1784
Langmuir 2007, 23, 1784-1791
Adsorption of N-Decyl-N,N,N-trimethylammonium Triflate (DeTATf), a Cationic Surfactant, on the Au(111) Electrode Surface Christa L. Brosseau,† Erin Sheepwash,† Ian J. Burgess,‡ Ewa Cholewa,† Sharon G. Roscoe,§ and Jacek Lipkowski*,† Department of Chemistry and Biochemistry, UniVersity of Guelph, Ontario, Canada N1G 2W1, Department of Chemistry, UniVersity of Saskatchewan, Saskatoon, Canada S79 5C9, and Department of Chemistry, Acadia UniVersity, NoVa Scotia, Canada B4P 2R6 ReceiVed August 2, 2006. In Final Form: NoVember 1, 2006 The adsorption behavior of the cationic surfactant N-decyl-N,N,N-trimethylammonium triflate (DeTATf) on the Au(111) electrode surface was characterized using cyclic voltammetry, differential capacity, and chronocoulometry. The thermodynamics of the ideally polarized electrode have been employed to determine the Gibbs excess and the Gibbs energy of adsorption. The results show that the adsorption of DeTATf has a multistate character. At low bulk DeTATf concentrations, the adsorption state is consistent with the formation of an adsorbed film of nearly flat molecules. At higher concentrations this film may represent a three-dimensional aggregated state. At negative potentials and charge densities close to 0 µC cm-2, the data suggest the formation of a film of tilted molecules oriented with the hydrocarbon tail toward the metal surface and the polar head toward the solution. A surprising result of this study is that DeTATf displays adsorption characteristics of a zwitterionic rather than a cationic surfactant. This behavior indicates that the adsorbed species is an ion pair.
Introduction Surfactant adsorption at the solid-liquid interface plays an important role in many industrial and technological applications, such as ore flotation, petroleum recovery, and detergency. An understanding of the surface aggregation behavior of ionic,1-7,9-12 zwitterionic,5,13-15 and neutral surfactants5,15,16 is therefore of considerable interest. Scanning probe microscopy has recently shown that surfactants may assemble at a solid-solution interface to form a variety of structures, including hemicylinders,1-8,10,15,16 hemispheres, spheres,2-3,9,10,15 and irregular globular aggregates.16 Furthermore, the shape of the surface aggregates depends on the interaction of the surfactant molecules with the two neighboring phases and results from the interplay of a number of factors including the following: (1) the hydrophobic attractive interaction between the hydrocarbon tails, (2) the repulsive lateral interaction between the ionic headgroups, (3) the interaction of the headgroups with the aqueous phase and the polarized surface, (4) geometric packing constraints, (5) the entropy of mixing, and (6) the hydrophilicity or hydrophobicity of the solid surface. The role of the various forces which influence the surface aggregation †
University of Guelph. University of Saskatchewan. § Acadia University. ‡
(1) Maquire, H. J.; Roscoe, S. G. Langmuir 1997, 13, 5962. (2) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (3) Manne, S. Prog. Colloid Polym. Sci. 1997, 103, 226. (4) Jaschke, M.; Butt, H.-J.; Gaub, H. E.; Manne, S. Langmuir 1997, 131, 381. (5) Wolgemuth, J. L.; Workman, R. K.; Manne, S. Langmuir 2000, 16, 3077. (6) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 3207. (7) Wanless, E. J.; Davey, T. M.; Ducker, W. A. Langmuir 1997, 13, 4223. (8) Liu, J.-F.; Ducker, W. A. Langmuir 2000, 16, 3467. (9) Liu, J. -F.; Min, G.; Ducker, W. A. Langmuir 2001, 17, 4895. (10) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 4447. (11) Schulz, J. C.; Warr, G. G.; Butler, P.; Hamilton, W. A. Phys. ReV. E 2001, 63, 041604. (12) Wanless, E. J.; Ducker, W. A. Langmuir 1997,13, 1463. (13) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915. (14) Ducker, W. A.; Grant, L. M. J. Phys. Chem. 1996, 100, 11507. (15) Grant, L. M.; Ducker, W. A. J. Phys. Chem. B 1997, 101, 5337. (16) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288.
