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Direct Visualization of the Potential-Controlled Transformation of Hemimicellar Aggregates of Dodecyl Sulfate into a Condensed Monolayer at the Au(111) Electrode Surface I. Burgess, C. A. Jeffrey, X. Cai, G. Szymanski, Z. Galus,† and J. Lipkowski* Guelph-Waterloo Center for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada Received August 12, 1998. In Final Form: January 21, 1999 Electrochemical measurements, atomic force microscopy, and scanning tunneling microscopy have been combined to present the first direct images of the potential-controlled phase transition between the hemimicellar and condensed states of a dodecyl sulfate (SDS) film at the Au(111) electrode surface. The adsorbed SDS forms stripe-shaped hemimicellar aggregates at small or moderate charge densities at the electrode. High-resolution STM images of these aggregates revealed that adsorbed SDS molecules are ordered and form a long-range two-dimensional lattice. A unit cell of this lattice consists of two vectors that are 4.4 and 0.5 nm long and are oriented at an angle of 70°. We propose that each unit cell contains two flat-laying SDS molecules stretched out along the longer axis of the cell with the hydrocarbon tails directed toward the interior of the cell. The remaining SDS molecules in the hemimicelle assume a tilted orientation. This long-range structure is stabilized by the interactions of sulfate groups belonging to the adjacent cells. The sulfate groups of the flat-laying SDS molecules are arranged into a characteristic (x3 × x7) structure in which the sulfate groups along the x7 direction are bridged by hydrogen-bonded water molecules. When the positive charge on the metal either becomes equal to or exceeds the charge of adsorbed surfactant, the surface aggregates melt to form a condensed film. The transition between the hemimicellar and condensed states of the film is reversible. The hemimicellar aggregates may be re-formed by decreasing the charge density at the electrode surface. The charging and discharging of the gold electrode can be easily controlled by a proper variation of the electrode potential.
Introduction The aggregation of surfactant molecules adsorbed at the solid-solution interface into assemblies called hemimicelles was first proposed by Fuerstenau in 1955.1 Since then, evidence for the formation of surface aggregates has been provided by fluorescence quenching,2 surface force microscopy,3 and neutron reflection4 measurements. The most convincing and direct proof of the formation of these surface aggregates has come from atomic force microscopy (AFM) images of hemimicelles obtained by Manne et al.5 and Ducker et al.6 Knowledge of the structure of films formed by surfactants at the solid-liquid interface is vital for many applications such as flotation, oil recovery, detergency,7,8 and templating.9,10 The aggregation of † Department of Chemistry, University of Warsaw, Warsaw 02093, ul.Pasteura 1, Poland.
(1) (a) Gaudin, A. M.; Fuerstenau, D. W. Trans. AIME 1955, 202, 958. (b) Wakamatsu, T.; Fuerstenau, D. W. Adv. Chem. Ser. 1968, No. 79, 161. (c) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 70. (d) Fuerstenau, D. W.; Wakamatsu, T. Faraday Discuss. Chem. Soc. 1975, 59, 157. (2) Chandar, P.; Somasundaran, P.; Turro, N. J. J. Colloid Interface Sci. 1987, 117, 31. (3) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169. (4) McDermott, D. C.; McCarney, J.; Thomas, R. K.; Rennie, A. R. J. Colloid Interface Sci. 1994, 162, 304. (5) (a) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (b) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (c) Manne, S. Prog. Colloid Polym. Sci. 1997, 103, 226-233. (d) Manne, S.; Schaeffer; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382-6387. (e) Jaschke, M.; Butt, H.-J.; Gaub, H. E.; Manne, S. Langmuir 1997, 13, 1381-1384. (6) (a) Ducker, W. A.; Grant, L. M. J. Phys. Chem. 1996, 100, 1150711511. (b) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 3207-3214. (c) Wanless, E. J.; Ducker, W. A. Langmuir 1996, 12, 59155920. (d) Wanless, E. J.; Ducker, W. A. Langmuir 1997, 13, 1463-1474. (e) Wanless, E. J.; Davey, T. W.; Ducker, W. A. Langmuir 1997, 13, 4223-4228.
surfactants at solid surfaces is therefore a subject of intensive research that has explored issues such as the influence of the surfactant’s molecular structure,5c,d hydrophilicity-hydrophobicity of the substrate surface,6a and ionic strength and the nature of the counterion.6d It has long been recognized that charging of the solid surface has a significant impact on the surface assembly of nonionic11 and ionic surfactants.2 Electrochemistry provides an excellent opportunity to study the effect of charge on the behavior of adsorbed surfactant molecules. The physics of the charge-driven surface aggregation of amphiphilic (nonionic) surfactants has recently been described by Bizzotto et al.11 through the use of electrochemical techniques and reflectance UV absorption and fluorescence spectroscopies. These studies demonstrated that micelles of nonionic surfactants adsorb at a moderately charged electrode surface to form hemimicellar aggregates. The hemimicelles spread to form a condensed film when the charge at the electrode surface approaches zero. The adsorption of an ionic surfactant such as dodecyl sulfate at a mercury electrode surface attracted significant attention from the electrochemical community.12-17 The adsorption of dodecyl sulfate has a multiple state char(7) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990. (8) Swalen, J.; Allara, D.; Andrade, D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932-950. (9) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138-1143. (10) Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 268, 1324-1327. (11) (a) Bizzotto, D.; Lipkowski, J. J. Electroanal. Chem. 1996, 409, 33. (b) Bizzotto, D.; Lipkowski, J. Prog. Colloid Polym. Sci. 1997, 103, 201-215. (c) Sagara, T.; Zamlynny, V.; Bizzotto, D.; McAlees, A.; McCrindle, R.; Lipkowski, J. Isr. J. Chem. 1997, 37, 197-211.
