Self-Assembled Monolayers of Symmetrical and Mixed Alkyl Fluoroalkyl Disulfides on Gold. 1. Synthesis of Disulfides and Investigation of Monolayer Properties

The synthesis of a variety of symmetrical bis(alkyl) and bis(fluoroalkyl) disulfides as well asmixed alkyl fluoroalkyl disulfides containing ester or amide groups is described. Self-assembled monolayers of these compounds on gold formed by spontaneous adsorption from solution are studied by contact angle measurementsandpolarizedgrazing incidenceFourier transforminfraredspectroscopy (FTIR). The contact angles indicate tight packing of the molecules and the incorporation of both alkyl and fluoroalkyl chains in the monolayers. By means of FTIR, the typical tilted orientation of the alkyl chains is confirmed for monolayers derived from symmetrical alkyl disulfides, whereas, in self-assembled monolayers of the symmetrical fluorocarbon and the mixed disulfides, the molecules are oriented much closer to the surface normal direction. The wetting properties and the FTIR spectra of monolayers formed by coadsorption of an n-alkanethiol and a perfluorinated thiol are comparable to those formed by the self-assembly of the corresponding mixed disulfide. There is no indication for phase separation in any of the systems studied at room temperature. The mixed alkyl fluoroalkyl disulfides promise to form suitable monolayers on gold for the investigation of heat-induced phase separation on the gold surface.