The dielectric continuum model has long been a major workhorse in Surface and Colloid Chemistry (e.g.: the Poisson-Boltzmann equation and simulations using the primitive model of electrolyte solutions). It is however somewhat surprising that in so many instances it is seen to hold down to very small length scales.<br/>In a series of studies we have performed Monte Carlo simulations of double layer forces between two like-charged plates in the primitive model (implicit solvent) and in a Stockmayer fluid (explicit molecular solvent).<br/>We first assessed that both systems display the same qualitative behaviour upon changing the electrostatic coupling, with an ion-ion correlation attraction minimum which is lost or reduced upon decreasing the counterion valency or the surface charge density or upon increasing the dielectric screening. In a molecular solvent the pressure curves display extra effects, namely packing oscillations. The analysis of the different pressure components and of interaction free energies aid in the interpretation.<br/>In a second stage, we have interpolated between the primitive model and a molecular solvent, having systems with solvent particles with progressively smaller radius and dipole moment, but increasing their number, keeping the dielectric properties of the solvent constant. For sufficiently small solvent size the interaction curves have clear similarities with those from the primitive model.<br/>All in all, we see that the primitive model reproduces qualitatively the dielectric screening by a dipolar solvent, which is not trivial.

Double layer forces and the dielectric approximation

255th National Meeting (2018)

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