Langmuir 2003, 19, 3357-3364
3357
Formation of n-Alkanethiolate Self-Assembled Monolayers
onto Gold in Aqueous Micellar Solutions of
n-Alkyltrimethylammonium Bromides
Dong Yan, Jacob L. Jordan, Vorakan Burapatana, and G. Kane Jennings*
Department of Chemical Engineering, Vanderbilt University, Nashville, Tennessee 37235
Received November 26, 2002. In Final Form: January 27, 2003
We have investigated the kinetics of formation for n-alkanethiolate self-assembled monolayers (SAMs)
onto gold in aqueous micellar solutions of n-alkyltrimethylammonium bromides (CmTAB; m ) 12, 14, 16,
and 18). The cationic micelles provide hydrophobic domains to solubilize the alkanethiols and facilitate
their delivery to the gold surface. The kinetics results for SAM formation in aqueous micellar solutions
of CmTAB are well described by a first-order Langmuir adsorption model. The measured rate constant
decreases exponentially with increasing hydrophobicity (chain length) of the alkanethiol adsorbate and
the CmTAB surfactant. The measured rate also decreases with increasingly positive potentials of the gold
electrode. A mechanism to describe SAM formation in CmTAB(aq) that is consistent with reported results
of solute exchange between ionic micelles in solution consists of (1) fragmentation of thiol-laden micelles
to produce submicelles, (2) incorporation of thiol-laden submicelles into adsorbed micelles (admicelles) at
the gold surface, (3) displacement of surfactants and counterions (rate-limiting step) by the alkanethiols,
and (4) chemisorption of the alkanethiol.
Introduction
There has recently been a growing interest in preparing
self-assembled monolayers (SAMs) from alternative solvents, such as aqueous micellar solutions1-3 and supercritical carbon dioxide.4-7 These solvents are environmentally friendly and enable the straightforward tuning
of solvent properties (the density of CO2 or the micellar
core size and volume fraction, etc.) to impact the formation
of SAMs. SAMs prepared from these alternative solvents
exhibit crystalline structures that provide exceptionally
high charge-transfer resistances.2,5 An important advantage of SAMs formed in aqueous micellar solutions is that
the kinetics of the assembly process can be monitored in
real time with electrochemical measurements.3 From a
fundamental standpoint, the measurement of kinetic rates
during SAM formation in micellar solutions can provide
mechanistic information concerning diffusion or reaction
rate limitations and/or the likely transfer of waterinsoluble solutes from micelles to physisorbed surface
micelles (admicelles).
We have recently reported the kinetics of formation for
n-alkanethiolate self-assembled monolayers (SAMs) onto
gold from aqueous micellar solutions of hexaethylene glycol
monododecyl ether (C12E6) and heptaethylene glycol
monododecyl ether (C12E7).3 In these nonionic surfactant
systems, micellar cores provide hydrophobic domains to
solubilize alkanethiols in aqueous solution and facilitate
* To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (615)343-7951.
(1) Liu, J.; Kaifer, A. E. Isr. J. Chem. 1997, 37, 235-239.
(2) Yan, D.; Saunders, J. A.; Jennings, G. K. Langmuir 2000, 16,
7562-7565.
(3) Yan, D.; Saunders, J. A.; Jennings, G. K. Langmuir 2002, 18,
10202-10212.
(4) Cao, C. T.; Fadeev, A. Y.; McCarthy, T. J. Langmuir 2001, 17,
757-761.
(5) Weinstein, R. D.; Yan, D.; Jennings, G. K. Ind. Eng. Chem. Res.
2001, 40, 2046-2053.
(6) Zemanian, T. S.; Fryxell, G. E.; Liu, J.; Mattigod, S.; Franz, J. A.;
Nie, Z. M. Langmuir 2001, 17, 8172-8177.
(7) Yan, S.; Jennings, G. K.; Weinstein, R. D. Ind. Eng. Chem. Res.
2002, 41, 4528-4533.
their delivery to the gold surface where they are incorporated into a growing molecular film. The kinetics of
SAM formation in C12E6(aq) and C12E7(aq) depends on
the micellar size, the concentration of solubilized alkanethiol, and various molecular factors that affect the
release of the alkanethiol from the micelle.3 The kinetics
data for SAM formation in aqueous micellar solutions of
C12E6 and C12E7 are best fit by a diffusion-limited, secondorder Langmuir adsorption model that accounts for
diffusion of the thiol-laden micelles from the bulk solution
to the proximity of the surface and release of the
alkanethiols from the micelle, most likely into admicelles
at the metal surface. The rate constant decreases exponentially with alkanethiol chain length, consistent with
an activated diffusion process for the release of the
alkanethiol from the micelle. The results in these nonionic
systems support a collision-induced mechanism for the
release of alkanethiols from solution-phase micelles to
admicelles before the alkanethiol is incorporated into the
SAM (Figure 1a).
If interactions between thiol-laden micelles and adsorbed micelles are important during the micelle-assisted
formation of SAMs, then the choice of surfactant could
greatly affect the transport of alkanethiols from the
micelles to the surface and therefore impact the kinetics
of assembly. Previous studies of solute exchange between
micelles in solution have shown that nonionic micelles
exchange insoluble solutes primarily by a collision-driven
fusion-fragmentation mechanism (Figure 1b, pathway
1).8-10 In contrast, electrostatic repulsion inhibits exchange
of solutes in ionic micelles.10,11 These charged micelles
are believed to exchange solutes by a fragmentationgrowth mechanism (Figure 1b, pathway 2) that is slower
(8) Rharbi, Y.; Li, M.; Winnik, M. A.; Hahn, K. G. J. Am. Chem. Soc.
2000, 122, 6242-6251.
(9) Rharbi, Y.; Winnik, M. A. Adv. Colloid Interface Sci. 2001, 8990, 25-46.
