Polarization behavior of water in extreme aqueous environments: A molecular dynamics study based on the Gaussian charge polarizable water model.

We study the polarization behavior of water under geologically relevant extreme aqueous environments along four equidistant supercritical isotherms, 773<or=T(K)<or=1373, and over a wide pressure range, 0<P(GPa)<or=30, by isobaric-isothermal molecular dynamics simulations of the Gaussian charge polarizable water model, to unravel and discuss the underlying link between two precisely defined orientational order parameters and the magnitude of the average induced dipole moment of water. The predicted behavior indicates an isothermal linear dependence (a) between the magnitude of the average induced dipole moment mu(ind) and the average system density rho, (b) between the magnitude of the average induced dipole mu(ind) and that of the total dipole mu(tot), resulting from (c), a compensating (inverse) dependence between the permanent-to-induced dipolar angle theta and the magnitude of the average induced dipole moment mu(ind). Moreover, we interpret this behavior in terms of the evolution of the state dependent tetrahedral order parameter q(T) and the corresponding bond-order parameter Q(6), supplemented by the microstructural analysis based on the three site-site radial distribution functions of water and the distance-ranked nearest-neighbor distributions. Finally, we show that while water exhibits a dramatic microstructural transformation from an open four-coordinated hydrogen-bonded network at normal conditions to a quasi-close-packed coordination, it still preserves a significant degree of hydrogen bonding.