SELF-ASSEMBLED OMEGA-HYDROXYALKANETHIOL MONOLAYERS WITH INTERNAL FUNCTIONALITIES - ELECTROCHEMICAL AND INFRARED STRUCTURAL CHARACTERIZATIONS OF ETHER-CONTAINING MONOLAYERS

The structure of omega-hydroxyalkanethiol monolayers containing an int ernal ether group is probed by traditional surface infrared reflection -absorption spectroscopy and a novel electrochemical method based on t he measured shifts in the potential of zero charge (pzc) upon introduc tion of the ether group. The analyses of the infrared and electrochemi cal data give complementary structural information. Vibrational bandwi dth measurements indicate that these monolayers exhibit crystalline pa cking. From a comparison of the integrated infrared absorption intensi ties between monolayer and bulk samples of the thiols, an average tilt angle of 29 degrees and an average twist angle of 40 degrees are dete rmined for the ether modified omega-hydroxyalkanethiol monolayers. The pzc of the ether modified monolayer coated Au electrodes is observed to shift by 130 +/- 40 mV from that characteristic of an unmodified om ega-hydroxyalkanethiol monolayer. A simple dipole analysis of the magn itude of these pzc shifts! assuming a twist angle of 40 degrees gives an average tilt angle of 23 +/- 8 degrees, in reasonable agreement wit h the infrared determination. In addition, the monolayers modified by an oxygen located at an even methylene site (from the S atom) are obse rved to give a positive pzc shift, while those located at odd sites gi ve strictly negative pzc shifts. Such an even/odd effect on the sign o f the pzc shift indicates that the S-C bond is directed away from the surface normal. This electrochemical approach to structural analysis i s generally applicable to crystalline monolayers which contain a net s tatic internal dipole.