Langmuir 2002, 18, 3191-3197
3191
Adsorbed Layer Structure of Cationic and Anionic Surfactants on Mineral Oxide Surfaces Jamie C. Schulz and Gregory G. Warr* School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia Received November 5, 2001. In Final Form: January 14, 2002 We report on the structure of adsorbed layers of sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB) on titanium dioxide (rutile) and kaolinite, and of TTAB and hexadecyltrimethylammonium bromide (CTAB) on quartz imaged by atomic force microscopy above their respective critical micelle concentrations (in the plateau adsorption region). The surfactants all form globular surface micelles on these three substrates, but under very different conditions. Adsorbed micelles of SDS are found at pH’s less than the point of zero charge of rutile, whereas TTAB and CTAB adsorbed micelles are found only above the points of zero charge of rutile and quartz, respectively. Adsorbed SDS micelles are observed on the terraced edges of kaolinite particles, which are positively charged under acidic conditions. In contrast, adsorbed TTAB micelles are observed only on the anionic kaolinite basal plane.
Introduction Ionic surfactant adsorption on mineral oxide surfaces plays an important role in many technological processes such as flotation,1 detergency,2 and tertiary oil recovery.3-5 This has resulted in a substantial amount of research being dedicated to studying the mechanism of surfactant adsorption on a wide range of mineral surfaces. The term mineral oxide here includes both metal oxides and aluminosilicate clays, which are the two main classes of minerals that are most often studied. Upon exposure of a hydrophilic mineral oxide surface to an ionic surfactant solution, surfactants adsorb to an oppositely charged surface. The driving force for this adsorption combines the electrostatic attraction between the charged headgroups of the surfactant and the oppositely charged surface and the hydrophobic attraction between the chains of the surfactant. Traditionally, adsorption isotherms have been used to quantify the amount of adsorbed surfactant as a function of concentration and hence deduce the adsorption mechanism and infer an adsorbed layer structure. Adsorption isotherms may be characterized by three (sometimes more) concentration regimes.6,7 The first regime occurs at concentrations much less than the critical micelle concentration (cmc). Here individual surfactant molecules adsorb primarily through electrostatic attraction to the surface. The surface charge density of the mineral remains approximately constant in this region, indicating that an ion exchange mechanism occurs. In the second regime, still at concentrations less than the cmc, adsorption is enhanced through association or aggregation of the hydrophobic chains of the adsorbed surfactant ions. It is marked by an abrupt increase in the slope of the adsorption isotherm. In this region, the original surface charge is neutralized by the oppositely charged * Author for correspondence. E-mail:
[email protected]. (1) Gaudin, A. M.; Fuerstenau, D. W. AIME Trans. 1955, 202, 958962. (2) Cutler, W. G., Kissa, E., Eds. Detergency: Theory and Technology; Marcel Dekker: New York, 1987; Vol. 20. (3) Gogarty, W. B.; Tosch, W. C. Trans. Soc. Pet. Eng. AIME 1968, 243, I-1407-I-1414. (4) Gale, W. W.; Sandvik, E. I. Soc. Pet. Eng. J. 1973, 13, 191-199. (5) Boneau, D. F.; Clampitt, R. L. J. Pet. Technol. 1977, 29, 501-506. (6) Wakamatsu, T.; Fuerstenau, D. W. Adv. Chem. Ser. 1968, 79, 161-172. (7) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 463-478.
