Indirect laser-induced temperature jump study of the chain-length dependence of the pKa's of ω-mercaptoalkanoic acid monolayers self-assembled on gold

The indirect laser-induced temperature-jump (ILIT) technique is used to measure the pK(a)'s of self-assembled monolayers (SAMs on gold electrodes) composed of dibutanoic acid disulfide (DBAD) and 6-mercaptohexanoic acid (MHA) in contact with 1.00 M ionic strength NaClO4 electrolyte solutions. The ILIT technique may be used to determine the pK(a) of a surface-attached acid because the size of the ILIT response is related to the potential drop across the electrode/electrolyte interface that is, in turn, related to the extent of ionization of the acid. The pertinent data are the potentials of zero ILIT response (E-pzr) versus pH. It is found that the measured E-pzr versus pH data set for the DBAD monolayers is (within experimental error) the same as that measured for the MHA monolayers. Additionally, an analysis of the combined DBAD/MHA data set gives a pK(a) (4.5 +/- 0.2) and a total concentration (Gamma(T)) of either DBAD or MHA comprising the SAM (Gamma(T) = (5.4 +/- 0.3) x 10(-10) mol/cm(2)) that (again within experimental error) are the same as those previously determined for SAMs composed of 11-mercaptoundecanoic acid (MUA) in contact with 1.00 M ionic strength electrolyte solutions. This result implies that, at least in the range of thickness defined by monolayers made from DBAD and MUA, the properties of the microenvironment of the SAM/electrolyte interface are independent of monolayer thickness.