Gas-induced drying of nanopores

Here, we investigate the role of a dilute hydrophobic gas on the phase behavior of water confined in hydrophobic nanopores. Molecular dynamics showed that gas atoms are hydrophobically attracted within the pores, where even a single particle is able to induce spontaneous drying of the whole pore. The drying process is rationalized in terms of its free-energy landscape, revealing that the penetration of a gas atom is able to suppress the drying free-energy barriers of hydrophobic pores. Results provide insights into the role of gases on the wettability of nanopores and evidence of a possibile physical mechanism for the action of volatile anesthetics on some kinds of ion channels. Results also indicate a novel, bioinspired strategy for controlling bubble formation in nanopores for sensing and energy applications

Intrusion and extrusion of liquids in highly confining media: bridging fundamental research to applications

Wetting and drying of pores or cavities, made by walls that attract or repel the liquid, is a ubiquitous process in nature and has many technological applications including, for example, liquid separation, chromatography, energy damping, conversion, and storage. Understanding under which conditions intrusion/extrusion takes place and how to control/tune them by chemical or physical means are currently among the main questions in the field. Historically, the theory to model intrusion/extrusion was based on the mechanics of fluids. However, the discovery of the existence of metastable states, where systems are kinetically trapped in the intruded or extruded configuration, fostered the research based on modern statistical mechanics concepts and more accurate models of the liquid, vapor, and gas phases beyond the simplest sharp interface representation. In parallel, inspired by the growing number of technological applications of intrusion/extrusion, experimental research blossomed considering systems with complex chemistry and pore topology, possessing flexible frameworks, and presenting unusual properties, such as negative volumetric compressibility. In this article, we review recent theoretical and experimental progresses, presenting it in the context of unifying framework. We illustrate also emerging technological applications of intrusion/extrusion and discuss challenges ahead

Protein-Water and Water-Water Long-Time Relaxations in Protein Hydration Water upon Cooling—A Close Look through Density Correlation Functions

We report results on the translational dynamics of the hydration water of the lysozyme protein upon cooling obtained by means of molecular dynamics simulations. The self van Hove functions and the mean square displacements of hydration water show two different temperature activated relaxation mechanisms, determining two dynamic regimes where transient trapping of the molecules is followed by hopping phenomena to allow to the structural relaxations. The two caging and hopping regimes are different in their nature. The low-temperature hopping regime has a time scale of tenths of nanoseconds and a length scale on the order of 2–3 water shells. This is connected to the nearest-neighbours cage effect and restricted to the supercooling, it is absent at high temperature and it is the mechanism to escape from the cage also present in bulk water. The second hopping regime is active at high temperatures, on the nanoseconds time scale and over distances of nanometers. This regime is connected to water displacements driven by the protein motion and it is observed very clearly at high temperatures and for temperatures higher than the protein dynamical transition. Below this temperature, the suppression of protein fluctuations largely increases the time-scale of the protein-related hopping phenomena at least over 100 ns. These protein-related hopping phenomena permit the detection of translational motions of hydration water molecules longly persistent in the hydration shell of the protein

Slow dynamics of hydration water and the trehalose dynamical transition

We present results from molecular dynamics simulations of a solution of water and trehalose, a cryoprotecting disaccharide, upon cooling. We focus our attention on both the dynamics of hydration water and of the trehalose. Hydration water presents two slow relaxations. One is the α relaxation typical of glass formers and the second one is a long relaxation that was also found in proteins hydration water and appears coupled to the movement of the surface of trehaloses. Below 280 K trehalose aggregates and upon further cooling we find a dynamical transition for the trehalose aggregate at around 250 K similar to the well known Protein Dynamical Transition. When this transition happens the long relaxation time has a dynamical crossover. We hypothesize that this dynamical transition is a general feature that can be found not only in proteins but also in aggregates that interact with water and that have a flexible structure. In fact this feature has already been found not only in proteins hydration water but also in a colloidal microgel. In the known cases, including the one that we present here, water enhances movements of the surface of these aggregates above a certain temperature. The temperature of this dynamical transition ranges between 260 K and 220 K in all known cases

Structure and slow dynamics of protein hydration water

We report results on the structure, local order and dynamics of water surrounding a lysozyme protein. The local order of water molecules is as much tetrahedral as in bulk water already at close vicinity of the protein but the number of hydrogen bonds depends more on the distance from the protein and gradually recovers bulk value upon moving outer. The dynamics of water seems in general to be more affected than its structure by the presence of the protein. An extremely long-relaxation detected in hydration water appears in the first monolayer around the protein, and the slow down is enhanced at low temperature. The dynamics of water within a layer of thickness 6 Å is sub-diffusive up to about ∼1 ns, above 1 ns we observe a crossover toward a hopping regime over a length-scale larger than that of nearest neighbors molecules. This hopping seems connected to transient trapping of water molecules on some specific protein domains

Mode coupling theory and fragile to strong transition in supercooled TIP4P/2005 water

We study by molecular dynamics simulations supercooled water with the TIP4P/2005 potential. This model is able to predict many properties of water in a large range of the thermodynamic space in agreement with experiments. We explore the dynamical behavior and, in particular, the self intermediate scattering function of the oxygen atoms. We find that the structural relaxation in the range of mild supercooling is in agreement with the Mode Coupling Theory (MCT). The ideal MCT crossover takes place at decreasing temperature with increasing density. Deviations from the MCT behavior are found upon further supercooling. A crossover from the MCT, fragile, regime to a strong, Arrhenius, regime is found and it is connected to the presence of a liquid-liquid phase transition and the Widom line emanating from the liquid-liquid critical point

Fragile to strong crossover and Widom line in supercooled water: A comparative study

The aim of this paper is to discuss the relationship between the dynamics and thermodynamics of water in the supercooled region. Reviewed case studies comprehend bulk water simulated with the SPC/E, TIP4P and TIP4P/2005 potentials, water at protein interfaces, and water in solution with electrolytes. Upon supercooling, the fragile to strong crossover in the α-relaxation of water is found to occur when the Widom line emanating from the liquid-liquid critical point is crossed. This appears to be a general characteristic of supercooled water, not depending on the applied interaction potential and/or different local environments