10.1021/la981023i CCC: $18.00 © 1999 American Chemical Society Published on Web 03/19/1999
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Figure 1. AFM images in air of the flame-annealed gold-coated glass slides: (a) image of a 1 × 1 µm section of the surface showing multiple grains; (b) zoomed-in image showing surface topology of a single grain; (c) atomic resolution image of a section of the single grain surface showing characteristic structure of the unreconstructed Au(111) surface. Since the images were acquired in air, the reconstruction of the Au(111) surface was apparently lifted by adsorption of impurities from ambient atmosphere.
acter, and the film of adsorbed molecules undergoes phase transitions as a function of the electrode potential. On the basis of theoretical arguments, Nikitas et al.13 postulated that this surfactant forms hemimicelles at a weakly charged surface which later are transformed into a condensed monolayer at moderately positive surface charge densities. We will show in this paper that on gold, this transition involves positive charging of the metal to an extent that is equal or superequivalent with respect to the charge on the adsorbed anionic surfactant. Recently, Cunha and Tao18 and Wandlowski et al.19 applied scanning tunneling microscopy (STM) to image charge induced order-disorder transitions of chemisorbed 2,2-bipyridine at the Au(111) electrode surface. In the present work we will combine AFM and STM with electrochemical measurements to present the first direct images of the potential-controlled transition between the hemimicellar and condensed states of a film of dodecyl sulfate at the Au(111) electrode surface. We will demonstrate that the coupling of electrochemistry with scanning (12) Wandlowski, T.; Hromadova, M.; de Levie, R. Langmuir 1997, 13, 2766-2772. (13) Sotiropoulos, S.; Nikitas, P.; Papadopoulos, N. J. Electroanal. Chem. 1993, 356, 201-223. (14) Damaskin, B. B.; Nikolaeva-Fedorovich, N. V.; Ivanova, R. V. Zh. Fiz. Khim. 1960, 34, 894. (15) Eda, K. Nippon Kagaku Zasshi 1959, 80, 349. (16) Eda, K. Nippon Kagaku Zasshi 1959, 80, 708. (17) Foresti, M. L.; Moncelli, M. R. J. Electroanal. Chem. 1978, 92, 61. (18) (a) Cuhna, F.; Tao, N. J. Phys. Rev. Lett. 1995, 75, 2376 1. (b) Cuhna, F.; Tao, N. J.; Wang, X. W.; Jin, Q.; Duong, B.; D’Agnese, J. Langmuir 1996, 12, 6410. (c) Tao, N. J. In Imaging of Surfaces and Interfaces; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1999. (19) (a) Dretschkow, Th.; Dakkouri, A. S.; Wandlowski, Th. Langmuir 1998, 43, 2991. (b) Dretschkow, Th.; Lampner. D.; Wandlowski, Th. J. Electroanal. Chem. 1998, 458, 121. (c) Dretschkow, Th.; Wandlowski, Th. J. Electroanal. Chem., in press.
