The anomalous properties of liquid water explained by a mixture model

A semiempirical theory, which is a modernization of the 'mixture model,' attempting quantitatively to correlate thermodynamic and dynamic effects of bulk and interfacial liquid water with various properties of ice polymorphs is proposed here. The basic concept rests on the disappearance, on the average, with increasing temperature or pressure, of open intermolecular tetrahedral bonding (Type-I) having a density similar to that of ordinary ice, in favor of compacdy bonded regions (Type-II) with a density near that of the dense ice polymorphs particularly, ice II, III, and V.

The mixture model is employed to explain quantitatively the origins of the 'anomalous' properties of liquid water - density maximum, isotope effects, thermal minimum in the isothermal compressibility curve. Strong support for this model can be found from an analysis of the accurate experimental density data of liquid H2O and D2O from the supercooled regime to about +70 °C. Published density data can be fit to this model with six- to seven-decimal-point accuracy, in the case of liquid H2O and to the reported fivedecimal-point precision, in the case of liquid DjO. The output parameters from the fits indicate the presence of capacious intermolecular bonding with a density extremely close to that of ordinary ice-Ih, intermixed with compactly bonded regions having a density near that of the common dense forms of ice. A quantitative assessment of the temperature dependence of the isothermal compressibility of liquid water at atmospheric pressure was carried out. The 'anomalous' minimum in this quantity near 50 °C is shown to emerge naturally. Independent support for this model has been provided by the differential x-ray scattering experiments of Bosio et al. Their resuhs clearly indicate that a dynamic, temperature dependent mixture of ice-I-, -II-, -III-, and -V-type bonding is present in the liquid in the manner expected for the model described in this work. Based on eariier x-ray scattering studies, Kamb reached a similar conclusion about these mixed bonding forms in liquid water. Recently, computational studies conducted by Cho, Singh and Robinson in our laboratory have indicated that the density anomaly of liquid water can be explained by utilizing this mixture model concept.

Ultrafast laser methods were used to analyze the properties of liquid water confined in small volumes. This study shows that interfacial water appears to be more structured and orientationally stiffer than bulk water.