The Faradaic impedance of a Au(111) electrode modified by three different self-assembled monolayers has been studied over a wide frequency range in aqueous NaClO4 solutions in the presence of 1 mM [Fe(CN)6] 4-/3-. The impedance characteristics of the bare electrode are compared with those of the same electrode modified with decanethiol, ω-hydroxydecanethiol, and 4′-hydroxy-4-mercaptobiphenyl. The 4′hydroxy-4-mercaptobiphenyl molecule is susceptible to π-π interactions and is of special interest as a mediator of electron transfer. In the case of the decanethiol and ω-hydroxydecanethiol systems the interfacial impedance can be modeled by the Randles circuit. However, for the 4′-hydroxy-4-mercaptobiphenyl system, the impedance behavior is more complex with a resistance in the monolayer parallel to and distinguishable from the expected Faradaic impedance. This behavior is quite different from that discussed to date in the literature and assumed in the interpretation of voltammetric data for modified electrodes. The parameters for these systems are reported here as a function of the type of electrode modification. The consequences of the present observations are discussed with respect to the phenomena observed.

The experimental determination of the ionic work function is briefly described. Data for the proton, alkali metal ions, and halide ions in water, originally published by Randles (Randles, J. E. B. Trans Faraday Soc. 1956, 52, 1573) are recalculated on the basis of up-to-date thermodynamic tables. These calculations are extended to data for the same ions in four nonaqueous solvents, namely, methanol, ethanol, acetonitrile, and dimethyl sulfoxide. The ionic work function data are compared with estimates of the absolute Gibbs energy of solvation obtained by an extrathermodynamic route for the same ions. The work function data for the proton are used to estimate the absolute potential of the standard hydrogen electrode in each solvent. The results obtained here are compared with those published earlier by Trasatti (Trasatti, S. Electrochim. Acta 1987, 32, 843) and more recently by Kelly et al. (Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2006, 110, 16066. Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2007, 111, 408). A comparison of the ionic work function with the absolute Gibbs solvation energy permits an estimation of the surface potential of the solvent. The results show that the surface potential of water is small and positive whereas the surface potential of the nonaqueous solvents considered is negative. The sign of the surface potential is consistent with the known structure of each solvent.

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Monte Carlo ͑MC͒ and density functional theory ͑DFT͒ results are reported for an electrolyte, consisting of charged hard spheres of diameter 3 Å with the solvent modeled as a dielectric continuum, near a charged flat uniformly charged electrode. These results are more interesting than the earlier MC results of Torrie and Valleau ͓J. Chem. Phys. 73, 5807 ͑1980͒; J. Phys. Chem. 86, 3251 ͑1982͔͒ for 4.25 Å spheres because the popular Gouy-Chapman ͑GC͒ theory is less successful for this system. The DFT results are in good agreement with the MC results. Both the MC and DFT results show particularly interesting features when the counterions are divalent. For such divalent counterions, the diffuse layer potential passes through a maximum magnitude, then declines, and ultimately has a sign that is opposite to that of the electrode charge. The consequences of this behavior are discussed. In contrast, the well-known GC theory consistently overestimates the magnitude of the diffuse layer potential, does not have any unusual behavior, and is in poor agreement with the simulation results.

The estimation of solvation parameters for monatomic ions is reviewed and new values for the Gibbs energy, enthalpy, and entropy of solvation are given on the basis of the most recent thermodynamic data. The results for alkali metal and alkaline earth metal cations, and for the halide anions together with the sulfide anion, are examined within the context of a model based on the mean spherical approximation (MSA) for the Gibbs energy of ion-solvent interactions.

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The impedance of a Au(111) electrode modified by three different self-assembled monolayers has been studied over a wide frequency range in aqueous NaClO4 solutions. The impedance characteristics of the bare electrode are compared with those of the same electrode modified with decanethiol, ω-hydroxydecanethiol, and 4‘-hydroxy-4-mercaptobiphenyl. In the case of the decanethiol system the interfacial impedance can be represented as a capacitor due to the self-assembled monolayer in series with the solution resistance. However, for the latter two systems, the impedance behavior is more complex with a high resistance in the monolayer parallel to the expected capacitance. This behavior is quite different than that discussed to date in the literature and assumed in the interpretation of voltammetric data for modified electrodes. The parameters for these systems are reported here as a function of electrolyte concentration at the point of zero charge on the bare Au(111). The consequences of the present observations are discussed with respect to the phenomena observed for these systems.