Langmuir 1994,10, 4286-4294
4286
Partial Electron Transfer in Octadecanethiol
Binding to Gold?
Pawel Krysinski,* Richard V. Chamberlain 11, and Marcin Majda*
Department of Chemistry, University of California in Berkeley, Berkeley, California 94720
Received July 12, 1994@
Langmuir-Blodgett transfer of octanedecanethiol(Cl&H) containingmonolayers onto gold-coatedglass
slides under potentiostatic conditions leads to a quantitative characterization of Cl&H chemisorption on
gold. The thiol-gold bond involves a partial electrontransfer from sulfur to the gold substrate. The CL$H
partial charge number was determined to vary linearly with the potential of gold substrates from 0.30 at
-0.30 V to 0.45 at 0.70 V vs SCE. Only ca. 81%of Cl&H molecules appear to chemisorb upon LB transfer
at 20 mNlm. This is likely a result of some orientational restrictions (e.g. tilt and registry with the gold
surface lattice) imposed on the system by the lateral pressure during LB transfer. The requirement of
monolayer charge neutrality, stemming from a negligible capacitance of the Au/LB monolayer interface,
results in a dissociation of thiol's proton to an extent equal to the charge transferred due to C&H
chemisorption.
Introduction
The well-ordered structure, simplicity of preparation,
and wide ranging applications of self-assembled alkanethiol monolayers on gold surfaces have attracted the
considerable interest of scientists from a spectrum of
backgrounds from chemical p h y s i ~ i s t s l - ~and elec~ - ~analytica120-24
~
chemists.
trochemists5-13to ~ r g a n i c l and
This relatively new area is already a subject of several
review^.^^-^^ Research in this area has been concerned,
* To whom all correspondence should be addressed.
+Dedicated to Professor Dr. Hab. Zbigniew Galus, Warsaw
University, on the occasion of his 60th birthday.
Permanent address: Department of Chemistry, Warsaw University, ul Pasteura 1, 02-093 Warsaw, Poland.
Abstract published inAdvance ACSAbstracts, October 15,1994.
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0743-7463/94/2410-4286$04.50l0
to a large extent, with structural characterization of the
monolayers of n-alkanethiols, disulfides, and their w-derivatives on gold, silver, and other metal
Related research involves earlier studies of self-assembled
alkyltrichlorosilane monolayer^^^,^^!^' and LangmuirBlodgett
These monolayer structureshave been
used to model and to investigate interfacial properties of
biological membranes and organic materials in processes
and phenomena such as molecular recognition, iontransport, wettability, adhesion, friction, biocompatibility,
and other^.^^-^^ Structural stability and molecular order
of alkanethiol monolayers on gold allowed systematic
studies of long range electron t r a r ~ s f e r ' ~ and
! ~ ~opened
-~~
a number of possibilities in designing chemical
sensors.21-24,44,46
Formation of the sulfur-gold bond in alkanethiol selfassembly is, naturally, the process of fundamental importance to a large fraction of research mentioned above.
Surprisingly, then, the understanding of some key mechanistic elements involved in this reaction still remains
incomplete and contr~versial.'~,~~,~~-~~
Specifically, XPS
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Wrighton, M. S. Science 1991,252, 688.
0 1994 American Chemical Society
Cl&H Chemisorption on Gold
Langmuir, Vol. 10,No. 11, 1994 4287
data supported by IR spectroscopy were used to suggest
that chemisorption of alkanethiols on gold(0) surface
results in the formation ofgold(1) thiolate denoted as either
RS-Au(1) or RS-Au+.14,31z47*49
It has then been postulated
that this general mechanism requires loss of hydrogen as
H2 (eq l ) , formation of water (eq 2), or of some other
products of the surface oxidation such as hydrogen
peroxide (eq
3).25947149950
-
+ l/,H,
BRSH + 2Au(O), + 1/20,
- 2RS-Au(I), + H,O
BRSH + 2Au(O), + 0, - 2RS-Au(I), + H,O,
RSH + Au(O),
RS-Au(I),
(1)
(2)
(3)
Indeed, H2Oz was detected in one study involving selfassembly of propanethiol on gold powder in an oxygensaturated aqueous solution.50 Its release was postulated
to be a result of 0 2 and H2reaction following reaction 1.
The mechanistic schemes presented above seem to be
supported by the measurements of surface potential4*and
by the electrochemical studies of alkanethiol reductive
d e s ~ r p t i o n . ~Recent
~ - ~ ~ laser desorption mass spectrometric studies point at the chemisorption of alkanethiolates
rather than a l k a n e t h i ~ l s .Temperature-programmed
~~
desorption spectra of monolayers of methanethiol and
dimethyl disulfide on gold(ll1) allowed estimation of the
binding energy of the former to be 12-14 kcal/mol
(indicating physisorption) while the S-Au bond strength
in chemisorption of the latter was estimated to be ca. 45
k c d m 0 1 . ~Overall
~
then, as much as the thiolate-gold
bond formation has been well established, the exact
character of this bond and the identity, or even presence,
of the products of the surface oxidation reaction (eqs 1-3)
a r e not well known.
In this report, we present a novel electrochemical
method which combines Langmuir-Blodgett transfer
technique with either potential (at open circuit) or current
measurements (under potentiostatic conditions). This
method allows us to determine that the primary step of
alkanethiol chemisorption involves a partial electron
transfer from thiol to gold. The partial charge number
(charge per thiol molecule) is a linear function of the
potential of gold substrates and varies from 0.30 electron
at -0.300 V to 0.45 electron at 0.700 V vs SCE. Consideration of a negligible capacitance of the gold/ClsSH
interface following LB transfer and an associated charge
neutrality requirement leads to a postulate of partial thiol
I
T
1
Initial
0.50 cm
final
Figure 1. Schematic diagram illustrating Langmuir-Blodgett
transfer of a monolayer from the aidwater interface onto a
gold-coated substrate with concurrent monitoring of the
substrate's potential. Below, an inset shows the pattern of the
vapor-deposited gold film. The central rectangular area (A =
0.20 cm2)is coated with an LB monolayer as the substrate is
withdrawn from the subphase. The two lines mark the initial
and the final positions of the triple-phase line in the LB
experiments.
grade, Aldrich),NaC104, KC1, and KOH (all from Fisher Scientific,
ACS grade). House distilled water was passed through a fourcartridge Barnstead Nanopure I1 purification train consisting of
Macropure pretreatment, Organics Free, two ion-exchange
columns, and 0.2-pm hollow fiber final filter for removing
particles. The final resistivity of water used in the experiments
was in the range 18.1-18.3 MQ cm.
