Application of Integral Equation Joined with the Chain Association Theory to Study Molecular Association in Sub- and Supercritical Water

Abstract

Abstract:In this report, the Percus-Yevick and the Ornestein-Zernike integral equations are solved simultaneously for the radial distribution functions of water at various state conditions, including sub-and supercritical states. The intermolecular potential function used in this study consists of an effective Kihara potential, which is derived for associated fluids. For derivation of the effective potential function, water is considered as a mixture of associated species due to hydrogen bonding. The contribution of hydrogen bonding is considered in the formulation of the effective Kihara potential parameters through the application of the analytic chain association theory. There is a good agreement between the present calculations and the experimental data in predicting the oxygen-oxygen radial distribution function near the critical point and at supercritical conditions for which experimental data are available. It is also concluded that at supercritical conditions a considerable degree of hydrogen bonding may be still present in the form of linear chain association. Therefore, the chain association model is valid near the critical point and at supercritical conditions instead of other structure models for the investigations on molecular structure of water.calculations, they ignored the temperature dependency of the intermolecular distances between the nearest neighboring molecules.On the basis of the reported experimental data, 2 it is evident that the intermolecular distances between the nearest neighbors are temperature-dependent. Then this temperature dependency should be accounted for in the potential function to be used in the RDF calculations. In this work, the complete form of the effective Kihara potential function, which includes the parameters for the temperature dependency of the nearest neighbors, is used to calculate the RDF (oxygen-oxygen radial distribution function) for water molecules. The method of RDF calculations in this work is based on the application of the integral equation, which, as the results obtained indicate, can be considered as an accurate approach and leads to evaluating the RDF in the whole range of intermolecular distance variations. Also, a new set of constants for the parameters of an effective Kihara potential function is proposed.From a structural viewpoint, the results of these calculations substantiate the previous conclusion by the other workers 2,7,9 that at supercritical conditions, hydrogen bonding in the form of linear chain associations exists between water molecules. In eq 1, R is the polarizability factor, µ is the dipole moment, and σ are energy and size parameters, and r is the intermolecular separation. In this potential function, the dielectric property of pure water has been noted, and the dielectric dipole moment (µ 4 /3kT) and induced dipole (2µ 2 R) terms have been introduced in the intermolecular potential function. This equation is used in our RDF calculations. The results (Figure 1) indicate is another alternative to represent the intermolecular pote...

Percolation Transition in Supercritical Water: A Monte Carlo Simulation Study

“…The behavior of water in thermodynamic states above the critical point is in the focus of scientific interest for more than a decade. − The scientific importance of this subject lies in the fact that various properties of water (e.g., dielectric constant, viscosity, solvating ability, etc.) can be continuously varied in the supercritical region from values characteristic of a strongly associating system to gaslike values by controlling its density and connectivity by pressure and temperature, respectively.…”