probe microscopy provides a unique opportunity to study the influence of surface charge on the aggregation of surfactants at the solid-liquid interface. We will show that by changing the potential applied to the electrode surface, one can follow the full evolution of the physical state of adsorbed surfactant molecules. It is hoped that this work will allow for a better understanding of the nature of the surface molecular assembly and will demonstrate the ability of electrochemistry to influence and control the molecular organization of surfactants at solid-liquid interfaces. Experimental Section SDS (99% Fluka) was recrystallized in ethanol and dried using an Albderhalden drying pistol. All solutions were prepared using Millipore (>18 MΩ) water. Images were captured using a Nanoscope E AFM (Digital Instruments, CA) using silicon nitride tips (Digital Instruments) which had a nominal spring constant of 0.06 N/m. The tips were exposed to ozone in a UV laminar flow cabinet for 30 min before use. All images were acquired in deflection mode, with both integral and proportional gains below 1, at scan rates between 7 and 12 Hz. No filtering of images was performed other than that inherent in the feedback loop. The STM images were acquired using a Nanoscope E instrument with an A scanner. The STM tips were electrochemically etched tungsten wires coated with polyethylene. All STM images were recorded in constant current mode with tunneling currents ranging between 0.2 and 20 nA. In AFM experiments, the working electrode consisted of a 2000 Å thick gold film vapor deposited onto a glass slide pretreated by a deposition of a 20 Å thick layer of chromium. The chromium layer was present to ensure a better adhesion of gold to the glass substrate. The electrode was annealed with a hydrogen torch before each experiment. Unannealed slides exhibited a rough surface that consisted of many small grains. Figure 1 shows that the flame treatment produced large grains with preferentially oriented flat (111) facets. A thin copper wire was attached to the gold-covered glass slide to provide electrical contact. The working electrode was isolated from the
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AFM scanner. The all-glass AFM cell was equipped with a gold oxide quasi-reference electrode and a platinum wire counter electrode. Before and after the experiment, the potential of the gold oxide quasi-reference electrode was measured versus a saturated calomel electrode (SCE), and all potentials are reported versus SCE. The AFM cell was connected to a glass reservoir using fine Teflon tubing. A glass reservoir of 100 mL of water and the AFM cell were purged with argon for 45 min before each experiment. Sufficient solid SDS to afford a 16 mM solution was added to the purged water. An argon blanket was placed over the bulk SDS solution to minimize oxygen contamination during the equilibration. The AFM cell was then filled with SDS solution and allowed to equilibrate for about 30-60 min until the open circuit potential reached a stable value (typically around -100 mV versus SCE). The working electrode used in STM experiments was a small bead produced by melting the end of a gold wire that was spot welded to a gold plate. The working electrode was cleaned by flame annealing before each experiment. A computer-controlled PAR model 173 potentiostat and a PAR model 5206 lock-in amplifier were used to perform the electrochemical experiments. These instruments were connected either to the AFM cell to control the potential applied to the working electrode and to perform the in-situ electrochemistry or to an independent threeelectrode cell used to study the properties of SDS at the Au(111) electrode surface. The data were acquired via a plug-in acquisition board (RC Electronics model ISC-16). In-house software was utilized to record the differential capacity curves C(E) (5 mV root mean square, 25 Hz ac) and to perform the chronocoulometric experiments. All measurements were conducted at a temperature of 20 ( 2 °C.
Results Electrochemical Studies. The adsorption of SDS onto the Au(111) electrode surface was initially characterized with the use of electrochemical techniques. The flameannealed gold-covered glass slide and a massive, rodshaped Au(111) single crystal electrode were used in these experiments. The electrochemical properties of the flameannealed slides were essentially identical to those of the massive Au(111) single crystal, and hence only the results for the gold-covered glass slides (with preferential Au(111) orientation after annealing) will be presented in this paper. All experiments with SDS were performed using a 16 mM solution, without any addition of a supporting electrolyte. This concentration is twice the critical micelle concentration (cmc), and hence the investigated solution was a mixture of micelles and the monomer of SDS. To compare the electrochemical behavior of SDS to the behavior of an inert electrolyte, the electrochemical measurements were also carried out in 16 mM KClO4 solution. Figure 2a shows differential capacity curves recorded for 16 mM SDS solution. For comparison, the curve for 16 mM KClO4 solution is also included in this figure. The data in Figure 2 cover potentials of the double layer region of the gold electrode, delimited by the hydrogen evolution reaction at the negative end and by the onset of gold oxidation at the positive end. In this region the metal-solution interface behaves as a capacitor. The differential capacity curves for SDS display features characteristic of a two-state adsorption. At the most negative potentials, the differential capacity curves for SDS merge with the curve recorded for the inert electrolyte and this feature indicates that the surfactant is desorbed from the electrode surface. Moving from left to right in Figure 2a, two peaks are observed that are separated by a long relatively flat section (-200 < E < 200 mV), in which the capacity assumes a value of ∼13 µF cm-2 (for the negative potential scan). This region is labeled as state I. At the positive limit of potentials (at E > 450 mV), the capacity drops to a very low value ∼5 µF cm-2. We denote this section as state II
Figure 2. Differential capacity of the Au(111) electrode recorded in 16 mM SDS solution: solid line, positive-going potential sweep; dashed line, negative-going potential sweep. The dotted line represents the differential capacity curve for 16 mM KClO4 solution. The curves were measured using 25 Hz, 5 mV root mean square ac perturbation, and a 5 mV/s potential sweep rate and were calculated assuming a series equivalent circuit. Charge density versus potential plots for: solid line, 16 mM SDS solution; dotted line, 16 mM KClO4 solution.
