Web Release Date: January 8,
D2O Water Interaction with Mixed Alkane Thiol Monolayers of Tuned Hydrophobic and Hydrophilic Character
Department of Chemistry, University of California Irvine, Irvine, California 92697-2025
Received: October 23, 2007
In Final Form: December 12, 2007
Abstract:
We have used temperature-programmed desorption (TPD) experiments to characterize the interaction between D2O water and a self-assembled monolayer (SAM) of mixed hydrophobic and hydrophilic thiols on Au(111). Two-component SAM surfaces with tuned hydrophobic and hydrophilic character have been formed by exposing gold samples to solutions of octanethiol and 6-mercaptohexanoic acid in varying millimolar concentrations. Water desorption spectra from the pure hydrophobic surface exhibit first-order desorption at low coverage with the onset of zero-order character at ~1 Langmuir with narrow peaks at ~145 K. In contrast, desorption of D2O water from the pure carboxylic acid-terminated surface exhibits broad peaks shifted to higher temperatures. Interestingly, water TPD spectra from a 50% octanethiol/50% 6-mercaptohexanoic acid surface closely resemble desorption from the purely hydrophobic octanethiol SAM. Increasing the surface acid fraction beyond 50% shifts the TPD profiles to higher temperatures with long, high-temperature tails that approach the behavior of water desorbing from the hydrophilic 6-mercaptohexanoic acid surface. We discuss the implications for water interaction with "textured" organic surfaces as well as with atmospherically relevant organic aerosol particles. In particular, the results suggest that significant surface oxidation is necessary to impact the interaction of water with organic aerosol surfaces.
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