Interaction sites dielectric properties

In this section, we review some of the important formal results in the statistical mechanics of interaction site fluids. These results provide the basis for many of the approximate theories that will be described in Section III, and the calculation of correlation functions to describe the microscopic structure of fluids. We begin with a short review of the theory of the pair correlation function based upon cluster expansions. Although this material is featured in a number of other review articles, we have chosen to include a short account here so that the present article can be reasonably self-contained. Cluster expansion techniques have played an important part in the development of theories of interaction site fluids, and in order to fully grasp the significance of these developments, it is necessary to make contact with the results derived earlier for simple fluids. We will first describe the general cluster expansion theory for fluids, which is directly applicable to rigid nonspherical molecules by a simple addition of orientational coordinates. Next we will focus on the site-site correlation functions and describe the interaction site cluster expansion. After this, we review the calculation of thermodynamic properties from the correlation functions, and then we consider the calculation of the dielectric constant and the Kirkwood orientational correlation parameters. 

In this section, we will review some of the results obtained for homogeneous fluids. The focus of the section strongly reflects the author s particular interest rather than a complete review of all work done in this area. To a large extent, we will concentrate on aspects that have not been reviewed previously, or on areas that developed since those reviews. The first section deals with the influence of electrostatic interactions on the structure factor, and we stress the decoupling of dipole-dipole interactions from the structure factor, although there is a strong effect on particular g y r) s. In Section V.B we consider the dielectric constant obtained from the CSL equation with particular reference to the influence of shape forces in the dielectric properties. Section V.C considers the application of interaction site theories to calculate thermodynamic properties and fluid phase equilibria. 

The reaction field of the highly polarisable solvent water decreases the field near the molecular surface of the solute. The earhest attempts to take into account solute-solvent interactions focussed on varying the dielectric constant in electrostatic energy terms used to describe pairwise interactions between species in the solute molecule, but, of course, there will be preferred hydration sites. Because of the intrinsic dielectric properties of the solute and the voids produced by unfavourable solute group-solvent interactions the local dielectric behaviour about the solute is... 

Polar solvents may interact strongly with a mineral oxide surface. In principle, the adsorption of die solvent must be considered. Claesson [13] studied the adsorption of fatty acids by sihca from solvents of various polarities. The results show that polar solvents compete with the solute for available sites on the surface, while nonpolar solvents show little competition. The polarity of the solvent is often determined from the measured dielectric properties. Krishnakumar and Somasundaran [13] studied surfactant adsorption on to silica and alumina from solvents with various dielectric properties. The aim of the study was to look at the effect of adsorbent and smfactant acidities and solvent polarity on the adsorption properties of the surfactant molecules. They used anionic and cationic surfactants as adsorption probes. The results show that polar interactions control the adsorption from solvents of low dielectric properties while hydrocarbon chain interactions with the surface play an important role in determining adsorption from solvents of higher dielectric properties. It was also found that an acidic surfactant interacts strongly with a basic adsorbent, and vice versa. One should be aware that the polarity of a molecule as measmed from the dielechic properties is not always eorrelated with the ability of the molecules to form ion pairs. For example, dimethylformamide and nihomethane have almost equal dielechic constants. However, the extent of ion pairing in nihomethane is much greater than that in dimethylformamide. Thus, the solvent acidity and basicity are the physical properties which can best characterize the ability of the solvent to compete with the solute for available sites on the mineral surface. 

The free energy changes accompanying the transfer of structures along the reaction coordinate from water to the ODCase active site were then computed using FEP methods.71 These computations employed a cutoff distance of 14 A for explicit electrostatic interactions, beyond which a shell (radius 14-16 A) of dielectric constant 4 was used to approximate the electrostatic properties of the remainder of the protein the area outside of this shell was treated with a dielectric constant of 78 to represent the electrostatic properties of the surrounding water. 

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