Local effects in hydrophobicity. Electrostatic interactions in the restructuring of highly polar solvent around less polar solute.

Two simplifying assumptions are frequently used in the biophysical chemistry of aqueous solutions: (i) a dielectric mediates the interactions of polar and ionic molecules in aqueous phases and (ii) the dielectric constant of this medium is high and uniform up to molecular surfaces. Because of their great utility in rationalizing simple electrostatic and dielectric effects in such polar systems, it is important to examine whether these assumptions also lead to deductions that are locally consistent with the solvent restructuring observed in hydrophobic phenomena. In this paper, using a model polar fluid system, these macroscopic assumptions are applied to the rigorous, microscopic nonlinear integral equation for Wki, the potential of mean force between two adjacent polar molecules. In systems of high dielectric constant, linearization of Boltzmann exponentials and approximation of three-molecule potentials of mean force by superposition of two-molecule potentials permit reduction to a linear integral equation for Wki. It is shown that the strictly local electrostatic contributions to Wki exert an effect that is qualitatively similar to the global screening effect of a dielectric medium. Through the relation between Wki and configurational probabilities, it is further found that reducing the polarity of a molecule in a polar fluid shifts local pair probability density from energetically unfavorable to energetically favorable two-molecule configurations. This general effect, which clearly promotes local structure, would augment more specific hydrophobic mechanisms in aqueous systems. Thus, the assumptions upon which the highly successful Debye-Hückel and Onsager models are supported lead also to deductions about local structure that are consistent with hydrophobic structure enhancement.