Langmuir 1995,11, 3337-3346
3337
Rheology of Cetyltrimethylammonium Tosilate-Water System. 1. Relation to Phase Behavior J. F. A. Soltero and J. E. Puig* Departamento de Ingenieria Quimica, Universidad de Guadalajara, B o d M. Garcia Barragan #1451, Guadalajara, Jalisco 44430, Mixico
0. Manero Instituto de Investigaciones en Materiales, Universidad Nacional Aut6noma de Mixico, Apdo. Postal 70-360, Mixico D.F. 04510, Mhxico
P. C . Schulz Departamento de Quimica y de Ingenieria Quimica, Universidad Nacional del Sur, Bahia Blanca 8000, Argentina Received September 28, 1994. I n Final Form: May 1, 1995@ The partial phase behavior of CTATIwater is investigated here as a function of temperature by WAXS, DSC, polarizing microscopy, conductometry, lH-NMR, and FTIR spectroscopy. Oscillatory strain and temperature sweeps are also reported. The Krafft temperature (TK) of CTATIwater is 23 "C. Below this value, triclinic crystals of CTAT coexist with an isotropic solution. Above TKand at low concentrations, spherical micellar solutions are Newtonian and exhibit low viscosities. At higher concentrations (ct), cylindrical micelles form and viscosity increases dramatically with CTAT concentration, but no elastic effects are noticed. When micelles are long enough to entangle (0.9-27 wt % at 25 "C), clear viscoelastic solutions form. At higher concentrations and up to 41 wt %, an hexagonal phase appears. This phase exhibits yield stress and viscoelasticity. At higher concentrations, a nonelastic, viscous solid paste forms. Micellar solutions and hexagonal phase depicts three regimes of viscoelasticity with temperature. These regimes are bounded by Tk and by the temperature (T,) at which the system exhibits its main relaxation time. T,moves to lower temperatures as CTAT concentration increases indicatingthat the main relaxation time decreases upon increasing concentration.
Introduction The physical properties, phase behavior, and rheology of aqueous solutions of cationic surfactants are very sensitive to changes in the molecular structure of the counteri~ns.l-~ For instance, all organic counterions cause a profound decrease in the cmc of cationic surfactants compared to that of the respective halide counterions.2 Also, large organic counterions (i.e., salicylate or tosilate) usually induce formation of long elongated micelles and viscoelasticity in the dilute regime (51wt % surfactant).8 The addition of salicylate (0-hidroxybenzoate) salts to CTAB micellar solutions produces viscoelasticity, but the addition of the m- and p-hydroxybenzoates does not, implying that small changes in the organic counterion structure can modify dramatically the rheological behavior.1° The explanation is that salicylate counterions embed in the CTAB micelle in a n orientation that allows the formation of long micelles, whereas the m- and p hydroxybenzoates cannot do this. The viscoelastic prop-
* Author to whom correspondence should be addressed. Abstract published inAdvance ACSAbstracts, August 1,1995. (1)Thurn,H.; Lobl, M.; Hoffmann, H. J . Phys. Chem. 1985,89,517. (2)Ohldendorf, D.;Interthal, W.; Hoffmann, H. Rheol. Acta. 1986, 25,468. (3)Shikata, T.;Hirata, H.; Kotaka, H. Langmuir 1987,3, 1081; Langmuir 1988,4,354;Langmuir 1989,5,398. (4)Rehage, H.; Hoffmann, H. J . Phys. Chem. 1988,92,4712. (5)Shikata, T.;Hirata, H.; Takatori, E.; Osaki, K. J.Non-Newtonian Fluid Mech. 1988,28,171. (6)Wunderlich, A.M.; Hoffmann, H.; Rehage, H. Rheol. Acta 1987, 26, 532. (7)Strivens, T. A. Colloid Polym. Sci. 1989,267, 269. (8)Gijbel, S.;Hiltrop, K. Prog. Colloid Polym. Sci. 1991,84,241. (9)Clausen, T.M.;Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J . Phys. Chem. 1992,96, 474. (10)Gravsholt, S. J . Colloid Interface Sci. 1986,57, 575. @
0743-746319512411-3337$09.0010
