Double-layer-capacitance titration of self-assembled monolayers of omega-functionalized alkanethiols on Au(111) surface

The acid-base equilibrium of omega-functionalized alkanethiol monolayers on Au(lll) has been studied using the change in double-layer capacitance acco mpanied by the protonation-deprotonation of omega-terminals. The pK of omeg a-carboxyl alkanethiols on Au(lll) increases by four pH units. The shift be comes greater as the alkyl chain length increases. The same magnitude of th e pK shift to the acidic side occurs in the monolayers of aminoethane thiol . The surface pK varies little with the electrode potential, whereas the in crease in the supporting electrolyte concentration slightly diminishes the pK shift. The drawn-out shape of the titration curves is consistent with th e mean-field model taking account of the repulsive interaction between adso rbed molecules, indicating the significance of the strong electrostatic rep ulsion between adsorbed thiol molecules in its charged states. The magnitud e of the pK shift is, more than that predicted by the mean-field model, how ever, and suggests the considerable contribution from other factors that st abilize the uncharged state, for example, hydrogen bonding and low dielectr ic constant in the vicinity of the self-assembled monolayer. The possibilit y of counterion binding when the degree of deprotonation is large has been suggested.