Cluster composition of liquid water derived from laser-Raman spectra and molecular simulation data

Abstract

Studying the behaviour of aqueous solutions of carbohydrates and other molecules commonly encountered in food science always leads to the fundamental question about the structure of bulk liquid water. The present study is an attempt towards the development of a mixture model of water structure based on information derived from experimental Raman spectra of water as well as molecular simulation data of isolated water clusters. The model assumes that liquid water is a thermodynamically ideal mixture of small water clusters in a state of chemical equilibrium. The equilibrium obeys the law of mass action and the van't Hoff temperature dependence of equilibrium constants. In order to validate the model and estimate its chemical equilibrium constants, Raman spectra of liquid water, predicted by the model, were compared with the isotropic components of carefully selected experimental spectra of pure liquid water, determined in the stretching vibration region at various temperatures. The theoretical Raman spectrum of bulk water was obtained by superposition of spectra of individual clusters, assuming their composition-based additivity. The spectra of small clusters were obtained from the Gaussian 98 programme using the 6-311+G(d,p) basis set together with the BLYP method. The model can reproduce the main features of the experimental Raman spectra of liquid water in the entire range of stretching vibration frequencies, at temperatures ranging from −30 to 300 °C. The resulting cluster composition of liquid water was found to be strongly temperature-dependent. At room temperature, water tetramer and pentamer are predominant, whereas the predicted concentrations of monomeric and dimeric water are very low. The latter gradually increase with increasing temperature at the expense of larger clusters. At its current stage of development, the model should be considered an alternative mathematical tool for more rational and rigorous analysis and interpretation of the Raman spectra of liquid water rather than a definite model of bulk water structure.