I. Yu. Shilov

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.

Gibbs energy of aggregation: An integral thermodynamic characteristic of the self-organization of equilibrium liquid systems

The Gibbs energy of aggregation, a new integral thermodynamic characteristic of the self-organization of equilibrium liquid systems due to specific interactions such as hydrogen bonds, is introduced. This function is defined as the difference between the Gibbs energy of the associated liquid system and the Gibbs energy of the hypothetical nonideal liquid system consisting of monomers at the same temperature and pressure. General expressions for molar Gibbs energies of aggregation of a pure liquid and a binary solution, taking into account the contributions from specific and universal (dipole, dispersion) interactions, are derived in the context of the quasi-chemical approach. An expression for the dependence of the Gibbs energy of aggregation of a pure liquid on the degree of chain aggregation is derived using the athermal associated solution model. In the same approximation, an expression for the dependence of the Gibbs energy of aggregation for a binary solution of an associating component in an inert solvent on the solution’s composition and the degree of the chain association of a pure liquid is derived. Results from numerical calculations of the Gibbs energies of aggregation of a pure liquid and a binary solution are reported. A correlation is made between the Gibbs energy of aggregation of a pure liquid and the standard Gibbs energy of formation of the dimer. It is shown that the Gibbs energy of aggregation versus the standard Gibbs energy of dimerization dependence is nonlinear in the range of low degrees of aggregation.