Stability and self-exchange in alkanethiol monolayers

Alkanethiols bearing radiolabeled (S-35) head groups have been incorporated into self-assembled monolayers (SAMs) on a variety of substrates to determine coverage, thermal- and photostability, and surface roughness. Spontaneous desorption of octadecanethiol SAMs immersed in solvent under ambient conditions is observed for all substrate/solvent combinations, including gold, silver, platinum, and copper and water, ethanol, tetrahydrofuran and hexane. A quantitative analysis of desorption suggests pseudo-first-order kinetics with rate constants in the range of 10(-5) s(-1) for THF. Evidence for multilayering on copper is found. Molecular hydrogen is not involved in the rate-determining step for desorption. Self-exchange of surface-labeled thiol with solution unlabeled thiol is also described by first-order kinetics. Both desorption and self-exchange experiments yield residual thiols at the surface which cannot be exchanged, and which are presumed to result from stronger binding at defect sites. Similarities in kinetics for exchange and desorption point toward a common mechanism for surface detachment, postulated to be a rate-limiting desorption step as a disulfide. Estimates for free energies of adsorption of a thiol generating molecular hydrogen and adsorption of a disulfide yield -5.5 and -24 kcal mol(-1), respectively. In conjuction with recent evidence that thiols are actually adsorbed as disulfides, RSSRAu(2(s)), the desorption is represented by RSSRAu(2(s)) --> RSSR + 2Au((s)) (slow). For exchange this is followed by 2RSH + 2Au((s)) --> RSSRAu(2(s)) + H-2 (fast), with some contribution from direct thiol/disulfide interchange for high concentrations of solution thiol, viz: RS*SRAu(2(s)) + RSH --> RSSRAu(2(s)) + R*SH. On clean gold surfaces, adsorption is shown to be diffusion limited. Finally, strategies for enhancing the stability of SAM monolayers made from thiols are discussed.