Water on Organic SAM Surfaces

Recent advances in atmospheric chemistry demonstrate that aerosol surfaces play a very important role in their chemistry. In particular, organic aerosol is aged and processed through photochemical and radical oxidation reactions. These reactions likely affect each aerosol particle uniquely, resulting in a patchwork of molecules and chemical functionalities even on originally homogeneous aerosol. The end result of these reactions is likely a surface with mixed hydrophilic and hydrophobic character.

The Hemminger group seeks to understand the interaction of water vapor with this patchwork aerosol surface on a fundamental molecular level. The degree of water interacting with a surface determines possible reaction pathways and the propensity for aerosol to act as cloud condensation nuclei (CCN). Self-assembled monolayers (SAMs) consisting of both hydrophilic and hydrophobic thiols simulate the patchwork aerosol surface. These thiols are allowed to adsorb onto gold from the gas phase which likely results in nanometer sized domains of hydrophobic and hydrophilic character. Following the formation of these monolayers, the surface is cooled and exposed to water which binds across the surface. As the sample is subsequently heated, water desorbs from the surface according to how tightly it binds to its specific location (it takes more heat to desorb from a hydrophilic region than a hydrophobic region!). We relate the desorption profile to the original surface to gain a more fundamental understanding of water's interaction with atmospheric aerosol.

This Table of Contents figure from our recent communication presents a number of relevant findings (J. Phys. Chem. C 2008. 112(4). 890-894). In the case of a 50% methyl-terminated / 50% carboxylic acid-terminated thiol surface, water exhibits weak interactions. This is weak adsorption energy is indicated by a low temperature desorption peak centered at ~140K, somewhat lower than water desorption from a purely methyl-terminated surface. As the surface concentration of carboxylic acid-terminated thiols increases, water desorption shifts to higher temperatures indicating progressively stronger water-surface interactions. Only at high relative surface acid concentrations does the water desorption resemble the desorption from the completely acid-terminated surface. In particular, the results suggest that significant surface oxidation may be necessary to impact the interaction of water with organic aerosol surfaces.