Advances in the study of supercooled water.

In this review, we report recent progress in the field of supercooled water. Due to its uniqueness, water presents numerous anomalies with respect to most simple liquids, showing polyamorphism both in the liquid and in the glassy state. We first describe the thermodynamic scenarios hypothesized for the supercooled region and in particular among them the liquid-liquid critical point scenario that has so far received more experimental evidence. We then review the most recent structural indicators, the two-state model picture of water, and the importance of cooperative effects related to the fact that water is a hydrogen-bonded network liquid. We show throughout the review that water's peculiar properties come into play also when water is in solution, confined, and close to biological molecules. Concerning dynamics, upon mild supercooling water behaves as a fragile glass former following the mode coupling theory, and it turns into a strong glass former upon further cooling. Connections between the slow dynamics and the thermodynamics are discussed. The translational relaxation times of density fluctuations show in fact the fragile-to-strong crossover connected to the thermodynamics arising from the existence of two liquids. When considering also rotations, additional crossovers come to play. Mobility-viscosity decoupling is also discussed in supercooled water and aqueous solutions. Finally, the polyamorphism of glassy water is considered through experimental and simulation results both in bulk and in salty aqueous solutions. Grains and grain boundaries are also discussed.

Structure and dynamics of nanoconfined water and aqueous solutions.

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed.

Computer simulation study of the structure of LiCl aqueous solutions: test of non-standard mixing rules in the ion interaction.

Aqueous solutions of LiCl have recently received much attention in connection with the study of the anomalies of supercooled water and its polyamorphism. From the point of view of computer simulation, there is need for a force field that can reproduce the structural and dynamical properties of this solution, and more importantly it is also simple enough to use in large scale simulations of supercooled states. We study by molecular dynamics the structure of the LiCl-water solutions with the force field proposed by Joung and Cheatham (J. Phys. Chem. B 2008, 112, 9020) appropriate for the water TIP4P-Ew model potential. We found that this force field does not reproduce the experimental ion pairing when the Lorentz-Berthelot (LB) rules are used. By incorporating deviations to the LB rules to obtain the crossed interactions between the ions, it is possible to get agreement with experiment. We have studied how the modification of the LB rule affects the structural and thermodynamic properties of the solution at increasing concentration of the solution from the low (around 2%) to medium (around 14%) concentration regimes. We also tested the transferability of the Joung and Cheatham force field to the water TIP4P/2005 model that works very well for supercooled water.