erties of such solutions are then the result of entanglements of the elongated chains, thereby immobilizing the constituents in solution and forming gel-like structures.11,12 In this paper, the phase behavior of cetyltrimethylammonium tosilate (CTAT) and water as a function of temperature is reported. The relationship between structure and structural changes with the rheological behavior is investigated. In two subsequent papers the linear and the nonlinear viscoelastic behavior of this system as a function of concentration and temperature will be presented.13J4 There are two important differences between this paper and previous reports on rheological behavior of CTATI water and similar ~ y s t e m s . ~First, ~ ~ ~the ~ Jrheological ~ behavior is examined here from dilute (1wt %) to highly concentrated regions (60 wt %) as a function of temperature; second, there are no extraneous electrolytes present in this study, such as NaBr whichis produced when sodium tosilate or salicylate is added to CTAB to induce formation of elongated micelles and viscoelasticity. This is very important because the rheology of such micellar solutions is very sensitive to the levels of extraneous electrolyte ~0ncentration.l~ (11)Manohar, C.; Rao, U. R. K.; Valaulikar, B. S.; Iyer, R. M. J . Chem. Soc.. Chem. Commun. 1986.379. (12) Rao; U. R. K.; Manohar, C.; Valaulikar, B. S.;Iyer, R. M. J . Phys. Chem. 1987,91,3286. (13)Soltero, J. F.A.; Puig, J. E.; Manero, 0.submittedforpublication in Langmuir. (14)Soltero, J. F.A.; Puig, J. E.; Manero, 0.submittedforpublication in Langmuir. (15)Gamboa, C.; Sepulveda, L. J . Colloid Interface Sci. 1986,113, 566.
0 1995 American Chemical Society
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Experimental Section Cetyltrimethylammonium tosilate or CTAT (Sigma) had a purity of 98%. It was purified by recrystallization from a chloroform-ethyl ether mixture (50:50 by volume). Water was doubly distilled and deionized. Samples were prepared by weighing appropriate amounts of CTAT and water in glass vials. These vials were placed in a water bath at 60 "C for a weekwhere they were frequently shaken. Then samples were allowed to reach equilibrium at the temperature of measurement. All samples were centrifuged to remove suspended air bubbles before being tested. X-ray diffraction of dry CTAT powder was done with a RigakuDenki diffractometer equipped with a n horizontal goniometer using Cu K a radiation and a Ni filter. For CTAT/water samples, diffractograms were obtained with a n horizontal goniometer and a Phillips 1140 generator operating a n X-ray tube a t 40 kV. The radiation was Cu Ka. A film of CTAT/water sample was placed in a thermostated home made cell. Diffractograms were analyzed with the method of Vand.16 To measure the density of CTAT, a picnometer was filled with a saturated solution of CTATin CCld and weighed with a Sartorius analytical balance (sensitivity of f O . O 1 g). Confidence levels were estimated a t 0.90. Conductivity was measured a t 25 k 0.1 "C with a n Orion 101 conductometer and a YSI immersion conductivity cell (cell constant = 1cm-l). The cell was calibrated with KC1 solutions of known cond~ctivity.'~ Thermograms were obtained with a Perkin-Elmer DSC-IV calorimeter. The instrument was calibrated with indium, water, and n-octane standards. Cooling scans below room temperature were done with a n Intracooler I refrigeration unit (Perkin-Elmer). All thermograms were obtained with heating and cooling rates of 10 "C/min. Aluminum sample pans for volatile samples (Perkin-Elmer) were used to minimize losses by evaporation. Samples were weighed before and after DSC runs. Results with samples that lost weight were discarded. Transition temperatures were determined (&0.5 "C) upon heating from the point where the base line changed slope significantly. IR spectra were taken in a Nicolet FTIR 5ZDA spectrometer. Samples were loaded in a ZnSe cell, which was then placed in a special cell holder with temperature control (k0.2 "C). Samples were also examined with a Meopta microscope equipped with cross polarizers, heating-and-cooling stage (- 15 to 300 "C), and digital temperature control ( f 0 . 1 "C). IH-NMR measurements of CTATD20 samples were carried out with a 300-MHz Varian VX NMR spectrometer operating in the Fourier transform mode. Temperature was controlled within 1"C. Visual appearance and textures under cross polarizers in the polarizing microscope of CTATD20 samples were identical to those observed in samples of similar composition prepared with water.Is Rheological properties were measured in a Rheometrics Dynamical Spectrometer RDS-I1 and a cone-and-plate geometry. The cone angle was 0.1 radians and its diameter was 2.5 cm. The transducer sensitivity was 2-2000 g cm. To prevent changes in composition for water evaporation during measurements, a humidification chamber was placed around the cone-and-plate fixture. This chamber contains a coil to circulate thermostated fluids to maintain temperature constant during measurements.
