The energy of intermolecular interactions in associated liquids

A model approach to calculating the contributions of intermolecular interactions to the thermodynamic functions of liquids taking into account their supramolecular structure was developed. The energy of intermolecular interactions includes the contributions of dispersion, repulsion, dipole, and specific interactions. Equations that successively included the contributions of coordination spheres were suggested for dispersion interactions. Computer simulation results were used to compare the "discrete" and "continuum" methods for estimating dispersion contributions. Coordination number singularities caused by H-bonding were observed in liquid methanol. The formation of H-bonds in methanol was shown to "split" the first coordination sphere characteristic of simple liquids such as argon into two spheres. As a consequence, the first and second coordination spheres of methanol gave comparable contributions to the dispersion component of intermolecular interaction energy. Dipole interactions were described within the framework of the generalized reaction field concept with the inclusion of the structure of supramolecular aggregates. The contributions of directional short-range non-covalent interactions to the internal energy of liquids were estimated in terms of a thermodynamic description of supramolecular aggregation. The intermolecular interaction energy and enthalpy of vaporization of liquid methanol were calculated for the supramolecular structure model comprising chain and comblike associates of arbitrary lengths with unit-length branches in the main chain. The dependences of intermolecular interaction energy and its components on the equilibrium constants of chain and branched association were analyzed. The intermolecular interaction energy of liquid methanol was shown to be largely determined by dispersion and specific interaction contributions and be weakly sensitive to the degree of branching of associates. The ratio between the contributions to interaction energy caused by the short- and long-range translational-orientational correlations of molecules in liquids was considered.