and packing geometry of surfactants has been recently discussed by Retter et al.17-20 Previously we have shown that surface aggregation of anionic surfactants such as sodium dodecyl sulfate (SDS) at metal surfaces is controlled by the electrode potential.21,22 This observation has been confirmed by Petri and Kolb23 and Tang and Wang.24 At small charge densities, SDS molecules aggregate into a wellordered state consisting of stripelike surface micelles (hemimicelles). The long-range order is stabilized by the interaction between the sulfate groups belonging to SDS molecules in adjacent rows. At large positive charge densities the hemimicellar aggregates melt to form a condensed film. The surface concentration of SDS molecules doubles upon transition from the hemimicellar state to the condensed state. The properties of the condensed film may be explained by a model of an interdigitated film in which half of the sulfate groups are turned toward the metal and half toward the solution.22 In addition, a zwitterionic surfactant, N-dodecyl-N,N-dimethyl3-ammonio-1-propanesulfonate (DDAPS) was studied in order to extend these studies to the adsorption behavior of neutral surfactant species.25 In this work the adsorption of DDAPS exhibited a multistate character, where the first two adsorption states occur at potentials close to zero charge. At low bulk DDAPS concentrations, these states corresponded to the formation of a film of nearly flat adsorbed molecules. At higher concentrations, this state is converted into a hemimicellar film. At more negative potentials another state was formed which corresponded to a (17) Israelachvili, J.; Mitchel, D.; Ninham, B. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (18) Anacker, E. W.; Ghose, H. M. J. Phys. Chem. 1963, 63, 1713. (19) Retter, U. Langmuir 2000, 16, 7752. (20) Retter, U.; Avranas, A. Langmuir 2001, 17, 5039. (21) Burgess, I.; Jeffrey, C. A.; Cai, X.; Szymanski, G.; Galus, Z.; Lipkowski, J. Langmuir 1999, 15, 2607. (22) Burgess, I.; Zamlynny, V.; Szymanski, G.; Lipkowski, J.; Majewski, J.; Smith, G.; Satija, S.; Ivkov, R. Langmuir 2001, 17, 3355. (23) Petri, M.; Kolb, D. M. Phys. Chem. Chem. Phys. 2002, 4, 1211. (24) Tang, Z.; Wang, E. J. Electroanal. Chem. 2001, 496, 82. (25) Cholewa, E.; Burgess, I.; Kunze, J.; Lipkowski, J. J. Solid State Electrochem. 2004, 8, 693.
10.1021/la062284s CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006
Adsorption of DeTATf on the Au(111) Electrode
Langmuir, Vol. 23, No. 4, 2007 1785 Scheme 1
film of tilted molecules oriented with the hydrocarbon tail toward the metal and the polar head toward the solution. The present study extends our previous work on the potential controlled aggregation of surfactants at electrode surfaces to a cationic species. The objective of this study was to develop a better understanding of how the nature of the polar headgroup influences both the interaction of the adsorbed surfactant molecule with the metal and the lateral interaction between the polar heads of adjacent molecules at the surface. In addition, we were interested to learn how this interaction can be controlled by the potential applied to the metal. N-Decyl-N,N,N-trimethylammonium trifluoromethanesulfonate (DeTATf) was the cationic surfactant molecule investigated in this project. The structure of DeTATf is shown in Figure 1. An important issue to consider when studying the adsorption of cationic species is the nature of the counterion. Halide salts are commercially available, but because halide ions strongly adsorb on gold,26 the occurrence of coadsorption would make them unsuitable for the present study. To alleviate this problem, a cationic surfactant with a trifluoromethanesulfonate (triflate) counterion was synthesized because we have previously demonstrated in our laboratory that triflate only very weakly adsorbs at a positively charged gold electrode surface. The results of this work constitute a basic model for the interpretation of the atomic force microscopy (AFM) imaging and infrared reflectionabsorption spectroscopy (IRRAS) studies of this system, which will be described in a future publication. Experimental Section
that the surface was free from contamination. A saturated calomel electrode (SCE) was used as the reference electrode (RE). The WE, CE, and RE were connected to a potentiostat (HEKA PG 590). The analog signals from the WE were transmitted to a data acquisition interface (NI-DAQ BNC-2090, National Instruments). The DAQ board digitized the analog signals from the potentiostat and transmitted the digital signals to a computer for analysis. Custom software (generously supplied by Prof. Dan Bizzotto, University of British Columbia) was used to collect data for the cyclic voltammetry (CV) and chronocoulometry experiments. CV was performed by applying a potential sweep to the electrode surface at a scan rate of 20 mV s-1. In the chronocoulometry experiments the electrode was held at a potential Eads, at which the surfactant molecules form a film at the electrode surface, for a period of time that was long enough for complete adsorption equilibrium to be established. Next the potential was stepped to Edes ) -1000 mV, where the film was totally desorbed from the electrode surface and the current transient corresponding to this desorption process was recorded as a function of time for 150 ms. The current-time curve was integrated to give the relative charge density on the metal surface as a function of the adsorption potential. The potential of zero charge (pzc), determined from an independent experiment as being equal to 290 mV versus SCE, was then used to determine the absolute charge densities. The Wilhelmy plate method was used to measure the surface pressure π ) (γo - γc) at the gas/solution interface, where γc is the surface tension at the gas/solution interface in the presence of surfactant molecules and γo is the surface tension in the absence of the surfactant molecules.27 The Wilhelmy plate was a piece of clean filter paper cut to the appropriate dimensions (19.6 mm wide × 21.8 mm long) and connected to a microbalance (KSV-Instruments). All experiments were carried out at room temperature (22 ( 2 °C).
