An indirect laser-induced temperature jump determination of the surface pK(a) of 11-mercaptoundecanoic acid monolayers self-assembled on gold

The indirect laser-induced temperature jump (ILIT) method is used to determ ine the acidity (pK(a)) of monolayers composed of 11-mercaptoundecanoic aci d (MUA) self-assembled on vapor-deposited gold film electrodes in contact w ith either 0.10 or 1.0 M ionic strength NaClO4 electrolyte solutions. The I LIT technique may be used to measure the pK(a) of a surface-attached acid b ecause the magnitude of the ILIT response is related to the potential gradi ent at the electrode/electrolyte interface which is, in turn, related to th e extent of ionization of the acid. The relevant data are the potentials of zero ILIT response versus pH. A simple two-layer model of the acid-modifie d electrode/electrolyte interface and Gouy-Chapman double layer theory (mod ified for the onset of dielectric saturation effected by the large concentr ation of charge present at this interface when a substantial fraction of th e carboxylic acid moieties are ionized) were employed to analyze these data . This analysis gives pK(a) = 5.7 +/- 0.2 at 0.10 M ionic strength and pK(a ) = 4.4 +/- 0.2 at 1.0 M ionic strength while the total concentration of MU A comprising the self-assembled monolayer is determined to be essentially t he same at both ionic strengths, averaging (5.4 +/- 0.3) x 10(-10) mol/cm(2 ). Reasons for the observed variation of pK(a) with ionic strength are disc ussed.