ELECTROCHEMICAL MEASUREMENT OF THE FREE-ENERGY OF ADSORPTION OF N-ALKANETHIOLATES AT AG(111)

The oxidative adsorption of n-alkanethiolates (CncH2nc+1S-) at Ag(111) in aqueous and methanolic solutions containing 0.5 M NaOH has been in vestigated by cyclic voltammetry and in-situ surface-enhanced Raman sp ectroscopy (SERS). Reversible adsorption of CncH2nc+1S- under active p otential control in solutions containing millimolar concentrations of CncH2nc+1S- provides a means to control the deposition of n-alkanethio late monolayers, and allows for the direct voltammetric measurement of the free energy of monolayer formation. Oxidative adsorption of short chain alkanethiolates (n(c) less than or equal to 6) in aqueous 0.5 M NaOH is characterized by two voltammetric waves, demonstrating that m onolayer formation involves at least two energetically distinct chemic al steps. The first voltammetric wave corresponds to the reversible an d rapid adsorption of CncH2nc+1S- at submonolayer coverages. The redox potential of this wave (E-1/2(I) = -1.19 +/- 0.02 V vs Ag/AgCl) is in dependent of n(c), suggesting that the interactions between adsorbed m olecules 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 elect rode potentials, corresponding to further adsorption of CncH2nc+1S- to yield a complete monolayer (Gamma(max) similar to 7.7 x 10(-10) mol/c m(2)). The redox potential for the second wave, E-1/2(II), is a functi on of chain length, shifting to more negative potentials with increasi ng n(c). The dependence of E-1/2(II) on n(c) reflects the influence of hydrophobic interactions and intermolecular forces between the hydroc arbon chains. For n(c) > 6, E-1/2(II) shifts to potentials negative of E-1/2(I), and the two voltammetric waves merge into a single wave, su ggesting that the more structurally ordered monolayer is energetically favored for longer chain lengths (i.e., n(c) > 6), In-situ SERS is us ed to establish the potential-dependent adsorption isotherm of n-hexan ethiolate adsorbed on roughened Ag electrodes, The potential dependenc e of the SERS intensities of the trans and gauche nu(C-S) stretching m odes provides a means to monitor the structural ordering of the alkane thiolate monolayer during electrochemical deposition. The electrochemi cal data are used to separate the total adsorption free energy (Delta G(ads)) into the individual contributions associated with the formatio n of the Ag(111)-S bond (-22.8 and -16.6 kcal/mol for the low- and hig h-density structures, respectively) and that associated with hydrophob ic interactions and intermolecular forces between hydrocarbon chains ( -1.02 +/- 0.04 kcal/mol per n(c)). Voltammetric data and Delta G(ads) values are also reported for the adsorption of CncH2nc+1S- (2 less tha n or equal to n(c) less than or equal to 16) onto Ag(111) from basic m ethanolic solutions.