DeTATf was synthesized via Scheme 1 (R ) (CH2)9CH3). The starting amine N,N-dimethyldecylamine (1; >98% purity) was purchased from Fluka, whereas methyl trifluoromethansulfonate (2) was purchased from Aldrich (99+ %). The starting materials were used as received. The methyl triflate was added dropwise to an approximately 20% excess of the starting amine in 150 mL of distilled toluene. A white precipitate was noted after the immediate addition of the methylating agent. The reaction mixture was left stirring at room temperature for 1 h and then allowed to stand overnight. The white precipitate was removed from the toluene by filtration through a fine sintered frit. High-purity pentane (ACROS, spectrophotometric grade, 99+ %) was then used to wash the precipitate and facilitate the drying process. The dried ammonium triflate salt was stored in an evacuated desiccator for several days before recrystallization in a hexane-ethyl acetate mixed solvent. The percent yield was 97% and the purity was >98% as determined by HPLC. The identity of the purified product was confirmed using 1D 1H NMR. Fresh solutions of DeTATf were prepared for all experiments in order to minimize contamination. The electrolyte (sodium fluoride salt, suprapur, > 99.99%, VWR Scientific) was cleaned in a UV-ozone chamber (Jelight, Irvine, CA). All solutions were prepared with Milli-Q ultrapure water (g18.2 MΩ cm). All glassware was cleaned in hot mixed acid (1:3 HNO3:H2SO4) for approximately 45 min and rinsed thoroughly with Milli-Q ultrapure water. The single-crystal Au(111) working electrode (WE) and gold wire counter electrode (CE) were flame-annealed using a Bunsen burner and quenched with Milli-Q ultrapure water to ensure
Critical Micelle Concentration. The critical micelle concentration (cmc) of DeTATf in 0.1 M NaF was determined by measuring the surface pressure at the air/solution interface as a function of DeTATf concentration, using the Wilhelmy plate method described above. Figure 2 plots the surface pressure as a function of the natural logarithm of the DeTATf concentration in 0.1 M NaF. The surface pressure initially rises but then levels off to a plateau. The plateau corresponds to the DeTATf concentration above the cmc, where the chemical potential becomes independent of the bulk surfactant concentration. Therefore, the cmc was obtained from the intersection of a polynomial fitted line extrapolated from the measurements obtained below the plateau and the straight fitted line for the plateau. The cmc value of DeTATf in 0.1 M NaF was found to be 0.410 ( 0.001 mM. The cmc value for DeTATf has not been previously reported in the literature, but the cmc values for the same cationic surfactant with different anions are well-established. Decyltrimethylammonium bromide (DeTAB), decyltrimethylammonium chloride (DeTAC), and decyltrimethylammonium sulfate (DeTAS), which have bromide, chloride, and sulfate counterions, respectively, have been reported to have cmc values ranging from 20 to 80 mM in supporting electrolyte, which is much higher than that observed in the present study for DeTATf.28
(26) Lipkowski, J.; Shi, Z.; Chen, A.; Pettinger, B.; Bilger, C. Electrochim. Acta 1998, 43, 2875.
(27) Shaw, D. J. Colloid and Surface Chemistry, 4th ed.; ButterworthHeinemann: Oxford, U.K., 1992; Chapter 4.
Results
1786 Langmuir, Vol. 23, No. 4, 2007
Brosseau et al.
Figure 1. Schematic structure of N-decyl-N,N,N-trimethylammonium triflate (DeTATf).
Figure 3. Relative Gibbs excess of DeTATf at the air/solution interface plotted versus the logarithm of the bulk DeTATf concentration at concentrations below the cmc.
Figure 2. Plot of the surface pressure at the air/solution interface versus the logarithm of the bulk DeTATf concentration. Data were fit to a Gaussian function (R2 > 0.99).
However, decyltrimethylammmonium decyl sulfate has been reported to have a much lower cmc value of 0.45 mM.29 These reported values indicate that the counterion plays an important role in determining the micelle properties of the surfactant system. In a similar manner, the trifluoromethanesulfonate group of DeTATf may play a role in substantially decreasing the cmc of DeTATf, especially if this triflate counterion is very strongly associated with the cationic surfactant. This is consistent with literature findings which suggest that the nature of the counterion strongly influences the concentration at which micelle formation is initiated.30 The less hydrated the counterion, the smaller the size of the anion, and therefore the closer the association with the surfactant headgroup. This leads to better neutralization of charge and less electrostatic repulsion between the surfactant molecules. Such a reduction in electrostatic repulsion leads to the formation of micelles at lower concentrations. For example, cetyltrimethylammonium tosylate (CTAT) has been reported to have a high degree of binding to the cationic surfactant due to poor hydration, resulting in a low observed cmc ) 0.23 mM.30 The data presented in Figure 2 can be used to determine the Gibbs excess of DeTATf at the air/solution interface with the help of the Gibbs equation:
Γ)
∂π RT ∂ ln cDeTATF
(1)
To apply eq 1, the surface pressure data below the cmc was fit to a Gaussian function and the Gibbs excess of DeTATf adsorbed at the air/solution interface was calculated from the derivative (28) Weers, J. G.; Rathman, J. F.; Axe, F. U.; Crichlow, C. A.; Foland, L. D.; Scheuing, D. R., Wiersema, R. J.; Zielske, A. G. Langmuir 1991, 7, 854.