(10) Rharbi, Y.; Winnik, M. A. J. Am. Chem. Soc. 2002, 124, 20822083.
(11) Malliaris, A.; Lang, J.; Zana, R. J. Phys. Chem. 1986, 90, 655660.
10.1021/la026912r CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/13/2003
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Langmuir, Vol. 19, No. 8, 2003
Yan et al.
Figure 1. (a) Proposed method that alkanethiols are transferred to the surface in nonionic micellar solutions of C12E6(aq) and
C12E7(aq) (see ref 3), from solution-phase micelles to admicelles prior to chemisorption of the thiol. (b) In nonionic micellar solutions,
exchange of insoluble solutes occurs primarily by a collision-driven fusion-fragmentation mechanism (pathway 1).8 The alternative
process in ionic micellar solutions involves solute exchange by a fragmentation-growth mechanism where a solute-containing
submicelle breaks apart from a normal-sized micelle and then grows or becomes incorporated into an empty micelle (pathway 2).10
Table 1. Surfactants Studied
micelle
cmc
(mM)12,13 structure Nagg14,15
surfactant
composition
C12TAB
C14TAB
C16TAB
C18TAB
CH3(CH2)11N+(CH3)3, BrCH3(CH2)13N+(CH3)3, BrCH3(CH2)15N+(CH3)3, BrCH3(CH2)17N+(CH3)3, Br-
Table 2.
16
3.5
0.9
0.3
spherical
spherical
spherical
spherical
53
77
144
1H
NMR-Determined Alkanethiol Concentration
(mM) in Aqueous Micellar Solutionsa
surfactant
Csurf - cmc
C10SH
C12SH
C12TAB
10
16
24
48
16
16
10
10
0.42
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.18
0.45
0.52
1.0
1.0
1.0
1.0
1.0
C14TAB
C16TAB
C12E6
C12E7
C16SH
numbers in comparison with those of nonionic surfactants.3 These surfactants do offer advantages over nonionic
surfactants such as C12E6 and C12E7, since they are
significantly less expensive and their behavior at metal
surfaces may be affected by applied potential.16 In this
paper, we use experimental variables such as surfactant
chain length, alkanethiol chain length, electrolyte addition, and applied potential to develop an enhanced
understanding of the factors that influence the micelleassisted formation of SAMs with cationic surfactants.
Results
0.17
1.0
0.30
a Enough thiol is added to form a 1 mM solution if all the thiol
is solubilized.
than the fusion-fragmentation mechanism exhibited in
nonionic systems.10,11 These reports suggest that the
mechanism of alkanethiol release in ionic micellar systems
may be altered from that in nonionic micellar systems
due to the presumably different pathways for solute
exchange between micelles and admicelles.
In this paper, we explore the formation of SAMs in
aqueous micellar solutions of cationic alkyltrimethylammonium bromides to gain additional insight toward the
mechanism of micelle-assisted SAM formation. By using
cationic micellar solutions, we seek to gain unprecedented
control over the formation of high-quality SAMs by
harnessing the combination of hydrophobic interactions,
reaction or diffusion limitations, and applied potential
that can be utilized in aqueous media. Table 1 lists various
surfactants studied in this work, their critical micelle
concentration (cmc),12,13 structure, and aggregation number.14,15 All these cationic surfactants tend to form
spherical micelles with higher cmc’s and lower aggregation
(12) Israelachvili, J. Intermolecular & Surface Forces; 2nd ed.;
Academic Press: San Diego, CA, 1992.
(13) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys.
Chem. 1993, 97, 6024-6033.
(14) Berr, S. S. J. Phys. Chem. 1987, 91, 4760-4765.
(15) Imae, T.; ABE, A.; Taguchi, Y.; Ikeda, S. J. Colloid Interface Sci.
1986, 109, 567-575.
Solubilization of Alkanethiols in Micellar Solutions. We have used 1H NMR to determine the solubility
of a series of even-chained n-alkanethiols in n-alkyltrimethylammonium bromides and poly(ethylene oxide)
dodecyl ethers. Table 2 displays the concentration of
solubilized alkanethiol in the micelles when enough thiol
is added to form a 1 mM solution, a typical concentration
used to study the formation of SAMs. As shown in Table
2, the thiols with longer chains are less soluble than those
with shorter chains in micellar solutions due to the greater
molecular volume required for solubilization.17 The concentration of solubilized thiols increases with surfactant
concentration and with surfactant chain length, since both
of these parameters increase the volume fraction of the
solubilizing micellar cores in the aqueous solution. By
comparing the same concentration of surfactant above
the cmc (Csurf - cmc ) 10 mM), Table 2 also shows that
C12E6 and C12E7 are superior to the cationic C12TAB in
solubilizing alkanethiols, due in part to their tendency to
form micelles with higher aggregation numbers and, thus,
larger micellar cores.
Effect of Surfactant Concentration on Kinetics
of SAM Formation. A notable advantage of forming
SAMs in aqueous solution is that electrochemical measurements can be used to monitor the formation process
in real time. We have used interfacial capacitance
measurements to assess the kinetics of SAM formation
from aqueous micellar solutions in situ.3 On the basis of
a parallel interfacial capacitance model,3 the transient
coverage (θ(t)) of the growing film can be calculated from
(16) Burgess, I.; Jeffrey, C. A.; Cai, X.; Szymanski, G.; Galus, Z.;
Lipkowski, J. Langmuir 1999, 15, 2607-2616.
(17) Weers, J. G.; Scheung, D. R. J. Colloid Interface Sci. 1991, 145,
563-580.
