John D Weeks, University of Maryland
Room 111, Physics Building
We discuss new and ongoing work that provides a unified theoretical description for mixtures of charged and polar fluids as well as apolar Lennard-Jones (LJ) mixtures. Our initial focus is on the special case of a two component WA solution where the solute component (A) may be charged or hydrophobic and is very dilute and the solvent W is water as described by the classical SPC/E water model. There can be very interesting behavior of the AA pair correlation function from solvent (W) induced effective interactions that presents difficult problems for both simulations and theory. The LMF theory used here introduces a general mapping that relates the structure of a nonuniform system with long-ranged Coulomb or van der Waals interactions to that of a simpler “mimic system” with renormalized short ranged interactions that accounts for the averaged effects of the long-ranged interactions in the full system of interest. We show that LMF theory can be viewed as a natural generalization of ideas used in the classical van der Waals equation for uniform simple liquids like Ar to nonuniform systems and mixtures with Coulomb interactions, where even more accurate results are found. The theory gives very accurate results for the solvation of both hydrophobic and hydrophilic solutes of varying sizes in water and provides qualitative insights into many other problems. We discuss in particular the very different behavior of the cation-anion pair correlation function in dilute aqueous solutions of NaCl and CaCl2 and use LMF theory to describe the association of model hydrophobic Argon solutes in water.