MOLECULAR INTERACTION OF DMMP AND WATER-VAPOR WITH MIXED SELF-ASSEMBLED MONOLAYERS STUDIED BY IR SPECTROSCOPY AND SAW DEVICES

Mixed self-assembled monolayers have been used as a model surface to s tudy the adsorption of DMMP (dimethylmethylphosphonate) and water vapo r under conditions relevant for sensor development. The use of -OH and -CH3 terminated thiols allows us to vary the polar character of the s urface in a wide range. Using a surface acoustic wave sensor it is fou nd that the adsorption equilibrium for both vapors are strongly affect ed by the surface hydrophilicity. Further analysis of the measured wat er vapor isotherms allows us to extract information about the strength of adsorption. It. is found that at a surface composition of approxim ately 1:1 a transition occurs. DMMP vapor at concentrations in the ppb range is found to produce adsorption isotherms with saturation corres ponding to less than one monolayer. In this case in-situ IR measuremen ts of the adsorbed molecules reveals that the strength of the hydrogen bonds involved is independent of surface -OH density.