Langmuir 2000, 16, 4447-4454
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Counterion Effects on Adsorbed Micellar Shape: Experimental Study of the Role of Polarizability and Charge Vivek Subramanian and William A. Ducker* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061 Received September 17, 1999. In Final Form: February 22, 2000 We have used atomic force microscopy (AFM) to measure the shape of micelles adsorbed to the interface between hydrophilic silica and solutions of hexadecyltrimethylammonium (CTA+) ions in the presence of various counterions. Spherical CTA+ surface micelles are adsorbed from solutions containing salts of CH3CO2-, CO32-, SO42-, SO32-, and HSO32-, even at high salt concentrations (>100 mM). Cl- and Brform slightly oblate micelles near their respective critical micelle concentrations. Addition of Br- causes a transformation to cylindrical micelles whereas addition of Cl- does not. The difference between Br- and Cl- is similar to the behavior observed previously in bulk solution. Addition of S2O32- or CS32- or HS-/S2transforms spherical micelles to cylinders. We rationalize these effects on the basis of the hard/soft (unpolarizable/polarizabile) nature of the ions. The ability to effect the sphere-to-cylinder transformation correlates with the availability of a soft anionic atom in the counterion. Presumably, this soft anionic atom promotes partitioning of the ion from the water to the micelle surface, thereby lowering the electrostatic repulsion between headgroups and effecting the sphere-to-cylinder transformation. Surface micelles are observed at concentrations below the critical micelle concentration. For CTABr, AFM can detect surface micelles at concentrations greater than half the critical micelle concentration.
Introduction The commercial utility and scientific interest in surfactants arises both because of the propensity of surfactants to modify interfacial forces and because of their ability to form mesoscopic structures such as micelles. There is now strong evidence that surfactants form micellelike structures at interfaces.1 The existence of clusters of surfactant molecules at solid-liquid interfaces (hemimicelles) was first proposed in 19552 and, more recently, Manne et al.3,4 showed that atomic force microscopy (AFM) can be used to image surfactant aggregates at the interface between aqueous surfactant solutions and a variety of substrates. The role of substrate,4-6 surfactant geometry,7,8,11 counterion type and concentration,9-11 and temperature8 in determining the shape of the surface aggregates have been examined. In this work, we examine the role of counterion charge and softness (polarizability) in controlling the shape of the surface micelle. The packing parameter model12 is the central idea in the theory of micellar shape. This model rests on the * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Surfactant Adsorption and Surface Solubilization; Sharma, R., Ed.; American Chemical Society: Washington, DC, 1995. (2) Gaudin, A. M.; Fuerstenau, D. W. Trans. AIME 1955, 202, 958962. (3) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (4) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (5) Ducker, W. A.; Grant, L. M. J. Phys. Chem. 1996, 100, 1150711511. Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. 1998, 102, 4288-4294. (6) Burgess, I.; Jeffry, C. A.; Cai, X.; Szymanski, G.; Galas, Z.; Lipkowsky, J. Langmuir 1999, 15, 2607-2616. (7) Manne, S.; Scha¨ffer, T. E.; Huo, W.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A.; Langmuir 1997, 13, 6382-6387. (8) Liu, J.-F.; Ducker, W. A. J. Phys. Chem., in press. (9) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160-168. (10) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602-7607. (11) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685-1692. (12) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1976, 72, 1525-1568.
concept that micellar shape is dictated by a balance between repulsive headgroup interactions and attractive forces arising from a need to minimize the exposure of the hydrophobic core to water. The counterion acts by altering one or both of these forces. Adsorption of the counterion to the micelle surface can (1) reduce repulsive headgroup interactions, thereby lowering the curvature of the micelle, (2) change the interfacial energy at the surface of the micelle, and (3) change the volume of hydrocarbon encapsulated (for large hydrophobic counterions). The counterions are not confined to the micellar surface; they can be distributed from solution to the micellar surface, or even into the hydrophobic core (e.g., catanionic surfactants13). Therefore, the two related questions are as follows: which forces are important in determining the distribution of counterions, and how do the counterions then affect micellar shape? Here, we study the shape of hexadecyltrimethylammonium (C16TA+) micelles adsorbed to silica. The role of counterions in the aggregation of quaternary ammonium surfactants in bulk solution has already been carefully considered. The shape of C16TA+ micelles can be changed by changing the counterion,14 the concentration of surfactant,14,15 or the concentration of added salt.16,17 At the cmc the micelles are spherical. A 0.1 M CTABr solution contains eccentric micelles (eccentricity ) 1.6), whereas a CTACl solution at the same concentration contains spherical micelles.14 The addition of more surfactant or salt causes the formation of cylindrical micelles.15 However, this transition requires about 20 times as much chloride as bromide. Hydroxides and small carboxylates form very small micelles.18 These “harder” ions have a greater affinity for water and experience a smaller attractive dispersion force with the micelle. These hard (13) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 6698-6707. (14) Berr, S.; Jones, R. R. M.; Johnson, J. S., Jr. J. Phys. Chem. 1992, 96, 5611-5614. (15) Imae, T.; Abe, A.; Ikeda, S. J. Phys. Chem. 1988, 92, 1548-1553. (16) Imae, S.; Ikeda, T. J. Phys. Chem. 1986, 90, 5216-5223. (17) Quirion, F.; Magid, L. J. J. Phys. Chem. 1986, 90, 5435-5441.