surfactant ions and then reversed, reflecting the charge on the surfactant. The third regime occurs at concentrations greater than the cmc. Here the adsorption isotherm plateaus at constant surface excess, referred to as a saturated surface. Many inferences regarding adsorbed layer structure have been drawn from adsorption isotherm data, particularly in the plateau region of the adsorption isotherm. Of these, the classical bilayer model of the adsorbed layer is the most common.8,9 This model considers the adsorbed layer to consist of three distinct regions: a surfactant headgroup region closest to the hydrophilic mineral surface, an (intercalated) tail region, and finally another headgroup region in contact with the bulk aqueous solution. The saturated surface coverage typically obtained from adsorption isotherms is rarely found to be unity, leading to the proposal of a partial or patchy bilayer model to account for this observation.10 Recent atomic force microscopy (AFM) results have prompted a re-evaluation of the classical view of the adsorbed layer. Saturated adsorbed surfactant films on a number of mineral surfaces, such as muscovite mica,11 silica,11,12 quartz,13 and sapphire,14 have all been shown to commonly consist of spherical or cylindrical surfactant aggregates. Complete bilayers have also been observed, but patchy bilayers have never been imaged as an equilibrium structure. The adsorbed layer structure is often found to parallel bulk surfactant aggregate geometries in micelles and liquid crystals, permitting aspects of bulk solution aggregation theory15 to be applied to the adsorbed layer morphology.16 (8) Harwell, J. H.; Hoskins, J. C.; Schechter, R. S.; Wadde, W. H. Langmuir 1985, 1, 251-262. (9) Wa¨ngnerud, P.; Jonsson, B. Langmuir 1994, 10, 3268-3278. (10) Chandar, P.; Somasundaran, P.; Turro, N. J. J. Colloid Interface Sci. 1987, 117, 31-46. (11) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (12) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548-2556. (13) Schulz, J. C.; Warr, G. G.; Hamilton, W. A.; Butler, P. D. Phys. Rev. E 2001, 63, 1604-1608. (14) Manne, S.; Warr, G. G. In ACS Symposium Series: Supramolecular Structure in Confined Geometries; Manne, S., Warr, G. G., Eds.; 1999; Vol. 736, pp 2-23. (15) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525-1568.
10.1021/la011647f CCC: $22.00 © 2002 American Chemical Society Published on Web 02/26/2002
3192
Langmuir, Vol. 18, No. 8, 2002
In this paper, we compare the adsorbed layer structures and adsorption mechanisms of one anionic surfactant and two cationic surfactants on three hydrophilic mineral surfaces. In particular, we examine the effect of pH on the adsorption of sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB) onto a single crystal of titanium dioxide, and of both TTAB and hexadecyltrimethylammonium bromide (CTAB) on quartz. We also examine the location of adsorption sites for these two surfactants on platelike kaolinite particles. The adsorption of surfactants onto all three substrates has been extensively studied. Primarily this effort has been motivated by their many important industrial applications. Quartz, or silica, is ubiquitous while rutile and kaolinite are both vital ingredients for the manufacture of pigments17,18 and in paper coating formulations.18 Mineral Oxide Surface Chemistry Surface charges on mineral oxide and clay surfaces arise by two very different mechanisms. The surface charge on mineral oxides originates from the protonation or deprotonation of surface hydroxyl groups. Both the sign and magnitude of the surface charge depend on the pH of the solution. At a particular pH called the point of zero charge (PZC), the relative population of positive and negative sites on the metal oxide surface is equal, which results in surface charge neutrality. Titanium dioxide (rutile) is a typical mineral oxide, which consistently exhibits a PZC in the pH range 5-6,19-21 while the PZC of quartz is below pH 2.22 Clay minerals are composed of well-defined, layered aluminosilicate sheets, which have two different surfaces: basal planes and edges. The basal planes have a pH-independent negative charge arising from isomorphous substitution of atoms within the aluminosilicate lattice. This results in a deficit of positive charge, which is balanced by interlamellar cations. On the external basal surfaces of clays, the balancing cations are accessible to the surrounding medium, and if this is an aqueous solution, an electrical double layer develops. At the edges of the clay platelets, exposed Al and Si centers are terminated by hydroxyls that can accept or donate protons. This results in a pH-dependent charge similar to that on mineral oxide surfaces. The kaolinite unit cell consists of a sheet of octahedrally coordinated aluminum bound to a sheet of silica tetrahedra through shared oxygens.23 The theoretical surface charge of the basal planes from isomorphous substitution is zero. However, it is generally accepted that a small negative basal surface charge does exist for kaolinite.24 The basal surface of kaolinite is also considered to be relatively hydrophobic.25 (16) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685-1692. (17) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, 1994. (18) Murray, H. H. Clay Miner. 1999, 34, 39-49. (19) Yates, D. E.; Healy, T. W. J. Chem. Soc., Faraday Trans. 1 1980, 76, 9-18. (20) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Am. Chem. Soc. 1993, 115, 11885-11890. (21) Machesky, M. L.; Wesolowski, D. J.; Palmer, D. A.; Ichirohayashi, K. J. Colloid Interface Sci. 1998, 200, 298-309. (22) Li, H. C.; DeBruyn, P. L. Surf. Sci. 1966, 5, 203-220. (23) Bailey, S. W. In Crystal Structures of Clay Minerals and Their X-ray Identification; Brindley, G. W., Brown, G., Eds.; Mineralogical Society: London, 1980; Vol. 5, pp 1-123. (24) Van Olphen, H. Clay Colloid Chemistry, 2nd ed.; Wiley: New York, 1977. (25) Brady, P. V.; Cygan, R. T.; Nagy, K. L. J. Colloid Interface Sci. 1996, 183, 356-364.