Results Phase Behavior. Figure 1 s h o w s an X-ray diffractogram of a n h y d r o u s C T A T taken at room temperature. Of the peaks detected, seven are due t o long spacing. The rest (13 p e a k s ) were available t o a n a l y z e the crystalline s t r u c t u r e of CTAT. The measured d e n s i t y of the CTAT was 1.025 & 0.004 g/cm3. C T A T is quite soluble in water above 23 i 1 "C. At temperatures lower than 23 "C, CTAT/water samples (with the exception ofvery diluted ones) exhibit a white, granular a p p e a r a n c e due to the presence of CTAT crystals dispersed in a viscous saturated s u r f a c t a n t solution. S a m p l e s (16)Vand, V. M. Acta Crystallogr. 1948, 1, 109, 290. (17) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 53rd ed.; CRC Press: Cleveland, OH, 1973;p D202. (18)Soltero, J. F. A. Tesis Doctoral, U.N.A.M., Mexico, 1995.
e Figure 1. Wide angle X-ray scattering of dry CTAT.
B
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60
SO
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30
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Figure 2. Wide angle X-ray scattering from a 40 wt % CTAT sample as a function of temperature: (A) 10 "C; (B) 20 "C; (C) 25 "C; (D) 30 "C. d e m o n s t r a t e an apparent yield stress that i n c r e a s e s with s u r f a c t a n t concentration. Samples with concentrations higher than 60% look like solid pastes. Wide-angle X-ray measurements as a function of temperature confirmed the p r e s e n c e of c r y s t a l s at temperatures below 25 "C for a 40 w t % CTAT sample (Figure 2A-C). The diffraction peaks widen and lose i n t e n s i t y as the temperature i n c r e a s e s d e m o n s t r a t i n g loss of crystalline s t r u c t u r e . Above 25 "C, only a v e r y b r o a d peak is d e t e c t e d indicating the d i s a p p e a r a n c e of the crystalline s t r u c t u r e (Figure 2D). The temperature at w h i c h crystalline diffraction patterns v a n i s h e d is a r o u n d 23 "C a n d i n c r e a s e s slightly with i n c r e a s i n g CTAT concentration, b e i n g near 28 "C for 50 w t % CTAT. Above 23 "C and for s u r f a c t a n t concentrations s m a l l e r than 0.9 wt %, CTAT/water mixtures f o r m isotropic and transparent solutions that have viscosities similar to that of w a t e r . The cmc was measured as a f u n c t i o n of temperature b y conductometry. P l o t s of conductivity vs s u r f a c t a n t c o n c e n t r a t i o n s h o w t w o d i s t i n c t "breaks" (Figure 3). The first b r e a k (cmc) signals the f o r m a t i o n of s p h e r i c a l micelles (most likely p r o l a t e micelles with small
Langmuir, Vol. 11, No. 9, 1995 3339
CTATI Water Phase Behavior 35
T=2S°C 30
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4
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7
C , d x 102 (wt%) 35
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C&x 102(wt%) 40 T=35'C
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Figure 3. Conductivity of CTAT solutions as a function of surfactant concentrationmeasured at different temperatures. The first "break" indicates the cmc; the second "break (ct) corresponds t o the transition to cylindrical micelles. aspect ratio).2 The second break (et) corresponds to a transition from spherical to cylindrical micelles.2 The cmc is quite insensitive to temperature, but ct shifts to higher concentrations as temperature is increased (Figure 3). From 1to 27%,samples are transparent (their turbidity is similar to that of pure water) although gel-like a t 25 "C. Under cross polarizers, samples in this concentration range are nonbirefringent, but they show strong streaming birefringence. When these micellar solutions are stirred with a rod, the "rod-climbing'' effect or Weissenberg effect is observed.lg Both, streaming birefringence and the Weissenberg effect decrease in intensity upon increasing temperature, and they disappear altogether above 40 "C. Above 27 wt % and up to 47 wt %, samples are birefringent with textures typical of the hexagonal phase. Figure 4 depicts photographs of a 30 wt % CTAT sample taken through cross polarizers in the polarizing microscope as a function oftemperature. Below 23 i1"C, interference colors typical of thick samples of crystalline structures are seen over a dark background which indicates the coexistence of crystals and an isotropic phase (Figure 4A). At 25 "C, crystals and a birefringent phase are clearly observed (Figure 4B). When the coverslip is pressed, the (19) Ferry, J. D. The ViscoelasticProperties ofPolymers;Wiley: New
York, 1980.