Electrode Fabrication Procedures. Electrodes were produced by vapor deposition of about 100 nm thick gold films
(99.95%Au, Lawrence Berkeley Laboratory) on microscopeglass
slides (Corning Plain Micro Slides). The rate of deposition was
ca. 5-10 k s . A 6-10 nm thick chromium underlayer was
evaporated first to provide good adhesion and stability of the
gold layer. The geometrical pattern of the vapor-deposited
electrodes was specially designed for the LB electrochemical
experiments and imprinted with the use of thin aluminum masks.
The pattern is shown in Figure 1(for a full description see text
in Results and Discussion). The surface area of its central
rectangular fragment was 0.20 cm2. Prior to the vacuum
depositionof Cr and Au films, glass slides were washed in chromic
dissociation upon chemisorption.
acid, rinsed thoroughly in Nanopure water, and then dried.
Subsequently, they were boiled in 50:30:20 2-propanoVmethanoV
Experimental Section
chloroform mixture, dried again, and transferred to the evaporator (Veeco 7700). After fabrication, vapor-deposited electrodes
Chemicals. 1-Octadecanol (Aldrich, 99%) and n-octadewere transferred and stored in a vacuum desiccator. They were
canethiol (Aldrich, 98%)were triply recrystallized from ethanol.
used in the Langmuir-Blodgett (LB) experiments typically
All other reagents were used as received from the supplier:
within 1 week from their fabrication. Immediately before the
chloroform (Fisher ACS certified), perchloric acid (70%reagent
LB experiments, gold electrodes were visually examined t o select
those free of any scratches or blemishes. The cleaningprocedure
(46) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J.Am. Chem. SOC.
involved rinsing with organic solvents (methanol, acetone,
1987,109, 733.
(47) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989,
2-propanol,chloroform)and Nanopure water. Subsequently, gold
5, 723.
electrodes were immersed into a fresh, hot (ca. 80 "C) chromic
(48) Evans, S. D.; Ulman, A. Chem. Phys. Lett. 1990,170, 462.
acid solution for about 25 s. The chromic acid treatment step
(49) Laibinis,P.E.;Whitesides,G.M.;Allara,D.L.;Tao,Y.-T.;Parikh,
was followed by an extensive rinsing with Nanopure water. Upon
A. N.; Nuzzo, R. G. J.Am. Chem. SOC.
1991,113, 7152.
completion of this step, the electrodes were dried in a stream of
(50) Widria, C. A.; Chung,
- C.; Porter, M. D. J. Electroanal. Chem.
argon and transferred immediately (in order to minimize their
1991, 310, 3$5.
(51) Walczak, M. M.: PoDenoe. D. D.: Deinhammer. R. S.:. LamD,
contamination) into the Plexiglas enclosure ofthe LB instrument.
- B.
D.; Chung, C.; Porter, M.D.Laigmuir 1991, 7, 2687.
Before each LB transfer experiment, the surface of a gold
(52) Weisshaar, D. E.; Walczak, M. M.; Porter,M. D.Langmuir 1993,
substrate was electrochemically reduced and conditioned by
9, 329.
voltammetric scanning in the potential range from -0.300 to
(53) Wiesshaar, D. E.; Lamp, B. D.; Porter, M. D. J.Am. Chem. SOC.
0.600 V at 50 mV/s directly in the subphase in the LB trough.
1992,114, 5860.
In the case of potentiometric experiments, the voltammetric
(54) Li, Y . ;Huang, J.; McIver, R. T., Jr.; Hemminger, J. C. J.Am.
Chem. Soc. 1992,114, 2428.
cycling was stopped at 0.0 V during an anodic scan, and then the
Krysinski et al.
4288 Langmuir, Vol. 10, No. 11, 1994
potentiostat was switched to open circuit conditions. The gold
substrates were allowed typically 10 min to attain a stable
potential value. Similarly, in the case of the potentiostatic
experiments, each potential value selected for a particular LB
transfer experiment was applied by interrupting an anodic scan.
Langmuir-Blodgett Procedures. The Langmuir-Blodgett
experiments were carried out with a KSVMinitrough instrument
operated under computer control. Routine Langmuir-Blodgett
procedures and the LB transfer protocol were described previously.55 C~~SWClsOH
monolayers were spread as chloroform
solutions at a surface concentration corresponding to 35 Az/
molecule. Following solvent evaporation, the monolayers were
compressed at a rate of ca. 4 &/(molecule/min) to a pressure of
20 mN/m. The LB transfers were carried out at that pressure
at a rate of 10 mm/min unless specified otherwise.
Instrumentation. The potential control of the gold electrode
substrates and current measurements during LB transfer
experiments were accomplished with a three-electrode potentiostat (CV 27 by BAS, Inc., West Lafayette, IN). The potentiometric measurements were done with the same instrument
operated at open circuit. The counter and reference (SCE)
electrodeswere placed in the LB trough behind the moving barrier
to prevent their interference with the monolayer compression
and transfer. In order to avoid C1- leakage, the reference
electrode was isolated from the subphase in the LB trough via
a salt bridge filled with the same electrolyte as that in the trough.
The current'potential, current'time and potentidtime data were
recorded on a X-Y recorder.
Results and Discussion
Langmuir-Blodgett Transfer under Potentiometric Conditions. The experiments described in this
and in the following sections concern standard LB
transfers of octadecane alcohol (CISOH) and mixed octadecane alcohol and octadecanethiol (C18SH) monolayers
from the water surface onto gold substrates fabricated by
vapor deposition of 100nm thick gold films on glass slides
(see Experimental Section). Behavior of Langmuir monolayers of ClBOH on the water surface is well-known in the
literature.26~2s
Pure ClaSH cannot be compressed on the
aqueous subphase above ca. 12 mN/m due to insufficient
polarity of the thiol
We showed earlier that CISSH and CISOH are fully miscible in their mixed monolayers
which combine the properties of the individual compone n t ~ Specifically,
. ~ ~
the collapse pressure of the mixed
system increases linearly with the increased mole fraction
of ClaOH. All LB transfer experiments discussed below
were carried out a t 20 mN/m. To accomplish this, the
ClSSH mole fraction must not exceed 70%.56
Figure 1shows schematically a n LB experiment with
potentiometric monitoring of the gold substrate. In a n
LB experiment, a solid substrate is slowly pulled vertically
up from the aqueous subphase across the water/air
interface. This results in a transfer of a monolayer from
the water surface onto the substrate's surface. During
the LB transfer, the monolayer on the water surface is
maintained a t a selected constant surface pressure by the
forward motion of the barrier of the Langmuir trough
controlled by the feedback mechanism of the instrument.