Schulz and Warr
Materials and Methods Titanium dioxide (rutile (100)) was obtained from Superconix Single Crystals (Minnesota). Quartz (SiO2) was a gift from Hoffman Materials Inc. (Philadelphia, PA) as large offcuts from pure Z, slow growth crystals, where Z denotes the surface perpendicular to the hexagonal prism faces of natural quartz. Kaolinite (KGa-1b), a clay standard, was obtained from the Clay Minerals Society of America.26,27 The surfactants sodium dodecyl sulfate (SDS), tetradecyltrimethylammonium bromide (TTAB), and hexadecyltrimethylammonium bromide (CTAB) were obtained from Aldrich at >99% purity and were used as received. All surfactant solutions were prepared in Milli-Q water at twice their critical micelle concentrations: 8.2 × 10-3 M, 3.6 × 10-3 M, and 9.0 × 10-4 M for SDS, TTAB, and CTAB, respectively.28 This produces a saturated adsorbed layer on the mineral surface. Surface Preparation. A 10 × 10 mm2 quartz piece was cut from a larger sample using a diamond tip saw used for cutting mineralogical thin section samples, taking care not to damage the surface to be used for imaging. The thickness and levelness of the base surface with respect to the top imaging surface were adjusted by further mechanical grinding. The quartz substrate was then thoroughly cleaned to remove all contaminants by soaking in ethanol for 24 h and repeated rinsing with deionized water. Titanium dioxide and cut quartz crystals were attached to a metallic disk, 10 mm in diameter, in the following manner: A small amount of thermosetting Epikote (Shell) resin (∼0.1 g) was placed on the disk, and the disk was heated on a heating plate. Once the resin had melted on the surface of the disk, the oxide crystal substrate was positioned. Then the disk was removed from the heat to allow the resin to set. Quartz and titanium dioxide surfaces were cleaned prior to use to remove organic and inorganic contaminants. First, a few drops of aqua regia (concentrated HNO3/H2SO4, volume ratio 1:3) was placed onto the substrate until its entire surface was covered, and was allowed to stand for 5-10 min. Next, the substrate surface was rinsed with copious amounts of deionized water. The substrate was then dried using filtered nitrogen and irradiated under a mercury vapor lamp for 20 min. Finally, the surface was again rinsed in deionized water, and then used immediately. The homoionic (sodium) kaolinite was prepared according to the method of Posner and Quirk29 by repeated rinsing, settling, and decantation of supernatant, initially consisting of 1 M NaBr at pH 3 (three washes) and finally Milli-Q water (five washes). The clays were immobilized onto freshly cleaved muscovite mica by deposition from a 0.01 wt %, slightly basified (3-5 drops of concentrated ammonia in 0.1 L) solution which had been sonicated for 5 min prior to deposition. The particles were dried at 130 °C for 15 min. Particles deposited in this way adhere to the mica by one face of the clay platelet, exposing the opposing face to the solution for imaging by AFM. Such deposited platelets occasionally exhibit steps and ledges on the exposed face. Atomic Force Microscopy. Adsorbed layer morphologies on the mineral surfaces were examined using a Digital Instruments NanoScope 3 by soft-contact imaging, following the same procedure as in previous work.16,30 High integral gains (∼3.0) were employed to obtain soft-contact images on kaolinite because of the range of height traversed during scanning. Low integral gains (∼0.3) were used for imaging adsorbed surfactant films on titanium dioxide. All experiments were performed at room temperature (22 ( 2 °C), and the structures observed within the adsorbed surfactant films were stable over the period of the experiment (several hours). (26) Clay Minerals Society of America, http://web.missouri.edu/ ∼geoscjy/SourceClay/. (27) Van Olphen, H., Fripiat, J. J., Eds. Data Handbook for Clay Minerals and Other Non-Metallic Minerals; Pergamon Press: Oxford, 1979. (28) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208-223. (29) Posner, A. M.; Quirk, J. P. Proc. R. Soc., A 1964, 278, 35-56. (30) Schulz, J. C.; Warr, G. G. Langmuir 2000, 16, 2995-2996.