of the film. The most negative peak corresponds to the adsorption/desorption of SDS and the second peak to the phase transition between states I and II of the film. For consistency with the literature they are denoted as Rand β-, respectively. The differential capacity curves recorded using positive-going and negative-going potential scans display a hysteresis. Its presence indicates that the adsorption/desorption and the phase-transition phenomena are slow. The adsorption of SDS at gold displays essentially the same features that were observed earlier for the adsorption of this surfactant at the mercury electrode.12,13 At mercury, the capacity curve also displays peaks R- and β-, separated by a flat section. In the region between the peaks R- and β-, the capacity is equal to 9 µF cm-2. At potentials greater than the potential of peak β-, a capacitive pit is formed in which C is equal to 4 µF cm-2. These values are only
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somewhat lower than capacities for the states I and II on gold. SDS molecules are also desorbed from the mercury electrode surface at the negative limit of potentials. Using theoretical arguments, Nikitas et al.13 suggested that the region between peaks R- and β- corresponds to the formation of hemimicelles and the region of potentials more positive than β- corresponds to the formation of a condensed monolayer. Consequently, the β- peak represents the phase transition from the hemimicellar aggregation to the condensed assembly of the surfactant molecules. It should be emphasized that no direct molecular level experimental data were available to support this hypothesis. For mercury, the surface concentration of SDS in the condensed monolayer was estimated to be 2.8 × 10-10 mol cm-2.13 This amount corresponds to an adsorbed charge density of about 27 µC cm-2. About 20% higher packing densities were reported for the saturation concentration of SDS at the water-air interface20 with the corresponding charge densities in the range of 32 µC cm-2. Obviously, the condensed monolayer of SDS can be formed only if the charge of adsorbed surfactant is effectively screened by the charge on the metal. It is therefore useful to determine the charge density for the SDS-covered gold electrode to acquire further insight into the surface properties of this surfactant. The chronocoulometric technique, described in detail in refs 21 and 22, was employed to determine the charge density for the Au(111) electrode in 16 mM SDS and 16 mM KClO4 solutions. Figure 2b shows the charge density curves determined from these experiments. We note that for each potential, the electrode was allowed to equilibrate with the bulk solution for a duration of 3 min. Consequently, the charge density data represent the state of adsorption equilibrium. The curve for SDS displays two steps separated by two sections in which the charge changes with potential in a quasi-linear fashion. This behavior is consistent with the two-state adsorption of this surfactant. The region of state I corresponds to charge densities ranging from -10 to 7 µC cm-2. The transition between states I and II is very steep and involves a significant charging of the metal side of the interface. The phase transition is apparently completed when the charge on the metal attains a value of about 33 µC cm-2. That amount is sufficient to screen the charge of a compact monolayer of adsorbed SDS. The formation of a condensed film of SDS at a potential more positive than peak β- is therefore quite plausible. Atomic Force Microscopy Studies. After electrochemically characterizing the system, we attempted to image the phase transition between states I and II of the film with AFM. To record the images, we followed the procedure described by Manne et al.5 and by Ducker et al.6 Initially, we recorded the force distance curves by measuring the deflection of the cantilever versus the position of the sample mounted on the piezoelectric translator. The force was then obtained by multiplying the deflection of the cantilever with the spring constant, and the tip-sample separation was calculated by adding the deflection to the position of the sample.23 Figure 3 shows representative force curves for potentials corresponding to states I and II of the film. Curve 1 was recorded (20) (a) Miles, D. G.; Shedlowsky, L. J. Phys. Chem. 1944, 48, 57. (b) Elsworthy, P. H.; Mysels, K. J. J. Colloid Interface Sci. 1966, 21, 331. (c) Bujake, J. E.; Goddard, E. D. Trans. Faraday Soc. 1965, 61, 190. (21) Richer, J.; Lipkowski, J. J. Electrochem. Soc. 1986, 133, 121. (22) Lipkowski, J.; Stolberg, J. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH: New York, 1992; pp 171-238. (23) Butt, H.-J.; Jaschke, M.; Ducker, W. Bioelectrochem. Bioenerg. 1995, 38, 191.
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Figure 3. Normal force as a function of separation as a silicon nitride tip approaches a SDS film covered Au(111) electrode surface in aqueous solution of pure 16 mM SDS recorded for the following electrode potentials: curve 1, E ) 50 mV; curve 2, 350 mV; curve 3, 50 mV (SCE).
