The density and temperature dependence of the structural relaxation time (τ) in water was determined by inelastic ultraviolet scattering spectroscopy in the thermodynamic range (P=1-4000 bars, T=253-323 K), where several water anomalies take place. We observed an activation (Arrhenius) temperature dependence of τ at constant density and a monotonic density decrease at constant temperature. The latter trend was accounted for by introducing a density-dependent activation entropy associated to water local structure. The combined temperature and density behavior of τ indicates that differently from previous results, in the probed thermodynamic range, the relaxation process is ruled by a density-dependent activation Helmholtz free energy rather than a simple activation energy. Finally, the extrapolation of the observed phenomenology at lower temperature suggests a substantial agreement with the liquid-liquid phase transition hypothesis.
F Bencivenga, A Cimatoribus, A Gessini, MG Izzo, C Masciovecchio. Temperature and density dependence of the structural relaxation time in water by inelastic ultraviolet scattering
published, J. Chem. Phys., 2009, 131