The Effect of Surface Preparation on Apparent Surface pK_a's of ω-Mercaptocarboxylic Acid Self-Assembled Monolayers on Polycrystalline Gold

We have measured the apparent surface p K 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/AgCl/1.0 M KCl) in 100 mM H 2 SO 4 with KCl concentrations ranging from 0.10 to 10 mM. The p K a 's are measured by determining the capacitance of the surface using electrochemical impedance spectroscopy in a range of controlled ionic strength (μ = 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 p K a . Over the range of KCl concentrations, the surface p K 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 p K a 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.