Gradual crossover in molecular organization of stable liquid H2O at moderately high pressure and temperature

Using the literature raw data of the speed of sound and the specific volume, the isothermal compressibility, kappa(T), a second derivative thermodynamic quantity of G, was evaluated for liquid H2O in the pressure range up to 350 MPa and the temperature to 50 degrees C. We then obtained its pressure derivative, d kappa(T)/dp, a third derivative numerically without using a fitting function to the kappa(T) data. On taking yet another p-derivative at a fixed T graphically without resorting to any fitting function, the resulting d(2)kappa(T)/dp(2), a fourth derivative, showed a weak but clear step anomaly, with the onset of the step named point X and its end point Y. In analogy with another third and fourth derivative pair in binary aqueous solutions of glycerol, d alpha(p)/dxG1(y) and d(2)alpha(p)/dxG1(y)(2), at 0.1 MPa (alpha(p) is the thermal expansivity and x(G1y) the mole fraction of solute glycerol) in our recent publication {[}J. Solution Chem. 43, 663-674 (2014); DOI:10.1007/s10953-013-0122-7], we argue that there is a gradual crossover in the molecular organization of pure H2O from a low to a high p-regions starting at point X and ending at Y at a fixed T. The crossover takes place gradually spanning for about 100 MPa at a fixed temperature. The extrapolated temperature to zero p seems to be about 70 80 degrees C for points X and 90 - 110 degrees C for Y. Furthermore, the midpoints of X and Y seem to extrapolate to the triple point of liquid, ice Ih and ice III. Recalling that the zero xG(1y), extrapolation of point X and Y for binary aqueous glycerol at 0.1 MPa gives about the same T values respectively, we suggest that at zero pressure the region below about 70 degrees C the hydrogen bond network is bond-percolated, while above about 90 degrees C there is no hydrogen bond network. Implication of these findings is discussed. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.