Surfactants on Mineral Oxide Surfaces
Langmuir, Vol. 18, No. 8, 2002 3193
Figure 1. Soft contact AFM images of CTAB on SiO2 (quartz) at (a) pH 11.3, (b) pH 5.7, and (c) pH 3.0. Table 1. Observed Periodicities ((0.5 nm) of Globular Ionic Surfactant Aggregates Adsorbed onto Quartz and Rutile As a Function of pH (See Text for More Precise pH Values)a pH 3
pH 4
pH 5
pH 6
pH 11
6.9 8.2
5.7 6.3
7.0 -
6.5
Quartz TTAB CTAB
8.1 10.0
TTAB SDS
7.0
a
9.0
Rutile
9.0
- denotes no adsorbed structure observed
Solution pH was adjusted by adding drops of concentrated aqueous solutions of HCl and NaOH to a stock of the surfactant solution; then the new surfactant solution at the desired pH was injected into the AFM liquid cell.
Results TTAB and CTAB Adsorption onto Quartz. AFM images of adsorbed CTAB films on the surface of quartz are shown in Figure 1 as a function of pH. At pH (a) 3.0, (b) 5.7, and (c) 11.3, a periodic and isotropic “orange peel” texture is observed. Similar images were obtained for TTAB over this pH range (not shown), as previously reported for TTAB on quartz13 and silica,11 and for CTAB on silica.12 Table 1 lists the nearest neighbor distances of the aggregates within the adsorbed layer, which decrease significantly with increasing pH for both CTAB and TTAB. The nearest neighbor spacings over the entire pH range are somewhat greater than the bulk micelle diameters of 5.1 ( 0.2 nm for TTAB and 5.3 ( 0.2 nm for CTAB measured by small angle neutron scattering.31 SDS and TTAB Adsorption onto Rutile. AFM images of adsorbed SDS films on the surface of titanium dioxide are shown in Figure 2 as a function of pH. At pH (a) 3.3, (b) 4.1, and (c) 5.2, a periodic texture is observed, indicating that SDS forms spherical or globular surfactant aggregates. The aggregates are ordered within the adsorbed layer in an approximately hexagonal symmetry (Fourier transforms not shown), indicating close packing.16 Parts c and d of Figure 2 show the TiO2 surface imaged in soft-contact mode, where the imaging force slowly increases throughout the scan (from top to bottom). In Figure 2c, the resolution of the adsorbed layer structure improves while moving down the image. Approximately halfway through acquisition of Figure 2d, the tip came into contact with the underlying titanium dioxide surface, so the remainder is a conventional liquid contact deflection image of the substrate. Some lateral structure is apparent in the soft-contact region of Figure 2d, especially just prior to the jump into contact. The size of these structures (31) Berr, S. S. J. Phys. Chem. 1987, 91, 4760-4765.
