The surface pKa of mercaptoundecanoic acid (MUA) in its self-assembled mono
layer (SAM) and in mixed monolayers with alkanethiol has been examined usin
g the surface mass titration method based on a quartz crystal microbalance.
The pKa value, determined from the pH at half the total mass change, shift
ed 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.