Paper SS-ThP10
Interaction of Water with Pure and Mixed Monolayers of Hydrophobic and Hydrophilic Terminated Alkanethiols on Au(111)

Thursday, October 23, 2008, 6:00 pm, Room Hall D

The interaction of water adsorbed on self-assembled monolayers (SAMs) with moieties that are either hydrophobic, hydrophilic, or mixtures of both were characterized by temperature programmed desorption (TPD). The SAMs were fabricated on Au(111) from dilute ethanolic solutions of alkanethiols terminated with either a methyl group (hydrophobic) or a polar group (hydrophilic). The goal of our experiments is to understand the interaction of water with localized regions of hydrophilic character on an organic surface as a function of the spatial extent of the hydrophilic region (in the 10’s of nanometer size regime). TPD experiments of water desorption from pure hydrophobic surfaces show first order desorption kinetics at low coverage with an activation of energy of ~34 kJ mol^-1. The peak temperatures seen in the pure hydrophilic TPD spectra are 10-60 K higher than that of the pure hydrophobic case, indicative of a stronger interaction with water. Surfaces functionalized with a mixture of the hydrophobic and hydrophilic terminated alkanethiols display different properties from either of the pure cases and are not well described by simple weighted addition of the two limiting cases. The aim of this work is to develop a well characterized model system that mimics the surfaces of organic-coated aerosols found in the troposphere. These aerosol surfaces are typically hydrophobic when the aerosol is first formed and are thought to become more hydrophilic as the aerosol undergoes varying degrees of oxidation due to reactions with atmospheric oxidizing species such as O₃ and OH radicals. We will discuss how our results can be useful in understanding water interactions with atmospheric aerosols.