D2O Water Interaction with Textured Carboxylic Acid-Terminated Monolayer Surfaces Characterized by Temperature-Programmed Desorption and Molecular Dynamics

We have used a combination of temperature-programmed desorption (TPD) experiments and molecular dynamics (MD) simulations to characterize interactions between water and carboxylic acid-terminated surfaces. In the TPD experiments, D2O water interacts with alkylthiol self-assembled monolayers (SAMs) comprised of 3-mercaptopropionic acid (C-3 acid), 15-mercaptopentadecaonic acid (C-15 acid), 16-mercaptohexadenoic acid (C-16 acid), or two-component monolayers of these constituents. Water TPD spectra exhibit broad, first-order desorption profiles with maximum desorption temperatures ranging from 168 K during desorption from the C-16 acid to a maximum desorption temperature of 200 K when water desorbs from the C-3 acid surface. For water desorption from the C-15 acid and for the two-component surfaces, desorption traces adopt intermediate profiles between these two extremes. Desorption activation energies range from 42 and 50 kJ mol(-1). In the MD studies, submonolayer concentrations of adsorbed water interact with slabs that simulate the one- and two-component SAMs employed in the TPD experiments as well as 4-mercaptobutanoic acid (C-4 acid). The MD simulations are characterized by distributions of the water-carboxylic acid interaction energies and the orientation of the carbonyl groups. The water-carboxylic acid interaction energy distributions show excellent qualitative agreement with the desorption activation energy values determined from the TPD experiments. Analysis of the carbonyl group orientation from the MD simulations shows strong effects of SAM chain length and whether the SAM contains ail odd or even number of carbon atoms. These "odd-even" and chain length effects support many of the results from the TPD experiments and the MD simulations. We discuss the effectiveness of employing MD simulations in concert with TPD studies as well as the atmospheric implications for the interaction of water with highly oxidized, multicomponent surfaces.