Tymofieiev M. Statistical theory of dilute water-alcohol solutions

The present work is devoted to investigation of the equation of state for aqueous solutions of alcohol, in particular, contraction of water-alcohol solutions, i. e. a value of relative deviation of solution total volume from sum of volumes for its components. The main attention is focused on the investigation of microscopic and averaged interaction potentials between molecules for components as well as the nature of H-bonds in water. Microscopic potentials of interparticle interaction in water and methanol vapor are built. Their distant asymptotic corresponds to the electrostatic multipole expansion of interaction energy. They reproduce main characteristics of isolated dimers and virial coefficients of vapor phase. Thermal rotation of molecules leads to self-averaging of microscopic interaction potentials on their angular variables. The thermodynamic properties of water and alcohols in liquid and vapor phases in wide temperature range are determined by averaged interaction potentials. Averaged potentials are obtained from the requirement that initial microscopic and averaged potentials should lead to the same configuration integral in a pair approximation. It is found that their structures are similar to the Lennard-Jones potential. Thus, it is possible to apply the similarity principle to determine their thermodynamic properties. It is shown that the similarity principle allows to estimate the critical temperatures of water and methanol, using argon as a basic system with satisfactory accuracy. The physical nature of the H-bonds is investigated. For this purpose, the degree of overlapping for electron shells of molecules forming water dimers is analyzed. It had been shown that the overlapping degree of electron shells does not exceed several percent (less than 4%). This indicates that the exchange effects lead to the same contribution in order of magnitude to the H-bond energy. Therefore, the H-bond is mainly caused by electrostatic forces. To describe the contraction and excess enthalpy of water-alcohol solutions, an equation of state based on the virial expansion is used. It is shown, that it allows to reproduce the most general features of the contraction and excess enthalpy of water-alcohol solutions. In particular, in this way, it is possible to reproduce the position, depth and the temperature dependence of the minimum on the concentration curves, as well as slopes of the contraction curves at small molar fractions of water and alcohol.