As we show below, the transfer of both CISOH and CISSWClsOH monolayers can be classified as, so-called,
reactive transfer57during which the surfactant molecules
interact directly with the substrate's surface at the
moment of transfer. The nonreactive transfer involves a
transfer of a monolayer onto a thin film of water which
subsequently is squeezed out as the monolayer is deposited
(55) Lindholm-Sethson, B.; Orr, J. T.; Majda, M. Langmuir 1993,9,
2161.
(56)Bilewicz, R.;Majda, M. Langmuir 1991, 7 , 2794.
(57)Gaines, G.L. Insoluble Monolayers at Liquid Gas Interfaces;
Interscience Publishers, J. Wiley C Sons: New York, 1966;Chapter 8,
pp 326-333.
0.150
-I
1
0.0
0
20
40
60
t/S
Figure 2. Typical potential-time transients recorded during
LB transfer of CISOHand C&H/ClsOH (70:30mol %) monolayers at 20 mN/m, from 0.05 M HC104 subphase onto goldcoated glass slides. LB transfer rate was 10 mm/min.
on the surface of the substrate. In the latter case, the
dynamic contact angle observed during the LB transfer,
8, is essentially zero while its value is significantly greater
than zero during a reactive transfer leading to a lower
water meniscus.57 The values of 6 observed on the gold
surface in our experiment were approximately 70" and
80"for ClsOH and C1BSWCl8OHmonolayers, respectively.
Since the dynamic contact angles on the glass surface are
close to zero, it is difficult to make a precise assessment
of 8 on gold. Nevertheless, the high values reported above
clearly demonstrate the reactive character of the monolayer transfer of gold.
The geometric pattern of the vapor-deposited gold films
on glass slides used in our experiments is shown in Figure
1. The lines coinciding with the top and the bottom edge
of the central rectangular part of the electrode mark the
initial and the final positions of the water meniscus in all
experiments. Thus, the top circular area was always kept
above the water surface and used to make the electrical
contact while the bottom circular area was always
submerged in the aqueous subphase. In the course of
each LB experiment, the rectangular part was withdrawn
above the water surface a t a constant rate (typically 10
m d m i n ) while coating it with a monolayer from the water
surface. Figure 2 shows two typical recordings of the
potential of the gold substrate monitored during the LB
transfer experiments carried out on 0.05 M HC104
(70:
subphase. In both cases of CisOH and C~~SWClaOH
30 mol %) monolayers, the initial open circuit potential
of the gold substrates was ca. 0.13 V vs SCE. Gold
substrates approximately reach this value in the course
of a 10 min equilibration in the subphase. As we show
in the next section, this value of a stable open circuit
potential is approximately 0.21 Vnegative ofthe potential
of zero charge (PZC) of the gold substrate.
Consider first the potential-time trace obtained during
LB transfer of CISOHmonolayer. Independent measurements showed that the transfer ratio of CISOH under these
conditions is 1.0 f0.05. A linear decrease ofthe electrode
potential is observed (Figure 2) during the withdrawal of
Langmuir, Vol. 10, No. 11, 1994 4289
CI&H Chemisorption on Gold
Scheme 1
1
alkanethiols is naturally expected to result in much
stronger interactions. Hence, in this case, we postulate
that a much larger decrease ofthe gold substrate potential
is due to two factors: (1) the double-layer charge “squeezing” as above and (2) electron transfer from C&H
molecules to gold substrate a s they bind to its surface.
Thus, we postulate that the primary event in the CISSH-gold bond formation can be represented by the
reaction
R-S
I
+ Au,
-
the rectangular section,58followed by a gradual increase
of the electrode potential aRer the completion of the LB
transfer when the entire rectangular area of the electrode
was brought above the water surface. Since the initial
potential of the gold substrate is negative of PZC, the
observed decrease of the potential is consistent with a n
increase of the negative excess charge density on the gold
surface remaining under water caused by a shift of the
excess negative charge from the upper part ofthe electrode.
In other words, as the CISOH monolayer is deposited on
the rectangular section of the gold substrate and the latter
is emersed dry from the electrolyte solution, the excess
negative charge stored at that interface is “squeezed”down
to the interface remaining under water and leads to the
observed decrease of the potential.
The double-layer charge “squeezing“ observed in this
experiment merely reflects the large difference between
the interfacial capacitance of the clean gold electrode
immersed in 0.05 M HClO4 (ca. 20pF/cm2)and that of the
gold surface coated with C180H monolayer in the air. The
latter can be assumed to be negligibly small. This
assumption is well justified since the interfacial capacitance of the gold surface coated with octadecanethiol or
similar monolayer assemblies in aqueous electrolytes is
only ca. 1 . 0 p F / ~ m ~ .Thus
~ J ~the
* ~ situation
~
at the end of
the experiment, when only the lower part of the electrode
remains under water and stores all the charge, can be
represented in terms of a n equivalent circuit in Scheme
1with C1<< C2 (since C1 G 0), where C1 and C2 represent
interfacial capacitances of the AdClsOWair and AdO.05
M HC1O4interfaces, respectively. Naturally, the amount
of charge accumulated in each of the two capacitors will
be directly proportional to the magnitude of their capacitances. Hence, essentially no charge can be stored a t the
AulCl8OHlair interface. Implications of this important
point will be seen in the sections below.
The gradual increase of the electrode potential observed
afier the LB transfer when the electrode is held in its
upper-most position (see insert in Figure 1) is due, most
probably, to a background faradaic process that slowly
consumes the negative excess charge. The very high input
impedance of the potentiometer (> lo9 S2) eliminates the
possibility of instrument “leakage” current as a reason
for the observed potential increase.
Let us consider now the second potential-time trace in
Figure 2 corresponding to the LB transfer of a mixed CISSWClsOH (70:30 mol %) monolayer. Unlike aliphatic
alcohols, for which weak physisorption on gold does not
involve partial electron t r a n ~ f e r , chemisorption
~~,~~
of
~~
(58)The magnitude of the potential decrease observed in such
experiment is independent of the presence or absence of oxygen in the
subphase electrolyte.
(59) Richter, J.; Lipkowski,J. J.Electroanul. Chem. 1988,251,217.
I
H
H
c, >> c1
+
R - S + ~ * * A U ~ 6e(4)
where 6 is thepartial charge number expressing the order
and the strength of this bond per alkanethiol molecule.61
Additional mechanistic steps accompanying the partial
electron transfer are discussed below. In view of the
notation used in eqs 1-3, it is perhaps worth pointing out
that no distinction is made between gold atoms on the
surface, as far as electron energetics are concerned,
whether they engage or do not engage in thiol-gold bond
formation. Electron energy at the gold surface is expressed
by the electrode potential.
