Electrode potential effect on the surface pK(a) of a self-assembled 15-mercaptohexadecanoic acid monolayer on a gold/quartz crystal microbalance electrode

Surface mass titrations of a self-assembled monolayer (SAM) of 15-mercaptoh exadecanoic acid on a gold/quartz crystal microbalance electrode have been performed at various electrode potentials. The apparent pK(a) of the monola yer, obtained from the pH value at half the total frequency change, shifted in a negative direction from 6.4 at -200 mV to 4.3 at -700 mV vs Ag/AgCl. When potentials more positive than 600 mV were applied, the pK(a) has a ten dency to shift in the positive direction. These results demonstrate that th e surface acid/base properties of the SAMs can be controlled by the electro de potential.