Electrophoretic mobility of aqueous colloidal suspensions in the gas and liquid-like phases

Abstract

New electrophoretic light scattering (ELS) measurements are reported on aqueous colloidal suspensions down to salt concentrations lower than n _s = 10⁻⁶ M, where the polyions are liquid-like ordered due to screened Coulomb forces. Mobility data are presented for spherical polystyrene latices of different radius a and rodlike tobacco mosaic virus (TMV) particles (length l = 300 nm and radius a = 9 nm), and they are discussed in relation to the small ion concentration or ka. The salt concentration n _s is determined via electric conductivity measurements. Static light scattering (SLS) combined with computer calculations are used to characterize the structure; they also yield the salt concentration of the suspensions. The suspensions of latex spheres show, with decreasing salt concentration, a minimum in mobility. For lowering hte salt content below 10⁻⁴ M an increase up to a factor of 3 is observed. The mobility of a dilute suspension of rodlike TMV is found to be about 5 μm/s/V/cm. Increasing the particle concentration leads to a decrease in mobility. In the limit of negligible salt concentration for a particle concentration below 0.1 mg/ml the mobilities lie above 5 μm/s/V/cm. At higher concentrations they are smaller than this value. A qualitative comparison with the theory is made.