Formation of n-Alkanethiolate SAMs onto Gold
Cd +
θ(t) )
1
2πfZ(t) sin Φ(t)
Cd - Cm
Langmuir, Vol. 19, No. 8, 2003 3359
(1)
where Cm is the final capacitance of the formed monolayer
(∼2 µF/cm2 for C12S/Au), Cd represents the interfacial
capacitance before the thiol molecules are introduced (∼20
µF/cm2) and is affected by the double layer capacitance
and preadsorbed surfactant molecules or admicelles (all
capacitances are per unit area), f is the frequency (100
Hz), Z(t) is the impedance modulus, and Φ(t) is the phase
angle. By comparing the rates of SAM formation under
different conditions, we can determine the effect of process
variables (surfactant concentration and chain length,
alkanethiol chain length, counterion, etc.) on the assembly
process.
We have investigated the in situ kinetics of assembly
for C12SH onto gold in aqueous solutions at different C12TAB concentrations from 8 to 80 mM. Figure 2a shows
the time dependence of coverage after a step-change in
thiol concentration from 0 to 1 mM. The solid lines in
Figure 2a represent theoretical fits of the kinetics data
with a first-order Langmuir adsorption model expressed
as18
dθ
) k1Lc(1 - θ)
dt
(2)
θ(t) ) 1 - e-k1Lct
(3)
and
where k1L is the rate constant, c is the solubilized
concentration of alkanethiols, and t is time.
The approximately good fits of the kinetics data with
this model are in contrast to the case of SAM formation
in C12E6(aq) and C12E7(aq), where a more complex secondorder, diffusion-limited model provides superior fits to
the data.3 These results suggest that the mechanisms for
SAM formation in nonionic and cationic micellar solutions
are different.
Figure 2a shows that SAMs can be formed quickly and
completely if the surfactant concentration is greater than
the critical micelle concentration (cmc ) 16 mM). Below
the cmc (c ) 8 mM), the slow increase in coverage with
time suggests that the concentration of solubilized alkanethiol is insufficient to allow the SAM to form rapidly.
Even after 100 h of adsorption, SAMs formed from surfactant solutions below the cmc did not exhibit the structural properties consistent with a well-ordered monolayer
film, as indicated by ex situ infrared spectroscopy. Clearly,
the presence of micelles is required to form high-quality
SAMs. These micelles provide hydrophobic cores to solubilize the alkanethiols and deliver them to the metal
surface.
The effect of C12TAB concentration on the first-order
Langmuir rate constant k1L′ ) k1Lc is shown in Figure 2b.
Below the cmc, the SAM does not form, but above the cmc,
the kinetic rate increases with surfactant concentration
as the greater concentration of micelles enhances the
solubilization of the thiol adsorbates. The rate exhibits a
plateau at a C12TAB concentration of ∼32 mM, well below
the surfactant concentration of 64 mM (Csurf - cmc ) 48
mM) required for complete solubilization of C12SH at 1
mM, as shown in Table 2.
(18) Dannenberger, O.; Buck, M.; Grunze, M. J. Phys. Chem. B 1999,
103, 2202-2213.
Figure 2. Effect of C12TAB concentration on the kinetics of
formation for SAMs upon exposure of gold to C12SH. The
concentration of C12SH is 1 mM if all the thiol is solubilized.
(a) Time-dependence of surface coverage at different concentrations of C12TAB. The curves represent best fits of the data
set by a first-order Langmuir adsorption model (eq 3). (b) Effect
of C12TAB concentration on the measured rate constant k1L′ )
ck1L. When no error bar is visible, the magnitude of the error
is estimated by the size of the symbol.
Figure 3. Poisson distribution of C12SH at different C12TAB
concentrations. The concentration of C12SH is determined from
Table 2.
To gain insight into why the onset of the plateau in
Figure 2b occurs at a C12TAB concentration in which C12SH is not completely solubilized, we have determined the
Poisson distribution of the number of thiol molecules per
micelle at different C12TAB concentrations.19 In this
analysis, we assume that the aggregation number of the
micelles is identical to that for empty micelles. Figure 3
shows that the fraction of empty micelles is greater and
that the fraction of micelles containing more than one
alkanethiol is smaller when the C12TAB concentration is
increased beyond 32 mM (Csurf - cmc > 16 mM). These
(19) On the basis of a Poisson distribution, the probability of a micelle
having s alkanethiols (p(s)) is given by p(s) ) e-λλs/s! where λ is the
average number of alkanethiols per micelle.
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Langmuir, Vol. 19, No. 8, 2003
Figure 4. (a) Effect of surfactant chain length on the kinetics
of SAM formation in aqueous CmTAB solutions containing 0.4
mM C12SH. The surfactant concentration was maintained at
a value of Csurf - Ccmc ) 16 mM. The curves represent best fits
of the data sets by a first-order Langmuir adsorption model (eq
3). Data for m ) 18 (not shown) showed no regular growth in
coverage over the first 180 s and could not be fit with any
common kinetic models. (b) Effect of surfactant chain length
(m) on the first-order Langmuir rate constant k1L. The line is
a least-squares fit to the data and has a slope that corresponds
to an activation energy of 0.38 kcal/mol per CH2 group.
empty micelles do not contribute to the transfer of thiol
molecules to the metal surface and may hinder the
transport of micelles that contain thiols. Likewise, release
of an alkanethiol from a micelle containing no other thiols
may be less energetically favorable than the release of a
thiol from a micelle containing multiple thiols, consistent
with the findings of a recent solubilization study of pyrene
in micellar solutions.20 Thus, as the C12TAB concentration
is increased above 32 mM, the increasing concentration
of empty micelles and/or the decreasing concentration of
micelles containing two or more alkanethiols may serve
to negate the effect of the increasing concentration of
solubilized alkanethiols.