birefringent areas flow and the birefringence persists. At higher temperatures (Figure 4C), striations in incipient geometric texture and some positive spherulites and air bubbles are observed. This texture is characteristic of the hexagonal (or middle) phase.20 Upon heating, the structure remains up to 40 "C. Above this temperature, the birefringence slowly disappears from the visual field of the microscope until only a dark field remains, demonstrating the transition into a n isotropic solution. When this isotropic sample is sheared by moving the coverslip, a n ordered structure develops as evident by the appearance of intense birefringence. The birefringence persists for a few seconds after shearing is stopped. The duration of this transient birefringence augments with CTAT concentration and decreases with increasing temperature. For concentrations higher than 50 wt % CTAT, samples are slightly turbid to the naked eye. The turbidity increases with CTAT concentration but decreases with increasing temperature. At these concentrations and a t room temperature, crystallites are observed in the microscope. At higher temperatures, a hexagonal phase forms and upon heating, the birefringence disappears slowly in the polarizing microscope a t around 80 "C indicating a transition into a n isotropic phase. Also, as temperature increases, samples become less elastic although they remained quite viscous, forming threads similar to those observed in solutions of high molecular weight polymers.20 Figure 5 depicts DSC thermograms of CTAT/water samples in the concentration range of 10 to 100%. A bulklike water fusion peak is detected a t 0 "C in samples containing up to 90 wt % CTAT. The water peak splits into two closed peaks at -2 and 0 "C for concentrations larger than 30 wt %. The peak at -2 "C is most likely associated with freezing (or melting) of water molecules interacting strongly with the tosilate group. Interfacial water freezing a t temperatures below 0 "C has been reported in other surfactant-based liquid crystalline systems.21 Another endothermic peak is observed a t 23 "C due to the melting of CTAT crystals (embedded in saturated aqueous solution below 23 "C) into a micellar solution or a liquid crystalline phase, as observed by light microscopy. This phase transition is seen in the whole concentration range except with dry CTAT, suggesting that it is induced by the presence of water. A third peak is seen around 37 "C in samples with concentrations higher than 50 %. This peak, which increases in intensity with CTAT concentration, may be related to a transition into a n isotropic phase. Several other peaks were noticed a t higher surfactant concentrations, but since these transitions lay outside the range where most rheological studies were conducted, they were not examined further. When pure CTAT is heated, three thermal transitions are detected a t 40,110, and 120 "C by DSC (thermogram for 100% CTAT in Figure 5); however, no changes were observed a t any of these temperatures in the polarizing microscope. Above 200 "C, pure CTAT decomposes. lH-NMR spectra were taken at different concentrations (10,20, and 60 wt % CTAT) as a function of temperature. The peak assignments were done by comparison to spectra of CTAB/Na salicylate.12 At 25 "C, all lH-NMR signals from CTAT molecules are broad and unresolved (Figure 6A). This type of spectra is typical of long elongated micelles.22Resonance peaks from ortho- and meta-protons as well as those ofthe aromatic CH3- a t the para-position shift to lower 6 values with respect to those in an aqueous (20) Rosevear, F. B. J . Am. Oil Chem. SOC.1954, 31, 628. (21) Schulz, P. C.; Puig, J.E.; Barreiro, G.; Torres,L. A. Thermochim. Acta 1994, 231, 239. (22)Ulmius, J.; Wennerstrom, H. J.Magn. Reson. 1977, 28, 309.