at a potential of 250 mV within the region of state I but just before the onset of the phase transition. Curve 2 was acquired after the electrode potential was moved through the phase transition to E ) 350 mV within the region of state II. The potential was then moved back to a value of E ) 50 mV, and curve 3 was recorded. Curves 1 and 3 are quite similar and this feature indicates that the state of the film can be controlled by moving the potential across the phase transition. The force-distance curves closely resemble the curves published in the literature.6b,d They consist of a repulsive section at moderate tip-surface separations where the force increases exponentially with the distance. At potentials corresponding to state I the curves have a discontinuity at a force of approximately 0.5 nN, caused by the displacement of the surfactant molecules by the tip. This “jump-in point” is rounded and observed at a higher force for curve 2, which was recorded at the potential corresponding to state II of the film. The section at forces higher than 1.5 nN corresponds to the direct interaction of the tip with the gold surface. Figure 3 shows a significant difference in the magnitude of the repulsive force between the tip and the surface at potentials of states I and II of the film. The sections of the curves at separations larger than 5 nm may be fit to the exponential function F ) A exp(-z/k). Forces measured at potentials corresponding to state I of the film have a decay length k ≈ 3 nm in good agreement with the estimated Debye length for the 16 mM SDS solution. This behavior suggests that the repulsive force is the doublelayer force.23 However, the force measured at 350 mV, within the range of state II of the film, has a longer decay length of ∼5 nm and an amplitude comparable to the force measured at much more negative potentials, corresponding to state I of the film. The silicon nitride tip is known to be negatively charged in a pure SDS solution.6b,e At separations comparable to the Debye length, the repulsive double-layer force is expected to dominate the interaction between the tip and the sample at potentials of state I, where the gold surface and the adsorbed film are negatively charged. However, an increase of the decay length and a negligible change of the pre-exponential factor for curve 2 in state II of the film are difficult to reconcile with the electrochemical data. The charge density curve
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Figure 4. AFM deflection images of stripe-shaped SDS aggregates adsorbed at a Au(111) electrode at the open circuit potential (∼-100 mV (SCE)): (a) 500 × 500 nm image; (b) 100 × 100 nm image of the well-ordered domain of SDS aggregates; (c) height versus distance profile measured in the direction normal to the stripes in Figure 4b.
in Figure 2a shows that the charge of adsorbed surfactant is either compensated or significantly screened by the charge on the metal. In contrast to the experimental result, the pre-exponential term of the double-layer force should decrease or even become attractive at potentials corresponding to state II of the film. At present, we do not know how to explain this contradiction. Consistent with the literature, all images were acquired using noncontact repulsive forces somewhat smaller than the value at which the discontinuity was observed on the force distance curve. Figure 4a shows a 100 × 100 nm image of the SDS film at the gold electrode captured at the open circuit potential. This image agrees very well with the result published by Jaschke et al.5e It shows stripeshaped surface aggregates formed by the anionic surfactant. The film has a mosaic of three domains of parallel stripes rotated by 60° or 120° separated by regions where the stripes are much less ordered. Figure 4b shows an enlarged section of a well-ordered domain, which consists of very long and very regular stripes. The periodicity of these stripes is shown in Figure 4c, which plots the height versus distance profile in the direction normal to the stripes. Within each domain, the distance between the stripes is equal to 4.5 ( 0.5 nm in good agreement with the results presented by Jaschke et al.5e for gold and Wanless and Ducker6b for a graphite substrate where values of 5 ( 0.5 nm were reported. The value of this distance depends somewhat on the orientation of the stripes with respect to the direction of the tip scan. Consistent with Wanless and Ducker6b the most reliable values were measured when the stripes were oriented in the direction normal to the movement of the tip. This distance is very close to the diameter of a spherical micelle
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formed by SDS in the bulk of the solution. The stripes are therefore interpreted as representing half-cylinder or cylinder-shaped aggregates of SDS lying on the substrate surface.5a-c,6b It should be emphasized that all these images display a periodic variation of the double-layer force across the surface, and the amplitude of the periodic deflection of the tip seen in Figure 4c should not be taken as the height of the SDS aggregate at the Au(111) surface. This point was discussed in detail by Wanless and Ducker.6b The AFM cell was then connected to the potentiostat and a potential of -100 mV (SCE), equal to the open circuit potential, was applied to the gold electrode. The connection to the potentiostat did not affect the AFM image of the film. Next, the electrode potential was changed to a value of 250 mV (SCE), just before the onset of the phase transition between phases I and II of the film. The AFM image of the film was then acquired at a frequency of 10.2 Hz, while simultaneously, the electrode potential was varied by applying a positive-going potential sweep at a scan rate of 5 mV/s. Consequently, during the capture of the AFM image the electrode potential was varied across the phase transition from a value of 250 to 500 mV (SCE). Knowing the slow scan axis frequency used to capture the image and the sweep rate of the potential applied to the electrode, we were able to calibrate the distance traveled by the tip with the change of the gold electrode potential. In this way, the changes of the film topology could be correlated to the change of the electrode capacity. Figure 5 compares the 400 × 400 nm image of the film to the change of the differential capacity of the interface. The AFM image in Figure 5b offers a direct visualization of the molecular reorganization that occurs as a function of the electrode potential. The image was collected with the tip moving top to bottom (fast scan axis) and progressing from left to right (slow scan axis) while the potential of the working electrode was swept through the phase transition. The left side of the image corresponds to potentials of state I of the film. The characteristic pattern of parallel stripes formed by the surface aggregation of SDS is clearly visible in this region. Moving from left to right, the stripes melt down and progressively convert into a smooth layer. On the right side of the image, when the potential has reached 500 mV (SCE), the surface is covered by a compact (condensed) layer and the characteristic striped pattern of the hemimicellar aggregates is totally lost. This smooth, homogeneous layer is formed at potentials corresponding to state II of the film. The melting of the striped aggregates occurs at potentials of the capacitive peak representing a phase transition from state I to state II of the film. Apparently, patches of the condensed film progressively develop and grow over the capacitive peak region. The transition between the hemimicellar and condensed states of the film is fully reproducible. If the direction of the voltage scan is reversed and the tip is scanned back from right to left, the hemimicellar phase is re-formed from the condensed film. Figure 6 shows the image of hemimicellar aggregates formed from the condensed film by sweeping the potential from 500 to 250 mV (SCE). This film has a very similar structure to the film formed by a spontaneous assembly of SDS at gold at the open circuit potential as seen in Figure 6b. It has the characteristic mosaic of parallel-striped domains rotated by a 60° angle. The size of these domains depends on the potential sweep rate. This behavior indicates that the rate of the phase transition is slow. The hysteresis on the differential capacity curve, seen in Figure 2b, has already indicated that the phase transition is sluggish. At mercury, the kinetics of the phase transition was investigated recently
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Figure 5. Differential capacity of the Au(111) electrode recorded in 16 mM SDS solution using 5 mV/s positive-going potential sweep, 25 Hz and 5 mV root mean square ac perturbation and calculated assuming a series equivalent circuit. 400 × 400 nm AFM deflection image recorded at a scan rate of 10.2 Hz progressing from left to right (slow scan axis). The potential of the working electrode was simultaneously swept at 5 mV/s.
