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 monolayer s of symmetrical and mixed alkyl fluoroalkyl disulfides on gold is rep orted. The monolayers were annealed in air at temperatures ranging fro m 60 to 110 degrees C for several hours and afterward characterized by contact angle measurements, polarized grazing incidence Fourier trans form infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (X PS), and atomic force microscopy (AFM). In mixed alkyl perfluoroalkyl systems evidence was sought for heat-induced phase separation of the a lkyl- and perfluoroalkyl-terminated molecules. The ester and amide lin kages of the fluorocarbon chains are shown to be less stable than thos e of the hydrocarbon analogues. Fluoro substituents in alpha-position to the ester linkage make the ester bond susceptible to hydrolytic cle avage at temperatures between 60 and 70 degrees C, whereas the corresp onding amide and the gamma-fluoro-substituted esters remain stable up to 80-100 degrees C. After annealing for 10 h or longer at these tempe ratures contact angle measurements, XPS, and FTIR reveal partial loss of fluorocarbon chains in monolayers of symmetrical fluoroalkyl disulf ides and mixed alkyl fluoroalkyl disulfides. In contrast, it is still possible to observe ordered domains with AFM in monolayers of mixed al kyl fluoroalkyl disulfides annealed at 100 degrees for 17 h. AFM scans in molecular resolution confirm that the lattice constant of the hexa gonal lattice remains unchanged (6.2 Angstrom). Since the monolayers o f both corresponding symmetrical disulfides show significantly smaller lattice constants, heat-induced phase separation can be excluded. The results indicate nonexisting to very low mobility of the molecules wi thin the layer at higher temperatures. Alternatively, the existence of disulfides rather than thiolates bound to the gold can explain the ob servations.