=--
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3340 Langmuir, Vol. 11, No. 9, 1995
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I
Figure 4. Photographs of a 30 wt % CTAT sample taken through cross polarizers in a polarizing microscope a t different temperatures: (A) 20 "C; (B) 25 "C; (C) 40 "C.
e n ~ i r o n m e nbecause t~~ they are in a less polar environment; i.e., most of the aromatic ring of the tosilate group is embedded in the micelle. At this temperature, the
sample is gel-like and viscoelastic. Remarkably, the proton signal from the water (-0.2% in DzO) is relatively sharp indicating that water moleculesremain mobile and
CTATI Water Phase Behavior
Langmuir, Vol. 11, No. 9, 1995 3341 A
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. 40
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. 60
-
. 80
.
.
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.
.-.
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Figure 5. Thermograms of CTAT/water samples as a function of CTAT concentration. The heating rate was 10 "Clmin.
free to reorient in these gel-like materials. When the sample is heated to 40 "C, resonance peaks sharpen, although they are still unresolved (Figure 6B). All peaks shift to higher 6 values. At 60 "C, resonance peaks from the hydrocarbon tail of CTAT become resolved (peak b splits into several ones which are related to different carbons of the tail) as well as those of the aromatic protons (peaks 2 and 3 split into doublets). Also, a small peak a t 3.15 ppm due to the a-methylene group appears (Figure 6C). The interpretation is that the surfactant tail and the head are more mobile and free to rotate causing a sharpeningofthe signals. Peaks 1,2,and 3 ofthe aromatic group keep shifting to higher 6 values suggesting that these protons are in a more polar environment. Finally, a t 80 "C, where the sample is very fluid and has almost no elasticity, all peaks are sharp and resolved (Figure 6D). 6 values of peaks 2 and 3 of the aromatic protons are similar to those of Na salicylate or Na tosilate in water12,23 suggesting that the aromatic ring has emerged from the micelle palisade into an aqueous environment. The water proton peak is very intense and sharp indicating bulklike water motion. Figure 7 shows IR spectra of several regions of a 50% CTAT sample as a function of temperature. Because of the strong absorption bands of water, some features are lost from the spectra. In the 3050-2700 cm-l region (Figure 7A), the asymmetric stretching mode of the headgroup (Nf)-CH3 is observed a t 3034 cm-l as a small shoulder on the strong absorption band of the OH group of water a t lower temperatures (15 "C). As temperature is increased, this band loses intensity (30 "C), and it vanishes a t higher temperatures (55 "C). This implies that a t lower temperatures the headgroup is "frozen" in a crystalline structure and that it is free to rotate a t higher temperature^.^^^^^ The strong bands a t 2918 and 2849 cm-l are assigned to the antisymmetric and symmetric CH2 stretching modes, respectively. At lower temperatures these bands are narrow, indicating lower mobility and ordered packing. At higher temperatures, both bands shift to higher wavenumbers and broaden noticeably, suggesting a more mobile environment. There are two strong bands in the 1500-1320 cm-l region (Figure 7B) a t 15 "C. One at 1487 cm-l is assigned to the asymmetric mode of the CH3-(N+) headgroupZ6 (23) Bunton, C.A.; Minch, M.;Hidalgo, J.;Sepulveda, L. J.Am.Chem. SOC.1973, 95, 3262. (24) Weers, J. G.; Scheuing, D. R. In FTIR Spectroscopy in Colloid and Interface Science;ACS Symposium Series 447;American Chemical Society: Washington D.C., 1990; Chapter 6. (25) Wang, W.; Li, L.; Xi, S. J. Colloid Interface Sci. 1993,155,369.
I !