by Wandlowski et al.12 However, investigations of the kinetics of the phase transition at the Au(111) electrode are beyond the scope of the present work. These results provide the first direct molecular level evidence that adsorption of SDS at metal surfaces involves formation of hemimicelles which later spread to form a condensed monolayer. They confirm the mechanism of adsorption of ionic surfactants proposed by Nikitas et al.13 They also support the model of surface aggregation of amphiphilic surfactants proposed recently by Bizzotto et al.11 The desorption of SDS molecules observed in Figure 2 at potentials ∼-400 mV, constitutes a second potentialcontrolled phase transition in the film of adsorbed SDS. For neutral amphiphilic surfactants, Bizzotto et al.11 proposed that by approaching the desorption potentials, surfactant molecules coalesce into nanometer-sized droplets. They envisaged that the contact angle between the
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Figure 6. AFM deflection images of stripe-shaped SDS aggregates adsorbed at a Au(111) electrode: (a) 250 × 250 nm image at E ) 250 mV after sweeping the electrode potential to E ) 500 mV (SCE) to form the condensed film and sweeping the potential back to more negative values to re-form the striped pattern of the film; (b) 200 × 200 nm image at the open circuit potential (E ∼ -100 mV (SCE)) before external potentials were applied.
droplet and the electrode surface increases when the potential is made more negative until at a sufficiently large value of the contact angle (sufficiently negative potential) the droplet is detached. The validity of this model was recently proven by Ivosevic and Zutic.24 To test the applicability of this model in describing the potentialcontrolled desorption of SDS, we have acquired images of the SDS-covered Au(111) surface at negative potentials. Figure 7a shows the AFM image for E ∼ -300 mV, just at the onset of the desorption. The image reveals that SDS molecules begin to coalesce at this negative potential. Nanometer-sized islands or droplike features could be seen (24) Ivoseviv, N.; Zutic, V. Langmuir 1998, 14, 231.
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Figure 7. Deflection AFM image of SDS aggregates adsorbed at a Au(111) electrode from 16 mM SDS solution at (a) E ∼ -300 mV and (b) E ∼ -400 mV (SCE). The images were acquired using a constant force ∼0.4 nN.
in those sections of the image where the film is disordered, while in the ordered domains, the stripes became smeared out and some of them are doubled in size, indicating a tendency to melt. Figure 7b shows an image of the surface at a potential of ∼-400 mV. Most of the SDS molecules are already desorbed. However, consistent with the model by Bizzotto et al.,11 one could clearly see randomly distributed, droplike islands that are about 10 nm in size. These droplike features disappear and the stripes of SDS aggregates appear when the electrode potential is moved back to a more positive value within the region of state I of the film. These images provide a visualization of the aggregation of surfactant molecules into nanodroplets as the first stage of the potential-controlled desorption. They support the predictions of the model by Bizzotto et al.11 that the desorption involves a potential-controlled detachment of the nanodroplets.
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Figure 8. STM images of hemicylindrical SDS aggregates formed at the Au(111) electrode surface at E ) 300 mV (SCE) from 16 mM SDS solution: (a) image of a 100 × 100 nm section of the electrode area; (b) unfiltered high-resolution image showing individual SDS molecules at the edge of hemicylindrical aggregates. The images were acquired with a constant tunneling current 0.2 nA and the tip voltage -200 mV.