Effect of Surfactant Chain Length on Kinetics of
SAM Formation. To explore the effect of surfactant chain
length on the assembly of molecular films, we have
investigated the kinetics of formation for SAMs formed
from C12SH (0.4 mM) onto gold in aqueous micellar
solutions of CmTAB (m ) 12, 14, 16) at room temperature
(Figure 4a). In this study, the concentration of surfactant
was maintained at 16 mM above the cmc (Csurf - cmc )
16 mM). As shown in Table 2, this concentration is
sufficient for all these micellar systems to solubilize C12SH completely at 0.4 mM. As m is increased, the kinetics
(20) Kim, J.-H.; Domach, M. M.; Tilton, R. D. Colloids Surf. 1999,
150, 55-68.
Yan et al.
of SAM formation in CmTAB(aq) is dramatically inhibited
(Figure 4a). In fact, the measured rate constant decreases
exponentially with increasing m (Figure 4b). The data for
all chain lengths are reasonably fit by a first-order
Langmuir adsorption model (eq 3).
Increasing the chain length of a CmTAB surfactant could
be expected to hinder SAM formation in the following
ways: (1) Longer chained surfactants produce larger
micelles that diffuse more slowly, perhaps reducing the
delivery rates of alkanethiols. (2) Longer chained surfactants may be more difficult to displace from the gold
surface by the alkanethiols.21 (3) Increasing the chain
length of a surfactant is expected to reduce the rate of
micellar fragmentation, since it also reduces the rate of
micellar dissolution.22 The slower rate of micellar fragmentation may affect the surfactant-assisted transfer of
alkanethiols from the micelles to the surface. The exponential effect observed in Figure 4b is much stronger than
that expected if the increase in m solely affects the
micellar diffusion, which scales roughly as 1/m on the
basis of the Stokes-Einstein equation. The stronger effect
observed here suggests that increasing m affects either
the rate of displacement of admicelles or the release/
transfer of the alkanethiol to the surface. Since both the
displacement of the admicelles by the alkanethiols and
the release of alkanethiols from the micelles can be viewed
as activated processes, the slope of the best-fit line in
Figure 4b would correspond to an activation energy of
0.38 kcal/mol per methylene unit in the surfactant.
We have used reflectance-absorption infrared spectroscopy to probe the effect of surfactant chain length on
the structural properties of SAMs prepared from C12SH.
Positions and line widths of bands that correspond to C-H
stretching vibrations in the infrared spectra provide
information on the relative extent of gauche defects within
the SAM while absorbances of these bands relate to the
average orientational conformation of adsorbates within
the SAM. Figure 5 shows the C-H stretching region of
reflectance infrared spectra for SAMs formed from C12SH
(0.4 mM) onto gold in aqueous micellar solutions of CmTAB (m ) 12, 14, 16, 18) at room temperature for 1 h. The
surfactant concentration was maintained at 16 mM above
the cmc (Csurf - cmc ) 16 mM). The positions of the
asymmetric methylene [νa(CH2)] stretching mode are
∼2918 cm-1 for m ) 12, ∼2919 cm-1 for m ) 14, ∼2921
cm-1 for m ) 16, and ∼2922 cm-1 for m ) 18, consistent
with an increased content of gauche conformers within
the C12S SAM as the chain length of the surfactant is
increased. The IR spectra for the SAMs formed in C12TAB(aq) and C14TAB(aq) exhibit markedly lower asymmetric and symmetric methylene intensities than those
for SAMs formed in aqueous solutions of longer chained
surfactants. The transition dipole moments of these
methylene modes are perpendicular to the axis of the
hydrocarbon chain, and their intensities in a reflectance
infrared spectrum are a function of the molecular tilt (cant
angle) and rotation of the chain axis (twist angle) relative
to the surface normal. The lower methylene intensities
for the SAMs formed in C12TAB(aq) and C14TAB(aq) are
consistent with a smaller average cant angle and a more
densely packed film compared to those formed in C16TAB(aq) and especially C18TAB(aq). The line widths of the
asymmetric methyl peaks at 2965 cm-1 increase with
(21) Dannenberger et al. (ref 18) have shown that increasing the
chain length of n-alkane solvents reduces the rate of SAM formation
in these solvents.
(22) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.;
Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem.
1976, 80, 905-922.
Formation of n-Alkanethiolate SAMs onto Gold
Figure 5. Reflectance infrared spectra of the C-H stretching
region for C12S SAMs on gold formed in aqueous CmTAB
solutions containing 0.4 mM C12SH for 1 h. The surfactant
concentration was maintained at a value of Csurf - Ccmc ) 16
mM. The dashed lines indicate the positions of the primary
methyl and methylene stretching modes for a trans-extended
monolayer with no defects: νa(CH3) ) 2965 cm-1, νa(CH2) )
2918 cm-1, νs(CH3) ) 2879 cm-1, and νs(CH2) ) 2851 cm-1. The
spectra for each chain length have been offset for clarity.
surfactant chain length, from a full-width at halfmaximum (fwhm) of ∼9 cm-1 for C12TAB to ∼13 cm-1 for
C18TAB, indicating that the C12S SAMs formed in aqueous
solutions of shorter chained surfactants have a more
homogeneous surface structure of methyl groups. On the
basis of this constant adsorption time of 1 h, these results
are consistent with a more complete, highly crystalline
SAM formed in aqueous solutions of the shorter chained
surfactants. The SAMs formed in C16TAB(aq) and C18TAB(aq) likely consist of some ordered domains of alkanethiolates but with a much higher density of defects
(domain boundaries, adsorbate vacancies, missing rows)
that are indicative of incomplete films. These ex situ IR
results are qualitatively consistent with the chain length
(m) dependence on in situ kinetics shown in Figure 4.
