Dw. Hatchett et al., ELECTROCHEMICAL MEASUREMENT OF THE FREE-ENERGY OF ADSORPTION OF N-ALKANETHIOLATES AT AG(111), Journal of the American Chemical Society, 120(5), 1998, pp. 1062-1069
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.