As explained above, charge stemming from both doublelayer charge “squeezing”and C d H binding accumulates
at the clean goldelectrolyte interface resulting in a more
significant negative shift of the potential as shown in
Figure 2.
Langmuir-Blodgett T r a n s f e r under C o n s t a n t
Potential Conditions: ClsOH Monolayers. Quantitative interpretation of the changes of the potential of the
gold substrate in order to obtain the partial charge number
of C&H on gold requires the exact value of the doublelayer capacitance of the goldelectrolyte interface and its
dependence on the electrode potential. It is therefore more
convenient to carry out the experiments of Figure 2 under
constant potential conditions. In those experiments, 2
three-electrode potentiostat was used. The gold substrate
was connected as a working electrode. As previously, LB
transfer of the CISOH monolayer results in “squeezing”of
the double-layer charge upon emersion of a dry gold
substrate. In this case, however, excess charge removed
from the emersed gold surface cannot accumulate at the
part of the electrode under water since its potential is
kept constant by the potentiostat. Instead, the doublelayer charge is collected and recorded as discharging
current, idis. A typical current-time trace recorded at
0.10 V vs SCE in the course of LB transfer of CISOH is
shown in Figure 3. The observed current can be simply
expressed by62
,i
=# W d t
(5)
where uM is the excess charge density on the gold surface
immersed in the electrolyte solution (before the LB
transfer) and W d t is the rate of surface emersion (in
cm2/s) during the LB transfer. Since the latter is a n
adjustable parameter, the observed current gives the
double-layer charge density. (In practice, uM was obtained
by integration of the current transient above background.)
As expected, idis is linearly proportional to the rate of
withdrawal of the electrode surface, dA/dt (see Figure 41,
as long as the LB transfer rate is less then 50 m d m i n .
(60) Lipkowski,J.; Stolberg, L. In Adsorption of Molecules at Metal
Electrodes, Frontiers in Electrochemistry Series; Lipkowski,J., Ross, P.
N., Eds.; VCH Publishers, Inc.: New York, 1992; Vol. I, Chapter 4, pp
171-238.
(61) Schultze, J. W.; Vetter, K J. J.Electroanal. Chem. 1973,44,63.
(62) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 1980;pp
145-157.
Krysinski et al.
4290 Langmuir, Vol. 10,No. 11, 1994
S.D.
a
a
00
%OH
l##
J
120flA
30
25
--
0
-15 --
0
0
.H
5.0
u.u
t
I
0
-0.2
1
I
1
1
1
0.1
0.4
0.7
,-..
1
I
1.3
E [VJvs. SCE
Figure 5. Plots of charge densities vs substrate potential
recorded in the course of the LB transfer experiments: open
circles, total charge recorded with ClsSWCl8OH (70:30mol %I
monolayers;squares, charge recorded with ClsOH monolayers
corresponding to the double-layer charge density at the gold/
solution (0.05M HC101)interface; closed circles, charge due to
ClsSH chemisorption and the apparent ClsSH partial charge
number. The former was obtained as a difference between the
total charge and the double-layer charge. The bar indicates
the average standard deviation, for all the measurements, of
1.25 NC/cm2.
0
20
2
-10
-0.5
I
I
10
20
I
30
40
50
60
LB Transfer Rate [mdminl
Figure 4. Plot of current vs transfer rate recorded during LB
transfer of ClsOH monolayers from 0.05 M HC104 onto 0.2 cm2
gold-coated glass substrates at 0.20 V vs SCE.
A negative deviation observed a t 50 m d m i n marks a
breakdown of this experiment, as some electrolyte is
entrapped on the gold surface during LB transfer.63 This
results in an incomplete removal of the double-layer
charge.
The plot of the double-layer charge density vs potential
obtained in these experiments is shown in Figure 5 (closed
squares). It is possible to approximate these data by a
straight line. The crossing point of the linear regression
line with the potential axis gives an estimate of PZC of
0.33 V vs SCE. This value is in a good agreement with
the literature data obtained for gold(ll1) surfaces in
nonadsorbing electrolyte^.^^^^^ This is perhaps unexpected but not surprising, since vapor-deposited gold films
on mica, glass, and other substrates have been known to
Our vaporexhibit predominantly (111)
(63) Srinivasan, M.P.;Higgins, B. G.; Strove, P.; Kowel, s.T. Thin
Solid Films 1988, 159, 191.
(64)Lecoeur, J.;Andro, J.; Parsons, R. Surf. Sci. 1982, 114, 320.
(65) Hamelin, A.In Modern Aspects ofElectrochemistry; Conway, B.
E., White, R. E., Bockris, J. OM., Eds.; Plenum Press: New York, 1985;
Vol. 16, Chapter 1, pp 1-101.
(66) Kolb, D.M.;Schneider, J. Electrochim. Acta 1986, 31, 929.
(67) Zei, M.S.;Naka, Y.; Lehmpfuhl, G.;Kolb, D. M. J . Electroanal.
Chem. 1983,150, 201.
deposited films are likely to be microcrystalline because
they were deposited a t rather fast rates (ca. 10 k s ) a t
room temperature on chromium-coated glass slides. STM
images ofour gold substrates show, indeed, about 10times
smaller crystal domains (ca. 20 nm in diameter) compared
to those found in Au films on mica.34 It is worth pointing
out that the uMvalues were impossible to obtain a t E >
0.4 V in the presence of even small quantities of C1- ions
in the electrolyte solution. Chloride adsorption in this
potential range apparently prevents close interactions of
C&H with the gold surface during LB transfer required
to observe the double-layer charge squeezing effect.
Although special measures were taken to eliminate
chloride ions from the system, it is, nevertheless, possible
that the uMvalues a t E > PZC are low. This could have
resulted in a positive error in our estimate of the PZC
value. Charge density at potentials more positive than
0.6 V was essentially impossible to measure accurately
due to a relatively high background current. In that
potential range, we approximated the double-layer charge
density by the linear regression line through the data a t
more negative potentials as shown in Figure 5.
The slope of the surface excess charge density vs
potential plot in Figure 5 provides the integral capacitance
value of ca. 14 pF/cm2. This value appears to be low
compared to ca. 20pF/cm2that could be obtained over the
same potential range from the literature data for single
crystal gold(111)surfaces.65 Knowing that double-layer
capacitance is very sensitive to the structure of the
electrode surface, traces of impurities, and the type and
concentration of the electrolyte, we believe that this
discrepancy is not unusual, considering all the differences
in the experimental conditions between our system and
the reference systems in the l i t e r a t ~ r e .Overall
~~
then,
our charge density data in Figure 5 are reasonably
consistent with the literature values. This, together with
the proportionality of with the LB transfer rate (Figure
4), assures us that the LB transfer of C180H results in a n
approximately complete removal of the double-layer
charge as gold substrates are emersed dry from the
electrolyte solution.