10.1021/la991245w CCC: $19.00 © 2000 American Chemical Society Published on Web 04/07/2000
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ions are therefore less concentrated at the micelle surface and each ion has a smaller effect on reducing the headgroup area. Often this is explained in terms of a lyotropic or Hoftmeister series. Recently, Ninham and Yaminsky19 have quantified this effect. “Soft” or polarizable ions have greater surface excess on the micelle compared to hard ions because of larger excess polarizability and therefore greater dispersion forces. Specific ion effects have also been reported in the study of microemulsions. Effective headgroup areas of quaternary ammonium surfactants were found to decrease when the counterion is changed from Cl- to Br- to I-.20 This was at the time rationalized by a decrease in the distance of closest approach of the ion to the micelle when the size of the hydrated ion decreases. The same trend is also explained by an increase in van der Waals force for the more polarizable ions. There have been few studies on the influence of divalent ions on the structure of CnTA+ ions. When image charges are ignored, the unfavorable electrostatic free energy, ∆Gel, for creating the double layer with monovalent ions is twice that for divalent ions, so one might expect divalent ions to be more concentrated at micelle surfaces and therefore to be more effective in promoting shape changes. However, Wennerstro¨m et al.21 report that SO42- is ineffective at inducing micellar growth of C16TA+. They attribute this to the greater repulsion of the highly charged ions from the low-dielectric interior of the micelles (image charge effects). Wennerstro¨m et al. show that the image charge repulsion always reduces the ratio of attractive electrostatic forces for divalent compared to monovalent ions to values less than 2. If a dianion approaches the micelle between the headgroups, the overall electrostatic interaction attraction of a monovalent ion is stronger than that for the divalent ion. This is used to explain why Br- is more effective than SO42- in binding to the CTA+ micelle and inducing growth. However, it is not clear why the divalent anion would approach a headgroup along an energetically unfavorable route. Alternatively, the importance of the hard/soft effect for monovalent ions suggests that the inability of SO42- to induce shape changes may arise because it is a hard molecule that retains water of hydration rather than closely associating with the micelle surface. This motivates us to study divalent ions of varying hardness (polarizability). We show that the softness of the surfactant counterion appears to be more important than the charge in determining the shape of C16TAB micelles. In related work,22 we examine the effect of changing the space between the charges in a dianion. We have used AFM to determine the shape of micelles adsorbed to the silica/water interface. AFM offers a convenient method for obtaining real-space information in situ. However, the adsorbent also plays a role in determining the structure, so the solid substrate must be chosen with care. We have chosen amorphous hydrophilic silica because past experience shows that C16TAB micelles have approximately the same shape (spherical micelles) at the silica-water interface as that in bulk solution.4,8 In contrast, C16TAB forms hemicylinders on graphite3 and bilayers on mica.8 The effects that we observe on the silica surface may thus be closely related to the effects in bulk. (18) Brady, J. E.; Evans, D. F.; Warr, G. G.; Grieser, F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 1853-1859. (19) Ninham, B. W.; Yaminsky, V. Langmuir 1997, 13, 2097-2108. (20) Chen, V.; Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1987, 91, 1823-1826. (21) Wennerstro¨m, H.; Khan, A.; Lindman, B. Adv. Colloid Interface Sci. 1991, 34, 433-449. (22) Subramanian, V.; Ducker, W. A., in preparation.
