Structure Dependence of the Surface pKa of Mercaptoundecanoic Acid SAM on Gold

The surface pKa of mercaptoundecanoic acid (MUA) in its self-assembled monolayer (SAM) and in mixed monolayers with alkanethiol has been examined using the surface mass titration method based on a quartz crystal microbalance. The pKa value, determined from the pH at half the total mass change, shifted in a positive direction with the increase in the surface coverage of MUA in both monolayers, showing the repulsive interaction between carboxylates. The pKa at zero coverage depends on the monolayer structure; 4.8 for the SAM and 5.6 for the mixed monolayer.