H2O and D2 mixtures under pressure: Spectroscopy and proton exchange kinetics

We have investigated the pressure-induced spectral changes and the proton exchange reactions of D-2-H2O mixtures to 64 GPa using micro-Raman spectroscopy. The results show the profound difference in the rotational and vibrational Raman spectra of hydrogen isotopes from those of the pure samples, showing the vibrational modes at higher frequencies and continuing to increase with pressure without apparent turnover. This indicates the repulsive nature of D-2-H2O interaction without hydrogen bonds between the two and, thus, interstitial fillings of D-2 molecules into the bcc-like ice lattice. The spectral analysis using the Morse potential yields a hydrogen bond distance of 0.734 angstrom at 6 GPa-slightly shorter than that in pure-attributed to the repulsive interaction. The pressure-dependent spectral changes suggest that the proton-ordering transition in the ice lattice occurs over a large pressure range between 28 and 50 GPa, which is substantially lower than that of pure ice (40-80 GPa). This again indicates the presence of high internal pressure arising from the repulsive interaction. The Raman spectra show evidences that the proton exchange occurs in various phases including in solid D-2 and H2O mixtures. Based on the time-dependent spectral changes, we obtained the proton exchange rates of k similar to 0.085 h(-1) at 0.2 GPa in fluid D-2 and water mixtures, k similar to 0.03 h(-1) and 0.003 h(-1) at 2 GPa and 4 GPa, respectively, in fluid D-2-ice mixtures, and k similar to 10(-3) h(-1) at 8 GPa in solid D-2 and ice mixtures. (C) 2011 American Institute of Physics. [doi:10.1063/1.3658485]