(68) Hallmark, V. N.; Chiang, S.;Rabolt, J. F.; Swalene, J. D.; Wilson,
1987, 59, 2879.
R. J. Phys. Rev. Lett.
Langmuir, Vol. 10, No. 11, 1994 4291
CI8SH Chemisorption on Gold
LB Transfer of Mixed ClsSWCisOH Monolayer
under Controlled Potential Conditions. Consider
now the results of the potentiostatic LB transfer experiments obtained with mixed Cd3-YC18OHmonolayers (see
Figures 3 and 5). As expected on the basis ofthe discussion
in the previous section, when these experiments are
carried out a t E < PZC, a substantially higher current is
observed. At constant potential, current due to ClsSH
binding to gold is added to the double-layer discharging
current. Naturally, a t the potentials positive of PZC, the
two current components are of the opposite sign. Using
the same procedure of data analysis as above, we obtained
the total charge collected during the LB transfer experiments (see Figure 5 open circles). By subtracting from
the latter series the values of the double layer charge
density, we obtained the dependence of the charge
transferred due to ClsSH binding to gold vs the potential
of the gold substrate (Figure 5, closed circles).
In order to obtain the apparent partial charge number
(d), i.e. the fraction of electron charge transferred by each
ClsSH molecule (assuming their 100% activity-see discussion below), we measured the geometric transfer ratio
(the ratio of the surface area of the monolayer removed
from the water surface to the geometric surface area of
the gold substrate coated with the monolayer). Large (3
in. x 1in.) glass slides coated with gold on both sides were
used in these experiments. A transfer ratio of 0.98 f0.06
was obtained by averaging data obtained with four
different substrates. Knowing the transfer ratio and the
mean molecular area ofthe molecules on the water surface
a t the LB transfer pressure (20.1 f 0.3 AVmolecule), we
obtained the apparent C&H partial charge number (see
Figure 5, closed circles and the right-hand side ordinate).
The latter exhibits a weak, roughly linear dependence on
the potential of gold substrates increasing from 0.25 a t
-0.3 V to 0.36 at 0.7 V vs SCE.
The data a t potentials greater than 0.7 V are strongly
affected by the formation of gold oxide species and by the
direct electro-oxidation of ClsSH. Thiol derivatives are
known to be electro-oxidized from a n adsorbed state on
gold electrodes in this range of potential^.^^,'^ Oxidative
desorption of propanethiol from gold yielding propanesulfinic acid was observed recently by Widrig and cow o r k e r ~ .Some
~ ~ additional experiments carried out in
this potential range will be presented in a separate report.
We consider now some mechanistic implications that
can be derived from the data presented above. The net
charge recorded in the potential range from -0.3 to 0.7
V (closed circles in Figure 5) reflects the partial electron
transfer from thiol to gold as stated in eq 4 above. Thus
we can say that under our experimental conditions ClsSH
chemisorption leads to the formation of a weak coordination type bond with the surface gold atoms, involving a
transfer of approximately 0.24 to 0.36 electron (depending
on the potential of the gold substrate, and assuming that
all ClsSH transferred chemisorb) per alkanethiol molecule.
It becomes important to realize that this would leave
approximately +0.3 charge on each thiol group as the
ClsSH monolayer is transferred onto the gold surface. In
view of the fact that the capacitance of the interface above
water is essentially zero (see Scheme 1 and the related
discussion above), a charge of this magnitude clearly
cannot exist a t the interface under these conditions.
Explicitly, the condition of negligible capacitance of the
interface means that no charge can be stored on either
the metal or the monolayer side of the interface. Since
~
(69)Koryta, J.;F’radac, J. J.Electroanal. Chem. 1968,17, 177.
(70)Reynaud, J.A.;Malfoy, B.; Canesson, P. J.Electroanal. Chem.
1980,114,195.
no counterions can be transferred with the ClsSWCleOH
monolayer to match this charge (perchlorate ions are the
only anions in the subphase and their transfer as
counterions is sterically impossible and inconsistent with
the transfer ratio reported above), we postulate that a n
equivalent quantity of protons is released from the
monolayer as ClsSH molecules are transferred and bound
to the gold substrate. This is the only way that the system
can maintain charge neutrality of the ClsSH monolayer
above the water surface. We note that the postulated
proton dissociation is consistent with the expected weakening of the -S-H bond upon thiol binding t o gold. It is
also consistent with several Raman,31”1IR,29and mass54
spectroscopic studies of self-assembled alkanethiol monolayer on gold showing absence of the thiol proton. In a
related Raman investigation, absence of the thiol proton
was also reported in chemisorption of 6-mercaptopurine
on gold electrode^.^^ We should stress again that in our
experiments the proton dissociation takes place primarily
to maintain the required charge neutrality of the monolayer, and, thus, is a consequence of, rather than a
prerequisite to, the thiol-gold bond formation. Its extent
is linked directly to the magnitude of the partial charge
number. The extent of proton dissociation during alkanethiol self-assembly experiments may be different or,
indeed, complete due to the differences in the prevailing
conditions of the interfacial capacitance, presence of traces
of ionic species, etc. Recent ab initio geometry optimization of CH3S on Au(ll1) and Au(100) surfaces by Ulman
and co-workers modeling alkanethiol chemisorption predicts -0.7 electron charge on sulfur atom bound to on-top
sites and -0.4 charge for those bound to 3-fold hollow
sites on g ~ l d ( l l l ) . ’ ~
Extent of Cl&H Chemisorptionupon LB Transfer
of ClsSWCleOH Monolayers on Gold. Considering the
mechanics ofthe potentiostatic LB experiments, it is only
fair to ask whether all C18SH molecules transferred from
the water surface bind to the gold surface on the time
scale allowed by the LB transfer rate (530s). In other
words: how well do we approximate the equilibrium
conditions of self-assembly? To answer these questions,
it is interesting to point out that LB transfer (under
“reactive”conditions)can be considered as a 2-D equivalent
of self-assembly. In the LB transfer, a one-dimensional
“section” of a surface or a “line” of gold is exposed to a
two-dimensional condensed system of the assembling
molecules. That exposure time is admittedly only ca. 6
ps (for a 10 wide ”line” with the transfer rate of 10
m d m i n used in these experiments). However, there are
two important characteristics of LB transfer that weight
heavily toward acceleration of the “2-D self-assembly”.