Effect of Alkanethiol Chain Length. Increasing the
alkanethiol chain length results in exponentially slower
rates of formation for SAMs in nonionic micellar solutions
of C12E6(aq) and C12E7(aq).3 We have also investigated
the effect of alkanethiol chain length on the kinetics of
SAM formation in cationic C12TAB(aq) solutions. Figure
6a shows the coverage versus time in 0.1 mM CnSH (n )
10, 12, and 16) and 40 mM C12TAB(aq). As shown in Table
2, each of these thiols can be completely solubilized at 0.1
mM concentration. Figure 6a shows that the kinetics of
SAM formation in C12TAB(aq) is dramatically affected by
the alkanethiol chain length, with shorter chained alkanethiols (n ) 10 and 12) assembling much more rapidly
than C16SH. For C16SH, the SAM is not formed, as
evidenced by significant transient capacitance changes
even after a 2-h exposure to the thiol-containing micellar
solution. Figure 6a shows that the data for all chain lengths
of alkanethiol at this lower concentration (0.1 mM) are
approximated by a first-order, diffusion-limited Langmuir
Langmuir, Vol. 19, No. 8, 2003 3361
Figure 6. (a) Kinetics of formation for CnSH (0.1 mM) on gold
in 40 mM C12TAB(aq). The curves represent best fits of the
data sets by a first-order, diffusion-limited Langmuir adsorption
model (eq 4). (b) Effect of alkanethiol chain length (n) on the
concentration-independent, diffusion-limited, first-order Langmuir rate constant (k1DL). The line is a least-squares fit to the
data and has a slope that corresponds to an activation energy
of 0.40 kcal/mol per CH2 group.
adsorption model, given as18,23
θ(t) ) 1 - e-2k1DLct
0.5
(4)
where k1DL is the rate constant, rather than a first-order,
nondiffusion-limited model, as was shown in Figure 2 for
higher concentrations of alkanethiol. At these lower
alkanethiol concentrations, the diffusion of thiol-laden
micelles likely becomes the rate-limiting step in the
assembly process.
Figure 6b shows the effect of alkanethiol chain length
on the concentration-independent rate constant (k1DL), as
determined by the fits of the kinetics data in Figure 6a.
The concentration-independent rate constant decreases
significantly (approximately exponentially) with increasing alkanethiol chain length and hydrophobicity, consistent with a mechanistic process where the alkanethiol
must transfer from the micellar core to a more polar
environment prior to adsorption. The probability (q) of
release of an alkanethiol from a micelle can be written as3
(
q ) exp -
E
κT
)
(5)
where E is the activation energy of the release process,
which is affected by the hydrophobicity of the releasing
molecule and the medium to which it releases, κ is the
Boltzmann constant, and T is the absolute temperature.
Assuming a linear relationship between the probability
for release and the measured rate constant, the slope of
the line in Figure 6b corresponds to an activation energy
(23) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731-4740.
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Langmuir, Vol. 19, No. 8, 2003
of ∼0.4 kcal/mol per CH2 group within the alkanethiol.
This activation barrier is significantly larger than that of
0.22 kcal/mol per CH2 group observed in C12E6(aq) and
C12E7(aq),3 suggesting a different release process that is
more dependent on the hydrophobicity of the alkanethiol.
Effect of Applied Potential and Salt Addition.
While the rate of formation for SAMs in C12E6(aq) and
C12E7(aq) is limited by diffusion of thiol-laden micelles,
the kinetics results for SAM formation in C12TAB(aq) are
consistent with a reaction limitation for sufficiently high
alkanethiol concentrations in which displacement of
adsorbed surfactant and/or counterions by the assembling
thiols limits the process. The effect of potential on the
rates of SAM formation in the presence of these counterions can reveal mechanistic aspects of the assembly
process. Parts a and b of Figure 7 show the effect of applied
potential on the kinetic rate of SAM formation in 1 mM
C10SH in 32 mM C12TAB(aq) and in 16 mM C12E6(aq),
respectively. The increased potential of the gold electrode
has little or no effect on the rate of formation in the nonionic
C12E6(aq) but greatly decreases the rate of formation in
cationic C12TAB(aq). We interpret this latter result as
being due to the increasing strength of interaction between
adsorbed bromide ions (Br-) and gold as the potential
becomes more positive, thereby reducing the displacement
rate of these ions by alkanethiols. At 0.6 V, the Gibbs free
energy of adsorption for bromide onto gold is ∼-40 kcal/
mol24 and is comparable to the estimated strength of the
Au-S bond (∼45 kcal/mol).25 The ability to inhibit and
even prevent SAM formation with applied potentials in
this aqueous process is remarkable and has attractive
utility in the selective modification of electrode arrays
that can impact applications in chemical sensing.26,27
The results in Figure 7a suggest that displacement of
counterions is the rate-limiting step in the formation of
alkanethiolate SAMs in aqueous micellar solutions of CmTAB. To test this hypothesis, we investigated the effect
that a small concentration of potassium iodide (KI) exhibited on the rates of C10S SAM formation in 32 mM C12TAB(aq). Since iodide ions bond more strongly to the gold
surface than bromide ions do,28 iodide should preferentially
adsorb to the gold surface. Displacement of adsorbed iodide
by the alkanethiols would be a slower process than displacing bromide and should therefore produce a measurable change in the rate of SAM formation if displacement
of adsorbed counterions is the rate-limiting step in the
process. As shown in Figure 7a, the addition of KI results
in a 5-fold decrease in the kinetic rate of SAM formation
at 0 V. In addition, the rate constant completely decays
at 0.2 V, a significantly smaller potential for decay in
comparison with the case of C12TAB(aq) that contains no
KI. Lipkowski et al.29 have shown that the Gibbs free
energy of adsorption for iodide onto gold becomes more
negative with increasing potential, thereby consistent with
a stronger interaction that is not easily disrupted by an
alkanethiol. This result provides supporting evidence that
displacement of adsorbed counterions is the rate-limiting
step in the formation of alkanethiolate SAMs in CmTAB(aq). Additional support of this rate-limiting step is that
(24) Shi, Z.; Lipkowski, J.; Mirwald, S.; Pettinger, B. J. Chem. Soc.,
Faraday Trans. 1996, 92, 3737-3746.
