Effects of omega-functional groups on pH-dependent reductive desorption of alkanethiol self-assembled monolayers

Self-assembled monolayers (SAMs) of alkanethiols having various terminal groups on their omega-positions were formed on a Au111 electrode, and their reductive desorption was studied by linear sweep voltammetry, focusing on effects of solution pH on the desorption behavior. The peak potentials (Ep) of cathodic waves representing reductive desorption were found to be reflected by the pKa value of the thiol group and were negatively shifted with an increase in pH of the electrolyte solution. The magnitude of the pH dependency of Ep was greatly influenced by the hydrophobicity of the terminal groups. In the cases of alkanethiol SAMs having pH-sensitive terminal groups such as carboxyl and amino groups, their basicity was estimated from bending points appearing in the pH titration profile of Ep. This method allows direct determination of not only the pKa value of the arrayed groups but also that of the groups dissolved in solution simultaneously. The pKa values of the arrayed carboxyl groups in SAMs were larger by ca. 3 pH units than their original ones, while those for amino groups were smaller by ca. 2 pH units.