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

Abstract

The oxidative adsorption of n-alkanethiolates (C n c H2 n c +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 C n c H2 n c +1S- under active potential control in solutions containing millimolar concentrations of C n c H2 n c +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 (n c ≤ 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 C n c H2 n c +1S- at submonolayer coverages. The redox potential of this wave ( = −1.19 ± 0.02 V vs Ag/AgCl) is independent of n c , 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 C n c H2 n c +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 n c . The dependence of on n c reflects the influence of hydrophobic interactions and intermolecular forces between the hydrocarbon chains. For n c > 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., n c > 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 (ΔG ads) 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 n c ). Voltammetric data and ΔG ads values are also reported for the adsorption of C n c H2 n c +1S- (2 ≤ n c ≤ 16) onto Ag(111) from basic methanolic solutions.

Communication: Reactions and adsorption at the surface of silver nanoparticles probed by second harmonic generation

“…26,[30][31][32][33] The adsorption free energy obtained in this work is close to the value deduced from the infrared adsorption study of benzenedithiol onto a Ag film in benzene (−9.4 kcal/mol). The adsorption energy was found to widely vary: as small as ∼−5 kcal/mol for alkanethiols on a microcrystalline gold surface and ∼−20 kcal/mol for nalkanethiolates on Ag(111), and should be examined further.…”

On the Contrasting Effect Exerted by a Thin Layer of CdS against the Passivation of Silver Electrodes Coated with Thiols

“…This result is in good agreement with the observation that n-alkanethiols with n > 6 could remain physisorbed on the terraces [36]. However, our results seem to contradict the possibility of completely removing the physisorbed micelles during electroreduction, at least for C 16 SH. The important findings of this experiment are that the criterion for SAM layer formation cannot be based on surface limited phenomena (UPD), because of the interactions with other species on the surface but must be based on the inhibition of the redox processes (Ru 3+ /Ru 2+ ) occurring in solution, which seems be related only to the area actually available for the reaction.…”

“…While the structure of thiols on Ag(III) is known to give well packed and ordered structures [1,2,16,17,31], none of the measurements presented here allow us to determine the exact structure or degree of crystallinity of the SAM layers obtained in the presence of CdS. Our results on the occurrence of electron transfer are purely phenomenological, since they are limited to the system studied, particularly Ag(III) covered by n-CdS using a reduction process such as that of Ru(NH 3 ) 6 3+ .…”

“…The closest approach of the redox species to the electrode surface is limited by the thickness of the insulating SAM layer [16,17]. This barrier decreases the electron transfer rate by increasing the separation between the electrode surface and redox molecules.…”

FITC functionalized magnetic core–shell Fe3O4/Ag hybrid nanoparticle for selective determination of molecular biothiols

“…Thus, the SCN-containing FITC molecule was able to be replaced by biothiol molecules and released from the surface of silver shell. Besides, it was also noteworthy that the energy of Ag S bond was larger than that of Ag-SCN [33], which suggested the stronger interaction tendency between the silver surface atoms and the thiol groups, and thus, the FITC molecules could be substituted by biothiol molecules. Therefore, a sensing system for chromogenic detection of biothiols was presented based on fluorescence recovery of FITC.…”