(25) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc.
1987, 109, 733-740.
(26) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc.
1992, 114, 5860-5862.
(27) Hsueh, C.-C.; Liu, Y.; Henry, M.; Freund, M. S. Anal. Chim.
Acta 1999, 397, 135-144.
(28) Chen, A.; Shi, Z.; Bizzotto, D.; Lipkowski, J.; Pettinger, B.; Bilger,
C. J. Electroanal. Chem. 1999, 467, 342-353.
(29) Lipkowski, J.; Shi, Z.; Chen, A.; Pettinger, B.; Bilger, C.
Electrochim. Acta 1998, 43, 2875-2888.
Yan et al.
Figure 7. Effect of applied potential on the rate of formation
for SAMs on gold from 1 mM C10SH in (a) 32 mM C12TAB(aq)
or (b) 16 mM C12E6(aq). The surfactant concentration in each
of these cases is ∼16 mM above the cmc. When no error bar is
visible, the magnitude of the error is estimated by the size of
the symbol.
the addition of 10 mM KBr to a nonionic micellar solution
of 10 mM C12E6 and 1 mM C16SH resulted in a 3-fold
decrease in the kinetic rate and a 4-fold increase in the
final capacitance as compared to the case of SAM formation
in the same solution without KBr.
Discussion
The results presented in the previous section reveal
important differences in the formation of SAMs from
cationic CmTAB(aq) versus nonionic C12E6(aq) and C12E7(aq).3 (1) The kinetics of SAM formation in CmTAB(aq)
are well described by a first-order Langmuir adsorption
process, except at low concentration of thiols (0.1 mM),
where the data are well fit by a first-order diffusion-limited
Langmuir process. In C12Ej(aq), the kinetics of SAM
formation is consistent with a second-order diffusionlimited process.3 (2) The rate of SAM formation in CmTAB(aq) decreases exponentially with increasing alkanethiol chain length, similar to the result in C12Ej(aq),
but the corresponding activation energy is a factor of 2
greater in CmTAB(aq). (3) Increasing the surface potential
inhibits SAM formation in CmTAB(aq) but not in C12Ej(aq). These differences suggest that the formation of SAMs
in cationic CmTAB(aq) occurs through a different mechanism from that in nonionic C12E6(aq) and C12E7(aq).
Release of Alkanethiols from Micelles. Our previous
results3 for the formation of SAMs in nonionic surfactants
C12Ej(aq) are consistent with a diffusion-limited process
where the alkanethiols are delivered to the surface via a
collision-induced release mechanism between micelles and
admicelles. Once in the admicelles, the alkanethiols
rapidly displace adsorbed surfactant molecules and chemi-
Formation of n-Alkanethiolate SAMs onto Gold
Langmuir, Vol. 19, No. 8, 2003 3363
sorb to the surface as thiolates. In CmTAB(aq), electrostatic
repulsion between the positively charged micelles and
admicelles likely prevents such a collision-induced release
of alkanethiols to the surface, consistent with studies on
the solute exchange in solution-phase ionic micelles.10,11
In addition, release of the alkanethiols directly to the
aqueous phase (“free” thiols) is ruled out due to the
extremely low concentration of free alkanethiols.30 A more
likely route for alkanethiol transport to the surface is
through a fragmentation process in which a few surfactant
molecules and an alkanethiol break away from a normalsized micellar carrier and form a submicelle that can
actively transport alkanethiols to the surface. Such a
fragmentation process has been concluded to drive the
exchange of insoluble solutes between ionic micelles in
solution, as shown in Figure 1b (pathway 2).10,11 The rate
of solute exchange in these fragmentation processes is
known to increase as the hydrophobicity of the solute is
reduced8,10 and the ionic strength of the solution is
increased.10,11 Since fragmentation relies on the dissociation of surfactants from the micelle, the rate of which
increases exponentially with decreasing surfactant chain
length,22 surfactants with shorter chain length should
fragment at a higher rate.
If such a fragmentation process occurs in the micelleassisted formation of SAMs, and the submicelles are the
active agents for the transport of alkanethiols to the
surface, we would expect the total concentration of alkanethiols in this submicellar phase to have a significant bearing on the rates of monolayer formation. Assuming that
the formation of a thiol-containing submicelle is an activated process where surfactants and an alkanethiol break
away from a regular-sized micelle, the concentration of
alkanethiols in the submicellar phase (csm) can be written
as
(
csm ) cq ) c exp -
E
κT
)
(6)
where c is the concentration of solubilized alkanethiols,
q is redefined as the probability of an alkanethiol and
accompanying surfactants to release from the micellar
environment to form a submicelle, and E is the activation
energy for thiol-laden submicellar formation, which
depends on the relative hydrophobicity of the alkanethiol
and the surfactant. Factors that increase the concentration
of alkanethiols in the submicelles should enhance the rates
of monolayer formation. Consistent with the literature11
and assuming the transfer of alkanethiols from micelles
to submicelles is surfactant-assisted, we would expect
increased hydrophobicity of the alkanethiol and the surfactant to reduce the concentration of submicelles and
the concentration of alkanethiols in the submicelles. The
results in Figures 4 and 6 are consistent with a fragmentation mechanism, since the measured rate of SAM growth
decreases exponentially as the chain lengths of the alkanethiol and the CmTAB surfactant increase. The larger activation barrier (∼0.4 kcal/mol per CH2 group) for this
process compared with that for release of alkanethiols in
C12Ej(aq) (∼0.2 kcal/mol per CH2 group) suggests that the
submicellar CmTAB environment containing the thiol is
more hydrophilic than the release site in C12Ej, the
admicelles, and is therefore less energetically favorable.