Figure 4. Cyclic voltammetry curves recorded for the Au(111) electrode in 0.1 M NaF without DeTATf (dotted line) and with selected DeTATf concentrations starting from 0.018 mM (light gray line) to 0.415 mM (black line).
of the resulting Gaussian function. The Gibbs excess values determined by this procedure are plotted versus the logarithm of the bulk surfactant concentration in Figure 3. The data show that at bulk concentrations of DeTATf approaching the cmc, the surface concentration of DeTATf reaches a maximum value of ∼4.2 × 10-10 mol cm-2. This value is slightly less than that observed for the zwitterionic surfactant DDAPS, which had a maximum surface coverage of 5.2 × 10-10 mol cm-2.25 Cyclic Voltammetry. Cyclic voltammetry was used to qualitatively characterize the electrochemical behavior of DeTATf at the metal/solution interface. Figure 4 shows the CV curves of several DeTATf concentrations in 0.1 M NaF supporting electrolyte recorded between -700 and 450 mV (vs SCE), with a sweep rate of 20 mV s-1. In this range, the Au(111) electrode is ideally polarized and the interface between the electrode and the solution behaves as an ideal capacitor. More positive potentials (29) Mukerjee, P.; Mysels, K. Critical Micelle Concentrations of Aqueous Sufactant Systems; National Standard Reference Data Series, NSRDS-NBS 36; U.S. National Bureau of Standards: Washington, DC, 1971. (30) Mata, J.; Varade, D.; Bahadur, P. Thermochim. Acta 2005, 428, 147.
Adsorption of DeTATf on the Au(111) Electrode
were not investigated in order to avoid complications due to OH- adsorption and the onset of oxide formation.31 At +400 mV the current increases steeply due to the initial onset of gold oxidation at this potential. Two pairs of peaks can be seen in the CV curve in the presence of the surfactant. The large rather reversible peaks at ∼-500 mV correspond to the adsorption/ desorption processes. These peaks are seen to progressively shift to more negative potentials with increasing surfactant concentration, indicating that DeTATf adsorption becomes thermodynamically favored as its concentration in the bulk solution is increased. In addition, the pair of small peaks observed at ∼150 mV may represent a phase change of the surfactant film. Surprisingly, adsorption at very negative potentials was not observed, which is not typical of the adsorption of cationic species, but rather resembles the adsorption of a neutral species. Cholewa et al.25 observed very similar behavior for the zwitterionic surfactant DDAPS, which indicates that the cationic surfactant DeTATf may in fact be strongly coupled to its counterion and may adsorb as an ion pair. Chronocoulometry. Chronocoulometry measurements were used to quantify DeTATf adsorption at the Au(111) electrode surface. Charge density data obtained using this technique were used for further thermodynamic analyses. The DeTATf concentrations used in this study ranged from 0.018 to 0.297 mM (below the cmc). In addition, two experiments were preformed at DeTATf concentrations of 0.415 mM (∼at the cmc) and 2.7 mM, which is well above the cmc of DeTATf. In chronocoulometry, the Au(111) electrode was maintained at a selected potential, Eads for 3 min to establish adsorption equilibrium. During this time, the solution was stirred to enhance mass transport. The stirrer was then turned off, and the solution was allowed to settle for 10 s before the potential was stepped to the desorption potential, Edes) -1000 mV. The current transient due to the desorption of the surfactant molecules and recharging of the double layer was measured and subsequently integrated to determine the difference between the charge density on the electrode surface at potentials Eads and Edes. This procedure was repeated by increasing the potential Eads from -950 to 450 mV in 50 mV increments. In an independent experiment, the pzc in NaF electrolyte was found to be 290 mV vs SCE as determined from the position of the diffuse layer minimum in the differential capacity curve of the electrode in 5 mM NaF. The absolute charge density at the electrode surface was then calculated from the measured difference of the charge densities and the pzc as described previously.32,33 The charge density curves for various concentrations of DeTATf in 0.1 M NaF supporting electrolyte are plotted as a function of potential in Figure 5. Consistent with the results obtained from CV measurements, the charge density curves for additions of DeTATf and the pure supporting electrolyte solutions merged at very negative potentials indicating that DeTATf is not adsorbed at these potentials. The charge density curves exhibit four distinct regions as indicated by the Roman numerals in Figure 5. In region I (-1 to -0.4 V) the curves initially coincide with the curve for the pure electrolyte, but then exhibit a sigmoidal inflection indicative of adsorption of the surfactant. Region II (-0.4 to +0.05 V) is characterized by a linear rise in the charge density with increasingly positive potentials. Region III (+0.05 to +0.3 V) also exhibits a linear charge dependence on the applied electrode potential, but the slope of the curve is larger compared to that of region II. In region IV, the charge density increases (31) Chen, A.; Lipkowski, J. J. Phys. Chem. 1999, 103, 682. (32) Richer, J.; Lipkowski, J. J. Electrochem. Soc. 1986, 133, 121. (33) Lipkowski, J.; Stolberg, L. Adsorption of Molecules at Metal Electrodes, Chapter 4; VCH: New York, 1992; p 171.