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7
6
5
4
b
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Figure 6. 'H-NMR spectra of a 10 wt % CTAT solution as a function of temperature: (A) 25 "C; (B) 40 "C; (C) 60 "C; (D) 80 "C. Numbers and letters of proton peaks in spectrumcorrespond to those in CTAT chemical representation.
and the other a t 1472 cm-l to CH2 scissoring mode.27r2s As temperature is increased, the headgroup band splits in a doublet a t 1490 and 1480 cm-l. The appearance of this doublet a t higher temperatures can be attributed to a relative disorder due to free rotation of the headgroup induced by electrostatic interactions in a more polar environment. The methylene scissoring band shifts slightly to lower wavenumbers (1468 cm-l) and broadens as temperature is raised. These changes in the spectrum suggest a phase transition from an ordered crystalline structure to a more fluid one. In fact, the scissoring band (26) Kawai, T.; Umemura, J.;Takenaka, T.; Kodama, M.; Ogawa,Y.; Seki, S. Langmuir 1982, 2, 732. (27) Umemura, J.;Cameron, D. G.; Mantsch, H. H.Biochim. Biophys. Acta 1980, 602, 32. (28) Snyder, R. G. J.Mol. Spectrosc. 1961, 7, 116.
3342 Langmuir, Vol. 11, No. 9, 1995
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80
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C surf W%) Figure 8. Temperature-composition phase diagram of binary mixtures of CTAT and water.
~ R M n7o
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Figure 7. Partial FTIR spectra of a 50 wt % CTAT sample taken at 15,25,30,and 55 "C: (A) 3200-2700 cm-l region; (B) 1780-1200 cm-l region; (C) 1240-890 cm-l region; (D) 864486 cm-l region.
a t 1472 cm-l a t low temperatures has been associated with methylene groups packed in a triclinic crystalline struct~re.~~ The bands of the SO3 of the tosilate counterion also give some evidence of structural transitions above 25 "C (Figure 7C). The symmetric stretching mode is observed as a
bimodal broad band around 1230-1150 cm-l a t low temperatures and broadens more and becomes less intense with increasing temperature. At 30 "C, only a very broad band is detected. The stretching mode of the head group around 966 cm-l also becomes broader as temperature increases (Figure 7C). The C-S stretching mode a t 687 cm-l (Figure 7D) broadens and loses intensity as temperature increases. The rocking mode of the methylene group appears as a doublet a t 723 and 714 cm-l a t 15 and 25 "C, and fuses into a single band a t higher temperatures (Figure 7D). The absorption bands associated with the aromatic ring also exhibit some changes upon increasing temperature. For instance, the out-of-plane deformation (820 cm-l) and the backbone mode (563 cm-l) become broader and less intense upon increasing temperature, particularly above 25 "C. These changes may be attributed to changes into a more polar environment or to a structural transition, or to both. The temperature-composition phase diagram of the CTAT/water binary system is shown in Figure 8. For temperatures between 0 and 23 "C, CTAT crystals and an isotropic solution coexist. Above 23 "C, a transparent nonbirefringent isotropic phase forms a t low to intermediate concentration. At temperatures above 80 "C, the isotropic phase extends almost to 90 wt % CTAT. At room temperature and above 27 wt % up to ca. 47 wt %, a hexagonal phase appears. The extent of this one-phase region diminishes as temperature is increased. At even higher concentrations, the hexagonal phase coexists with a n isotropic phase and then with CTAT crystals. Phase boundaries were not determined exactly because of the lack of resolution of the techniques employed. A similar phase diagram for equimolar mixtures of cetylpyridinium chloride and sodium salicylate in DzO was reported elsewhere.29 However, a lamellar phase and a viscous isotropic one-phase region appear in that phase diagram. The existence of a one-phase viscous-isotropic (VI)region a t high surfactant concentrations and temperatures cannot be ruled out in the system examined here; however, the lack of sensitive techniques, such as low-angle X-ray scattering, did not allow us the identification of this kind of liquid crystalline phase. On the other hand, a lamellar phase, which is easily detectable by polarizing microscopy, was not observed. This mesophase may have been induced by the presence of extraneous electrolytes, as in the system reported by Monduzzi et aLZ9 Rheological Measurements. Figure 9 shows strain sweeps made a t a frequency (0) of 10 r a d s and 22,30, or 40 "C for a 20 wt % CTAT sample. At low temperatures (Figure 9A), both the elastic ( G I and the loss (G') moduli decrease monotonically with deformation strain from the (29)Monduzzi, M.; Olsson,U.; Soderman, 0. Langmuir 1993,9,2914.
