Experimental and Theoretical Characterization of Adsorbed Water on Self-Assembled Monolayers: Understanding the Interaction of Water with Atmospherically Relevant Surfaces

A combination of experiments and molecular dynamic (MD) simulations has been applied to elucidate the nature of water on organic self-assembled monolayers (SAMs) before and after oxidation. SAMs mimic organics adsorbed on environmental urban surfaces. Water on clean or SAM-coated borosilicate glass surfaces was measured at equilibrium as a function of relative humidity (RH), using transmission Fourier transform infrared (FTIR) spectroscopy at 1 atm and 22 +/- 1 degrees C. The SAMs included C18 and C8 alkanes, as well as the C8 terminal alkene. Oxidation of the terminal alkene SAM was carried out with either KMnO(4) solution or gaseous O(3). The FTIR data showed at least two distinct peaks due to water on these surfaces, one at approximately 3200 cm(-1), which dominates at low RH (20%), and one at approximately 3400 cm(-1) at high RH (80%), which is similar to that in bulk liquid water. Temperature-programmed desorption (TPD) experiments showed that oxidation leads to more strongly adsorbed water. However, the amount of water in equilibrium with water vapor on the oxidized alkene was not significantly different from that on the unoxidized SAM, although there was a change in the relative intensities of the two contributing infrared peaks at 80% RH. MD simulations with hydrogen bond analysis suggest that molecules on the surface of small water clusters that dominate on SAM surfaces at low RH have fewer hydrogen bonds, while those in the interior of the clusters have three and four hydrogen bonds similar to bulk liquid water. Taken together, the experimental infrared data and MD simulations suggest a correlation between the relative intensities of the 3200 cm(-1)/3400 cm(-1) bands and the hydrogen-bonding patterns of the water on the surface and in the interior of clusters on the SAM surfaces. These studies suggest that water clusters will be present even on hydrophobic surfaces in the atmosphere and hence are available to participate in heterogeneous chemistry. In addition, oxidation of organic coatings on atmospheric particles or surfaces in the boundary layer may not lead to enhanced water uptake as is often assumed.