Langmuir, Vol. 23, No. 4, 2007 1787
Figure 5. Charge density versus electrode potential plots for the Au(111) electrode in contact with 0.01 M NaF solution (dotted line) and with selected DeTATf concentrations ranging from 0.018 (light gray line) to 2.7 mM (black line).
rapidly as a function of potential. As the pH of the supporting electrolyte is close to 9, hydroxide adsorption and pre-oxidation of the gold surface start in this range of applied potentials,31 making it impossible to separate these processes from any structural changes taking place in the adsorbed DeTATf layer. The chronocoulometric data provide information on the orientation of the surfactant molecules in regions II and III. The charge density curve shows that the potential of zero charge is shifted in the presence of DeTATf. The change in the potential of zero charge (∆Epzc) due to the displacement of surface water by a film of adsorbed molecules is described by:33,34
∆Epzc ) Γmax(µorg - nµw)/
(2)
in which Γmax is the maximum surface concentration of the organic molecules, µ org and µw are the average components of the permanent dipole moment in the direction normal to the surface of the organic molecule and water, respectively, n is the number of water molecules displaced from the electrode surface by one adsorbed organic molecule, and is the permittivity of the inner layer. Determination of ∆Epzc for region III is readily obtained from the intersection of the charge density plot with the zero line of the ordinate axis. Epzc ranges from ∼0.33 V for the lowest concentration of DeTATf to ∼0.27 V for the highest concentration of DeTATf, meaning the value of ∆Epzc changes from ∼ +0.04 to ∼-0.02 V with the bulk DeTATf concentration (the latter value is observed for the solution with a DeTATf concentration much higher than the cmc). This small shift indicates that the adsorbed ion must be an ion pair that resembles a “zwitterionic” molecule rather than a positively charged surfactant. We will therefore discuss the polarity of this molecule in terms of its dipole moment rather than in terms of the charge on the polar head. Inspection of eq 2 reveals that the term in the brackets dictates the sign of ∆Epzc. At the pzc, water molecules are known to be weakly preferentially oriented with the oxygen atom turned toward the Au (111) surface.35 At the pzc of a polycrystalline gold electrode, Beccuci et al.36a estimated the surface potential of oriented water molecules to be -640 mV. The (111) surface of gold is more hydrophobic than the polycrystalline surface,36a (34) Trasatti, S. J. Electroanal. Chem. 1974, 53, 335. (35) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664. (36) (a) Becucci, L.; Moncelli, M. R.; Guidelli, R. Langmuir 2003, 19, 3386. (b) Trasatti, S.; Parsons, R. J. Electroanal. Chem. 1986, 205, 359. (37) Lipkowski, J.; Nguyen Van Huong, C.; Hinnen, C.; Parsons, R.; Chevalet, J. J. Electroanal. Chem. 1983, 143, 375.
1788 Langmuir, Vol. 23, No. 4, 2007
Figure 6. Differential capacity curves calculated by differentiation of the charge density (dashed line) and from cyclic voltammetry (solid line) for the 0.297 mM DeTATf addition.
and hence this value must be smaller in magnitude for Au(111). In order for ∆Epzc to be small, µ org must be small as well. Consequently, in region III, because the values of ∆Epzc are small, DeTATf molecules replacing the solvent on the electrode surface must be oriented such that their dipole moment forms a small angle with the interfacial plane with the negative pole of the dipole facing the metal. To determine ∆Epzc in region II, a line is carried through the charge densities in region II and extrapolated to σm ) 0. Depending on the bulk concentration of DeTATf, Epzc changes from ∼+0.47 V (lowest concentration) to ∼+0.55 V (highest concentration). Consequently, ∆Epzc ranges from 0.17 to 0.26 V and is a positive value compared to the ∆Epzc in region III. Such ∆Epzc values indicate that in region II the DeTATf molecules must assume an orientation which gives a significant average dipole moment in the direction of the surface normal with the positive pole of this dipole turned to the electrode and the negative pole toward the solution (for the sign convention, see ref 36b). Clearly a change of the electrode potential causes a significant reorientation of DeTATf molecules at the electrode surface. In conclusion, the analysis of the charge density data indicates that the adsorption of DeTATf has a multistate character. Each of the states is characterized by a different value for the pzc, but all have similar minimum values of the capacity. Each state corresponds to a different orientation of DeTATf molecules at the gold electrode surface. The transition between these states is potential-controlled. Differential Capacitance. To further analyze the adsorption behavior of DeTATf molecules, the differential capacities of the Au(111) electrode in the presence of varying amounts of DeTATf were determined. The differential capacity for the 0.297 mM addition was calculated from the cyclic voltammetry data by dividing the current by the scan rate and also by numerical differentiation of the charge density curves, and these data are compared in Figure 6. For high bulk concentrations, the numerically differentiated curve may be considered the zero frequency capacity representing the adsorption equilibrium. The shape of the capacity curve calculated from the charge density curve agrees reasonably well with the capacity determined by single-frequency ac impedance. The differences between the two curves are relatively small, with the exception of the desorption peak at -550 mV. There, the error of differentiation of the chronocoulometry curve may be relatively large due to too large of a potential difference between adjacent points.
