Replacement of adsorbed alkanethiolate on Au with carboxyl-terminated thiol in solution: effect of alkyl chain length

We have investigated the effect of the alkyl chain length on the replacement of the self-assembled alkanethiolate on Au surface with carboxyl-terminated thiol dissolved in solution. Immersion time-dependent replacement of self-assembled alkanethiolates on Au substrate was monitored by lateral force microscope (LFM), contact angle measurement, and infrared spectroscopy (IRS). Both the self-assembled monolayers (SAM) of 1-octadecanethiol (ODT) and octanethiol (OT) were reacted with carboxyl-terminated 11-mercaptoundecanoic acid (MUA) molecules in ethanol. OT SAM was rapidly replaced by MUA molecules. More than 50% of OT molecules were replaced by MUA within the initial 50 h. In contrast, less than 10% of ODT molecules were replaced by MUA within the same time scale. This can be explained in terms of the relative stability between methyl- and carboxyl-terminated thiolates on Au surface, depending upon the chain length and the lateral hydrogen bonding.