Marco G. Mazza

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.

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.

Hydrophobic nanoconfinement suppresses fluctuations in supercooled water

We perform very efficient Monte Carlo simulations to study the phase diagram of a water monolayer confined in a fixed disordered matrix of hydrophobic nanoparticles between two hydrophobic plates. We consider different hydrophobic nanoparticle concentrations c. We adopt a coarse-grained model of water that, for c = 0, displays a first-order liquid-liquid phase transition (LLPT) line with negative slope in the pressure-temperature (P-T) plane, ending in a liquid-liquid critical point at about 174 K and 0.13 GPa. We show that upon increase of c the liquid-gas spinodal and the temperature of the maximum density line are shifted with respect to the c = 0 case. We also find dramatic changes in the region around the LLPT. In particular, we observe a substantial (more than 90%) decrease of isothermal compressibility, thermal expansion coefficient and constant-pressure specific heat upon increasing c, consistent with recent experiments. Moreover, we find that a hydrophobic nanoparticle concentration as small as c = 2.4% is enough to destroy the LLPT for P ≥ 0.16 GPa. The fluctuations of volume apparently diverge at P ≈ 0.16 GPa, suggesting that the LLPT line ends in an LL critical point at 0.16 GPa. Therefore, nanoconfinement reduces the range of P-T where the LLPT is observable. By increasing the hydrophobic nanoparticle concentration c, the LLPT becomes weaker and its P-T range smaller. The model allows us to explain these phenomena in terms of a proliferation of interfaces among domains with different local order, promoted by the hydrophobic effect of the water-hydrophobic-nanoparticle interfaces.

Effect of hydrophobic environments on the hypothesized liquid-liquid critical point of water

The complex behavior of liquid water, along with its anomalies and their crucial role in the existence of life, continue to attract the attention of researchers. The anomalous behavior of water is more pronounced at subfreezing temperatures and numerous theoretical and experimental studies are directed towards developing a coherent thermodynamic and dynamic framework for understanding supercooled water. The existence of a liquid–liquid critical point in the deep supercooled region has been related to the anomalous behavior of water. However, the experimental study of supercooled water at very low temperatures is hampered by the homogeneous nucleation of the crystal. Recently, water confined in nanoscopic structures or in solutions has attracted interest because nucleation can be delayed. These systems have a tremendous relevance also for current biological advances; e.g., supercooled water is often confined in cell membranes and acts as a solvent for biological molecules. In particular, considerable attention has been recently devoted to understanding hydrophobic interactions or the behavior of water in the presence of apolar interfaces due to their fundamental role in self-assembly of micelles, membrane formation and protein folding. This article reviews and compares two very recent computational works aimed at elucidating the changes in the thermodynamic behavior in the supercooled region and the liquid–liquid critical point phenomenon for water in contact with hydrophobic environments. The results are also compared to previous reports for water in hydrophobic environments.

Natural selection of academic papers

Academic papers, like genes, code for ideas or technological innovations that structure and transform the scientific organism and consequently the society at large. Genes are subject to the process of natural selection which ensures that only the fittest survive and contribute to the phenotype of the organism. The process of selection of academic papers, however, is far from natural. Commercial for-profit publishing houses have taken control over the evaluation and access to scientific information with serious consequences for the dissemination and advancement of knowledge. Academic authors and librarians are reacting by developing an alternative publishing system based on free-access journals and self-archiving in institutional repositories and global disciplinary libraries. Despite the emergence of such trends, the journal monopoly, rather than the scientific community, is still in control of selecting papers and setting academic standards. Here we propose a dynamical and transparent peer review process, which we believe will accelerate the transition to a fully open and free-for-all science that will allow the natural selection of the fittest ideas.

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.

Understanding the role of open peer review and dynamic academic articles

We welcome the commentary by L. Egghe (Scientometrics, this issue) stimulating discussion on our recent article “Natural selection of academic papers” (NSAP) (Scientometrics, 85(2):553–559, 2010) that focuses on an important modern issue at the heart of the scientific enterprise—the open and continuous evaluation and evolution of research. We are also grateful to the editor of Scientometrics for giving us the opportunity to respond to some of the arguments by L. Egghe that we believe are inaccurate or require further comment.

Heterogeneities in the Dynamics of Supercooled Water

Publisher Summary This chapter investigates the dynamics of liquid water by performing molecular dynamics (MD) simulations using the SPC/E model. The chapter identifies clusters of mobile molecules indicating the presence of spatially heterogeneous dynamics. Following this, the chapter examines the time and temperature dependence of the cluster size and illustrates that clusters are larger at the time when molecules start to diffuse and grow as temperature decreases. The average mass of mobile particle clusters, n*, is related to the diffusion constant and the configurational entropy. It is found that n* can be interpreted as the mass of the “cooperatively rearranging regions” that form the basis of the Adam-Gibbs theory of the dynamics of supercooled liquids. In the context of the potential energy landscape (PEL) approach, the diffusion of molecules is related to change of basins. By studying the dynamics of the system on the PEL, the chapter identifies clusters formed by the molecules with larger displacements between neighbor basins. These transitions between basins of the PEL are the elementary steps of diffusion and therefore of the spatially heterogeneous dynamics observed in the real MD-trajectory.