Molecular aspects of aqueous interfacial structures

Abstract

The structure of water at the interface between a solid and an aqueous phase is different from the structure of bulk water. Somewhat surprisingly, structural similarities have been found between water adjacent to a purely hydrophobic surface and water adjacent to a hydrophilic surface. In the presence of an insoluble monolayer on water, the vicinal water structure resembles the structure near a water-solid interface. Anomalies in the thermal properties of vicinal water are frequently observed and interpreted as higher order phase transitions in the structured entities at the interface. Water structures near a solid surface (or near an insoluble monolayer) may be stabilized by an ‘energy delocalization effect.’ At the pure air-water interface, no such stabilization is likely to occur; hence the surface tension of pure water is not expected to reveal thermal anomalies. This explanation reconciles previous anomalous results with more recent accurate measurements of surface tension carried out by Gittens, by Johanssen and Eriksson, and by Cini and his co-workers. A notable anomaly in interfacial properties of water occurs at around 29°–32°C. Assuming that natural sea water may be partially (or completely) covered by an insoluble monolayer, we expect that the air-sea interfacial properties may, therefore, also show an anomaly in this temperature range. The large surface entropy of vicinal water probably reflects an increased number of monomeric molecules. This anomaly is reflected both in the viscosity and the mechanical disjoining pressure of water between two solid surfaces (Peschel and Adlfinger) and in the ‘mechanical coupling’ between a solid surface and the vicinal water (data by Forslind and by Kerr and Drost-Hansen). We tentatively propose that this phenomenon may affect the rate of vaporization of sea water from a partially or completely covered air-sea interface. It is interesting to note that hurricanes appear to be spawned in the tropical ocean where the surface temperature exceeds approximately 30°C! A cause and effect relationship between this observation and the anomaly in surface entropy is suggested.