Scanning Tunneling Microscopy Studies. In AFM experiments the force between the tip and the SDS-covered Au(111) surface is significantly different for potentials of states I and II of the film. Consequently, it is not certain whether the disappearance of the characteristic pattern of stripes during the phase transition imaged in Figure 5 is due to the formation of a disordered condensed layer or to a loss of contrast caused by a change of the force. We have learned that aggregates of SDS may be imaged by STM from Schneweiss,25 and hence we have employed this technique to acquire additional images of the hemimicellar and condensed states of SDS adsorbed at Au(111). Figure 8a shows an STM image of the SDScovered gold surface at a potential slightly more negative than the potential of the phase transition. The metal (25) Schneeweiss, M. A. Personal communication at the Conference on Electrified Interfaces, Portugal, July 5-11, 1998.
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Figure 9. (a) Model of the long range ordering of SDS aggregates built around a unit cell having vectors of 4.4 and 0.5 nm orientated at an angle of 70°. (b) Model of the water-bridged sulfate groups arranged into (x3 × x7) cells. Adopted from ref 28. (c) Model of the cross section of the hemimicelle along the direction of the longer vector of the unit cell.
surface is already positively charged at this potential. Consistent with the AFM results, the image shows a characteristic pattern of stripes arranged in domains rotated by 60° or 120°. The stripes have periodicity of 4.4 nm, which is the same periodicity of the stripes seen previously in the AFM images shown in Figures 4 and 6. The image shows that the gold surface is covered by a number of gold islands of monatomic height. The adsorbed SDS molecules form the stripelike aggregates on the top of these islands as well. Figure 8b shows a high-resolution image of the stripes. This image reveals stripes with a frizzy interior and an ordered array of individual SDS molecules at the edges. Apparently, the surfactant molecules are immobilized at the edge of the stripe while they are relatively mobile in the stripe interior. The molecules belonging to the edges of two neighboring stripes are apparently correlated so that the border between the two stripes has a characteristic “zipperlike” appearance. The distance between the spots on the rim of a stripe amounts to 5.0 ( 0.4 Å, and the distance between spots belonging to the nearest rims of the zipper amounts to 7.6 ( 0.4 Å. These dimensions correspond well to the size of the so-called (x3 × x7) structure formed by sulfate adsorbed at the Au(111)
electrode surface.26 In fact one could easily see that positions of SDS molecules belonging to different zippers are also correlated to the extent that adsorbed molecules form a long-range two-dimensional lattice. A unit cell of this lattice consists of two vectors that are 4.4 and 0.5 nm long and are oriented at an angle of 70°. We can now use this lattice to build a model of the hemimicelle shown in Figure 9a. In each unit cell, we place two flat-laying SDS molecules which are stretched out along the longer axis of the cell and are directed with their hydrocarbon tails toward the interior of the cell. In this packing the sulfate groups from four adjacent cells are arranged into the characteristic (x3 × x7) structure. It is well-known that adsorbed sulfate ions form the (x3 × x7) ordered adlayers at (111) surfaces of many metals.26c It is also known that this particular structure is stabilized by water molecules bridging neighboring sulfate ions through hydrogen bonding.27 We propose that the same interaction stabilizes the sulfate groups of the flat adsorbed SDS molecules. We (26) (a) Magnussen, O. M.; Hageboeck, J.; Hotlos, J.; Behm, J. Faraday Discuss. Chem. Soc. 1992, 94, 329. (b) Eden, G. J.; Gao, X. J. Electroanal. Chem. 1994, 375, 357. (c) Nichols, R. J.; Li, W. H. J. Electroanal. Chem. 1998, 456, 153. (27) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951.
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Figure 10. Unfiltered high-resolution STM image of a reconstructed Au(111) electrode surface at E ) -200 mV (SCE) in 0.1 M KClO4 solution. The image was acquired with a tunneling current of 1 nA and a tip voltage of 0 mV.
also propose that the characteristic pattern of long stripelike surface aggregates of SDS results from the interaction between the sulfate groups from adjacent hemicelles. The sulfate groups are ordered in an array of (x3 × x7) cells forming a “zipper” which cements the large unit cells of the SDS aggregates. Such a molecular model of the water-bridged sulfate groups, adopted from the work by Ataka and Osawa,27 is shown in Figure 9b. In this model, the position of the hydrocarbon tails relative to gold atoms on the surface is shown schematically. The remaining molecules in the hemimicelle aggregate on the top of the flat adsorbed molecules to form a hemicylindrical shape as proposed by Manne5c and Wanless and Ducker.6b The cross section of the hemimicelle along the direction of the longer vector of the unit cell is schematically shown in Figure 9c. Following Wanless and Ducker,6b we have assumed that seven SDS molecules may be accommodated into the hemimicelle. The resolution of STM images is much better than the resolution of images acquired by AFM. Consequently, STM images provide details of the molecular structure of the hemimicellar aggregates formed by SDS unknown to the previous AFM studies of this system. At negative charge densities the Au(111) electrode surface is known to form the so-called (x3 × 23) reconstructed surface. The topography of the reconstructed surface consists of characteristic stripes quite similar in appearance to the stripes formed by SDS aggregates.28,29 However, the two patterns display significant differences in detail such that the images of the SDS aggregates can be distinguished from the images of the reconstructed surface. Figure 10 shows a high-resolution image of a reconstructed Au(111) surface in 0.1 M KClO4 solution. The stripes in this image are different in appearance. They are wider, their periodicity is ∼6.5 nm, and they show well-resolved individual gold atoms. An important question is whether the SDS aggregates are formed at the reconstructed or unreconstructed surface. We have made attempts to image the gold surface under the SDS aggregates by increasing the tunneling currents up to 20 (28) Kolb, D. M. In Structure of Electrified Interfaces; Lipkowski, J., Ross, P. N., Eds.; VCH: New York, 1993. (29) Kolb, D. M. Prog. Surf. Sci. 1996, 51, 109.
