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2008, 112, 890-894 Published on Web 01/08/2008
D2O Water Interaction with Mixed Alkane Thiol Monolayers of Tuned Hydrophobic and Hydrophilic Character Ronald L. Grimm, Nicole M. Barrentine, Christopher J. H. Knox, and John C. Hemminger* Department of Chemistry, UniVersity of California IrVine, IrVine, California 92697-2025 ReceiVed: October 23, 2007; In Final Form: December 12, 2007
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.
Introduction Surface and interfacial water influences reactivity and functionality in biochemistry,1 atmospheric chemistry,2,3 and catalysis.4 Recent emphasis on atmospherically relevant heterogeneous reactions on organic or organic-coated particles highlights the need for greater molecular-level understanding of the hydrophilic and hydrophobic character of these vapor/condensed phase interfaces.5,6 In particular, interfacial water influences the chemical and physical properties of aerosol particles. Recent studies from this laboratory and others highlight atmospherically relevant reactions that occur in thin water films on surfaces. It has been proposed that both nitrogen dioxide hydrolysis and gas-phase nitric acid uptake occur in thin water films on surfaces and produce HONO, a major source of urban hydroxyl radical (OH).7,8 X-ray photoelectron spectroscopy experiments on ozone-induced oxidation of halide surfaces indicate water-vapordependent oxidation.9,10 Interfacial water similarly affects organic aerosol particles. Laboratory experiments demonstrate that bulk water in miscible droplets11 and the relative humidity surrounding immiscible organic droplets each affect gas-phase hydrogen halide uptake.12,13 Humidity also affects ozone uptake and reactivity14-16 as well as secondary organic aerosol formation and growth.17-19 Understanding the interactions between water and organic aerosol surfaces will further elucidate the details of these atmospheric heterogeneous reactions. * Corresponding author. E-mail:
[email protected].
10.1021/jp710257q CCC: $40.75
Organic aerosol particles are processed and aged by chemical oxidation and photolysis reactions, resulting in a chemically “textured” surface of hydrophobic and hydrophilic functionalities.20,21 Recent ultrahigh vacuum (UHV) experiments provide molecular-level insight into the interactions between adsorbed water and organosulfur self-assembled monolayers (SAMs) with various chemical functionalities that serve as proxy surfaces for atmospheric aerosol. Temperature-programmed desorption (TPD) spectroscopy22,23 as well as combined TPD/infrared reflectionadsorption spectroscopy (IRAS) investigations24-28 highlight different adsorbed water structures as the organic surface changes hydrophobic or hydrophilic character. These studies provide significant insight into water interactions with singlecomponent organic surfaces, but only a few studies have considered water interactions with multicomponent surfaces.26,27 Studying the interplay between surface hydrophilicity and water is critical for applying laboratory-based investigations to heterogeneous reactions in the atmosphere. Such studies motivate the present investigation. We employ TPD spectroscopy to characterize the interaction between D2O water adlayers and organosulfur SAMs of varying hydrophobic or hydrophilic character. Solution-phase techniques are used to functionalize a gold substrate with octanethiol and 6-mercaptohexanoic acid that serve as the hydrophobic and hydrophilic species, respectively. Previous microscopy studies of methyl-terminated and methyl ester-terminated thiols of similar chain length demonstrate that such mixtures randomly assemble into nanometer sized-domains with a fractional surface coveragecomparabletotherelativesolution-phaseconcentration.29-31 In the experiments, there is no indication of mesoscale or larger © 2008 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 112, No. 4, 2008 891
phase separation of the two thiols. Monte Carlo simulations on mixed methyl-terminated thiols similarly showed only nanometer-scale domain formation.32 Following SAM formation, we use UHV TPD experiments to characterize the water-organic interaction as a function of initial water coverage and the surface fraction of hydrophobic and hydrophilic headgroups. The desorption data is interpreted with the aid of a rectangular lattice model that qualitatively approximates the morphology and domain size for randomly assembled two-component thiol SAMs in the absence of inter-thiol interactions. Experimental Section Sample Preparation. A polished single-crystal Au(111) disk (MaTecK GmbH, Julich, Germany) 1 cm in diameter with