Brosseau et al.
Figure 7. Plots of the surface pressure of DeTATf at the gold/ solution interface versus the electrode potential for various bulk DeTATf concentrations: 0.018 (-9-), 0.048 (-0-), 0.059 (-b-), 0.089 (-O-), 0.237 (-2-), 0.297 (-4-), and 2.70 mM (-g-).
The plots of differential capacity (Figure 6) clearly demonstrate that two minima regions exist within the potential region of DeTATf adsorption at the Au(111) electrode surface. In the first region (-400 mV < E < -100 mV) the capacity reaches a minimum of ∼17 µF cm-2. The second minimum capacity region (200 mV < E < 300 mV) has a value of ∼35 µF cm-2. A broad peak centered at ∼+150 mV corresponds to the phase transition between the two states. Surface Pressure and Gibbs Excess. The surface pressure due to the adsorption of DeTATf at the gold/solution interface was calculated by integrating the charge density versus electrode potential plot obtained from the chronocoulometry experiments as described in
π ) γ0 - γ ) (
∫EE
des
σM dE)[DeTATf] - (
∫EE
des
σM dE)[DeTATf])0 (3)
where γo and γ represent the surface energy of the surfactant free and the surfactant covered electrode, respectively. We note that eq 3 is derived by integration of the well-known Lippmann equation.33 The surface pressure is a measure of the energetics of DeTATf adsorption. Figure 7 plots the surface pressure, π versus the electrode potential for several bulk DeTATf concentrations. The surface pressure data are observed to take the form of bell-shaped curves, and the maximum of the curve defines the potential of maximum adsorption (Em). These potentials correspond to the intersection points of the charge density curve for a DeTATf solution with the charge density curve for the pure electrolyte. For all DeTATf concentrations the value of Em is confined within region III. For all concentrations of DeTATf studied, the potential of maximum adsorption is dependent on the surfactant concentration, with Em for the lowest concentration being 0.25 V and Em for the highest concentration being 0.30 V. The potential of maximum adsorption is related to the ∆Epzc by the following equation:34,38
(Em - Epzc) ) -Cθ)1∆Epzc/(C0 - Cθ)1) (38) Parsons, R. Proc. R. Soc. London, Ser. A 1961, 261, 79.
(4)
Adsorption of DeTATf on the Au(111) Electrode
Langmuir, Vol. 23, No. 4, 2007 1789
Figure 8. Plots of the surface pressure of DeTATf at the gold/ solution interface versus charge density at the metal surface for various bulk DeTATf concentrations: 0.018 (-9-), 0.048 (-0-), 0.059 (-b-), 0.089 (-O-), 0.237 (-2-), 0.297 (-4-), and 2.70 mM (-g-).
where Cθ)1 and C0 are the capacities of the gold electrode fully covered and DeTATf free, respectively. Consistent with eq 4 and the earlier discussion of the ∆Epzc, the Em values are negative with respect to Epzc for all concentrations below the cmc (where ∆Epzc is positive) and positive with respect to Epzc for 2.70 mM (above the cmc), where ∆Epzc is negative. Therefore, any minimal dependence of Em on the DeTATf concentration is caused by reorganization of the adsorbed DeTATf molecules as discussed earlier. Independently, the Parsons’ function ξ ) σME + γ 39 has been calculated from the experimental data, and the film pressure at a constant charge Φ ) ξ0 - ξθ 40 has been determined. The calculated values of Φ are plotted against the charge density in Figure 8. For DeTATf, the shape of the Φ versus σM plots can be seen as being composed of two overlapping bell shape curves with maxima at ∼-10 µC cm-2 and close to the zero charge, respectively. As the concentration is increased, these two peaks tend to separate slightly from one another. Clearly, the analysis based on charge as the independent electrical variable also shows that DeTATf adsorption at the Au(111) electrode surface has a multistate character. By differentiating the Gaussian function obtained from fitting the surface pressure versus the natural logarithm of the bulk DeTATf concentration at constant E, the relative Gibbs surface excess Γ can be determined.
