4.6 Article

Supramolecular Ion-Pair Interactions To Control Monolayer Assembly

Journal

LANGMUIR
Volume 25, Issue 4, Pages 2159-2165

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/la803057x

Keywords

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Funding

  1. NSF [CMMI-0748094]
  2. U.S. Department of Homeland Security graduate fellowship
  3. 3M Corp
  4. MRSEC
  5. Directorate For Engineering
  6. Div Of Civil, Mechanical, & Manufact Inn [0748094] Funding Source: National Science Foundation

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We demonstrate that noncovalent ion-pair interactions in solution can be employed to control the molecular spacing of thiols in a self-assenibled monolayer (SAM) on gold. Ion-pairs formed between the carboxylate tail-group of 16-mercaptohexadecanoic acid (MHA) and tetraalkylammonium (TAA(+)) hydroxide salts of various alkyl side-chain lengths remain intact during chemisorption of the thiol on gold. The resulting ion-pair SAMs exhibit a 1:1 molar ratio of MHA:TAA(+) on the surface and are covalently bound to the gold surface through the thiol headgroup of MHA. We hypothesize that the incorporation of the bulky TAA(+) group competes with the strong tendency of the thiols to organize into an ordered monolayer, which highlights the strength of the ion-pair complexes. The ion-pair films can he 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 and X-ray spectroscopy (XPS) confirm the stoichiometry and covalent attachment of the monolayers. XPS analysis and contact angle measurements indicate that die surface density of bound MHA decreases with increasing size of the TAA(+) cation. These results suggest that steric hindrance created by the bulky side-chains of the TAA(+) cation dictates the lateral spacing of MHA chains on the surface.

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