Friction titration measurements of electrochemically generated mixed alkylthiol monolayers on polycrystalline gold

Abstract

An electrochemical procedure was used to generate a heterogeneous mixed monolayer composed of 1-undecanethiol (UDT) and 11-mercaptoundecanoic acid (MUA) on polycrystalline gold and the interface was characterized over a range of solution pH using Lateral Force Microscopy (LFM). To generate the heterogeneous mixture, a single component layer of UDT was formed on the gold electrode and portions were removed by reductively desorbing molecules selectively from the Au(111) facets. The voids created by this method were backfilled with MUA using either short-term passive incubation or electrochemically assisted deposition and LFM was used to gather friction maps of the surface in pH adjusted KClO4. From pH 3 to 8, the friction of both UDT and MUA zones in the mixed layer were relatively constant but above this range, the friction of the MUA regions increased significantly. The elevated friction was attributed to the ionization of the MUA groups since pH induced frictional differences were not observed when backfilling with the non-ionizable thiol 11-mercapto-1-undecanol (MUD). The relative difference in friction between the MUA and UDT zones were fit to a modified Henderson-Hasselbach equation to extract the apparent surface pKa (or pK1/2) of MUA bound to the Au(111) facets. The results were similar for both passive and electrochemically backfilled MUA and rendered a pK1/2 of 10.3 ± 0.8. The elevated pK1/2 was attributed to hydrogen bonding of the COOH groups when tightly packed on Au(111) as well as the presence of UDT, which created a local hydrophobic environment near the MUA molecules.