Microscopic origin of the surface tension anomaly of water

We investigate the hydrogen bonding percolation threshold of water molecules at the surface of the liquid-vapor interface. We show that the percolation temperature agrees within statistical accuracy with the high-temperature inflection point of the water surface tension. We associate the origin of this surface tension anomaly of water with the sudden breakup of the hydrogen bonding network in the interfacial molecular layer

Applications of computational geometry to the molecular simulation of interfaces

The identification of the interfacial molecules in fluid-fluid equilibrium is a long-standing problem in the area of simulation. We here propose a new point of view, making use of concepts taken from the field of computational geometry, where the definition of the "shape" of a set of point is a well-known problem. In particular, we employ the $\alpha$-shape construction which, applied to the positions of the molecules, selects a shape and identifies its boundary points, which we will take to define our interfacial molecules. A single parameter needs to be fixed (the "$\alpha$" of the $\alpha$-shape), and several proposals are examined, all leading to very similar choices. Results of this methodology are evaluated against previous proposals, and seen to be reasonable.Comment: 22 pages, 8 figure

How Is the Surface Tension of Various Liquids Distributed along the Interface Normal?

The tangential pressure profile has been calculated across the liquid - vapor interface of five molecular liquids, i.e., CCl 4 , acetone, acetonitrile, methanol, and water in mole cular dynamics simulations using a recently developed method. Since the value of the surface tension is directly related to the integral of this profile, the obtained results can be interpreted in terms of the distribution of the surface tension along the interface normal, both as a function of distance, either from the Gibbs dividing surface or from the capillary wave corrugated real, intrinsic liquid surface, and also in a layerwise manner. The obtained results show that the surface tension is distributed in a 1 - 2 nm wide range along the interface normal, and at least 85% of its value comes from the first molecular layer of the liquid in every case. The remaining, roughly 10% contribution comes from the second layer, with the exception of methanol, in whic h the entire surface tension can be accounted for by the first molecular layer . C ontributions of the third and subsequent molecular layers are found to be already negligible in every case

Polarization effects at the surface of aqueous alkali halide solutions

The polarizability of ions, with its strong influence on their surface affinity, is one of the crucial pieces of the complex puzzle that determines the surface properties of electrolyte solutions. Here, we investigate the electrical and structural properties of alkali halide solutions at a concentration of about 1.3 M using molecular dynamics simulations of polarizable water and ions models. We show that capillary fluctuations have a dramatic impact on the sampled quantities and that without removing their smearing effect, it would be impossible to resolve the local structure of the interfacial region. This procedure allows us to investigate in detail the dependence of the permanent and induced dipoles on the distance from the interface. The enhanced resolution gives us access to the surface charges, estimated using the Gouy-Chapman theory, despite the Debye length being shorter than the amplitude of capillary fluctuations

Dynamics of the Water Molecules at the Intrinsic Liquid Surface As Seen from Molecular Dynamics Simulation and Identification of Truly Interfacial Molecules Analysis

Dynamic properties at the liquid-vapor interface of water are investigated at 298 K on the basis of molecular dynamics simulations and intrinsic surface analysis. The mean surface residence time and diffusion coefficient of the molecules as well as H-bond lifetimes are calculated at the liquid surface and compared to the bulk values. It is found that surface molecules have a non-negligible diffusion component along the surface normal, although this component is limited in time to 7-15 ps, a value comparable with the mean surface residence time. It is also seen that interfacial molecules move considerably faster, and their H-bonds live shorter, than in the bulk liquid phase. This finding is explained by the relation between the number of H-bonded neighbors and mobility, namely that molecules being tethered by more H-bonds move slower, and their H-bonds live longer than in the case of molecules of less extensive H-bonding. Finally, it is found that molecules residing long at the surface are clustering around each other, forming more and longer living H-bonds within the surface layer, but much less outside this layer than other interfacial molecules, indicating that longer surface residence is related to weaker interaction with the subsurface region. © 2016 American Chemical Society

Properties of the Liquid-Vapor Interface of Acetone-Water Mixtures. A Computer Simulation and ITIM Analysis Study

Molecular dynamics simulations of the liquid-vapor interface of acetone-water mixtures of different compositions, covering the entire composition range have been performed on the canonical (N, V, T) ensemble at 298 K, using a model combination that excellently describes the mixing properties of these compounds. The properties of the intrinsic liquid surfaces have been analyzed in terms of the Identification of the Truly Interfacial Molecules (ITIM) method. Thus, the composition, width, roughness, and separation of the subsurface molecular layers, as well as self-association, orientation, and dynamics of exchange with the bulk phase of the surface molecules have been analyzed in detail. Our results show that acetone molecules are strongly adsorbed at the liquid surface, and this adsorption extends to several molecular layers. Like molecules in the surface layer are found to form relatively large lateral self-associates. The effect of the vicinity of the vapor phase on a number of properties of the liquid phase vanishes beyond the first molecular layer, with the second subsurface layer already part of the bulk liquid phase in these respects. The orientational preferences of the surface molecules are governed primarily by the dipole-dipole interaction of the neighboring acetone molecules, and hydrogen bonding interaction of the neighboring acetone-water pairs. (Figure Presented). © 2015 American Chemical Society

Single Particle Dynamics at the Intrinsic Surface of Various Apolar, Aprotic Dipolar, and Hydrogen Bonding Liquids As Seen from Computer Simulations

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