Dewetting phenomenon: interfacial water structure at well-organized alkanethiol-modified gold-aqueous interface

The interfacial properties at well-ordered short-chain alkanethiol monolayer-aqueous interfaces are probed to understand the water structure near a hydrophobic surface. Monolayers of hexanethiol on highly oriented gold substrates have been prepared by various methods such as adsorption from alcoholic solution of the thiol, adsorption from neat thiol, and potential-controlled adsorption. The compactness and crystallinity of the monolayer have been probed using reflection-absorption infrared spectroscopy (RAIRS), atomic force microscopy (AFM), quartz crystal microbalance (QCM), and electrochemical techniques. The presence of a thin layer of solvent with reduced density/dielectric constant (termed "drying transition") close to the methyl groups is identified. This is based on reduced interfacial capacitance observed in the presence of an aqueous electrolyte solution as compared to the expected value for a well-ordered monolayer-aqueous interface. Atomic force microscopy allows the determination of the variation in the dielectric constant of the solvent medium as a function of distance from the monolayer head group. The thickness of the transition layer (interphase) is found to be approximately 2 nm. The phenomenon of drying transition is not unique to water; preliminary studies indicate that formamide, which has a two-dimensional hydrogen-bonded network, shows similar characteristics.