The effect of surface preparation on apparent surface pKa's of ω-mercaptocarboxylic acid self-assembled monolayers on polycrystalline gold

We have measured the apparent surface pK(a) values of self-assembled monolayers (SAMs) formed from 3-mercaptopropionic acid (3-MPA) and thioctic acid (TA) on evaporated gold film electrodes with a range of surface roughness (R-a from 1.3 to 6.3 nm as measured by AC mode atomic force microscopy). The surface roughness is induced and controlled with electrochemical etching via multiple potential cycles between 0.2 and 1.5 V (versus Ag/AgCI/1.0 M KCl) in 100 mM H2SO4 with KCl concentrations ranging from 0.10 to 10 mM. The pK(a)'s are measured by determining the capacitance of the surface using electrochemical impedance spectroscopy in a range of controlled ionic strength (mu = 0.25 M) buffer solutions. The results for 3-MPA are striking, exhibiting a strong logarithmic correlation (R-2 = 0.988) between the KCl concentration used in the electrochemical etching solutions and the apparent surface pK(a). Over the range of KCl concentrations, the surface pK(a) of 3-MPA shifts from 6.5 to 8.4. However, the relationship between the KCl concentrations in the electrochemical etching solutions and the apparent surface pKa of TA is flat at 7.3 +/- 0.2 with an R-2 value of less than 0.3 for both linear and logarithmic relationships.