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CTATI Water Phase Behavior
I 4 0
10 0.1
1
10
100
loo0
y Figure 10. Strain sweeps made at 10 rads and 30 "C as a functionof CTAT concentration. Surfactantconcentrationsare indicated in the figure. %
-a2 11 k
*
e
** ........................... I *
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2
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Figure.9. Strain sweeps of a 20 wt % CTAT sample, made at 10 rads at: (A) 22 "C; (B)30 "C;(C) 40 "C.
lowest strain deformation examined here (0.1%). Also G is larger than G at all strains. Above 23 f 1"C, both G and G are strain-independent (linearviscoelastic region) up to about 100% deformation (Figure 9B). Now, the sample is predominantly elastic a t all strains, i.e., G > G a t 30 "C (Figure 9B). At 40 "C, G and G are still strain-independent in the whole range but G"is closer to G (Figure 9 0 . At even higher temperatures (not shown),18 G becomes larger than G at all strain amplitudes; i.e., viscous behavior dominates at higher temperatures. Similar results were obtained a t other concentrations. Figure 10 depicts strain sweeps made at w = 10 rads at 30 "C as a function of CTAT concentration. For concentrations between 5 and 25% CTAT, the linear viscoelasticregionspans the whole strain amplituderange. For higher concentrations, where the hexagonal phase appears, the strain-independent region becomes smaller with increasing surfactant concentration. For instance, the linear viscoelastic region for the 40% sample goes only to 4% strain. However, the linear region shifts to higher y values with increasing temperature. Figure 11presents oscillatorytemperature sweeps made at 10 rads in the linear viscoelastic range of strain deformations for several concentrations. The rheological behavior is predominantly viscous for temperatures below
ca. 23 "C (TKin Figure 11).At this temperature, G and G"cross over. Above TKand up to the temperature where G and G cross over again ( T J ,the sample response is dominantly elastic. Above T, (Figure 11) the sample becomes predominantly viscous again. This response is observed up to concentrations of 55 wt % CTAT. At low CTAT concentrations (5 wt %), the complex viscosity depicts a minimum at 20 "C, then increases up to 24 "C, where a shallow maximum is observed followed by a shear thinning region (Figure 11A). G follows the behavior of Iv*l. G , on the other hand, goes through a minimum a 24 "C, then increases with temperature up to 42 "C (T,), where it accomplishes a maximum (which coincides with the onset of the shear thinning region), and then it decreases again with further increase in temperature. The linear viscoelastic functions of samples with higher concentrations (15, 25, and 40 wt %) first rise with temperature and then go through a maximum, followed by a continuous decrease a t higher temperatures. The maximum shifts to higher temperatures with increasing concentration (Figure 11B-D). Above 15 wt % CTAT, there is a minimum in the rheological functions located a t 28 "C which does not appear a t lower concentrations (Figure 11C-E). The minima become deeper with increasing surfactant concentration. Throughout the whole concentration range, the maximum in the moduli and viscosity is always detected a t temperatures higher than TK. As concentration increases, the temperature range over which elasticity dominates (interval between T x and T, in Figure 11)diminishes, and vanishes entirely a t 60 wt % CTAT-at this concentration, the systems becomes a solid paste with dominant viscous behavior over the whole temperature range, i.e., G and G"do not crossover a t all (Figure 11E). Figure 12 reports the zero-frequency complex viscosity Il;l*l~ as a fknction of surfactant concentration at several temperatures. Viscoelastic behavior is first detected at around 1wt % at 25 "C. This lower limit shifts to higher concentrations as the temperature is raised. Viscosity increases sharply with surfactant concentration and reaches a constant value between 2 and 4 wt %, depending on temperature. Viscosity also decreases sharply upon increasing temperatures. Discussion At room temperature, dry CTAT is a triclinic crystal with four molecules per elementary cell. The crystallographic parameters of CTAT estimated from X-ray diffraction (Figure 1)16 are the following: a = 1.379 f 0.03 nm, b = 1.343 f 0.03 nm, c = 3.434 & 0.007 nm; a = 117.9 f 2.4", = 117.2 f 2.4", y = 31.9 f 0.8". The length of a CTAT molecule (L)measured parallel to the
3344 Langmuir, Vol. 11, No. 9, 1995
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60
T("C)
Figure 11. Temperature sweeps made at 10 rads within the linear viscoelastic region for different CTAT concentrations: (A) 5 wt %; (B)15 wt %; (C) 25 wt %; (D)40 wt %; (E) 60 wt %.
