Electrophoretic mobility and primary electroviscous effect of dilute “hard” prolate ellipsoids

Electrophoretic mobilities and primary electroviscous coefficients are determined for “hard” prolate ellipsoids of axial ratio less than or equal to 4 in KCl, NaCl, and Tris-glycine salt solutions. Account is taken of the steady state distortion of the ion atmosphere around the ellipsoid by numerical solution of the coupled Poisson, low-Reynolds-number Navier-Stokes, and ion transport equations. This is accomplished by a boundary element procedure. Results are presented as ratios of mobilities and primary electroviscous coefficients of ellipsoids to those of corresponding spheres. It is shown that the electrophoretic mobility of an ellipsoid is very similar to that of a sphere under similar conditions of size, ionic strength, salt type, and zeta potential. Other factors being equal, shape has little effect on electrophoretic mobility. For the primary electroviscous coefficient, on the other hand, there is a substantial shape effect. It is argued that the complementary techniques of electrophoresis and viscosity together provide an effective means of studying the size, charge, and shape of macroions and colloidal particles.