Subramanian and Ducker
At this point it is worth considering the limitations of our technique. The AFM measures forces between the micelles and the AFM tip in solution. Because the forces have short range, the images are dominated by the solution face of the surface micelle and yield little or no information about the structure closer to the silica surface. Chemical considerations, surface force measurements,23 and neutron reflectivity measurements24 suggest that there is a layer next to the silica with headgroups oriented toward the silica and a layer adjacent to the solution with headgroups oriented toward the solution. For simplicity, we will describe the structures as if there is a mirror plane between the first and second layers, but the data are also consistent with hemispherical and hemicylindrical layers on a (continuous) planar inner layer. The existence of a planar inner layer could be justified on the grounds that interactions with the silica layer reduce the headgroup spacing of the inner layer. In that case, our arguments about the influence of solution anions apply only to the outer surfactant layer. Rutland and Parker23 provide a lengthy discussion and review on the organization of CTABr adsorbed to glass surfaces, based mainly on their adhesion and potential measurements. The large adhesion between CTABr-coated glass surfaces that they measure suggests that an inner hydrophobic layer forms when the outer layer of the surfactant is removed. It is not clear whether this layer exists in the unperturbed state. Ideally, for this study, a series of CTA+ salts with varying counterions should have been prepared. However, for convenience, most experiments were done at 10 mM CTAAcetate (CTAAc) and the desired anion concentration. CTAAc was chosen because previous work showed that acetate did not affect the micelle shape.18 We observed that LiAcetate (LiAc) concentrations up to 500 mM also did not change the shape of micelles adsorbed to the silica surface and concluded that micellar shape was indifferent to the presence of acetate. Later, we show that this assumption is not entirely correct. Wherever possible we used the Li+ salt of the counterions because this strongly hydrated (hard) ion should have the lowest affinity for the micelle and silica surfaces. Experimental Section Samples and Substrates. Silica plates (Hareaus Amersil Inc.) were cleaned by soaking in hot concentrated sulfuric acid (95.7%, analytical grade, Baker, USA) for more than 12 h. The plates were then thoroughly rinsed with water. They were accepted for adsorption measurements if a uniform thin film of water formed on the surface when exposed to steam (the “steam test”).25 Silica surfaces passing this test were completely wet by water. The force between a clean silicon AFM tip and silica substrates thus cleaned was always repulsive at small separations (99%), potassium trithiocarbonate (K2CS3‚H2O 99%), and sodium metabisulfite (Na2S2O5 99%) were obtained from Aldrich and used without further purification. Atomic Force Microscopy. Images were captured in situ in solutions using a Nanoscope III AFM26 (Digital Instruments, CA) using silicon cantilevers (Part Scientific, CA) with nominal spring constants of 0.06 N/m. Spring constants were not independently calibrated because the force also depends on the radius of the tip and this was not known. Prior to use, the cantilevers were irradiated in a laminar flow cabinet with ultraviolet light (∼9 mW/cm2 at 253.7 nm) generated from a PENRAY Lamp (UVP, Inc., Upland, CA). The irradiation time was about 40 min for the first use and 10 min for subsequent use. The solution was held in a fluid cell and sealed by a silicone O-ring. For each experiment, the fluid cell was first cleaned with water and then distilled ethanol (Aaper Alcohol Chemical Co., USA). Images record cantilever deflection while integral and proportional gains were in the range 0.5-1 and scan rates were in the range 5-10 Hz. Distances in lateral dimensions were calibrated by imaging a diffraction grating replica of a 2160 lines/mm waffle pattern. Distances normal to the surface were calibrated by measuring the depth of the bars of the grid pattern (31-nm deep). AFM images are unfiltered deflection images, except Figure 3a,b as noted in the caption. However, it should be mentioned that some long-wavelength features are removed by the AFM feedback loop. When determining the shape of objects by AFM, it is very important to minimize the effects of drifts that might distort the measured shape. The following steps were taken to eliminate drift in the slow scan axis. First, a new O-ring was used for each experiment. O-rings were immersed overnight in sodium dodecyl sulfate solution before each experiment and rinsed thoroughly with water before use. Ethanol was never used to rinse the O-ring because exposure to ethanol caused it to adhere to the sample. This accentuated the drift and allowed the introduction of contamination. Second, the piezoelectric scanner was left scanning over an area of (10 µm)2 for periods ranging from 1 to 3 h. This procedure was always successful in eliminating the drift. Images were only recorded after measurable drift was eliminated. Images were usually captured at the maximum force that would allow stable imaging before the characteristic instability that is associated with displacement of the surfactant film (see Figure 1b). As the force was decreased from this point (i.e., increased separation), the image became less clear.