This barrier is still significantly smaller than that
expected for release of alkanethiols directly into water,
(30) As discussed in ref 3, the concentration of free alkanethiols would
be extremely low (∼10-9 mM), since the micelle-water partition
coefficient for molecules of similar hydrophobicity to those of these
alkanethiols is ∼109.
on the basis of the measured partitioning of alkanes from
micelles to water (0.72 kcal/mol per CH2 group),31,32 and
for surfactant dissociation from micelles (∼0.7 kcal/mol
per CH2 group).22,33,34
An increase in ionic strength has been shown to enhance
the fragmentation rates in micellar solutions of dodecyltrimethylammonium chloride11 and sodium dodecyl sulfate.10 This enhanced extent of fragmentation has been
attributed to the presence of more polydisperse micelles
at high ionic strength,11 which favors the fragmentation
process, or to micellar surface fluctuations10 that may
increase the probability of pinching off a submicelle. As
further evidence for a fragmentation delivery mechanism,
the rate of SAM formation increases as the ionic strength
increases. We have observed that addition of 200 mM NaF
to a 0.4 mM solution of C12SH in 32 mM C12TAB increases
the rate of SAM formation by a factor of ∼6. This result
is consistent with a higher concentration of thiols in
submicelles at higher ionic strength.
The trends in Figures 4 and 6 are also partially consistent with a mechanism where alkanethiols are released
from micelles to admicelles via collisions, similar to the
mechanism proposed for alkanethiol release in the C12Ej(aq) system.3 However, this “sticky” collision process would
be extremely slow in the absence of added salt and is not
consistent with the exchange of solutes between ionic
micelles in solution. Rharbi and Winnik 10 and Malliaris
et al.11 determined the exchange of pyrene derivatives
between ionic micelles to be first-order in micellar concentration and independent of the concentration of empty
micelles, thereby inconsistent with a collision-induced
process. Therefore, we feel the use of a fragmentation
mechanism to transfer alkanethiols to the surface is much
more consistent with the literature describing the exchange of insoluble solutes in micellar solutions.8-10
On the basis of the above discussion, we postulate that
alkanethiols are transferred to the admicellar region of
the gold surface via submicelles. These thiol-containing
submicelles most likely transfer directly into the admicelles35 by exchange with admicellar surfactants, thereby
transferring the alkanethiol to the surface. A similar
process has been concluded to occur in solution where
fragmented submicelles fuse with other micelles.8,9 The
energetic penalty that arises from uniting these likecharged species is outweighed by the factors favoring
micellization.
Interactions at the Surface: Displacement of
Surfactant Molecules and Counterions. Upon transfer
to the surface, alkanethiols must displace adsorbed
surfactants and bromide counterions as they chemisorb
onto the remaining available sites. In CmTAB(aq), wormlike admicelles are bound to the surface via electrostatic
interactions with adsorbed bromide ions, consistent with
in situ AFM images and subsequent interpretation by
Jaschke et al.35 The negatively charged bromide surface
interacts with only a small portion of the cylindrical
aggregate, leading to relatively poor stability of these
wormlike admicelles.35 Such a weak surface interaction
indicates that the displacement of these wormlike admicelles by the adsorbing alkanethiols should not be rate
limiting.
(31) Woodrow, B. N.; Dorsey, J. G. Environ. Sci. Technol. 1997, 31,
2812-2820.
(32) Prak, D. J.; Abriola, L. M.; Weber, W. J.; Bocskay, K. A.; Pennell,
K. D. Environ. Sci. Technol. 2000, 34, 476-482.
(33) Bolt, J. D.; Turro, N. J. J. Phys. Chem. 1981, 85, 4029-4033.
(34) Tanford, C. The Hydrophobic Effect; Wiley-Interscience: New
York, 1973.
(35) Jaschke, M.; Butt, H.-J.; Gaub, H. E.; Manne, S. Langmuir 1997,
13, 1381-1384.
3364
Langmuir, Vol. 19, No. 8, 2003
A more physically realistic rate-limiting step is the
displacement of adsorbed bromide ions by the alkanethiols.
Chemisorption of halides, particularly bromide and iodide,
occurs spontaneously on the gold surface in aqueous
solution.36-38 Bromide ions bind to gold with a strength
that increases with potential.24 The slower rate of SAM
formation with increased potential (Figure 7a) is consistent
with the required displacement of bromide or iodide ions
that are bound more strongly by the gold surface at higher
potentials. Since the Gibbs energy of adsorption for
bromide onto Au at 0.6 V (or that for iodide at 0.2 V)
exceeds the strength of the Au-S bond, the alkanethiols
cannot effectively displace the adsorbed counterions and
the SAM is not formed.
The displacement of adsorbed counterions as the ratelimiting step in SAM formation from CmTAB(aq) is also
consistent with the observed first-order Langmuir kinetic
behavior of this system. The displacement at the surface
can be expressed as
CnSH* + Br--Au f CnS-Au + Br- + 1/2H2 (7)
where CnSH* indicates physically adsorbed thiols at or
near the surface. If the displacement of chemically
preadsorbed counterions is the rate-limiting step (dissociative mechanism), the concentration of physically
adsorbed alkanethiols [CnSH*] can be assumed as a
pseudosteady constant. The reaction rate is dominated
by the dissociation of Br- at available Au sites that scale
as (1 - θ), and the physically adsorbed alkanethiols are
in excess. Thus, the overall kinetic rate is consistent with
a pseudo-first-order process
rate ) k[CnSH*][Br--Au] = k′[Br--Au] ∝ k′(1 - θ)
(8)
where k′ is the measured rate constant. The concentration
of physically adsorbed alkanethiols [CnSH*] is imposed
by the concentration of alkanethiols in the submicelles
(csm) and, thus, is greatly affected by the hydrophobicities
of the surfactant and alkanethiol. A more complex secondorder, diffusion-limited kinetics was observed in nonionic
micellar solutions, since [CnSH*] is not in excess, as it is
proposed to be here.3
Conclusions
The formation of a SAM from aqueous CmTAB solutions
is consistent with a series of steps in which (1) the micelles
solubilize the alkanethiols, (2) fragmentation of thiol-laden
micelles results in smaller submicelles that transfer
alkanethiols to the surface, and (3) the thiols displace the
adsorbed surfactant molecules and counterions and (4)
chemisorb to the gold surface. By considering the preparation of SAMs in aqueous micellar solutions of C12Ej and
CmTAB, the mechanism for the micelle-assisted formation
of SAMs depends on the selection of surfactant, the
micellar size and charge, and micellar interactions in
solution and at the surface. The use of cationic micelles
as delivery vehicles provides a unique way of controlling
the kinetics of SAM formation in aqueous solutions by
using applied potential.
