THE SURFACE-CHARGE OF ALKALI-HALIDES - CONSIDERATION OF THE PARTIAL HYDRATION OF SURFACE LATTICE IONS

Recently, nonequilibrium electrokinetic measurements using laser-Doppl er electrophoresis were reported for alkali halides and the sign of th eir surface charge was thus determined. These surface charge results f or alkali halides are generally as expected from simplified lattice io n hydration theory with some exceptions, specifically LiF, KCl, NaCl, KBr, RbBr, KI, and Rbl. 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 o f the surface charge must include consideration of the partial hydrati on of surface lattice ions. In this regard, ion-dipole interaction ene rgies have been taken into consideration for three cases, based on ste ric and hydration effects, in order to predict more accurately the sur face charge of alkali halides. The calculations indicate that the sign of the surface charge of alkali halides can be accurately predicted f rom an extended lattice ion hydration theory which involves the partia l hydration of surface lattice ions. (C) 1997 Elsevier Science B.V.