The impact of ionic solvation energy and water structure on forces

The structure of water in an interfacial region differs from bulk due to surface-induced water ordering. The difference is seen both in the local water density and in the water polarisability, particularly orientational polarisability as water dipoles are oriented towards the interface. The impact of this phenomenon in continuum models is to replace the dielectric constant of the medium with a position-dependent spatial dielectric function. The relationship that this kind of dielectric function bears with the electrostatic potential is known in the Poisson equation. But an additional effect of the spatially dependent dielectric function on ionic solvation energies (including a change in the Born energy of the ion) has not been widely recognised. The dielectric constant in an interfacial region has been reported to fall to values around 5, far from the bulk value of 78. The corresponding spatially-dependent ionic solvation energy therefore introduces a strongly repulsive ion-surface interaction which must be included as an additional “nonelectrostatic potential” in the Boltzmann factor, determining ion concentrations in a Poisson-Boltzmann model. Consequently a strongly repulsive surface force—a primary hydration force—is obtained with a range corresponding to the range of the surface-induced water ordering, usually the thickness of several water layers.