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Adsorbed Layer Structure of Cationic Surfactants on Clays (Mica Is Not a Typical Substrate for Adsorption Studies) Jamie C. Schulz and Gregory G. Warr* School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia Received September 24, 1999. In Final Form: January 31, 2000 Solution atomic force microscopy imaging of the adsorbed layer of tetradecyltrimethylammonium bromide on kaolinite and montmorillonite particles deposited onto mica shows a strong dependence of film morphology on clay substrate. Whereas stripes (cylindrical micelles) are observed on mica, spherical structures are seen on both kaolinite and montmorillonite particles under identical solution conditions.
Much interest has been generated by recent atomic force microscopy (AFM) investigations of the structure of adsorbed surfactants films on solid substrates. On the most frequently studied substrates, graphite and mica,1,2 AFM has revealed the dominant geometry in the adsorbed layer to be cylindrical. Neither the planar geometry of the substrate nor the spherical geometry of the surfactant micelles in solution is observed in the adsorbed layer. Instead straight, parallel, half-cylinders are observed on graphite and meandering full cylinders are seen on mica. The special nature of the interactions between surfactant tails and the cleavage plane of graphite has been invoked to explain this unusual result,2 and studies of hydrophobically modified silica showing an unstructured layer tend to support this.3 Mica however is regarded as a more typical hydrophilic substrate and is implicitly used as a model for other clay minerals and even for mineral oxides. Mica has been employed as a model for clays in swelling studies,4 showing many similarities but also some important differences in hydration behavior. Evidence commonly cited for the bilayer interpretation of adsorbed layer structure on hydrophilic substrates includes surface force apparatus results for hexadecyltrimethylammonium bromide (CTAB) on mica.5 AFM images of surfactants adsorbed on mica typically show stripes which are interpreted as adsorbed full cylinders, even though they are in equilibrium with spherical micelles in bulk.2 Fairly unusual conditions are required before adsorbed spherical micelles can be generated on mica, such as highly charged surfactants or large headgroups6,76,7 or surface modification by ion exchange.8 But is mica a good model for drawing general conclusions about adsorbed layers on clays or oxide minerals? In this paper we report findings which clearly demonstrate that adsorbed layers of cationic surfactants on mica have * To whom correspondence may be addresses. Phone: (+61 2) 9351 2106. Fax: (+61 2) 9351 3329. E-mail: g.warr@ chem.usyd.edu.au. (1) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349. Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160. (2) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (3) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288. (4) Pashley, R. M.; Quirk, J. P. Colloids Surf. 1984, 9, 1. (5) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169. (6) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685. (7) Manne, S.; Schaffer, T. E.; Huo, Q.; Hansma, P. K.; Morse, D. E.; Stucky, G. D.; Aksay, I. A. Langmuir 1997, 13, 6382 (8) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602.
Figure 1. 500 nm × 500 nm AFM image of the TTAB adsorbed layer on mica and kaolinite. Insets show two-dimensional Fourier transforms of the mica and kaolinite regions of the image.
decidedly different morphologies from those on other typical clays, kaolinite and montmorillonite. Kaolinite (KGa-1b) and sodium montmorillonite (SWy-2), both of which are clay standards, were obtained from the Clay Minerals Society of America.9 The homoionic clays were prepared according to the method of Posner and Quirk10 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) 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 (9) http://web.missouri.edu/∼geoscjy/SourceClay/ (10) Posner, A. M.; Quirk, J. P. Proc. R. Soc. London, Ser. A 1964, 278, 35.
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platelets occasionally exhibit steps and ledges on the exposed face (more commonly for kaolinite than montmorillonite); however large, flat areas of order 100 nm × 100 nm are commonly obtained and imaged. All AFM images were recorded on a Digital Instruments NanoScope 3 following the same procedure as in previous work.6 Tetradecyltrimethylammonium bromide (TTAB, Aldrich) was used as received. All solutions were prepared in Milli-Q water at twice their critical micelle concentration, so that the adsorbed layer represents surface saturation by the adsorbed surfactant. Figure 1 shows a 500 nm × 500 nm deflection image of a kaolinite particle deposited onto mica and immersed in an 8 mM aqueous solution of TTAB. The top section shows mica and the bottom section shows part of a deposited kaolinite platelet. Spherical adsorbed micelles are clearly visible on kaolinite coexisting under identical solution conditions with cylindrical adsorbed micelles on mica as the AFM tip traverses a step from mica up onto kaolinite. Two-dimensional Fourier transforms, shown as insets, confirm the presence of different lateral structures on the two clay surfaces. Spheres are also observed for TTAB adsorbed onto montmorillonite particles prepared in this way. When mica itself is ground up and deposited, TTAB cylinders are observed on both the substrate and deposited particles. (11) Mehrian, T.; de Keizer, A.; Korteweg, A. J.; Lyklema, J. Colloids Surf., A 1993, 73, 133. (12) Bailey, S. W. Structures of Layer Silicates; Brindley, G. W., Brown, G., Eds.; Mineralogical Society: London, 1980; Vol. 5, p 1.
Letters
As previously observed for mica and silica,2 the adsorbed layer of TTAB on kaolinite and montmorillonite consists of discrete aggregates and is not a laterally unstructured bilayer.11 Two differences noted between silica (where adsorbed TTAB spheres are reported) and mica are charge density and crystallinity. Mica’s higher charge density has been proposed as the cause of the interfacial sphereto-cylinder transition, and the crystal lattice implicated in preferentially orienting the adsorbed cylinders. The surface charge densities of the basal planes derived from isomorphous substitution on kaolinite and montmorillonite are much lower than those obtained on mica,12 lending support to the proposition that surface charge density is a determining factor in adsorbed layer morphology.2 Mica is not a typical substrate for surfactant adsorption, and full cylinders are not normally expected as the adsorbed layer morphology. Generalizations to other minerals based on observations on mica should thus be made with caution. The adsorption of ionic surfactants onto these and other clays under various solution conditions is under further investigation. 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. LA9912747