suggests that they may be due to adsorbed aggregates on the surface at large separations. However, it is also possible that these features are simply due to surface heterogeneity (regions of differing height or surface charge), resulting in different interactions between the tip and the surface, and that their size is a tip convolution artifact. Image acquisition at low pH was relatively easy because of a strong electrostatic attraction between the anionic surfactant and cationic titanium dioxide surface, and a consequent strong electrostatic repulsion between the adsorbed surfactant and the AFM tip. However, as pH was increased, these interactions were weakened by the reduced surface charge on TiO2, making images more difficult to acquire. No adsorbed film could be detected at pH values above 6.6 from either the images or the force curves. The nearest neighbor distances obtained from the softcontact images are 7.0 ( 0.5 nm at pH 3.3, 9.0 ( 0.5 nm at pH 4.1, and 9.0 ( 0.5 nm at pH 5.2. These values are all substantially greater than the bulk micelle diameter obtained from small-angle neutron scattering (SANS) of 4.3 ( 0.2 nm32 and are consistent with the expected mutual electrostatic repulsion that aggregates within the adsorbed surfactant film would experience. Figure 3 shows soft-contact AFM images of titanium dioxide in contact with TTAB solution at pH (a) 11.2, (b) 6.6, and (c) 5.5. The adsorbed film structures in parts a and b are similar to those in Figure 1 and are also interpreted to be globular surfactant aggregates. Figure 3c shows an image of rutile exposed to a TTAB solution at pH 5.5, with the majority of the image (bottom) obtained in soft-contact mode. This image is similar in appearance to Figure 2d, showing some structure just prior to the tip making contact with the titanium dioxide surface, and may be similarly interpreted. No TTAB adsorbed layer could be detected when the pH was less than 5.5. As with SDS, the observed nearest neighbor distances for TTAB on rutile of 6.5 ( 5 nm at pH 11.2 and 7.0 ( 0.5 nm at pH 6.6 are greater than the bulk micelle diameters. SDS and TTAB Adsorption on Kaolinite. Figure 4a shows a contact image of a kaolinite platelet deposited onto mica in contact with a 8.2 × 10-3 M SDS solution at pH 3. This image is consistent with a recent comparative AFM and X-ray diffraction study of similarly prepared KGa-1b kaolinite, which yielded a platelet diameter and thickness of approximately 600 and 40 nm, respectively.33 Microvalleys are clearly observed in the clay platelet, and these are typical of the KGa-1b clay.25,33-35 It is also apparent from this image that a large fraction of the surface of the clay platelet is due to edges, also consistent with previous studies.25 (32) Berr, S. S.; Jones, R. R. M. Langmuir 1988, 4, 1247-1251.
3194
Langmuir, Vol. 18, No. 8, 2002
Schulz and Warr
Figure 3. Soft contact AFM adsorbed layer images of TTAB on TiO2 at (a) pH 11.2, (b) pH 6.6, and (c) pH 5.5. The bottom portion of part c was obtained in soft-contact mode, and the top quarter of the image, in contact mode, after the tip drifted into contact.
Figure 2. Soft contact AFM images of SDS on TiO2 (rutile) at (a) pH 3.3, (b) pH 4.1, (c) pH 5.2, and (d) pH 6.6. The top half of part d was obtained in soft-contact mode, and the bottom half of the image, in contact mode, after the tip drifted into contact.
Figure 4b shows an expanded contact image of the highlighted region of the kaolinite platelet in Figure 4a.
Flat areas corresponding to the basal planes are seen at the top of the crystal (upper left corner), and terraced steps are also observed (indicated by the arrow in Figure 4b). Parts c-e of Figure 4 show soft-contact AFM images of kaolinite in contact with a 8.2 × 10-3 M SDS solution at pH 3. The images obtained in soft-contact mode are distinctly different from the contact image in Figure 4b and display a periodic pattern on the terraced steps and edges, similar in both texture and dimensions to those seen on quartz and TiO2. We interpret this periodic texture (33) Zhou, Q. Surface Characteristics and Dissolution Kinetics of Two Standard Kaolinites. M.Sc. Thesis, Kent State University, OH, 1996. (34) Maurice, P. A. Colloids Surf., A: Physiochem. Eng. Aspects 1996, 107, 57-75. (35) Zbik, M.; Smart, R. S. Clay Clay Miner. 1998, 46, 153-160.