The first one is the concentration of ClsSH in a monolayer
a t 20 mN/m which is effectively several orders of magnitude larger than those used typically in solution selfassembly experiments. The second, even more important
factor, is the fact that in the LB process, the surface is
exposed to a n already well-ordered array of ClsSH
molecules. In the conventional self-assembly experiments
carried out in 1mM ethanol solutions of octadecanethiol,
Bain and co-workers observed close to the limiting contact
angles on gold surfaces in less than 1min and proposed
that the self-assembly process proceeds in two distinct
phases.14 This was recently confirmed by near-edge XAFS
a
(71)Garrell, R. L.,private communication, 1994.
(72) Taniguchi, I.; Higo, N.; Umekita, K.; Yasukouchi, K. J. Electroanal. Chem. 1986,206,341.
(73)Sellers, H.;
Ulman, A.; Shnidman, Y.; Eilers, J. E. J.Am. Chem.
SOC.1993,115,9389.
4292 Langmuir, Vol. 10,No.11, 1994
500
400
1
e
t
Krysinski et al.
0
Table 1. Dependence of the C&H Partial Charge
Number and the Cl~SWClsOHLB Transfer Ratio on the
Roughness of Gold Substrates
120
v-
Qo*dep
o*de(n)/ LB transfer
QLB?
substrate type fiC/cmZ%,dl)
ratio
&/cmZ
1. Aulglassd
352
1.00 0.98f 0.06 15.7f 0.8 0.29i 0.02
2.Au foild
100
t
385
unroughened
3. Au foil'
428
roughened
4. Au foil'
619
roughened
5. Au foil'
1150
roughened
5
0
0.00
10
0
20
30
40
50
60
LB Transfer Rate [mdmin]
Figure 6. Plots of current and the corresponding chargedensity
vs transfer rate recorded during LB transfer of ClsSWClsOH
(70:30mol %) monolayers from 0.05 M HClO4 at 0.0 Vvs SCE.
I S.D.
1.09
0.99i 0.04 17.5f 0.8 0.32i 0.02
1.22
1.13f 0.08 22.0f 1.5 0.34f 0.03
1.76
1.35f 0.08 29.02c 1.5 0.39 0.03
3.27
1.38f 0.08 28.0 f 1.5 0.36i 0.03
a Charge due to electroreduction of the surface gold oxide film
produced in an anodic voltammetric scan extended to +1.3 V vs
SCE in 0.05 M HC104. Charge due to C&H chemisorption
obtained during potentiostatic LB experiments at +0.30 Vvs SCE.
Apparent value of ClsSH partial charge number. Data in this
row are the averages of four experiments done with different
substrates. e Roughening of the gold plate substrates involved
voltammetric cycling between -0.3 V and 1.2V vs SCE in 0.1 M
~ ~ 1 . 7 5
15
s p e c t r o ~ c o p y .The
~ ~ arguments presented so far would
suggest that all C18SH molecules could bind to gold.
To examine this issue further, we have carried out three
additional sets of experiments. Figure 6 shows the
dependence of current and charge recorded during the LB
transfers of C18SWClsOH monolayers as a function of the
substrate withdrawal rate. Linearity ofthis plot for rates
below 50 m d m i n (where one begins to transfer molecules
on a thin film of water rather than directly onto gold
surface), shows that the time scale of the experiment does
not affect the efficiency of Cl8SH bonding to gold. In the
second set of experiments, we measured the total charge
collected during LB transfer a t 0.0 V vs SCE as a function
of Cl8SH mole fraction in mixed Cl~SWCl8OHmonolayers.
The linearity of the observed charge, shown in Figure 7,
proves that all or a constant fraction of C&H molecules
transferred bind to gold.
In the third set of experiments, we have examined the
effect of roughness of the gold substrateson the LB transfer
ratio and on the charge due to C&H binding to gold. The
latter issue directly addresses the question whether all
C18SH molecules transferred onto gold in the LB experiments bind to its surface. Self-assemblyof octadecanethiol
on gold(ll1) surfaces is known to result in surface
monolayers exhibiting a ca. 30" tilt and a specific registry
with respect to the surface gold atoms.27 A different
monolayer structure may be obtained in the case of the
LB deposition experiments since the ClsSH molecules are
transferred in a compressed state where their projected
area per molecule on the water surface is only 20.1
molecule.
The following potentiostatic LB transfer experiments
were carried out at 1.5cm2thin gold foil substrates. Their
original surface was smooth but not optically flat. To
further increase the extent of surface roughness, we
followed a procedure involving repetitive voltammetric
cycling of the gold substrates between +1.2 V and -0.3
V vs SCE in 0.1 M KC1 solution.75 (The same procedure
is used to produce substrates for surface enhanced Raman
spectroscopic studies.) Roughened and unroughened
substrates were then pretreated chemically and electrochemically using the same procedures as those used for
the vapor-deposited gold on glass substrates. Subsequently, the relative extent of surface roughness was
determined by integrating current due to reduction of gold
oxide formed anodically by extending voltammetric scan
to 1.3V vs SCE in 0.05 M HC104. The transfer ratio and
the potentiostatic C1&3WClsOH LB transfer experiments
were done a t 0.30 V as described above. The substrate
potential was selected to coincidewith the experimentally
obtained value of the PZC to avoid double-layer charge
corrections. The results are collected in Table 1.
The gold oxide charge (Qodae) in the second column
illustrates a n increase of true surface roughness of the
gold substrates. The third column lists surface roughness
relative to Adglass. As expected, the LB transfer ratios
and the charge due to C&H chemisorption (QLB)increase
with the substrate roughness. This is consistent with the
reactive nature of the LB transfer of the ClsSH/Cl@H
monolayers. However, comparison of the transfer ratios
and the QLB values for the two most roughened substrates
(entries 4 and 5 ) indicates that monolayer transfer on
excessively rough surfaces no longer follows the microscopic contour of the surface.
The variation of the apparent partial charge number
( 6 )with surface roughness is the key feature of the data
in Table 1. It is clear that 6 is higher on rough surfaces
compared to relatively smooth Adglass substrates for
which roughness factor is known to be ca. 1.2.60 In view
of the finite precision of these measurements, we believe
that there are no significant differences between the 6
values for the entries 3, 4, and 5 in Table 1. Therefore,
we will use their average of0.36 & 0.03. Since this average
(74)Hahmer, G.; Woll, C.; Bach, M.; Grunze, M. Langmuir 1993,9,
1955.
(75)Gao, P.; Gosztola, D.; Leung,
Electroanal. Chem. 1987,233,211.
0.0
r
!