Experimental Section
Materials. Gold shot (99.99%) and silicon (100) wafers were
obtained from J&J Materials (Neptune City, NJ) and Silicon
(36) Tao, N. J.; Lindsay, S. M. J. Phys. Chem. 1992, 96, 5213-5217.
(37) Wandlowski, T.; Wang, J. X.; Magnussen, O. M.; Ocko, B. M. J.
Phys. Chem. 1996, 100, 10277-10287.
(38) Paik, W. K.; Genshaw, M. A.; Bockris, J. O. J. Phys. Chem. 1970,
74, 4266-4275.
Yan et al.
Sense (Nashua, NH), respectively. All chemicals, including
n-alkanethiols (Aldrich), dodecyltrimethylammonium bromide
(C12TAB; Aldrich), tetradecyltrimethylammonium bromide (C14TAB; Sigma), hexadecyltrimethylammonium bromide (C16TAB;
Fluka), octadecyltrimethylammonium bromide (C18TAB; Fluka),
poly(oxyethylene) monoalkyl ethers (C12E6 and C12E7; Fluka),
sodium fluoride (Fisher), potassium iodide (Merck), 100% ethanol
(AAPER), and deuterium oxide (D2O; 99.9% atom D; Aldrich)
were used as received. Deionized water (16.7 MΩ) was purified
with a Modu-Pure system.
Sample Preparation. Gold substrates were prepared by
evaporating 1000-1500 Å of gold at a rate of 3-5 Å/s onto silicon
[Si(100)] wafers inside a diffusion-pumped chamber with a base
pressure of 3 × 10-6 Torr. Prior to the evaporation of gold, a
100-Å layer of chromium was evaporated onto silicon to serve as
a primer.
SAM Preparation for Ex Situ Studies. To prepare SAMs,
gold substrates were first rinsed with ethanol, dried in a stream
of nitrogen, and immersed into solutions containing 0.4 mM C12SH in aqueous micellar solutions of CmTAB (m ) 12, 14, 16, 18)
at room temperature for 1 h. The concentration of surfactant
was maintained at 16 mM above the cmc (Csurf - cmc ) 16 mM).
Upon removal, the samples were rinsed with fresh solvent (CmTAB(aq)) and water and dried under a stream of nitrogen.
Reflectance-Absorption Infrared Spectroscopy. IR spectra were obtained in a single reflection mode with a Bio-Rad
Excalibur infrared spectrometer equipped with a universal
sampling accessory. The polarized light was incident at 80° from
the surface normal. The reflected light was detected with a
narrow-band MCT detector cooled with liquid nitrogen. Spectral
resolution was 2 cm-1 after triangular apodization. Spectra were
referenced to those of SAMs prepared on gold from octadecanethiol-d37, and 1000 scans of both sample and reference were
collected.
In Situ Capacitance Measurements. Capacitance measurements were taken with a CMS300 electrochemical impedance
system (Gamry Instruments) interfaced to a personal computer.
Measurements were taken inside a Teflon cell containing a goldcoated silicon wafer as a working electrode with a 1 cm2 fixed
area, a gold-coated silicon wafer as a counter electrode, and a
Ag/AgCl/saturated KCl reference electrode. The gold-coated
wafers were rinsed with ethanol and dried in a stream of nitrogen
prior to use in the cell. The cell initially contained 1 mL of the
desired concentration of surfactant (CmTAB) in an aqueous solution with or without salt (NaF or KI). After a stable capacitance
was obtained, a 6-mL aqueous solution containing alkanethiol
at the desired concentration was added. This aqueous solution
contained the same concentrations of surfactant and salt as those
of the initial solution to ensure that no changes in ionic strength
occurred. Capacitance readings were recorded every 3 or 6 s and
were obtained from the imaginary impedance at 100 Hz.
Imaginary impedance data were converted to coverage using
eq 1, and the resulting coverage data were fit by either a firstorder Langmuir adsorption model (eq 3) or a first-order, diffusionlimited Langmuir model (eq 4) to determine a rate constant.
Error bars on the plots of rate constant versus concentration,
potential, and chain length represent standard deviations
obtained from fits of at least three independent runs of coverage
versus time.
Nuclear Magnetic Resonance. The 1H NMR measurements
were performed by using a Bruker DRX 400 MHz NMR
spectrometer. The spectra were recorded over 128 scans using
5-mm tubes with D2O as a solvent. The characteristic peak for
alkanethiols is that due to -CH2SH (chemical shift: 2.4), that
for C12Ej is that due to -(OCH2CH2)n (chemical shift: 3.3-3.7),
and that for C12TAB is that due to CH2-N-(CH3)3 (chemical
shift: 2.9-3.6). The characteristic peaks of the alkanethiols and
each surfactant are clearly separated. Since the concentration
of surfactant is known, the concentration of solubilized alkanethiol is determined from the ratio of the integrated areas
of the characteristic peaks.
Acknowledgment. We gratefully acknowledge financial support from the National Science Foundation (Grant
CTS-9983966).
LA026912R