Double Layer Forces: the Role of Molecular Solvents

Abstract: The dielectric continuum model has rightfully been and continues to be a major workhorse for theory and modelling in Surface and Colloid Chemistry. Due to the implicit description of water, entering only as a scaling constant for charge-charge interactions, one would not expect it to work for short distances. However, considerable evidence has been accumulated over the years which shows that this is not the case, and the dielectric continuum model gives a reasonable description sometimes on lengthscales which approach the size of a water molecule! A first part of this thesis concerns theoretical modeling of the salt dependent water uptake of a complex between DNA and CTA (hexadecyltrimethylammonium). The electrostatic component of the free energy was treated at the Poisson-Boltzmann level. Despite the demanding conditions, with high electric fields, low water content and complex geometry, the dielectric description of water works. The agreement between theory and experiment is quantitative. To try to understand why the primitive model is working under such conditions, double layer forces with an explicit description of the solvent have been studied through Monte Carlo simulations. A simple and previously well-characterized system of two infinite, like-charged plates with only neutralizing counterions was chosen. The solvent was treated as Lennard-Jones dipoles (Stockmayer fluid) and, for comparison, also at the primitive model level. The forces between like-charged plates in the primitive model agree qualitatively with the molecular solvent ones and the implicit solvent gives a reasonable description of the dielectric screening. As expected, the molecular solvent introduces extra effects, namely packing. The phenomenon of ion-ion correlation attraction, i.e., the attraction of the two like-charged plates, is also reproduced in a molecular solvent. One of the studies includes an analysis of the regime between the dielectric continuum model and the full explicit molecular solvent. This is done by progressively reducing the size of the molecular solvent, doubling its number density, and keeping the dielectric properties constant. The comparison between the two solvent models requires being able to calculate the dielectric constant for a general dipolar fluid. A simulation methodology has also been developed and tested for this purpose.