Electrophoretic mobility of colloidal particles in salt-free media

摘要

It is shown that the electrophoretic mobility μ of dilute spherical colloidal particles of radius a, carrying charge Q in a salt-free medium (containing only counteri-ons) is expressed generally as μ = Q_(eff)/D_H, irrespective of the particle type, where Q_(eff) is the effective particle charge and D_H is the drag coefficient of the particle. There is a certain critical value of Q separating two cases: the low-charge case and the high-charge case. In the low-charge case, Q_(eff) equals the real particle charge Q, while in the high-charge case, Q_(eff) becomes constant independent of Q. In the high-charge case, in which case counter-ion condensation occurs in the vicinity of the particle surface, there are two kinds of counterions: bound and free counterions. Bound counterions condensed at the particle surface do not contribute to the electrophoretic mobility so Q_(eff) and μ become constant independent of Q.