Kevin Stokely

Effect of hydrogen bond cooperativity on the behavior of water

Four scenarios have been proposed for the low-temperature phase behavior of liquid water, each predicting different thermodynamics. The physical mechanism that leads to each is debated. Moreover, it is still unclear which of the scenarios best describes water, because there is no definitive experimental test. Here we address both open issues within the framework of a microscopic cell model by performing a study combining mean-field calculations and Monte Carlo simulations. We show that a common physical mechanism underlies each of the four scenarios, and that two key physical quantities determine which of the four scenarios describes water: (i) the strength of the directional component of the hydrogen bond and (ii) the strength of the cooperative component of the hydrogen bond. The four scenarios may be mapped in the space of these two quantities. We argue that our conclusions are model independent. Using estimates from experimental data for H-bond properties the model predicts that the low-temperature phase diagram of water exhibits a liquid–liquid critical point at positive pressure.

Pressure effects in supercooled water: Comparison between a 2D model of water and experiments for surface water on a protein

Waters behavior differs from that of normal fluids, having more than sixty anomalies. Simulations and theories propose that many of these anomalies result from the coexistence of two liquid phases with different densities. Experiments in bulk water confirm the existence of two local arrangements of water molecules with different densities, but, because of inevitable freezing at low temperature T, cannot ascertain whether the two arrangements separate into two phases. To avoid the freezing, new experiments measure the dynamics of water at low T on the surface of proteins, finding a crossover from a non-Arrhenius regime at high T to a regime that is approximately Arrhenius at low T. Motivated by these experiments, Kumar et al (2008 Phys. Rev. Lett. 100, 105701) investigated, by Monte Carlo simulations and mean field calculations on a cell model for water in two dimensions (2D), the relation of the dynamic crossover with the coexistence of two liquid phases. They show that the crossover in the orientational correlation time r is a consequence of the rearrangement of the hydrogen bonds at low T, and predict that: (i) the dynamic crossover is isochronic, i.e. the value of the crossover time τ L is approximately independent of pressure P; (ii) the Arrhenius activation energy E A (P) of the low-T regime decreases upon increasing P; (iii) the temperature T* (P) at which r reaches a fixed macroscopic time τ* ≥ τ L decreases upon increasing P; in particular, this is true also for the crossover temperature T L (P) at which τ = τ L . Here, we compare these predictions with recent quasi-elastic neutron scattering (QENS) experiments performed by Chu et al on hydrated proteins at different values of P. We find that the experiments are consistent with these three predictions.

Cluster Monte Carlo and numerical mean field analysis for the water liquid-liquid phase transition

Article history: Using Wolffs cluster Monte Carlo simulations and numerical minimization within a mean field approach, we study the low temperature phase diagram of water, adopting a cell model that reproduces the known properties of water in its fluid phases. Both methods allow us to study the thermodynamic behavior of water at temperatures, where other numerical approaches - both Monte Carlo and molecular dynamics - are seriously hampered by the large increase of the correlation times. The cluster algorithm also allows us to emphasize that the liquid-liquid phase transition corresponds to the percolation transition of tetrahedrally ordered water molecules. Published by Elsevier B.V.

Phase transitions and dynamics of bulk and interfacial water

New experiments on water at the surface of proteins at very low temperature display intriguing dynamic behaviors. The extreme conditions of these experiments make it difficult to explore the wide range of thermodynamic state points needed to offer a suitable interpretation. Detailed simulations suffer from the same problem, where equilibration times at low temperature become extremely long. We show how Monte Carlo simulations and mean field calculations using a tractable model of water help interpret the experimental results. Here we summarize the results for bulk water and investigate the thermodynamic and dynamic properties of supercooled water at an interface.

Effect of pressure on the anomalous response functions of a confined water monolayer at low temperature.

We study a coarse-grained model for a water monolayer that cannot crystallize due to the presence of confining interfaces, such as protein powders or inorganic surfaces. Using both Monte Carlo simulations and mean field calculations, we calculate three response functions: the isobaric specific heat C(P), the isothermal compressibility K(T), and the isobaric thermal expansivity α(P). At low temperature T, we find two distinct maxima in C(P), K(T), and ∣α(P)∣, all converging toward a liquid-liquid critical point (LLCP) with increasing pressure P. We show that the maximum in C(P) at higher T is due to the fluctuations of hydrogen (H) bond formation and that the second maximum at lower T is due to the cooperativity among the H bonds. We discuss a similar effect in K(T) and ∣α(P)∣. If this cooperativity were not taken into account, both the lower-T maximum and the LLCP would disappear. However, comparison with recent experiments on water hydrating protein powders provides evidence for the existence of the lower-T maximum, supporting the hypothesized LLCP at positive P and finite T. The model also predicts that when P moves closer to the critical P the C(P) maxima move closer in T until they merge at the LLCP. Considering that other scenarios for water are thermodynamically possible, we discuss how an experimental measurement of the changing separation in T between the two maxima of C(P) as P increases could determine the best scenario for describing water.

Metastable Water Under Pressure

We have summarized some of the recent results, including studies for bulk, confined and interfacial water. By analyzing a cell model within a mean field approximation and with Monte Carlo simulations, we have showed that all the scenarios proposed for waters P-T phase diagram may be viewed as special cases of a more general scheme. In particular, our study shows that it is the relationship between H bond strength and H bond cooperativity that governs which scenario is valid. The investigation of the properties of metastable liquid water under pressure could provide essential information that could allow us to understand the mechanisms ruling the anomalous behavior of water. This understanding could, ultimately, lead us to the explanation of the reasons why water is such an essential liquid for life.