pK(a) of an omega-carboxylalkanethiol self-assembled monolayer by interaction model

An interaction model of omega-carboxylalkanethiol monolayers adsorbed on th e Au(lll) surface is presented in order to explain the shift of the sigmoid al pH-titration curve from the titration curve of acetic acid. The model is based on the six-coordinated Coulomb interaction between the nearest neigh boring deprotonated carboxylic ions. The pK(1/2) value (pH of the solution at which adsorbed calboxyl groups are half-ionized) calculated thermodynami cally from the Coulomb interaction is smaller than the experimental value. The small shift is ascribed to stabilization of the protonated carboxyl gro ups 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 t o take into account the difference between the six-coordination of the Coul omb interaction and the four-coordination of the hydrogen bonding. Better f itting 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 interacti on 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 experimen tal titration curve well. (C) 1999 Elsevier Science S.A. All rights reserve d.