Statistical Mechanics of Fluids and Solutions

Abstract

Biological systems function in watery environments. Biological fluids are either water solvent or various aqueous solutions and suspensions of ions and macromolecules, with which virtually all chapters of this book are concerned. In this chapter we start with a review of how the canonical ensemble method of statistical mechanics can be used to derive some basic properties of simple, classical fluids that consist of small molecules. We derive the well-known thermodynamic properties of non-interacting gases either in the absence or in the presence of external forces. For dilute and non-dilute fluids, we study how the inter-particle interactions give rise to the spatial correlations in the fluids, which affects the thermodynamic behaviors. These results, which are essential for a simple fluid for its own, can be extended to aqueous solutions of colloids and macromolecules; e.g., the results of dilute simple gas can be directly applied to dilute solutions. We outline coarse-grained descriptions in which the solutions are treated as the fluids of solutes undergoing the solvent-averaged effective interactions. As a particularly simple but useful variation we shall introduce the lattice model.