2004
Langmuir 2002, 18, 2004-2012
Temperature-Induced Transitions from Rodlike to Globular Micellar Aggregates in Aqueous Cetyltrimethylammonium Bromide in the Presence of 9-Anthrylalkanols Roderich Bott Dr. Bott KG, Ortsstrasse 37, D-07426 Unterhain, Germany
Thomas Wolff* Technische Universita¨ t Dresden, Institut fu¨ r Physikalische Chemie und Elektrochemie, D-01062 Dresden, Germany
Karl Zierold Max-Planck-Institut fu¨ r Molekulare Physiologie, Postfach 50 02 47, D-44202 Dortmund, Germany Received August 13, 2001. In Final Form: December 14, 2001 The flow behavior of aqueous solutions of cetyltrimethylammonium bromide was investigated as a function of temperature (15-50 °C) in the presence of various amounts of three 9-anthrylalkanols: 9-anthrylmethanol; 9-(1-(1-hydroxy)ethyl)anthracene; 9-(1-(1-hydroxy-2,2,2-trifluoro)ethyl)anthracene. Changes from non-Newtonian flow to Newtonian flow were observed upon increasing the temperature or decreasing the content of the 9-anthrylalkanols. The temperatures (between 291 and 321 K) and concentration ratios, at which these changes were observed, are distinct for the three solubilizates. Evaluations of rheological and light scattering data are in accordance with the presence of very big globular micellar aggregates (diameters up to 35 nm) in the Newtonian and long rodlike aggregates (lengths up to 800 nm) in the non-Newtonian cases. Electron micrographs confirm this interpretation.
1. Introduction A manifold of influences that ionic or nonionic solubilized additives can have on size and shape of micellar aggregates in water has been reported in the literature. These range from ordinary salt effects1 to very specific interactions of solubilizate and micelles resulting in unique chemical and photochemical reactivity2 or in the induction of unusual flow properties, even in the absence of added salt.3,4 Among the latter there are light-induced changes of viscosity and flow behavior originating from effects that solubilized anthracene derivatives and their in situ photodimerization have on size and shape of micellar aggregates as well as * Corresponding chemie.tu-dresden.de.
author.
E-mail:
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(1) (a) Pilpel, N. Trans. Faraday Soc. 1954, 50, 1369. (b) Miller, D. J.; Klein, U. K. A.; Hauser, M. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 1135. (2) (a) Knoche, W., Schoma¨cker, R., Eds. Reactions in compartmentalized liquids; Springer: Berlin, 1989. (b) Kalyanasundaram, K. Photochemistry in microheterogeneous systems; Academic Press: Orlando, FL, 1987. (c) Pileni, M. P. Structure and reactivity in reverse micelles; Elsevier: Amsterdam 1989. (3) (a) Nash, T. J. Colloid Sci. 1958, 13, 134. (b) Wan, L. S. C. J. Pharm. Sci. 1966, 55, 1395. (c) Gravsholt, S. J. Colloid Interface Sci. 1976, 57, 575. (d) Hyde, A. J.; Johnstone, D. W. M. J. Colloid Interface Sci. 1975, 53, 349. (e) Ulmius, J.; Wennerstro¨m, H.; Johansson, L. B. A.; Lindblom, G. Gravsholt, S. J. Phys. Chem. 1979, 83, 2232. (f) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933. (g) Makhloufi, R.; Cressely, R. Colloid Polym. Sci. 1992, 270, 1035. (4) (a) Wolff, T. In Structure and flow in surfactant solutions; Herb, C. A., and Prud’homme, R. K., Eds.; ACS Symposium Series 578; American Chemical Society: Washington, DC, 1994; p 181. (b) Wolff, T.; Klaussner, B. Adv. Colloid Interface Sci. 1995, 59, 31. (c) Wolff, T.; Emming, C.-S.; von Bu¨nau, G.; Zierold, K. Colloid. Polym. Sci. 1992, 270, 822. (d) Fro¨mmel, J.; Wolff, T. J. Colloid Interface Sci. 1998, 201, 86. (e) Lehnberger, C.; Wolff, T. J. Colloid Interface Sci. 1999, 213, 187.
on micellar hydration shells4 (photorheological effects). Despite only small variations in the substituents, very specific influences of the three anthryl alkanols 9-anthrylmethanol (AM), 9-(1-(1-hydroxy)ethyl)anthracene (THAE), and 9-(1-(1-hydroxy-2,2,2-trifluoro)ethyl)anthracene (TFAE) on the flow of aqueous cetyltrimethylammonium bromide (CTAB) solutions at room temperature were reported:5 when AM and THAE were solubilized in 250 mmol/dm3 CTAB (at this concentration small spherical micelles are present in pure aqueous CTAB), an increase of bulk viscosity by orders of magnitude was induced. The flow behavior, however, remained Newtonian throughout. In contrast, solubilized TFAE induced highly non-Newtonian, strongly viscoelastic behavior under the same conditions, i.e., in the absence of salt.6
In this paper we will show that a common and consistent view of the differing features observed in these three CTAB-solubilizate systems may arise when experiments are carried out at various temperatures. Results from light scattering and rheological experiments are reported for (5) Wolff, T.; Bott, R.; Kerperin, K. J. Colloid. Polym. Sci. 1992, 270, 1222. (6) Wolff, T.; Kerperin, K. J. J. Colloid Interface Sci. 1993, 157, 185.