Figure 11. STM images of a condensed film of SDS formed at the Au(111) electrode surface at E ) 400 mV (SCE) from 16 mM SDS solution: (a) image of a 40 × 40 nm section of the electrode area; (b) unfiltered high-resolution image showing individual SDS molecules at the edge of a small gold island The images were acquired with a constant tunneling current of 0.2 nA and a tip voltage of -200 mV.
nA. Our attempts were not successful, and we were not able to determine the structure of the gold surface at potentials where the surface aggregates of SDS are formed. However, two pieces of information may suggest that the gold surface is reconstructed at potentials corresponding to state I where the hemimicelles are formed. First, the ordered (x3 × x7) structure of adsorbed sulfate is always observed at an unreconstructed (111) surface. Second, in the presence of SDS, the STM images show a number of islands of monatomic height (see Figure 8a). Such islands are formed when the reconstruction of the Au(111) surface is lifted.28,29 Nevertheless, surface X-ray scattering experiments of the type described in refs 30 and 31 need to be performed to resolve this issue. (30) Wandlowski, Th.; Ocko, B. M.; Magnussen, O. M.; Wu, S.; Lipkowski, J. J. Electroanal. Chem. 1996, 409, 155. (31) Wu, S.; Lipkowski, J.; Magnussen, O. M.; Ocko, B. M.; Wandlowski, Th. J. Electroanal. Chem. 1998, 446, 67.
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Figure 11a shows a 40 × 40 nm image of the SDScovered Au(111) surface at a potential within the region of the condensed film. Consistent with the results of AFM studies, the characteristic pattern of stripelike aggregates is lost. The film is apparently disordered or fluidlike. An interesting feature to note is the decoration of steps of monatomic islands of gold by SDS molecules. This point is illustrated by Figure 11b, which shows poorly resolved but discernible short rods at the edge of the island. These short rods are about 5 Å apart, and this feature suggests that they correspond to immobilized SDS molecules rather than to the step frizziness caused by the mobility of gold atoms. In general, the STM images confirm the conclusion of the AFM experiments that the phase transition between states I and II of the film of SDS involves a transformation of hemimicelles into a condensed, structureless layer. The STM images provided more detail concerning the molecular structure of surface aggregates. However, we have not succeeded in imaging the formation of nanodroplets at the negative desorption potentials using STM. These features are better imaged with the noncontact AFM. Conclusions At moderately charged Au(111) electrode surfaces, SDS molecules form aggregates (hemimicelles) arranged in a long-range two-dimensional lattice. A unit cell of this lattice consists of two vectors that are 4.4 and 0.5 nm long and are oriented at an angle of 70°. Each unit cell contains two flat-laying SDS molecules stretched out along the longer axis of the cell with the hydrocarbon tails directed toward the interior of the cell. The remaining SDS molecules in the hemimicelle assume a tilted orientation. This long-range structure is stabilized by the interactions of sulfate groups belonging to the adjacent cells. The sulfate groups of the flat-laying SDS molecules are
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arranged into a characteristic (x3 × x7) structure in which the sulfate groups along the x7 direction are bridged by hydrogen-bonded water molecules. At positive charge densities, sufficient to screen the charge of adsorbed SDS, the hemimicellar aggregates melt to form a condensed film. We have provided the first direct visualization of the potential-controlled transformation of hemimicellar aggregates of SDS into a condensed film, most likely a monolayer. These images constitute the first molecular level evidence that surface aggregates of SDS are indeed formed in the region between the capacitive peaks R- and β- and that the film formed at potentials more positive than the potential of the peak β- has a condensed nature. Our data show that the hemimicellar aggregates are formed at small or moderate absolute charge densities and that the condensed film is formed when the amount of positive charge on the metal is either equal to or superequivalent with respect to the charge of adsorbed surfactant. These results demonstrate a tremendous potential of electrochemistry to study the influence of the charge on the solid on the surface aggregation of surfactants. Electrochemistry provides a unique opportunity to study continuously the different stages involved in the interaction of surfactants with solid surfaces such as adsorption of micelles, formation of hemimicelles, and spreading of hemimicelles into a condensed monolayer. Acknowledgment. This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. Supporting Information Available: Color images of Figure 1, 4-8, 10, and 11. This material is available free of charge via the Internet at http://pubs.acs.org. LA981023I
