of zero concentration. At the limit of infinite dilution then, the hydrodynamic size is representative of the physical size of the particle in the absence of any concentration effects, e.g. particle interactions, restricted diffusion, etc.
Figure 1 shows the effects of sample concentration on the hydrodynamic size of lysozyme at 4 ionic strengths or salt concentrations. The effects of sample concentration on the measured size are evident at all ionic strength values, but are much more pronounced at the lower ionic strength values. It is in fact, this ionic strength dependence of the "apparent size" that has led to the misconception that the measured size must include the double layer, or at least a portion of the double layer. However, if the measured particle size from a dynamic light scattering measurement included the double layer, then one would expect to see a distinct ionic strength dependence of the hydrodynamic size. As evident in Figures 1 & 2 though, the primary particle size is independent of the ionic strength. In addition, since the Debye length (double layer thickness) increases with decreasing ionic strength, any incorporation of the double layer into the measured hydrodynamic size would lead one to expect an apparent size that increased with decreasing salt. This expectation is in sharp contrast to the observed effects of salt on the apparent size shown in Figure 1. In other words, at fixed sample concentration, the measured lysozyme size is smaller at lower salt concentrations, even though the double layer is larger.
The obvious auxiliary question to the one addressed in this FAQ is "if the apparent size does not include a portion of the double layer, why does one observe an ionic strength dependence?" This in fact, is the more important question, and is still under examination by researchers in both the dynamic and electrophoretic light scattering fields. One area of current interest is the distortion of the double layer at low ionic strength, arising from the particle motion under the influence of an applied electric field. This distortion is proposed to have an influence on the frictional coefficient of (or apparent drag on) the particle, which would of course be manifested by a change in the measured diffusion coefficient. See White-Mangelsdorf model for additional details.
Figure 1: Effects of sample concentration and ionic strength on the apparent particle size of lysozyme in NaCl solutions, measured with a Zetasizer Nano system.
Figure 2: Debye length effects of the hydrodynamic size of lysozyme in NaCl solution.
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