Single ionic activities and the thermodynamics of charged interfaces

Abstract

An expression given previously for single ionic activities (SIAs) is combined with the Gibbs adsorption isotherm for charged interfaces to obtain a general expression for isothermal changes in ψ, the potential drop across a charged interface. Also a general criterion for the absence of specific adsorption (SA) near the zero point of charge (z.p.c.) is forwarded. It is shown that (1) at the z.p.c., ψ is independent of solution composition when SA does not occur near the z.p.c., (2) SIAs can be estimated from studies of charged interfaces at the z.p.c. when there are good grounds for believing that SA is negligible near the z.p.c.; (3) calculated values of liquid junction potentials for non-ideal solutions are not greatly in error and (4) for non-ideal solutions non-ideal values of the double layer transport numbers are compatible with the absence of SA near the z.p.c.

Finally the role of SIAs in understanding real double layers and in interpreting surface tension data for solutions of ionic surfactants containing added electrolytes is outlined.