Results and Discussion Effect of Monovalent Ions. The principal monovalent ions studied were Br-, CH3COO- (Ac-), and Cl-. Of these, the Br- is “borderline” soft whereas the latter two are relatively hard.27 Figure 1a shows a 300-nm image of micelles with an approximately circular cross section at the interface between silica and 10 mM CTABr solution. The Fourier transform of the image is a circular ring which indicates that there is a preferred distance between the centers of the micelles, but no preferred orientation. The maximum intensity in the Fourier transform of the AFM image, R, is 9.3 ( 0.4 nm. R corresponds to the sum of the micelle diameter and the intermicellar separation. The height of the adsorbed layer cannot be obtained directly from the image for two reasons. First, the image signal is the tip deflection captured with gain, and second, the tip has similar dimension to the gaps between the micelles, so it may not reach the substrate. Nevertheless, the thickness of the adsorbed layer can be estimated from curves of force (F) versus separation (S) between the tip and the substrate (see Figure 1b). As S decreases, the force becomes more repulsive until S ≈ 4.7 nm. At this (26) Binnig, G.; Quate, C.; Gerber, G. Phys. Rev. Lett. 1986, 56, 930933. (27) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry; HarperCollins College Publishers: New York, 1993; Chapter 9.
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separation, the repulsive force exerted by the tip is large enough to displace the adsorbed surfactant (sideways or into solution) and the tip jumps toward the silica substrate. The separation at which the jump occurs (shown in the figure by the start of the arrow) is a rough measure of the adsorbed layer thickness, T. (For example, T might be smaller than the jump distance if an attractive force at finite separation causes the tip instability, or greater than the jump distance if the film is compressible.) Our measured value of T is slightly larger than twice the length of an extended CTABr molecule (4.5 nm) and much larger than the separation at which the jump occurs in the surface forces apparatus (3-3.5 nm).23 The fact that the applied strain at which the surfactant was displaced depends on the lateral extent of the interaction zone suggests that the micelles may move sideways in the AFM experiment. When the load applied by the tip is decreased, an adhesive force is measured as observed previously.23 C16TAB micelles are cylindrical in solutions containing 50-300 mM NaBr and 10 mM CTABr and also in solutions containing 50-100 mM LiBr and 10 mM CTABr. Figure 1c shows an image of cylindrical micelles in 10 mM CTABr and 300 mM NaBr. Both the image and its Fourier transform show that the cylinders are aligned roughly parallel to each other. However, we did not observe a specific orientation relative to the substrate. Patches of a single orientation (domains) of at least 500 nm were distributed across the interface. In 300 mM NaBr, R ) 7.4 nm, which is ∼2 nm less than that for the spherical micelles in Figure 1a. This probably reflects a reduction in separation between micelles. An increase in the concentration of Br- between and on the micelles should decrease the electrostatic repulsion between micelles, allowing the separation between micelles to decrease. Experiments were also conducted with CTABr alone. Cylinders formed at concentrations at or above 18 mM. The growth into cylindrical micelles at the silica-water interface is analogous to the growth into cylindrical micelles previously observed in bulk solution. An increase in NaBr concentration at the micelle surface reduces the electrostatic repulsion between headgroups and favors less curved micelles. The sphere-cylinder transition at the silica surface occurs at a lower bulk Br- concentration (chemical potential) than the corresponding transition in bulk. This is analogous to the observation that surface micelles form at a lower chemical potential than bulk micelles and implies that the surface must decrease the chemical potential of the surface cylinders by more than it decreases the chemical potential of surface spheres. In seeking a mechanism we must remember that data in the AFM image is dominated by the solution face of the micelle and we cannot distinguish between full cylinders and hemicylinders on a flat layer. For full cylinders, a reasonable mechanism is that electrostatic interactions between the cationic surfactant and the anionic silicasurface make a larger negative contribution to the chemical potential of molecules that are in close proximity to the negative silica sites. A cylinder has a greater fraction of headgroups in contact with the silica, so has a greater reduction in energy than a sphere when it adsorbs. For the adsorption of hemistructures on a hydrophobic monolayer, the adsorption of hemicylinders is also favored because they cover more of the hydrophobic surface at high packing densities. Thus, we would expect the spherecylinder transition to be decreased for both hemistructures and full structures. Images in 10 mM CTACl solution were similar to those in 10 mM CTABr solution: spherical aggregates were observed with R ) 9.2 ( 0.3 nm. Addition of 250 mM LiCl,
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b
c
Figure 1. (a) 300-nm AFM image of the interface between amorphous silica and 10 mM CTABr solution showing the organization of the surfactant into spherical aggregates. The Fourier transform (bottom left) is a circular ring indicating randomly oriented micelles with a mean center-to-center spacing, R ) 9.4 nm. (b) Force between a silicon AFM tip and silica in 10 mM CTABr solution. The arrow shows the range of mechanical instability as the tip approaches the silica. The tip displaces some adsorbed surfactant during this mechanical instability. The force is reversible up to the instability, but after the instability the force is irreversible when the load is decreased. The adhesion was 5.2 nN. In the AFM technique, there is no unambiguous method for assigning a zero of separation, S. The convention is to assign S ) 0 to a region of high stiffness. The large adhesion at S ) 0 suggests that there is still a hydrophobic layer on both the tip and silica plate. The instability occurs at roughly the same distance for CTACl or CTAAc. With the addition of other monovalent and divalent counterions, the shape of the curve is similar; only the force barrier becomes higher. (c) 300-nm AFM image showing long cylinders in 10 mM CTABr and 300 mM NaBr. Note that the Fourier transform is not a complete circle but two arcs indicating that the cylinders axis only undergoes small variation in direction over the scale of the image. R ) 7.4 nm.