10.1021/la011277v CCC: $22.00 © 2002 American Chemical Society Published on Web 02/21/2002
Transitions in Micellar Aggregates
Langmuir, Vol. 18, No. 6, 2002 2005
the three systems, which reveal size and shape of the aggregates and which are supported by electron micrographs of the solutions containing AM and TFAE. It will be shown that transitions from solutions with long rodlike aggregates (exhibiting non-Newtonian flow) to solutions with large globular surfactant aggregates (exhibiting Newtonian flow) take place at distinct transition temperatures. 2. Experimental Section Materials. Cetyltrimethylammonium bromide (Merck, pa) was recrystallized three times from acetone-methanol (9:1 mixture). 9-Anthrylmethanol (AM, Aldrich, 97%) was chromatographed on basic alumina and recrystallized from ethanol. 9-(1(1-Hydroxy-2,2,2-trifluoro)ethyl)anthracene (TFAE; Aldrich, racemic mixture, 98%) was used as supplied. 9-(1-(1-Hydroxy)ethyl)anthracene (THAE) was prepared by following a literature procedure.7 Light Scattering and Rheology. The homemade apparatus for low-angle static and dynamic light scattering6,8,9 and the equipment for rotational viscometry5 were described previously together with evaluation procedures. Flow curves as displayed in several figures were measured in Mooney-Eward geometry throughout. They were obtained in that shear rates were increased linearly from 0 to the maximum value within 50 s, kept there for 10 s, and decreased linearly within 50 s to 0 s-1. Slowing down the time ramp did not lead to significant changes in zero shear viscosities η0 (initial slopes of flow curves). For diluted, low viscosity solutions (η < 10 mPas) the Haake ME 15 measuring system was exchanged by a homemade system similar to the Haake ME 45. Solutions. Micellar solutions were prepared using triply distilled water which was filtered through 50 nm filters (Sartorius). A stock solution (containing 250 mmol dm-3 CTAB for rheological experiments and 5 mmol dm-3 for light scattering experiments) was weighed according to the highest desired ratio of solubilizate and surfactant. The solubilizate concentrations of actual solutions then were adjusted by appropriate dilution with water or with CTAB solution. The solubilization was accelerated by heating to 60 °C and periodic sonication. In the preparation of solutions for light scattering experiments, all glassware was washed three times with triply distilled and 50 nm filtered water prior to use. Solutions prepared in this way were filtered once more through 100 nm one way filters when transferred to the light scattering cell. The concentrations given pertain to the temperature of sample preparation (293 K); for simplicity concentration changes due to temperature variations are not mentioned explicitly in the following text. All solutions containing anthracene derivatives were stored and handled in the dark to prevent photoreactions such as endoperoxide formation or photodimerization.5 Samples showed identical flow curves after 1 year when treated as described above. Electron Micrographs. Cryotransmission electron micrographs were produced via the freeze fracture method as described previously.4c Special care was necessary to transfer samples to the shock-freezing equipment under temperature control.
3. Results The following experiments with aqueous solutions containing cetyltrimethylammonium bromide (CTAB) and 9-anthrylalkanols were performed at temperatures between 288 and 323 K. The lower temperatures are near or below the Krafft point of the pure water-CTAB system (for the phase diagram see ref 4b). Nevertheless, neither a precipitation at 288 K nor a macroscopic phase separa(7) Fieser, L.; Hartwell, J. J. Am. Chem. Soc. 1938, 6, 2555. (8) Wolff, T.; Suck, T. A.; Emming, C. S.; von Bu¨nau, G. Prog. Colloid Polym. Sci. 1987, 73, 18. (9) Wolff, T.; Emming, C.-S.; Suck, T. A.; von Bu¨nau, G. J. Phys. Chem. 1989, 93, 2065.
Figure 1. Flow curves (shear stress τ vs shear rate dγ/dt) of aqueous solutions containing 250 mmol dm-3 CTAB and 50 mmol dm-3 AM at 201 K (a), 292 K (b), 294 K (c), 298 K (d), and 303 K (e). Thin lines: curves for increasing shear rates. Thick lines: curves for decreasing shear rates.
tion at higher temperatures was observed in the presence of 9-anthrylalkanols, even in solutions handled for several days. 3.1. 9-Anthrylmethanol (AM). Rheology. Flow curves of 250 mM aqueous solutions of cetyltrimethylammonium bromide (CTAB) containing 9-anthrylmethanol (at a molar ratio of 25:5) at various temperatures are depicted in Figure 1. Actual viscosities (slopes in flow curves) are in the range of several Pa‚s to be compared with