Surfactants on Mineral Oxide Surfaces
Langmuir, Vol. 18, No. 8, 2002 3195
Figure 4. (a) Contact AFM image of a kaolinite platelet deposited onto mica. (b) Enlargement of the box area in part a, showing terraced steps on the particle edges (arrow). (c-e) Soft-contact AFM images of spherical SDS micelles adsorbed on the clay platelet edges. The circles in parts d and e show the basal planes of kaolinite and mica, respectively, which are bare of SDS.
as adsorbed globular SDS aggregates. The adsorbed aggregates are not as well resolved as those in Figures 1-3 because of the high integral gains used to obtain the images. Note, however, that the adsorbed layer structure of SDS on the terraced edges is much larger than the size of the steps. In contrast to the lateral structure evident on the edges, no structure can be detected on the kaolinite basal plane (circle in Figure 4d). Likewise, there is no structure observed on the mica cleavage plane (circle in Figure 4e). The absence of structure on the basal planes of both kaolinite and muscovite mica suggests that little or no SDS adsorption occurs in these regions. This is also verified by the force curves obtained in these regions. Figure 5 shows a soft-contact AFM image of a kaolinite platelet deposited onto mica in contact with a TTAB solution at pH 7. The top half of the image is the kaolinite particle, and the bottom is mica. The kaolinite crystal is well cleaved and exhibits a large basal area when compared to that for the particle in Figure 4a. Within the adsorbed TTAB film, spherical aggregates are seen on the surface of kaolinite and cylindrical aggregates on the surface of mica.30 Unlike the case for SDS, no aggregates were observed on the terraced edges of kaolinite particles deposited on mica, indicating that TTAB does not adsorb on the edges of the plates. Discussion The AFM results show that the adsorbed surfactant films consist of globular surfactant aggregates on all three substrates examined. This structure is also similar to AFM imaging results previously reported on silica11,12 and on quartz13 and similar to the bulk solution aggregate shape under the same conditions. However, the results differ markedly from those for the adsorbed cylinders of TTAB and similar cationic surfactants repeatedly observed on mica.11,16
Figure 5. Soft-contact AFM images of TTAB on the basal planes of mica (cylinders, bottom of image) and a deposited kaolinite crystal (spheres, top). No structure is seen on the steps of the kaolinite particle with TTAB.
This presence of such aggregates on the surface is consistent with recent adsorption isotherm studies of both TTAB36,37 and SDS38 on rutile that have shown the plateau surface excess to be too low for a complete bilayer. Similar inferences have been drawn for the adsorption of CTAB on silica.39 Surface micelles have also been reported to form on various clays surfaces but below the plateau adsorption region.40 (36) Vanjara, A. K.; Dixit, S. G. Adsorpt. Sci. Technol. 1996, 13, 397407. (37) Luciani, L.; Denoyel, R. Langmuir 1997, 13, 7301-7303. (38) Imae, T.; Muto, K.; Ikeda, S. Colloid Polym. Sci. 1991, 269, 4348. (39) Bijsterbosch, B. H. J. Colloid Interface Sci. 1973, 47, 186-198.
