TEMPERATURE-PROGRAMMED DESORPTION AND INFRARED STUDIES OF D2O ICE ON SELF-ASSEMBLED ALKANETHIOLATE MONOLAYERS - INFLUENCE OF SUBSTRATE WETTABILITY

This paper examines the relationship between the thermal desorption of thin overlayers of condensed D2O ice and the wettability properties of the supporting substrate surface. Mixed self-assembled monolayers (SAMs) on gold with controlled chemical composition and wettability (-0.4 < cos theta < 1.0, where theta represents the static contact angle with water) derived from HS(CH2)(16)OH and HS(CH2)(15)CH3 were used as model surfaces. The D2O ice overlayers were prepared on these substrates by dosing of 0.1-30 langmuirs of D2O in ultrahigh vacuum at 80-120 K and characterized with temperature-programmed desorption (TPD). Infrared reflection-absorption spectroscopy (IRAS) was also used to characterize the structural progressions within the overlayers during the course of the TPD experiments, as well as at selected temperatures before and after annealing of the overlayer structure. The IRAS data show that amorphous-like ice is formed at sufficiently low temperatures (less than or equal to 100 K) on all mixed SAMs, regardless of their wettability. A structural transition of the D2O ice from amorphous-like to polycrystalline-like is observed above 100 K. The exact onset of the transition is strongly dependent on the wettability and varies from about 110 K on the extreme hydrophobic (CH3) substrate to 145-150 K on the hydrophilic (OH) substrate. On the most hydrophilic substrates, the strong hydrogen bond interaction with surface hydroxyls prevents completion of the structural transition before desorption of the D2O overlayer. This type of pinning of the D2O molecules to the substrate surface is most likely responsible for the sharp increase in desorption energy of similar to 0.2 kcal/mol which is seen at cos theta approximate to 0.6, a value defining the hydrophilicity limit above which, for our set of experimental parameters, the transition is no longer completed. The TPD data also support a model of the D2O overlayer as forming clusters of very different shape depending on substrate wettability-flat, two-dimensional clusters on hydrophilic SAMs and dropletlike, three-dimensional clusters on hydrophobic SAMs.