however, did not induce any shape transformations. The only effect was a reduction to R ) 6.9 nm, in accordance with a decrease in electrostatic repulsion between micelles. As mentioned in the Introduction, a difference in effect between Cl- and Br- has been observed in bulk solutions.16 Similar behavior has also been observed by Velegol et al. at the solid-liquid interface.28 This may be attributed to the fact that Cl- is harder than Br- and hence compared to Br- has a greater affinity for water than the quaternaryammonium-coated micelle surface. Hence, Cl- may not give up its water of hydration and so is less effective at reducing repulsive interactions between headgroups. Figure 2 shows a 200-nm image of CTAAc micelles on silica. At 10 mM CTAAc alone, the symmetry is similar to CTABr and CTACl. The thickness (measured from the force curve) is similar for all three surfactants but R ) 7.4 ( 0.3 nm for CTAAc, or about 2 nm smaller than that for (28) Velegol, B. S.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548-2556.
CTABr or CTACl,. There was no change in micellar symmetry upon addition of 500 mM LiAc but R decreased to 6.5 nm (in response to a decrease in intermicelle repulsion). If we take the diameter of the micelle as the extended length of two CTA+ molecules (4.5 nm), this leaves about 2 nm for two acetate ions and water. This water could be thought of as the “hydration” shell of the micelle because of its association with the micelle, even in relatively high salt concentration. Uncertainly in interpretation of our AFM data prevents us from exactly determining the symmetry of the approximately spherical micelles. However, the Cl- and Brmicelles are more oblate than the acetate micelles. A model that fits our data is spherical acetate micelles (with a radius of about 4.5 nm), with the intermicelle separation dependent on the bulk concentration of acetate. The halide micelles are then oblate with an eccentricity of ∼1.5 if we assume that the separation between micelles is the same for halides and the acetate. (The eccentricity might be
Counterion Effects on Adsorbed Micellar Shape
Langmuir, Vol. 16, No. 10, 2000 4451 Table 2. Effect of Divalent Anions on the Shape of CTA+ Aggregates on Silicaa
Figure 2. 200-nm AFM image of the interface between silica and 10 mM CTAAc solution showing spherical micelles with R ) 7.3 nm. Table 1. Effect of Monovalent Anions on the Shape of CTA+ Aggregates on Silica
a
anion
hard/soft27
aggregate shape
ClAcBrHSO3HS-
hard hard borderline borderlinea softa
spheres (oblate) spheres cylinders spheres cylinders
Our designations, based on SO32- and S2-.