3196
Langmuir, Vol. 18, No. 8, 2002
In a pioneering study, Welzen et al.41 examined the plateau adsorption of SDS and CTAB onto kaolinite as a function of pH. Significant adsorption of SDS was observed at pH 3, whereas the amount adsorbed at pH 10 was insignificant. In contrast, CTAB adsorbs at both pH values, with greater adsorption at pH 10. The adsorption behavior of SDS was attributed to the charge on the edges of kaolinite particles (positive at pH 3 and negative at pH 10). The results shown in Figures 4 and 5 directly show the different adsorption sites for cationic and anionic surfactants, confirming the earlier inferences from adsorption studies, as well as the persistence of globular aggregates in the adsorbed film in the plateau region. Adsorbed layer structure may be characterized by two parameters, the adsorbed aggregate morphology and the fractional surface coverage, which is equivalent to the number density of aggregates or nearest neighbor spacing if the layer is not continuous. Our results show that little effect of pH upon the morphology of the adsorbed aggregates appears on these substrates; they remain globular over the entire range examined. However, the interaggregate spacing and even the presence of an adsorbed layer are sensitive to pH. The approximate location of the PZC of the rutile surface may be deduced from the presence or absence of adsorbed surfactant in Figures 2 and 3. A TTAB adsorbed layer is present at pH 6.6 but not pH 5.5, and an adsorbed SDS layer is present at pH 5.2 but not at pH 6.6. This yields a PZC between pH 5.5 and 6.6, consistent with previous studies.19-21 Similarly, the presence of TTAB and CTAB adsorbed layers implies that the PZC of quartz is below pH 3, consistent with expectations.22 Previous adsorption studies of cationic surfactants on rutile37 and silica39 as a function of pH have also shown that the plateau surface excess decreases as the PZC is approached from high pH. This is consistent with the observed decrease in number density of aggregates (Figures 1 and 3) or an increase in the interaggregate spacing (Table 1), and may be rationalized in terms of the changing density of charged sites on mineral oxides with pH. There have been two main proposals to account for the different adsorbed layer structures observed by AFM on various hydrophilic mineral surfaces. These concern the roughness and the charge density of the mineral surface.42 Mica has come to be widely regarded as a model for both clays and other metal oxide surfaces, primarily as a result of its extensive use in the surface forces apparatus.43 Typically, mica has a greater number of adsorption sites than most mineral oxide surfaces, which is reflected by its high negative surface charge density.44 AFM images of many cationic surfactants adsorbed on mica show stripes that are interpreted as adsorbed full cylinders (e.g. see Figure 5f) even though they are in equilibrium with spherical micelles in bulk.11 Recent AFM results of adsorbed cationic surfactants on a wide range of mineral surfaces11,14,30,45 including ion exchanged mica46 have (40) Kunyima, B.; Viaene, K.; Khalil, M. M. H.; Schoonheydt, R. A.; Crutzen, M.; De Schryver, F. C. Langmuir 1990, 6, 482-486. (41) Welzen, J. T. A. M.; Stein, H. N.; Stevels, J. M.; Siskens, C. A. M. J. Colloid Interface Sci. 1980, 81, 455-467. (42) Manne, S. Prog. Colloid Polym. Sci. 1997, 103, 2226-233. (43) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169187. (44) Nishimura, S.; Tateyama, H.; Tsunematsu, K.; Jinnai, K. J. Colloid Interface Sci. 1992, 152, 359-367. (45) Schulz, J. C. Interfacial Structures in Thin Surfactant Films. Ph.D. Thesis, University of Sydney, 2000. (46) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602-7607.