Γ)
(
∂π RT ∂ ln cDeTATf
)
(5)
E
The results of the differentiation are illustrated in Figure 9A, where Γ is plotted as a function of the electrode potential for the DeTATf concentrations studied. All DeTATf concentrations show a similar dependence of the Gibbs excess on the electrode potential. These curves are characterized by a slightly sloped plateau in the Γ(E) plot for potentials corresponding to region II followed by an ascending Gibbs excess as the potential is increased into the domain of (39) Parsons, R.; Trasatti, S. J. Electroanal. Chem. 1986, 205, 359. (40) Damaskin, B. B.; Petrii, O. A.; Batrakov, V. V. Adsorption of Organic Compounds on Electrodes; Nauka: Moscow, 1968.
Figure 9. (A) Plot of the Gibbs excess of DeTATf versus the electrode potential for various bulk DeTATf concentrations. (B) Plot of the Gibbs excess of DeTATf versus charge density at the metal surface for various bulk DeTATf concentrations.
regions III and IV. The maximum Gibbs excess in the region of the plateau is 1.7 × 10-10 mol cm-2. With increasing potential, the Gibbs excess is observed to be nearly twice as high as the Gibbs excess in the plateau. These higher Gibbs excesses indicate that there may be a transition from a two-dimensional to a threedimensional adsorption in region III as the surfactant concentration is increased. The underlying, horizontal DeTATf molecules may serve as a template for the three-dimensional aggregation of structures as previously visualized using STM and AFM imaging for SDS.21,22 Such changes in the surface composition can be described with the help of theory developed by Israelachvili et al. and later expanded on by Retter et al.17,19-20 In these works, the importance of packing constraints on the self-assembly of amphiphilic molecules is considered. Such constraints allow one to predict the favored aggregate structures for a particular surfactant system. The favored aggregate structure (i.e., micelles, bilayers, and hemicylinders, etc.) will depend on the optimal area per molecule (a0), the volume of the hydrocarbon chain (υ), and the maximum effective length that the chain can assume (lc). The critical packing parameter is then defined as (υ/a0lc). This dimensionless parameter can then be used to determine whether the surfactants will form spherical micelles (υ/a0lc < 1/3), nonspherical micelles (1/3 < υ/a0lc < 1/2), vesicles or bilayers (1/2
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this case unit mole fraction of organic species in the bulk of the solution and unit coverage (θ ) 1) by an ideal monolayer of noninteracting molecules. The Henry isotherm could be applied only to describe DeTATf adsorption at the most negative potentials corresponding to region II. Therefore, in the calculation of ∆G°ads, the maximum surface coverage of 1.3 × 10-10 mol cm-2 was used for Γmax. The ∆G°ads values are plotted against the electrode potential in Figure 10A. The Henry adsorption isotherm, as presented above is valid only when the film pressure is found to depend linearly on the bulk concentration of the adsorbing species. More complex adsorption isotherms are required to describe adsorption phenomena which deviates from linearity. Hence, the Gibbs energy was determined only in the initial region of the adsorption isotherm where there is a linear dependence of surface pressure on bulk concentration. Similarly, the film pressures at constant charge and eq 6 were used to calculate the zero coverage Gibbs energies of adsorption for charge used as the independent electrical variable. These values of ∆G°ads are plotted against the charge densities in Figure 10B. The Gibbs energies at constant potential and constant charge vary from ∼-28 to ∼-33 kJ mol-1. These values are characteristic for a physical adsorption of an organic molecule at a gold surface34 and indicate that in region II the interactions between the adsorbed molecule and the gold surface are weak. Interestingly, comparable magnitudes of the Gibbs energies of adsorption were observed for SDS and DDAPS adsorption at the gold electrode surface.22,25
Summary and Conclusions
Figure 10. (A) Zero coverage Gibbs energies of adsorption plotted as a function of the electrode potential. (B) Zero coverage Gibbs energies of adsorption plotted as a function of the charge density on the metal surface.
