Formation of n-Alkanethiolate Self-Assembled Monolayers onto Gold

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Langmuir 2003, 19, 3357-3364

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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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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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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.

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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

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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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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.

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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-

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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.

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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