Abstract

Recently, nonequilibrium electrokinetic measurements using laser-Doppler electrophoresis were reported for alkali halides and the sign of their surface charge was thus determined. These surface charge results for alkali halides are generally as expected from simplified lattice ion hydration theory with some exceptions, specifically LiF, KCl, NaCl, KBr, RbBr, KI, and RbI. Thus the sign of the surface charge can not be predicted solely from consideration of the hydration energies of the gaseous ions which constitute the ionic lattice. Accurate prediction of the surface charge must include consideration of the partial hydration of surface lattice ions. In this regard, ion-dipole interaction energies have been taken into consideration for three cases, based on steric and hydration effects, in order to predict more accurately the surface charge of alkali halides. The calculations indicate that the sign of the surface charge of alkali halides can be accurately predicted from an extended lattice ion hydration theory which involves the partial hydration of surface lattice ions.