< υ/a0lc < 1), or “inverted” structures (υ/a0lc > 1). The packing parameter, υ/a0lc,17 for DeTATf (assuming an aggregation number of 59 18) is calculated to be 0.38, which is within the range expected for hemicylindrical surface micelles.19 Independently, the Gibbs excess at constant charge was calculated by plotting the Parsons’ function ξ ) σME + γ 38 and differentiating the relative ξ versus the logarithm of the bulk DeTATf concentration. The relative Gibbs excesses are plotted as a function of the electrode charge density in Figure 9B. For low bulk DeTATf concentrations the Gibbs excess plots display two maxima, one at ∼-20 µC cm-2 and the second at ∼-2 µC cm-2. As the concentration is increased, only one major maxima is observed at -10 µC cm-2. This may indicate that at lower concentrations a cationic form is adsorbed and that with increasing concentration coadsorption with the anion proceeds. Gibbs Energy of Adsorption. The standard Gibbs energy of adsorption, ∆G°ads at zero coverage, has been determined from the initial slopes of the surface pressure π Versus the bulk concentration of DeTATf plots using the Henry’s Law isotherm:31
c π ) RTΓβ 55.5
(6)
where β ) exp((-∆G°ads)/(RT)). The standard state assumed for
The chronocoulometric experiments provided charge density data that allowed us to describe the thermodynamics of DeTATf aggregation on the Au(111) electrode surface. We have used charge densities to determine the surface pressure of the film covered electrode surface, Gibbs excesses, and the zero coverage energies of adsorption. An unexpected and interesting result of this study is that the cationic surfactant behaves as a dipolar molecule rather than a cation at the electrode surface. This behavior strongly suggests that the anion is coadsorbed with the surfactant molecule. Our results indicate that DeTATf molecules start to adsorb onto the electrode surface at potentials between -600 and -400 mV, depending on the bulk concentration of DeTATf. The DeTATf molecules exhibit multiple states of adsorption. In the region of potentials between ∼-600 mV < E < ∼-100 mV the surfactant molecules assume a tilted orientation in which the hydrocarbon tails are directed toward the metal and the polar head toward the solution. The zero coverage Gibbs energies of adsorption determined in this region are close to -31 kJ mol-1 and hence are characteristic of a physisorbed state. At more positive electrode potentials ∼100 mV < E < ∼300 mV, the structure of the film depends strongly on the bulk DeTATf concentration. When the surfactant concentration is low, the molecules are oriented almost horizontal to the electrode surface. At higher bulk concentrations of DeTATf, the Gibbs excess is much higher than the value expected for a horizontal orientation. We propose a three-dimensional aggregation, or micellization, to explain this result. With only data from the electrochemical experiments, it is impossible to determine the exact geometry of the aggregate but the small shift in Epzc implies that the average permanent dipole moment of the adsorbed molecules must be nearly parallel to the electrode surface. Possible geometrical aggregates that satisfy this observation include full cylinders as observed by Ducker and Wanless for DDAPS adsorption on pyrolitic graphite and hemicylindrical aggregates with the
Adsorption of DeTATf on the Au(111) Electrode
individual surfactant molecules aligned colinear with the longaxis of the cylinder.13 The triflate counterion is believed to play an important role in the adsorption of the surfactant and the formation of such multidimensional aggregates. An increase in the counterion concentration causes a decrease in the electrostatic repulsion between charged headgroups, thus favoring both the formation of aggregates over monomers and closer packing of the headgroups, leading to less curved and more closely packed aggregates.41,42 The surface is also known to have an important role in determining the adsorption behavior of ionic surfactants.41 For adsorption of anionic surfactants at silica and mica, which are both negatively charged, hydrophilic surfaces, a closer arrangement of surfactant molecules is observed due to the interaction of the charged head groups with the charged surface. In contrast, on hydrophobic graphite, surface templates are observed to be the initial adsorption phase due to the good fit between the alkyl chains and the graphite lattice.41 Such flat-lying templates can in turn influence the type of aggregate structure which is then formed as the second layer. The adsorption of ionic surfactants at a Au surface is more complicated, because, in general, Au is not entirely hydrophobic nor hydrophilic.4 In fact, the nature of the polarity of the Au surface is largely determined by the type of surface adsorbed species. Therefore the aggregate morphology will depend on the (41) Liu, J.-F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 8558. (42) Lamont, R. E.; Ducker. W. A. J. Am. Chem. Soc. 1998, 120, 7602.
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relative interaction strengths of the alkane-Au physisorption vs the counterion-Au chemisorption. In the present study, the adsorption of DeTATf was examined under conditions of high salt concentration (supporting electrolyte) and this may encourage the formation of low curvature aggregates. In region IV, the interaction between the metal and the cationic surfactant should be repulsive. This repulsive interaction could be weakened by the formation of an ion pair between the decyltrimethylammonium surfactant and the triflate ion. The most reasonable aggregate structures for DeTATf to adopt in region IV would be hemimicelles or hemicylindrical/cylindrical stripes. AFM and IRRAS studies of DeTATf adsorption at Au(111) to be completed in the near future may provide insight into whether or not this is the case. Acknowledgment. We would like to thank Professor Dan Bizzotto (University of British Columbia) for supplying us with the software used to collect the electrochemical data. In addition, the authors would like to thank Allan McAlees for the initial suggestion of using triflate as the counterion and for assistance with the synthesis of DeTATf. Also, we thank M. A. Monteiro and A. Rullo (University of Guelph) for providing the 1D 1H NMR of DeTATf. This work has been supported by a grant from the Natural Sciences and Engineering Research Council of Canada. J.L. acknowledges the Canada Foundation for Innovation for the Canada Research Chair Award. C.L.B. acknowledges the OGSST award from the Government of Ontario. LA062284S
