Energy-representation theory of solutions: Its formulation and application to soft, molecular aggregates

Abstract

The energy-representation theory of solutions is developed to address the dissolution of a molecule in homogeneous fluid as well as the partitioning into such nanoscale structures as micelle and lipid membrane and the physisorption onto gasliquid and solid-liquid interfaces in a unified manner as solvation in an extended sense. The present review describes the formulation of the solution theory with illustrative applications to the peptide configuration in lipid membrane, the water dissolution into polymer, and the physisorption on urea crystal in contact with liquid water. The solution theory in the energy representation is a density-functional scheme formulated by adopting the solute-solvent pair interaction energy as a one-dimensional coordinate for distribution functions and provides an approximate functional for the solvation free energy in terms of energy distribution functions in the reference-solvent and solution systems of interest. Each of the solute and solvent molecules is treated as a single unit as a whole, and due to this feature, a species with intramolecular flexibility and a solvent system with nano-scale inhomogeneity or interface can be analyzed in a common framework. The role of water is pointed out in determining the configuration of a peptide in lipid membrane, and the dissolution of water into polymer medium is described at chemical accuracy. Some directions of future developments are also discussed.