화학공학소재연구정보센터
Journal of Chemical Physics, Vol.107, No.22, 9559-9568, 1997
The local order in liquid water studied through restricted averages of the angular correlation function
A structural study of TIP4P [W. L. Jorgensen et al., J. Chem. Phys. 79, 926 (1983)] water is presented. The method of structural analysis is based on the choice of restricted ranges of the pair angular configuration space. Such ranges, referred to as states or configurations Gamma, are used to obtain restricted averages, g(Gamma)(r), of the angular correlation function g(r, omega(1), omega(2)). Eulerian angles are used to define molecular orientations. This allows one to analyze all the geometries of the configuration space and to pay due attention to the nonhydrogen bonded configurations. The local structures and their temperature evolution are studied using the restricted distribution functions of oxygen-oxygen, g(OO)(Gamma)(r), and of oxygen-hydrogen g(OH)(Gamma)(r) of the different confrgurations. As the temperature rises, the local population of nonhydrogen bonded configurations increases owing to the breakdown of the tetrahedral network. By comparing the g(OO)(Gamma)(r) to the g(r) of simple fluids, scaled from liquid argon, the evidence of a residual tetrahedral network in liquid water at high temperature is obtained. Some restricted averages g(OO)(Gamma)(r) reveal the existence of long-range direct correlation, masked by the orientational averages of the total distribution function g(OO)(r) The comparison between g(OO)(Gamma)(r), evaluated at the liquid density and in the limit of zero density, shows that the direct correlation extends beyond four molecular diameters (about 12 Angstrom for water) as in simple polar liquids. Finally, the dimer geometries contributing to form the first peak in g(OH)(Gamma)(r) are identified. A practical method for determining the coordination numbers of hydrogen and nonhydrogen bonded molecules from the experimental data is suggested. Applied to TIP4P liquid water at various temperatures, the method yields coordination numbers in agreement with the random network model predictions. (C) 1997 American Institute of Physics.