Effect of chain length on the surface properties of omega-carboxy alkanethiol self-assembled monolayers

Self-assembled monolayers (SAMs) of thioglycolic acid, 3-mercaptopropionic acid, 11-mercaptoundecanoic acid, 16-mercaptohexadecanoic acid and bis(carb oxydecyl) disulfide are formed on gold electrodes, respectively. The surfac e pK(a)s of the omega -terminal acid groups in these monolayers are determi ned using electrochemical titration, to be 6.1, 5.3, 7.3, 7.9 and 7.3, resp ectively. The effect of chain length on the surface pK(a) values of omega - carboxy alkanethiol SAMs is described. The SAM has a strong influence on th e heterogeneous electron transfer rate constants k(s), of the electroactive probe Fe(CN)(6)(3-). The k(s) values of Fe(CN)(6)(3-) determined with cycl ic voltammetry at the pH values of the inflection point of the electrochemi cal titration curves for the four monolayers with carbon number 2, 3, 11 an d 16, are calculated to be 7.4 x 10(-3), 4.6 x 10(-3), 4.4 x 10(-3) and 2.6 x 10(-3) cm s(-1), respectively. The effect of time of immersion for monol ayer formation on the defects present in the SAMs formed is discussed. The SAMs of bis(carboxydecyl) disulfide and 11-mercaptoundecanoic acid show the same surface pK(a) and k(s) values and thus possess the same surface prope rties.