underestimated because the halide ions are probably more effective in reducing the intermicelle separation.) So as not to distract from our focus on the sphere-cylinder transition, in this paper we use the word “sphere” and “cylinder” to include oblate spheroid and oblate cylinder structures that occur with Cl- and Br- ions. The difference in aggregate geometry between acetate and halide surfactants at the silica surface may be explained on the basis of earlier observations with bulk micelles. Brady et al.18 showed that the hydroxide and acetate micelles in bulk are smaller and more highly charged than are halide micelles. The size and charge was found to be consistent with the positioning of the counterion about 0.5 nm further from the micelle core than for Cl- or Br-. Later, this was explained in terms of a smaller dispersion force between the acetate or hydroxide ions and the micelle than for halides in water because of the almost zero excess polarizability of these ions.19 The shape transformation results obtained with monovalent anions are summarized in Table 1. Our measurements of micelle size and shape at the silica-water interface show similar trends to those previously observed in bulk: acetate micelles are smaller than Cl- or Br-, and Br- induces growth of the micelle but Cl- or acetate alone do not. Cylindrical micelles were also observed in 100 mM HS- solution. HS- may be considered soft because of the presence of the large polarizable sulfur atom. The results shown in Table 1 suggest that soft polarizable monovalents induce a growth from spherical to cylindrical micelles whereas the hard ones do not. A notable exception to this idea is the report that NO3- is effective in producing eccentric CnTAB micelles in solution.14 Some of the anions we examined are basic (e.g., CH3COO-), so their addition to solution will change the pH. Because H+ is also a potential determining ion for silica (via the reaction -SiOH / -SiO- + H+), there is
a The asterisk indicates our designation. The dagger shows that the numbers on the sulfur atoms indicate the calculated charge30,31
the possibility that the addition of basic ions will affect the adsorbed aggregate structure by modifying the silica. We examined the effect of changes in pH by observing surfactant structure upon addition of hydroxide salts. Addition of NaOH to pH 12 did not change the structure of aggregates in 10 mM CTAB (oblate spheroids) and addition of LiOH to pH 10.2 did not change the structure of aggregates in 10 mM CTAAc (spheres). Effect of Divalent Anions. We also measured the shape of surface micelles in solutions of dianions of varying hardness. The results show that soft/polarizable dianions induce a change from spherical to cylindrical micelles, but that hard dianions do not. The magnitude of the anion charge appears to be less critical. Table 2 lists the dianions, and the effect on the shape of CTA+ surface micelles in CTAAc solution. The first divalent anion studied was SO42-, which has been shown not to cause any micellar growth of CTA+ in bulk. Wennerstro¨m et al.21 have argued that this is because of the image charge effect, which reduces the electrostatic attraction between the SO42- ion and the CTA+ micelle. Our results with adsorbed micelles are similar to the observations of Wennerstro¨m et al. on bulk solutions. No sphere-to-cylinder transition was observed at 10 mM CTAAc and 25-500 mM Li2SO4. R was constant at 6.5 ( 0.3 nm for all Li2SO4 concentrations, which is similar to the limiting value in the presence of acetate. Thus, SO42behaves similarly to the hard monovalent anions. Nevertheless, from this result alone it is not clear whether the hardness or the image charge is responsible for the inability of SO42- to effect changes in micellar shape. To examine softer anions, sulfurous atoms were substituted for oxygen in common dianions (e.g., S2O32instead of SO42-; CS32- instead of CO32-). These substitutions had a large effect on the micellar shape, which will be detailed below. The dianions that altered the micelle shape also affected our ability to image. Exposure of the AFM cantilever to these ions caused an immediate and rapid drift in the diode signal corresponding to a bending of the gold/silicon cantilever toward the gold film. Although this bending settled to a steady state after about 3 h, our ability to image was severely degraded after this initial change. Nevertheless, the shape transitions that occurred were unmistakable. The most obvious comparison in Table 2 can be made
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Figure 3. 200-nm AFM images showing the coexistence of different micelle geometries in two different positions at the interface between silica and 10 mM CTAAc + 5 mM Na2S2O3 solution: (a) long cylinders and (b) short rods/spheres. R ∼ 6.4 nm for both images. The long wavelengths that represent substrate features have been removed from both (a) and (b).
between CO32- and CS32-. The former is a hard anion;27 we designate CS32- as soft because the sulfur atoms are larger and more polarizable than oxygen. The effect of CO32- was the same as that for SO42-. No shape change was observed, and R decreased from 7.5 to 6.0 nm in solutions containing 100-400 mM CO32-. In contrast, cylindrical micelles formed in 5 mM K2CS3. The cylinders were long and continuous on a 200-nm scale, with R ) 9.1 nm. The Fourier transform was not a continuous ring but was similar to that in Figure 1c. Solutions containing 10 mM CTACl and 5 mM K2CS3 were very also very viscous, which suggests that the solution micelles are also very long (cf. Br-15 and salicylate29). Comparing the effects of CO32- and CS32- suggests that softer dianions are more effective in inducing micellar growth. Returning to Table 2, we see that although hard SO42forms spheres, soft S2O32- forms cylinders. Cylinders were observed at 5, 20, and 30 mM S2O32-. Figure 3a,b shows two typical images captured in the same experiment in 10 mM CTAAc and 5 mM S2O32- solution. Figure 3a shows cylinders whereas 3b shows short rods or spheres, demonstrating that the adsorbate organization was not (29) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474-484.
