Redox-Induced Ion Pairing of Anionic Surfactants with Ferrocene-Terminated Self-Assembled Monolayers: Faradaic Electrochemistry and Surfactant Aggregation at the Monolayer/Liquid Interface

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• 作者：Eric R. Dionne ; Tania Sultana ; Lana L. Norman ; Violeta Toader ; Antonella Badia
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：November 20, 2013
• 年：2013
• 卷：135
• 期：46
• 页码：17457-17468
• 全文大小：767K
• 年卷期：v.135,no.46(November 20, 2013)
• ISSN：1520-5126

文摘

Oxidoreduction of self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate on gold in aqueous solutions of surface-active sodium n-alkyl sulfates (NaC_nSO₄) of 6, 8, 10, and 12 carbons is investigated by cyclic voltammetry and surface plasmon resonance. The effects of surfactant micellization and alkyl chain length on the redox response of the surface-tethered ferrocenes are examined. The SAM redox electrochemistry is sensitive to the surfactant aggregation state in solution. The nonideal behavior of the sodium alkyl sulfates at concentrations above the critical micelle concentration leads to a non-Nernstian variation of the SAM redox potential with concentration. The presence of micelles in solution results in decreased anodic-to-cathodic peak separations and anodic peak full widths at half-maximum. A longer alkyl chain length results in an increased ability of the alkyl sulfate anion to ion pair with the SAM-bound ferrocenium, resulting in oxidation of the ferrocene at lower potential. A comparison of the SAM redox potential at a fixed surfactant concentration of ideal behavior suggests a 4.5 脳 10⁴ difference in the ion-pairing abilities of the shorter-chain C₆SO₄^{鈥?/sup> and longer-chain C₁₂SO₄鈥?/sup>. One-half of the available SAM-bound ferrocenes are oxidized in the NaC_nSO₄ electrolyte. Surfactant anions adsorb and assemble onto the SAM surface by specific ion-pairing interactions between the sulfate headgroups and oxidized ferrocenium species, forming an interdigitated monolayer in which the surfactant anions alternate between a heads-down and heads-up orientation with respect to the SAM. The work presented points to applications of ferrocenylalkanethiolate SAMs as anion-selective membranes, probes of micelle formation, and surfaces for the electrochemically switchable assembly of organosulfates.}