Mixture model description of the T-, P dependence of the refractive index of water

In this paper the temperature/pressure dependence of the refractive index of liquid water is analyzed using the two-state outer-neighbor mixed bonding structural model. So far, this theoretical model has been successful in reproducing, usually within the experimental uncertainty, the temperature/pressure dependence of the density, the viscosity, and the oxygen-oxygen pair correlation functions, in addition to the isothermal compressibility and isotope effects of this important substance. The philosophy of the present paper is to use the high accuracy of refractive index measurements to further test this model. It is shown that a very simple linear dependence with respect to temperature and pressure of the specific refractions L-I and L-II of the two contributing structural components in this two-state model is sufficient to give better than 5-decimal-point agreement with the experimental refractive index data at low pressures and temperatures between about -10 degreesC and +70 degreesC. The maximum in the refractive index between -5 degreesC and +5 degreesC is reproduced to an even better precision. Generally better than 4-decimal-point accuracy is achieved for higher pressures, where the experimental data are less accurately known. The pressure dependence considered here also allows the calculation of the isothermal piezo-optic coefficient as a function of temperature, which exhibits a minimum near 50 degreesC. (C) 2001 American Institute of Physics.