Makhlaichuk P. Role of the hydrogen bonds in the formation of water properties.

The work is devoted to the analysis of the thermodynamic properties of water on the basis of the microscopic representation of the intermolecular interaction. Thereby an interaction potential based on the sum of the repulsive, dispersive (Van-der-Waals) and electrostatic terms as well as the actual energy of the hydrogen bond is constructed. The non-model potential is introduced, which has its electrostatic part represented as a multipole series while repulsive and dispersive terms take common expressions. The properties of water dimer are studied carefully. It is shown that there are two dimer configurations that have close ground state energies. The estimate value of the hydrogen bond contribution to the interaction potential is obtained. It is shown that H-bond energy does not exceed , where is the melting temperature of the hexagonal ice. Nevertheless, weak hydrogen bonds are fundamentally necessary for the qualitative description of the anomalous value of the heat capacity of water. Based on the structure of the introduced potential the averaged interaction potential is constructed. It is shown that in the liquid water and its vapor the averaged potential is satisfactorily approximated with the Lennard-Jones potential. All the main thermodynamic properties are determined with the averaged interaction potential. This fact is naturally confirmed by the quasi-free type of the thermal rotational motion of water molecules. From the experimental data for the heat capacity of water the picture of the cluster structure of its liquid and vapor states is obtained. It is shown that for the vapor on its coexistence curve the fraction of higher order clusters grows with the rise of the temperature. On the other hand the cluster structure of liquid water shows opposite behavior.