Liquid acetone and chloroform: a comparison between Monte Carlo simulation, molecular Ornstein-Zernike theory, and site-site Ornstein-Zernike theory

Abstract

Site-site distribution functions, internal energies, compressibilities, and dielectric constants of liquid acetone and chloroform are computed and compared. The hypernetted chain approximation is used for the molecular Ornstein-Zernike (MOZ) and site-site Ornstein-Zernike (SSOZ) theories. Both solvent molecules are described as sets of interaction sites, which are the centres of Lennard-Jones potentials and which carry partial electric charges. How the approximations inherent in both OZ theories affect the calculated liquid properties is discussed. In contrast to SSOZ theory, MOZ theory yields a correct description of the liquid structure as defined by the site-site distribution functions of simulations. The MOZ results for the dielectric constants are in good agreement with simulation, but both OZ theories slightly underestimate the internal excess energies. In contrast to the SSOZ formalism and to the simulations, the molecular pair distribution of MOZ theory can be computed readily for all two-molecule configurations, allowing a detailed study of the complex relief of this distribution. A search was performed for configurations that are highly probable in the liquid. They can be assigned to a small number of peaks of the molecular distribution function. These peaks have major contributions to the Kirkwood factors, which measure the orientational order in the liquid, and to the electrostatic part of the excess energies. The partitioning of the whole two-molecule configuration space makes it possible to study the details of the site-site distribution functions. For instance, a sharp peak in the oxygen-oxygen distribution in acetone results from many superimposed contributions of acetone-acetone arrangements of low and medium statistical weights, and not from the highly probable arrangements.