Water Structuring at Non-Polar Fluid Interfaces

Abstract

The structuring of water molecules at the water/vapour interface is an object of scientific interest for decades. After the first successful attempts to explore liquid water with the help of theoretical chemistry, the number of studies on this topic grows progressively. Most of them are focused on bulk water but there is still need of a more detailed research on surface water. In addition, interfaces with alkanes are interesting as being instructive from both biological and industrial perspectives. Since in both bio- and industrial applications water/air and water/oil interfaces are mediated by amphiphiles, the role of a surfactant monolayer on surface water structuring deserves more attention as well. In the present study several atomistic water models were chosen—non-polarisable (SPC, TIP3P, and TIP4P) and polarisable (SW-RIGID-ISO, SWM4-NDP, and COS/G2) and classical molecular dynamics simulations were carried out on bulk water, water/vapour and water/alkane (from pentane to nonane) systems, as well as on water/DLPC/vapour and water/DLPC/octane models. In all cases the temperature was kept at 298 K. Several structural properties of bulk and surface layers were examined by means of radial distribution functions and Voronoi analysis. Dipole moments, surface tension and hydrogen bonding were addressed too. The objective was to estimate the impact of accounting for polarisability on the water properties of interest and to select a cost-efficient water model for describing them, as well as to add new data to the existing knowledge about interfacial water structuring.