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homogeneous. The spacing of both the short rods/cylinders was ∼6.2 nm. SO32- does not cause a transformation to cylindrical micelles, even though it is classified as borderline soft.27 Br- is also borderline soft and does cause the transformation. Is this because of image charge effects or is it simply the coarseness of our hard/soft classification? We could not find a suitable anion with two different charge states (in contrast to the abundance of cations), so the next best option was to lower the pH of S2O32- solutions and examine HSO3-. HSO3- also formed spheres at all concentrations. Thus, electrostatic charge was not the deciding factor in determining micelle shape, even for a borderline hard/ soft ion. This leads us to speculate that perhaps one needs to consider the internal structure of polyatomic ions, and specifically, that the presence of a negatively charged soft atom induces the formation of cylinders. Thus, the failure of HSO3-/ SO32- to effect a shape transformation may arise from the fact that the soft sulfur atom is positively charged (see Table 2) and the negative charge is distributed on the hard oxygen atoms.30 The same is true for SO42-. (For SO42-, approach to the S atom is blocked.) In contrast, Br-, CS32-, and S2O32- all have negatively charged soft atoms. Comparing SO42- and S2O32-, we see that placement of a single (soft) S atom on the exterior of the ion produces a large increase in the ability to transform the shape of CTA+ micelles. This is probably because the extra S atom in the latter is negatively charged.31 We also find that cylindrical micelles are produced in a 1:1 mixture of HS-:S2-, both of which have an accessible soft anion. Thus, our results are consistent with the idea that the spherecylinder transition is effected by a soft negatively charged site on the counterion. This negatively charged ion has low hydration and can interact with the micelle. Divalent versus Monovalent Ions in Bulk Solution. One of the objectives of this work was to examine the idea that image charge repulsion may reduce the relative affinity of divalent anions compared to monovalent ions for a cationic micelle.21 Our experiments on surface micelles suggest that the charge is relatively unimportant but that softness is an important factor. Also, no evidence was found to suggest that the charge affected the concentration at which counterions effect shape change: Br-, CS32-, and S2O32- all caused transformations at only 5 mM. We did find some evidence to suggest that the binding of a dianion to a CTA+ micelle may be more favorable than binding of a monoanion in bulk solution. We titrated a solution of HCS3- ions with 10 mM CTACl while measuring the pH. (5 mM KHCS3 was prepared by mixing equimolar K2CS3 and HCl.) When CTACl micelles were added to solution, the pH dropped, so CTA+ induced the formation of CS32- from HCS3-. Similar changes in pH were not observed upon addition of CTACl to water. A quantitative measure of the difference in binding energies of HCS3- and CS32- to the micelles can be obtained by considering the following cycle: We used the pH and the initial concentrations in our titration to calculate the pK for the deprotonation of HCS3in the presence and absence of CTA+(7.74 vs 8.18) and therefore obtained ∆G3 + ∆G4. From the cycle this energy is equal to -(∆G1 + ∆G2), which is the difference in energy when CS32- binds to a micelle compared to when HCS3(30) Knop, O.; Linden, A.; Vincent, B. R.; Choi, S. C.; Cameron, T. S.; Boyd, R. J. Can. J. Chem. 1989, 67, 1984-2008. (31) Knop, O.; Boyd, R. J.; Choi, S. C. J. Am. Chem. Soc. 1988, 110, 7299-7302.
Counterion Effects on Adsorbed Micellar Shape
binds. The free energy drops by an additional ∼1 RT when the divalent CS32- ion binds to the micelle. (We have assumed that all the HCS3- and CS32- ions are bound to the micelle surface and have ignored effects due to the change in ionic strength.) Other Observations. Ac- and Cl- Effect Cylinder Formation in CTABr Solution. CTAAc or CTACl micelles do not transform to cylinders, even at very high concentrations of Ac- and Cl-, respectively. However, as little as 5 mM LiAc or LiCl added to 10 mM CTABr causes the transformation of spherical-to-cylindrical surface micelles. The sphere-cylinder transition on the silica surface with CTABr alone occurs between 10 and 18 mM, so surface micelles in 10 mM CTABr are very close to the shapetransformation boundary. In mixed counterion systems such as the one above, shape transitions may be accompanied by an unequal distribution of counterions among different locations on the micellar surface. For example, cylindrical micelles could form with more bromide counterions on the cylindrical walls and chloride on the spherical caps. Furthermore, the aggregate shape also depends on intermicellar forces, which are influenced by Cl- or Ac- counterions relatively distant from the micelles. Surface Aggregates of CTABr in the Absence of Salt. The structure of CTABr aggregates was studied as a function of the CTABr concentration. Surface micelles were clearly observed in solutions below the cmc, at 0.5-0.8 cmc. Dissociation of surface silanol groups renders the silica negatively charged, leading to an attraction between surfactant molecules and the surface, which causes a surface excess. This additional (favorable) term in the chemical potential of surface micelles also means that surface aggregates can form at a lower chemical potential than bulk aggregates. In the range 0.5-10 mM, the aggregates were spherical with a constant period of R ) 9.3 ( 0.4 nm. At 18 mM CTABr, a mixture of short rods and cylinders were observed with R ∼ 8.0 nm. At concentrations >50 mM, there was no change in the observed geometry, but R decreased to ∼7.3 nm. The decrease in R in 10-50 mM solutions is probably due to increased screening of repulsive electrostatic forces between micelles. The constancy of R in the range 0.5-10 mM while the ionic strength increases suggests that the micelle diameter increases in this range. Neutron scattering data have indicated that bulk CTABr micelles remain fairly spherical up to 50 mM.14 Thus, the surface causes a shift in the sphere-cylinder transition to a lower concentration. When one performs AFM of delicate samples, there is always concern that the force between the AFM tip and the sample will affect the structure of the sample. At concentrations greater than 10 mM CTABr, the force at small separations is a strong function of CTABr concentration, even while the chemical potential remains almost constant (see Figure 4). This implies that desorption of CTABr occurs as a function of separation.32 This is a situation where one needs to take care to determine whether the structure also is a function of applied force. We monitored the structure as a function of force in the (32) Pethica, B. A. Colloids Surf. A 1995, 105, 257-264.
