Effect of surface hydrophobicity on the dynamics of water at the nanoscale confinement: A molecular dynamics simulation study

Computational discovery of chemically patterned surfaces that effect unique hydration water dynamics

The interactions of water with solid surfaces govern their apparent hydrophobicity/hydrophilicity, influenced at the molecular scale by surface coverage of chemical groups of varied nonpolar/polar character. Recently, it has become clear that the precise patterning of surface groups, and not simply average surface coverage, has a significant impact on the structure and thermodynamics of hydration layer water, and, in turn, on macroscopic interfacial properties. Here we show that patterning also controls the dynamics of hydration water, a behavior frequently thought to be leveraged by biomolecules to influence functional dynamics, but yet to be generalized. To uncover the role of surface heterogeneities, we couple a genetic algorithm to iterative molecular dynamics simulations to design the patterning of surface functional groups, at fixed coverage, to either minimize or maximize proximal water diffusivity. Optimized surface configurations reveal that clustering of hydrophilic groups increases hydration water mobility, while dispersing them decreases it, but only if hydrophilic moieties interact with water through directional, hydrogen-bonding interactions. Remarkably, we find that, across different surfaces, coverages, and patterns, both the chemical potential for inserting a methane-sized hydrophobe near the interface and, in particular, the hydration water orientational entropy serve as strong predictors for hydration water diffusivity, suggesting that these simple thermodynamic quantities encode the way surfaces control water dynamics. These results suggest a deep and intriguing connection between hydration water thermodynamics and dynamics, demonstrating that subnanometer chemical surface patterning is an important design parameter for engineering solid-water interfaces with applications spanning separations to catalysis.

A review of the structure and dynamics of nanoconfined water and ionic liquids via molecular dynamics simulation

During the past decade, the research on fluids in nanoconfined geometries has received considerable attention as a consequence of their wide applications in different fields. Several nanoconfined systems such as water and ionic liquids, together with an equally impressive array of nanoconfining media such as carbon nanotube, graphene and graphene oxide have received increasingly growing interest in the past years. Water is the first system that has been reviewed in this article, due to its important role in transport phenomena in environmental sciences. Water is often considered as a highly nanoconfined system, due to its reduction to a few layers of water molecules between the extended surface of large macromolecules. The second system discussed here is ionic liquids, which have been widely studied in the modern green chemistry movement. Considering the great importance of ionic liquids in industry, and also their oil/water counterpart, nanoconfined ionic liquid system has become an important area of research with many fascinating applications. Furthermore, the method of molecular dynamics simulation is one of the major tools in the theoretical study of water and ionic liquids in nanoconfinement, which increasingly has been joined with experimental procedures. In this way, the choice of water and ionic liquids in nanoconfinement is justified by applying molecular dynamics simulation approaches in this review article.

Slow dynamics of water confined in Newton black films

Slowdown of translational and reorientational dynamics of water confined in Newton black films (NBFs) is revealed by molecular dynamics simulations. As a film becomes thinner, both translational and reorientational dynamics become slower. The polarization of water molecules in the macroscopic electrostatic field across the NBF and the coordination of Na(+) ions and surfactant anionic groups around water molecules concertedly lead to slowdown of water dynamics. The polarization effect is obvious for water not coordinated by Na(+) ions, which exhibits reorientational dynamics depending on initial dipole orientations. Na(+) ions and surfactant anionic groups retard dynamics of surrounding water by decreasing the hydrogen bond exchange probability and increasing the viscosity of water. The dependences of translational and reorientational dynamics on coordination environments of water are similar. Dynamics of water in positions close to the interfaces of NBFs are mainly retarded by Na(+) ions and surfactant anionic groups, while the macroscopic polarization effect plays the main role in influencing water dynamics in positions far from the interfaces. This study sheds light on the improvement of knowledge about the water dynamics slowdown mechanism in similar environments like reverse micelles and lamellar structures.

Wetting behavior of nonpolar nanotubes in simple dipolar liquids for varying nanotube diameter and solute-solvent interactions

Atomistic simulations of model nonpolar nanotubes in a Stockmayer liquid are carried out for varying nanotube diameter and nanotube-solvent interactions to investigate solvophobic interactions in generic dipolar solvents. We have considered model armchair type single-walled nonpolar nanotubes with increasing radii from (5,5) to (12,12). The interactions between solute and solvent molecules are modeled by the well-known Lennard-Jones and repulsive Weeks-Chandler-Andersen potentials. We have investigated the density profiles and microscopic arrangement of Stockmayer molecules, orientational profiles of their dipole vectors, time dependence of their occupation, and also the translational and rotational motion of solvent molecules in confined environments of the cylindrical nanopores and also in their external peripheral regions. The present results of structural and dynamical properties of Stockmayer molecules inside and near atomistically rough nonpolar surfaces including their wetting and dewetting behavior for varying interactions provide a more generic picture of solvophobic effects experienced by simple dipolar liquids without any specific interactions such as hydrogen bonds.