The determination of very small electrophoretic mobilities in polar and nonpolar colloidal dispersions using phase analysis light scattering

Abstract

An apparatus is described that can determine electrophoretic mobilities of polar and nonpolar colloidal dispersions down to 1 × 10 −12 m 2 s −1 V −1, with typical resolutions of 0.5 to 5%, depending on the nature of the dispersion being studied. The diffusion coefficient and settling convection velocity of the sample may be determined simultaneously with the electrophoretic mobility in real time. The technique, phase analysis light scattering (PALS), is based upon classical laser-Doppler electrophoresis, but employs signal processing of the time domain phase information within the scattered light signal, rather than analysis of its frequency spectrum. This allows much smaller electric field strengths to be employed, thereby alleviating the usual heating problems associated with electrophoretic studies of nonpolar dispersions. PALS measurements of typical aqueous latex dispersions with large mobilities and some nonpolar dispersions with very small mobilities are presented to illustrate the versatility of this technique.