Kinetics of exchange of alkanethiol monolayers self-assembled on polycrystalline gold

We report on the exchange between a hydrophilic thiol (11-mercapto-l-undecanol) in a liquid or gas phase and a hydrophobic thiol (dodecanethiol) of similar length self-assembled on a polycrystalline gold surface for a wide range of temperatures and times. The molecular composition of the mixed monolayers is determined by the static water contact angle and X-ray photoelectron spectroscopy measurements. Atomic force microscopy in lateral force mode is used to characterize the molecular domains at the nanometer level. The exchange first occurs rapidly at the gold grain boundaries, with an activation energy of about 66 +/- 4 kJ/mol. Then, boundaries of ordered thiol domains are progressively replaced, and the exchange is slowed because only regions of increasing perfection are left untouched. Higher temperatures lead to faster kinetics of replacement and the removal of larger amounts of the original thiol. No significant difference could be detected between exchange occurring in an ethanol solution or in the gas phase, and the initial rate of exchange was found to be similar for the displacement of dodecanethiol by 11-mercapto-1-undecanol molecules and for the converse displacement.