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

Self-assembled monolayers (SAMs) of alkanethiols having various terminal groups on their to-positions were formed on a Au(111) electrode, and their reductive desorption was studied by linear sweep voltammetry, focusing on effects of solution pH on the desorption behavior. The peak potentials (E-p) of cathodic waves representing reductive desorption were found to be reflected by the pK(a) 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 E-p 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 E-p. This method allows direct determination of not only the pK(a) value of the arrayed groups but also that of the groups dissolved in solution simultaneously. The pK(a) 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.