Dynamics and thermodynamics of water

On decreasing the temperature T, the correlation time tau of supercooled water displays a dynamic crossover from non-Arrhenius dynamics (with T-dependent activation energy) at high T to Arrhenius dynamics (with constant activation energy) at low T. Simulations for water models show that this crossover occurs at the locus of maximum isobaric specific heat in the pressure-temperature (P-T) plane. Results of simulations show also that at this locus there is a sharp change of local structure: more tetrahedral below the locus, and less tetrahedral above it. Furthermore, in water solutions with proteins or DNA, simulations show that in correspondence with this locus there is a crossover in the dynamics of the biomolecules, a phenomenon commonly known as the protein glass transition. To clarify the relation of the dynamic crossover with the thermodynamics of water, we study the dynamics of a cell model of water which can be tuned to exhibit: (1) a first-order phase transition line that separates the liquids of high and low densities at low temperatures; this phase transition line terminates at a liquid-liquid critical point (LLCP), from which departs the Widom line T-W(P), i.e. the line of maximum isobaric specific heat in the P-T plane; (2) the singularity-free (SF) scenario, under which the system exhibits water-like anomalies but with no finite temperature liquid-liquid critical point. We find that the dynamic crossover is present in both the LLCP and the SF cases. Moreover, on the basis of the study of the probability p(B) of forming a bond, we propose and verify a relation between dynamics and thermodynamics that is able to show how the crossover is a consequence of a local relaxation process associated with breaking a bond and reorienting the molecule. We further find a distinct difference in pressure dependence of the dynamic crossover between the LLCP and SF scenarios, which may help in resolving which of the scenarios correctly explains the anomalous behavior of water.