Solvent structure effects in the macroscopic theory of van der Waals forces

Abstract

The theory of van der Waals forces is generalized in order to include the effects of solvent structure by explicitly dealing with the spatial correlations between the solvent molecules. This is achieved within the framework of nonlocal dielectric response theory, with the specular reflection approximation to apply the nonlocal response of the structured solvent in the bulk to give the response of such solvent intercalated between the interacting macroscopic surfaces. A closed form expression for the zero-order contribution to the van der Waals free energy is derived for two semi-infinite dielectric regions (characterized by a standard, local dielectric response and fixed dielectric constant), interacting across a layer of a structured solvent described by a nonlocal dielectric response function. The nonlocal character of the latter gives rise to several quantitatively and qualitatively new features of the dispersion interaction; the most important ones are similar in consequences to the effects of spatial variation of the Hamaker coefficient, as seen from the particular case of solvents with an exponentially decaying spatial intersolvent correlations.