The determination of very small electrophoretic mobilities of dispersions in non-polar media using phase analysis light scattering.
An apparatus is described that can determine electrophoretic
mobilities of polar and non-polar colloidal dispersions down to
-12 2 10 msV -1 -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 in real time with the electrophoretic
mobility. The technique, phase analysis light scattering (PALSY,
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 non-polar dispersions. PALS measurements
of typical aqueous latex dispersions with large mobilities and
non-polar dispersions with very small mobilities (down to
5x 10_12 m2 s-1 V-1) are presented to illustrate the versatility
of the technique.