Surface Bound Counterions

Could similarly charged surfaces be attracted to one another given the presence of a third, oppositely charged species? At first glance this would seem unlikely since counterions would at most completely compensate the charge, leaving two neutral surfaces. The classic Poisson-Boltzmann approach, in which solvated electrolytic species are assumed uncorrelated, predicts only repulsion between like charged objects in an electrolytic solution. However, there are examples where charge overcompensation may occur. A spectacular recent example of this is the layer-by-layer method of growing thin films by exposing a substrate alternatively to solutions of polyanions and polycations (Decher and Schlenoff 2003). Although not fully understood theoretically, the idea is that for entropic reasons, charged sites on the polyions exchange nearly completely with smaller ions at the surface. However, in addition to sites contacting the surface, the polyions generally contain additional charged sites that extend away from the surface, thus resulting in charge overcompensation. However, this type of charge overcompensation would not be expected from the relatively small ionic species present in most protein adsorption systems.

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