Schulz and Warr
instead shown globular adsorbed aggregates such as those reported here. The morphology of adsorbed surfactant aggregates has been shown to be sensitive to the same parameters as bulk micelles. Changes in the structure of the surfactant, such as increasing headgroup area either sterically16 or electrostatically,47 increase the curvature of adsorbed aggregates on mica and in bulk. Increasing alkyl chain length decreases curvature on mica and silica,48 as it does in bulk. A more strongly bound counterion or an increase in electrolyte concentration decreases aggregate curvature on both mica16 and quartz,13 just as it does in bulk solution. These changes are all understandable within the general framework of surfactant packing considerations.15 However, within the adsorbed surfactant film, electrostatic interactions between the surfactant and the surface must be considered, as must interactions between neighboring adsorbed aggregates. These factors dictate the maximum surface coverage at which either spherical or cylindrical aggregates may be observed,49 and have recently been used to interpret the neutron reflectivity of adsorbed cationic surfactant films on quartz.13 As surface charge density is increased, the number density of adsorbed globular aggregates should likewise increase until they become (almost) close-packed. The relative magnitude of the interaggregate repulsion compared with the remaining surface charge will then determine whether they undergo a sphere to cylinder transition or simply reach a maximum coverage. The surface charge on titanium dioxide is relatively low (ζ potential < 30 mV) below its PZC.20 The interaggregate spacings of adsorbed SDS micelles decrease with increasing distance from the PZC. Above its PZC, the spacing of TTAB aggregates is less sensitive to pH. At pH 6.6 the ζ potential of similar, single-crystal TiO2 is about -30 mV,20 and here the adsorbed aggregates are quite close together (Table 1). Increasing pH may decrease the spacing, but this is at the limit of detection. Silica surfaces exhibit much wider pH and ζ-potential ranges.22 At pH 3, where the ζ potential is below -20 mV, nearest neighbor distances are much greater than spherical micelle diameters. At the highest pH examined, the ζ potential is greater than -100 mV. Here TTAB and CTAB micelles adsorbed on quartz are approaching close packing. However, the surface charge density is (still) not high enough to induce a transformation into cylinders. Liu and Ducker48 have suggested instead that the surface roughness hinders the ability of an adsorbed surfactant to form cylindrical micelles. A cylindrical micelle has a large contact area with the surface and when placed on a rough substrate must bend to accommodate surface features, resulting in an energy penalty. However, the adsorption of a spherical micelle does not incur the same energy penalty as the adsorption of a cylindrical micelle, because of the small attachment area on a surface. It has therefore been suggested that a surfactant that forms cylinders on a flat substrate like mica will form spheres on a rough surface. If this were true, then globular aggregates are observed here on quartz, rutile, and kaolinite because the surfaces are rough compared with mica. Figure 5 demonstrates that spherical aggregates of TTAB form over large areas of the kaolinite basal plane, which is of comparable (47) Manne, S.; Scha¨ffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382-6387. (48) Liu, J. F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 85588567. (49) Schulz, J. C.; Warr, G. G.; Hamilton, W. A.; Butler, P. D. J. Phys. Chem. B 1999, 103, 11057-11063.
Surfactants on Mineral Oxide Surfaces
roughness to the cleaved mica. Also, ground mica deposited onto a cleaved surface has previously been shown to exhibit adsorbed cylinders, despite any possible additional roughness.30 We have previously shown that spherical TTAB aggregates on quartz may be transformed into cylindrical aggregates by the addition of electrolyte to modify curvature, but with no change in surface roughness.13,45 This sphere to rod transition occurs at a lower concentration than the corresponding bulk solution sphere to rod transition, reflecting the influence of the surface charge upon the adsorbed film morphology. We note also that the variation in spacing between aggregates with pH previously observed on silica11 and quartz45 and reported here underscores the role of surface charge. Were these aggregates a result of the too-high energy cost of bending cylinders to accommodate a rough surface, we would not expect a pH-dependent spacing because the surface roughness is not pH dependent. We therefore conclude that the primary factor driving adsorbed film structure on minerals is the surface charge density, with the surface roughness playing a minor role, at least on the scales examined here.
Langmuir, Vol. 18, No. 8, 2002 3197
Conclusions In this work we have investigated the adsorbed layer structure of ionic surfactants on quartz, rutile, and kaolinite. It was shown that the adsorbed films consist of spherical or globular aggregates under all conditions examined. The results, and particularly the interaggregate spacings, suggest that the adsorbed aggregates are similar to bulk solution micelles. This is noticeably different from adsorption on mica, which significantly modifies the aggregate morphology by its high surface charge density. Adsorption of cationic surfactants occurs primarily on the basal planes of kaolinite. At low pH, adsorption of anionic surfactants primarily occurs on particle edges. Acknowledgment. This work was funded by the Australian Research Council. J.C.S. acknowledges receipt of a Henry Bertie and Florence Mabel Gritton Postgraduate Scholarship from the University of Sydney. LA011647F
