Electrochemical Measurement of the Free Energy of Adsorption of n-Alkanethiolates at Ag(111)

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Abstract

The oxidative adsorption of n-alkanethiolates (CncH2nc+1S-) at Ag(111) in aqueous and methanolic solutions containing 0.5 M NaOH has been investigated by cyclic voltammetry and in-situ surface-enhanced Raman spectroscopy (SERS). Reversible adsorption of CncH2nc+1S- under active potential control in solutions containing millimolar concentrations of CncH2nc+1S- provides a means to control the deposition of n-alkanethiolate monolayers, and allows for the direct voltammetric measurement of the free energy of monolayer formation. Oxidative adsorption of short chain alkanethiolates (nc ≤ 6) in aqueous 0.5 M NaOH is characterized by two voltammetric waves, demonstrating that monolayer formation involves at least two energetically distinct chemical steps. The first voltammetric wave corresponds to the reversible and rapid adsorption of CncH2nc+1S- at submonolayer coverages. The redox potential of this wave ( = −1.19 ± 0.02 V vs Ag/AgCl) is independent of nc, suggesting that the interactions between adsorbed molecules are minimal at low surface coverages and that the energetics of adsorption are determined primarily by the strength of the Ag(111)−S bond. A second voltammetric wave is observed at more positive electrode potentials, corresponding to further adsorption of CncH2nc+1S- to yield a complete monolayer (Γmax ∼ 7.7 × 10-10 mol/cm2). The redox potential for the second wave, , is a function of chain length, shifting to more negative potentials with increasing nc. The dependence of on nc reflects the influence of hydrophobic interactions and intermolecular forces between the hydrocarbon chains. For nc > 6, shifts to potentials negative of , and the two voltammetric waves merge into a single wave, suggesting that the more structurally ordered monolayer is energetically favored for longer chain lengths (i.e., nc > 6). In-situ SERS is used to establish the potential-dependent adsorption isotherm of n-hexanethiolate adsorbed on roughened Ag electrodes. The potential dependence of the SERS intensities of the trans and gauche ν(C−S) stretching modes provides a means to monitor the structural ordering of the alkanethiolate monolayer during electrochemical deposition. The electrochemical data are used to separate the total adsorption free energy (ΔGads) into the individual contributions associated with the formation of the Ag(111)−S bond (−22.8 and −16.6 kcal/mol for the low- and high-density structures, respectively) and that associated with hydrophobic interactions and intermolecular forces between hydrocarbon chains (−1.02 ± 0.04 kcal/mol per nc). Voltammetric data and ΔGads values are also reported for the adsorption of CncH2nc+1S- (2 ≤ nc ≤ 16) onto Ag(111) from basic methanolic solutions.

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