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

Self-assembled monolayers (SAMs) of thioglycolic acid, 3-mercaptopropionic acid, 11-mercaptoundecanoic acid, 16-mercaptohexadecanoic acid and bis(carboxydecyl) disulfide are formed on gold electrodes, respectively. The surface pK_as of the ω-terminal acid groups in these monolayers are determined using electrochemical titration, to be 6.1, 5.3, 7.3, 7.9 and 7.3, respectively. The effect of chain length on the surface pK_a values of ω-carboxy alkanethiol SAMs is described. The SAM has a strong influence on the heterogeneous electron transfer rate constant, k_s, of the electroactive probe Fe(CN)₆^3-. The k_s values of Fe(CN)₆^3-, determined with cyclic voltammetry at the pH values of the inflection point of the electrochemical titration curves for the four monolayers with carbon number 2, 3, 11 and 16, are calculated to be 7.4 × 10^-3, 4.6 × 10^-3, 4.4 × 10^-3 and 2.6 × 10^-3 cm s^-1, respectively. The effect of time of immersion for monolayer 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 properties.