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Ion-Pair Assisted Self-Assembly at the Solid-Liquid Interface

Gloria Olivier, Jonathan Brian Gilbert, and Joelle Frechette. Chemical and Biomolecular Engineering, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218

We demonstrate that non-covalent ion-pair interactions in solution can be employed at a solid-liquid interface, to control the lateral spacing of thiols in a self-assembled monolayer (SAM) on gold. Ion-pairs formed between the carboxylate tail-group of 16-mercaptohexadecanoic acid (MHA) and tetralkylammonium (TAA+) hydroxide salts of various alkyl side-chain lengths remain intact during self-assembled monolayer formation and result in a loose packing of chemisorbed MHA chains on the surface, despite the strong tendency of thiols to organize into a tightly-packed monolayer at the gold-solution interface. The ion-pair film can be converted into a loosely-packed MHA monolayer, by rinsing the SAM with a solution of potassium perchlorate, which releases the TAA+ from the surface. Contact angle measurements confirm that the ion-pair SAMs do not resemble fully-packed pure MHA SAMs and appear to have a mixture of methylene and methyl groups exposed at the surface due to the presence of TAA+ cations. X-ray photoelectron spectroscopy shows that the ion-pairs self-assemble in a 1:1 molar ratio of MHA:TAA+ on the surface and are covalently bound to the gold surface through the thiol head-group of MHA. Analysis of the Au4f, O1s, C1s, and N1s intensities of the ion-pair SAMs indicates the surface density of MHA decreases with increasing size of the TAA+ cation. These results suggest that steric hinderance created by the bulky side-chains of the TAA+ cation dictates the lateral spacing of MHA chains on the surface. Lastly, we show that increased lateral spacing of MHA chains afforded by the ion-pair SAMs may be important for the creation of molecular recognition devices, as these SAMs exhibit remarkably stronger binding between the carboxylate group of MHA and the quaternary ammonium ions compared to traditional densely-packed MHA monolayers.