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Figure 4. Force between tip and sample as a function of CTABr concentration. There was no measurable change in micellar geometry as a function of separation, even though adsorption was a function of separation. For each concentration the force is reversible for separations greater than the instability point. There is a large hysteresis at smaller separations, resulting in adhesion of 5.7, 4.7, 4.4, and 4.4 nN for the 2, 20, 50, and 100 cmc solutions, respectively. The force curves for 5 and 10 cmc were the same as those for 2 cmc. All forces were measured with the same tip and cantilever.
Figure 5. Fourier transform of a 300-nm AFM image showing hexagonal packing of spherical micelles at the interface between silica and 10 mM CTAAc + 100 mM Li2CO3. Six patches of high intensity can be seen, indicating three preferred directions for neighboring micelles, each inclined at ∼60°. There was no particular orientation with respect to the substrate. Similar images were obtained in the presence of Li2SO4.
concentration range 10-100 cmc but did not see any change in structure. Ordering of Micelles. Figures 1a and 2a show that the spherical aggregates of CTABr and CTAAc on the silica surface are oriented randomly. At high concentrations of Cl- or Ac-, the micelles also showed no long-range orientational order. However, micelles in SO42- or CO32solution (two of the non-shape-transforming divalent ions) exhibited orientational order. Figure 5 is a Fourier transform of an image obtained at 10 mM CTAAc and 100 mM Li2CO32-. The six bright spots at approximately 60° spacing show that the micelles are packed in a hexagonal array with R ) 6.3 nm. Images at different positions on
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the same substrate showed the same symmetry but with different alignment relative to the substrate. Therefore, the symmetry does not appear to be directly induced by the substrate. (Also, in some 500-nm images, two lattices at different angles were observed). A high degree of ordering implies that, in a curve of energy as a function of separation between aggregates, a large gradient exits near the potential minimum. This may have occurred for the divalent counterions because of (a) a shorter Debye length or (b) attractive ion-correlation forces. Effect of Salt on R. At high counterion concentrations, the center-to-center distance between micelles (R) was 6.3 ( 0.2 nm for all ions except Br- (RBr ) 7.4 nm) and Cl- (RCl ) 6.9 nm). The aggregate shape did not seem to affect this number; both spheres and cylinders tended toward this value. Thus, 6.3 nm probably represents the minimum center-to-center spacing for the surface micelles separated by two hydrated counterions. Conclusions Soft monoanions and dianions are much more effective than hard anions at inducing a sphere-to-cylinder trans-
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formation in CTA+ micelles adsorbed to the silica-water interface. More specifically, molecules with a soft negative atom effect the transformation. Presumably, this is because the soft atoms prefer to bind with the quaternary ammonium headgroup of CTA+ rather than water. This reduces the repulsive interaction between headgroups and lowers the energy of the less-curved cylindrical portion of the micelles. A comparison between the effect of monovalent and divalent ions was inconclusive because of our inability to isolate charge from the dominant effect of hardness. Surface micelles were observed below the critical micelle concentration. Acknowledgment. This work was funded by the Thomas F. Jeffress and Kate Miller Jeffress Memorial Trust (J-472) and by an Aspires Grant from Virginia Tech. Our thanks go to Paula Godfry for preparing the CTAAcetate. LA991245W
