Self-Assembled Monolayers of Symmetrical and Mixed Alkyl Fluoroalkyl Disulfides on Gold. 2. Investigation of Thermal Stability and Phase Separation

The investigation of the thermal stability of self-assembled monolayers of symmetrical and mixed alkyl fluoroalkyl disulfides on gold is reported. The monolayers were annealed in air at temperatures ranging from 60 to 110 °C for several hours and afterward characterized by contact angle measurements, polarized grazing incidence Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). In mixed alkyl perfluoroalkyl systems evidence was sought for heat-induced phase separation of the alkyl- and perfluoroalkyl-terminated molecules. The ester and amide linkages of the fluorocarbon chains are shown to be less stable than those of the hydrocarbon analogues. Fluoro substituents in α-position to the ester linkage make the ester bond susceptible to hydrolytic cleavage at temperatures between 60 and 70 °C, whereas the corresponding amide and the γ-fluoro-substituted esters remain stable up to 80−100 °C. After annealing for 10 h or longer at these temp...