The voltammetric response of a Ag(111) electrode coated with a mixed m
olecular film of 11-mercaptoundecanoic acid and 1-decanethiol is prese
nted. The voltammetric current arises from the reversible, electric fi
eld-driven deprotonation of carboxylic acid groups. Voltammetric peak
heights for interfacial protonation and deprotonation are a function o
f solution pH, obtaining maximum values near the pK(1/2) (similar to 8
.5) of the molecular film.