Phase separation of alkanethiol self-assembled monolayers during the replacement of adsorbed thiolates on Au(111) with thiols in solution

Replacement of self-assembled monolayers (SAM) of hexadecanethiol (HDT) on the Au( 111) surface with 12-mercaptododecanoic acid (MDDA) in ethanol solution has been studied using voltammetry of the reductive desorption and scanning tunneling microscopy (STM). The exchange of adsorbed HDT molecules with MDDA dissolved in ethanol proceeds domainwise, that is, not in a random fashion. Two well-separated peaks, corresponding to the desorption of MDDA-rich domains and HDT-rich domains, appear in a voltammogram over the replacement process. The peak potential associated with the desorption of NDT-rich domains remains unchanged in the course of the replacement, indicating that the solubility of MDDA in HDT-rich domains is very small. STM imaging of the substrate shows that domains whose sizes are greater than 15 nm(2) are predominant and occupy 85% of the area of HDT-rich domains. The entire exchange process is pseudo-first-order with the rate constant being 9.1 x 10(-3) h(-1) in 1 mmol dm(-3) MDDA in ethanol at 31 degrees C. The reverse process, i.e., the replacement of adsorbed MDDA with dissolved HDT in ethanol, is much slower, suggesting the stabilization of MDDA monolayers by lateral hydrogen bonding. A significant shift in the peak potential of MDDA-rich domains during the replacement indicates the considerable dissolution of HDT in MDDA domains.