I
0
20
I
I
40
60
I
I
80
Mol % of CWSH
Figure 7. Plot of the total charge collected during LB transfer
(10 d m i n ) of ClsSWC18OH monolayers from 0.05 M HClO4
on gold substrates at 0.0 V vs SCE as a function of ClsSH mole
fraction. The average standard deviation bar is 1.0 pC/cm2.
L.-W. H.; Weaver, M. J. J.
CI&H Chemisorption on Gold
Langmuir, Vol. 10,No. 11, 1994 4293
99
n cc
no longer depends on the increasing roughness of the gold
S.D.
surface, it can be considered to be the true partial charge
number of C18SH chemisorption at 0.30 V. This value of
20
6 is 24%higher than the apparent partial charge number
obtained on the vapor deposited Adglass substrates. Since
the 6 values were calculated under the assumption of 100%
E
0.45 fi
reactivity of the transferred Cl8SH molecules, the lower
value of 6 on Adglass substrates indicates that only ca.
0
81%of C&H molecules actually chemisorb on gold upon
0.40 2
16
LB transfer. The true partial charge number can,
therefore, be obtained by multiplying the values in Figure
B
e
e
e
B
5 by 1.24. Thus, they increase from 0.3 at -0.3 V to 0.45
0
.
3 12
14
5 q
14 a
at 0.7 V vs SCE.
0
2
4
6
8
1
0
:
This important conclusion requires some discussion.
PH
Since it is apparent that the higher surface roughness
Figure 8. Plot of the charge-transferred due to ClsSH
leads to a n increase of the fraction of the thiol molecules
chemisorption on gold, and the ClsSH partial charge number
capable of chemisorption, we postulate that the LB transfer
(correctedfor incomplete chemisorption)as a function of pH of
the subphase in the LB transfer experiments. Data were
of a monolayer a t high pressures is followed by pressure
obtained at -0.20 V vs SCE in HC104, NaC104, and KOH
relaxation and partial respreading of the monolayer before
electrolytes.
These data represent the difference between the
chemisorption of Cl8SH. These events may, indeed, be
total charge and the double-layer charge (see Figure 5). The
necessary to allow the molecules to adopt appropriate
bar indicates the average standard deviation of 0.80 pClcm2.
surface orientation andor registry with respect to the
surface gold atoms. Such conditions for chemisorption
Schneider and B ~ t t r ythat
, ~ ~Porter’s interpretation is
would be met more readily on substrates with certain
burdened with a positive error since a significant portion
minimum microscopic surface roughness. Conversely,
of the voltammetric “desorption”in Porter’s experiments
these conditions may be lacking on surfaces composed,
is undoubtedly due to a double-layer charge that has to
partially, of atomically flat terraces over which monolayer
flow to the interface, since desorption of a surface
relaxation and respreading may be difficult. It appears
monolayer results in a large increase of its capacitance.
that the vapor-deposited Adglass substrates represent
Nevertheless, we attempted to carry out our measurethe latter type of surface. The dominance of the 111
ments in more alkaline electrolytes (see data in Figure 8).
domains on vapor-deposited Au films on glass and mica
Unfortunately, our experiments could not be done a t pH’s
surfaces is well-known in the l i t e r a t ~ r e (see
~ ~ ,also
~ ~the
, ~ ~ above 13 due to partial solubility of dissociated octadediscussion of the experimentally determined PZC value
canethiol from Cl8SWCl8OHmonolayers on very alkaline
above).
subphases. However, considering the rate of the increase
of the partial charge number in Figure 8 (corrected for
The preceding discussion leads to a prediction that a
incomplete chemisorption of ClsSH as described above),
higher extent of CleSH chemisorption might also be
we do not think that its magnitude a t pH 14 would be
observed when C18SWCl8OH LB films are transferred a t
much above its largest value obtained a t pH 13. It is
a lower surface pressure. Under such conditions, surface
interesting to point out that the observed small increase
monolayers are in a slightly more expanded state; for
of the partial charge number at and above pH 12 coincides
example, C&H mean molecular area increases from 20.1
with a n expected value of C18SH pKa77,78and could
A21moleculeat 20 mN/m to 21.3 A21moleculeat 8 mN1m.
therefore be explained by a n induction effect that allows
We have repeated the potentiostatic LB transfer experifor a larger fraction of a n electron to be donated to gold.
ments on Adglass substrates at 0.3 V at 8 mN1m and
This is not inconsistent with our earlier postulate of the
found, indeed, that a higher charge was transferred
coupling via charge-neutrality requirement between the
compared to 20 mN1m (19.2 f 1.2 pClcm2 was obtained
partial
electron transfer and the extent of proton disas an average of six runs compared to 15.7pClcm2in Table
sociation.
1). In view of the fact that the LB transfer ratio remained
Overall, the mechanism of C18SH chemisorption on gold
unchanged, this yields a partial charge number of 0.37 f
presented in this report is consistent (at least qualitatively)
0.02, a value essentially identical to that in Table 1 afier
with Porter’s electrodesorption experiments. Our results
correction for partial chemisorption. This result gives
could also be compared with the detailed studies of
strong support to our explanation of the partial chemichemisorption of sulfide ions by Schultze and c o - w ~ r k e r s ~ ~
sorption of C&H molecules in LB films transferred at
and hydrosulfide ions carried out by Briceno and Chandhigh pressures. We should point out that the higher
er.80 Schultze et al. reported the electrosorption valency
surface pressure conditions are, nevertheless, more adof S2- on gold to be -2 in the potential range -0.6 to 0.4
vantageous as far as precision and reproducibility of the
V vs SCE. The same value was obtained for HS- on gold
results are concerned.
by Briceno and Chander who also postulated a concurrent
We move now to a discussion of a pH effect on alkanethiol
dissociation of the hydrosulfide’s proton. The transfer of
chemisorption. In recent reports, Porter and co-workers
two electrons from sulfide to gold (observed in 1 M NaOH
described a series of electrochemical experiments aimed
solution) compared with the 0.3 to 0.45 electron transfer
a t desorption of alkanethiols from gold e l e ~ t r o d e s . ~ O - ~ ~accompanying ClsSH chemisorption reflects reasonably
Their experiments conducted in 0.5 M aqueous or ethanolic
(76)Schneider, T.W.; Buttry, D. A. J.Am. Chem. SOC.1993,115,
KOH solutions showed well-developed surface voltam12391.
metric peaks a t negative potentials (e.g. -1.35 V vs Agl
(77)Dalman, G.;Gorin, G. J. O g . Chem. 1961,26,4682.
AgCl for octadecanethiol). The authors assigned the
(78)Irving, R.J.; Nelander, L.; Wadso, I. Acta Chem. Scund. 1964,
18.
