PKa of an ω-carboxylalkanethiol self-assembled monolayer by interaction model

Abstract

An interaction model of ω-carboxylalkanethiol monolayers adsorbed on the Au(111) surface is presented in order to explain the shift of the sigmoidal pH-titration curve from the titration curve of acetic acid. The model is based on the six-coordinated Coulomb interaction between the nearest neighboring deprotonated carboxylic ions. The pK1/2 value (pH of the solution at which adsorbed carboxyl groups are half-ionized) calculated thermodynamically from the Coulomb interaction is smaller than the experimental value. The small shift is ascribed to stabilization of the protonated carboxyl groups by the hydrogen bonding. The hydrogen bonding interaction is introduced by use of a statistical mechanical approach for the cooperative phenomena. The calculation is carried out based on the Bethe approximation in order to take into account the difference between the six-coordination of the Coulomb interaction and the four-coordination of the hydrogen bonding. Better fitting to the experimental values is obtained by use of the usual values of the hydrogen bonding energy. As a result, the sum of the Coulomb interaction and the hydrogen bonding interaction, each of which is evaluated to be 3.65 pH units, equivalent to 0.22 V or 21 kJ mol−1, explains the experimental titration curve well.