AVS 46th International Symposium
    Surface Science Division Monday Sessions
       Session SS3-MoM

Paper SS3-MoM4
Kinetics of Water Uptake on a Hydrophobic Surface Studied by UHV-TPD

Monday, October 25, 1999, 9:20 am, Room 612

Session: Water-Surface Interactions
Presenter: T.R. Linderoth, Chalmers University of Technology, Sweden
Authors: T.R. Linderoth, Chalmers University of Technology, Sweden
P. Löfgren, Chalmers University of Technology, Sweden
V.P. Zhdanov, Boreskov Institute of Catalysis, Russia
B. Kasemo, Chalmers University of Technology, Sweden
Correspondent: Click to Email
Thin water (ice) films are currently receiving much attention. The motivations are both intrinsic scientific interest and their importance in areas such as astrophysics, biology and atmospheric chemistry. As a model of water adsorption on a very hydrophobic surface, we have investigated the uptake of water onto an octane (C@sub 8@H@sub 18@) covered Pt(111) surface and compared the results with previous studies on hydrophilic clean Pt(111). Our adsorption experiments were executed at substrate temperatures (T) in the range 100-120 K. A water-doser provided precise control of dosage flux (F) and exposure time (t). The adsorbed amount of water (@theta@) was determined by thermal desorption spectroscopy (TDS). In contrast to the case of hydrophilic metal substrates, we find that the apparent sticking (condensation) coefficient is well below unity for adsorption on the octane film (typically <<1 ML is adsorbed for exposures up to 10 ML). Furthermore, the adsorbed amount of water at a given exposure is strongly dependent on dosage flux and substrate temperature, being larger for higher F or lower T. Qualitatively, these findings can be understood if the binding of a single water molecule to the octane surface is so weak that adsorbed, diffusing water molecules can re-desorb before they manage to nucleate with other adsorbed molecules or incorporate into already stable water islands. A quantitative treatment of the corresponding kinetic equations yields: @theta@(t)~F@sup 3@exp[(3E@sub d@-2E@sub a@)/k@sub B@T]t@sup 2@ (E@sub d@/E@sub a@ are activation energies for desorption/surface diffusion of water molecules). The experimental data agree well with the predicted time and temperature dependences: We find that @theta@~t@sup 2.3@ and from the Ahrrenius analysis 3E@sub d@-2E@sub a@=0.20 eV is obtained. The observed flux-dependence of @theta@~F@sup 0.6@ is not in agreement with the model, however. Further investigations of the system are currently being undertaken.