769.
charge associated with these peaks to be due to breaking
(79)Wierse, D.G.;Lohrengel, M. M.; Schultze, J. W. J . Electroanul.
of the thiolate-gold bond. Its value was 1.0 electron per
Chem. 1978,92,121.
alkanethiol, a value substantially larger than that re(80) Briceno,A.; Chander,S. J.Appl. Electrochem. 1990,20,506
and
512.
ported above. It is almost certain, as shown recently by
--
1
I
t
F
3
4294 Langmuir, Vol. 10,No.11, 1994
well the chemical differences between these two sulfur
species. Proton dissociation (and not HZrelease) observed
in the chemisorption of HS- 8o is fully consistent with our
postulate of partial proton dissociation.
Partial Charge Number and Electrosorption Valency. Chemisorption and physisorption at the electrode/
solution interface has been an area of considerable
importance and a subject of extensive investigations in
electrochemistry.81 The parameter describing the charge
supplied to the electrode surface from the external circuit
upon adsorption of a species is electrosorption valency61
or, accordingto a recent IUF'AC recommendation,82formal
partial charge number, ya, a quantity defined as
where uM is the excess charge density at the electrode
surface and rais the surface concentration ofthe adsorbed
species. In general, adsorption of a species a t the electrode
surface may result in a change of the excess charge density
for two reasons: one stems from a n inevitable change
(usually a decrease) of differential capacitance of the
electrodeholution interface, and the other related to charge
transfer between the adsorbing molecule and the electrodeS6l The IUPAC recommended term for the latter,
which may be negligible (as in physisorption), is partial
charge number.82 It is important to note that the formal
partial charge number is the only quantity accessible
experimentally. This is because chemisorption of molecules at the electrodeholution interface simultaneously
causes both effects mentioned above. In other words, the
partial charge number can be measured only when
adsorption does not affect the structure of the doublelayer-a virtually impossible scenario.82 We believe that
the LB transfer experiment carried out under potentiostatic conditions described in this report creates experimental conditions that allow, for the first time, a direct
determination of thepartial charge number independently
of the double-layer capacitance component. This is
accomplished by allowing chemisorption to occur only at
the triple-phase contact line as the electrode surface is
emersed dry from the electrode solution (see Figure 1).
Thus the structure and the capacitance of the electrode/
solution interface are never perturbed. The double-layer
charge ofthe electrodeholution interface, as demonstrated
above, can also be measured independently in a parallel
experiment involving physisorbed species (Cl8OH). The
measurements of the double-layer charge density by the
LB transfer of Cl8OH monolayer, followed by the total
charge measurements with mixed CleSWCl8OH monolayers, and a calculation ofthepartial charge number for
Cl8SH adsorption on gold as a difference of the two
constitute a n illustration of this approach. The only
limitation of this approach, besides its precision which is
admittedly not as high as impedance or chronocoulometric
methods,60 is the requirement that the adsorbent be
insoluble in water. This would require one to work, for
example, with long alkyl chain derivatives of adsorbents
of interest.
Conclusions
We have developed a coulometric technique that combines Langmuir-Blodgett monolayer transfer with constant potential current measurements. This technique
(81)See, for example, a series of recent reviews in Adsorption of
Molecules at Metal Electrodes. Frontiers in Electrochemistry Series;
Lipkowski, J.,Ross, P. N., Eds.;VCH Publishers, Inc.: NewYork, 1992;
VOl. 1.
(82) Trasatti, S.; Parsons, R. J.Electroanal. Chem. 1986,205, 359.
Krysinski et al.
can be used to measure charge associated with a transfer
of a monolayer from the aidwater interface onto a
conducting substrate under potentiostatic conditions. In
cases of weak monolayerlsubstrate interactions (physisorption), this charge corresponds to the double-layer
charge density on the substrateholution interface before
the LB transfer. In cases of chemisorption, a sum of the
double-layer charge and the charge transferred between
the molecules in the monolayer and the substrate is
obtained. The latter is illustrated in this report by LB
transfer of C~~SWC18OH.
In this case, the difference
between the total charge and the double-layer charge,
which can be obtained independently in the experiments
involving weakly interacting C18OH, gives the partial
charge number of the chemisorbed species, C&H. Thus
this technique allows one, for the first time, to obtain
partial charge number under conditions where chemisorption and the measurements of the related charge
transfer are not perturbed by the otherwise inevitable
changes of the double-layer capacitance that normally
accompany adsorption at the electrodeholution interface.
We have investigated the potential dependence of Cl8SH chemisorption on vapor-deposited gold films on glass.
The C&H partial charge number increases from 0.30 at
-0.30 V to 0.45 a t 0.70 V vs SCE. Thus C&H binding
to gold involves formation of a coordination type bond
whose character depends weakly on the potential of the
gold substrate. Clearly, this mechanism does not involve
gold oxidation, as postulated in the l i t e r a t ~ r e ,nor
~~~~~,~~
does it postulate reduction of proton or oxygen (eqs 1-3).
I t is well-known that in the potential range investigated
in this study the gold surface is in a reduced state.6O~6~-6~
To maintain electroneutrality of the surface monolayer
upon its transfer on gold and above the water surface, we
postulated that the charge transferred due to chemisorption is coupled to partial proton dissociation of the thiol
groups. This is due to the negligible capacitance of the
gold-Cl8SWCl8OH interface above water. The release
of protons is also consistent with weakening of the S-H
bond caused by thiol binding to gold but its extent is
governed quantitatively by the magnitude of the partial
charge number via the electroneutrality requirement.
Investigations of the effect of the surfaces roughness of
the gold substrates on the LB transfer ratios and the
apparent partial charge number revealed that only
approximately 81% of the Cl8SH chemisorb to gold when
a compressed Ci&WC180H LB monolayer is transferred
onto vapor-deposited Aufglass substrates. We postulate
that inability ofthe Cl8SH molecules that were transferred
in a n ordered array onto a flat gold substrate to assume
appropriate orientation and/or registry with the surface
gold atoms may be responsible for this incomplete chemisorption. Electrochemical roughening of the surface
apparently alleviates this problem by allowing the molecules to respread and readjust their registry and tilt with
respect to the substrate surface due to its microscopic
roughness. Consistent with this postulate, we determined
that LB transfer done a t a lower surface pressure also
results in a n increase of the partial charge number and
in the full extent of chemisorption. The values of the
partial charge number given above include the correction
for the incomplete chemisorption on Aufglass substrates.
Acknowledgment. We thank the National Science
Foundation for supporting this research under Grant
CHE-9108378. We also acknowledge and thank Dr.
Steven Feldberg for a very insightful discussion of some
double-layer problems related to this project.