f6.3"with respect to the (001)plane. The area per polar group, S, was estimated as 0.49 f 0.01 nm2 with the equation31
1000
100 h
1
t
10
E .100 1
0.1
1
10
C,,rl(wt
"/.I
Figure 12. Plateau complex viscosity measured at the low frequencylimit as a function of CTAT concentrationmeasured at several temperatures. Temperatures are indicated in the figure. Solid lines are an aid for the eye.
hydrocarbon chain is 3.18nm. The following equation:30
i!A
s i n t = - sinp shows that the CTAT molecule has a n inclination t = 54.4 (30)Schulz, P. C.;Puig, J. E. Langmuir 1992,8, 2623.
where M is the molecular weight, N AAvogadro's number, and the crystallographic density (=1.0205 f 0.0038 g/cm3). The exceptionally large value of S (compared to 0.304nm2for sodium dioctylph~sphinate,~~ 0.281nm2for cesium soaps,31 0.289 nm2 for disodium dodecane phoshate^^ and 0.200nm2for potassium soaps33)is probably a consequence of the large size of the tosilate and the trimethylammonium groups. The value of S sin z (which it is usually taken as the value of the transversal area of the hydrocarbon chain) is 0.235 f 0.005 nm2 which is comparable to those reported for other surfactants (0.21 nm2).32 CTAT is very soluble in water above 23 "C (Krafft temperature or TK).This value was determined with a battery of techniques. X-ray diffraction patterns of CTAT/ water samples (Figure 2)correspond to a triclinic struc(31)Gallot, B.;Skoulios, A. E. Kolloid 2.1968,213, 143. (32)Schulz, P.C.An. Asoc. Quim. Argent. 1983, 71, 271. (33)Gallot, B.;Skoulios, A. E. Kolloid 2.1986,210, 143.
CTATI Water Phase Behavior
Langmuir, Vol. 11, No. 9, 1995 3345
40 "C, depending on concentration, the elastic modulus ture16 below 23 f 1 "C. Above this temperature, the can be as much as 20 times higher than the viscous diffraction peaks become much broader and less intense modulus. All these samples exhibit apparent yield stress indicating the disappearance of the crystalline structure. (measured in increasing-decreasing shear stress ~ y c l e s ) ~ ~ J ~ Direct evidence of the Kr& transition was obtained by polarizing microscopy (Figure 4). Melting of the crystals and their shear viscosity at low shear rates is always larger to produce a micellar solution or a hexagonal phase was than the plateau or Newtonian complex viscosity, 1 ~ * 1 0 also detected by DSC as a sharp endothermic transition (cf. Figure 12 and Table I11 of Gamboa and Sepulveda15) at 23 "C that shifisslightly to higher values with increasing and, in general, the Cox-Merz rule is not followed-these CTAT concentration (Figure 5). results are discussed in part 3.14J8 This is typical of FTIR spectroscopy gives abundant information about materials with yield stress.36 Upon heating, complex structure and phase transitions. The existence of triclinic viscosity decreases rapidly (Figure 12); streaming birecrystals ofCTAT at low temperature (15 "C)was confirmed fringence loses intensity and decays faster. Also, at by the presence of the scissoring band of the methylenes temperatures larger than the crossover of G and G ' at 1472 cm-l (Figure 7B) and by the doublet assigned to (Figure 111, the elasticity of the sample has almost the rocking of the -CH2 groups of the hydrocarbon tails vanished and G becomes much larger than G . Also, the at 723 and 714 cm-l (Figure 7D). The spectrum also Weissenberg effect is not longer observed. At the same changes substantially from 15to 25 "C. Furthervariations time, lH-NMR bands are sharp and well-resolved a t T > are noticed at 50 "C, all indicating a more fluid, disordered T, (Figure 6) which indicates that the long cylindrical phase. In particular, the bands associated with the SO3 micelles no longer exist at these temperatures. In fact, group and the benzene ring broaden and shift to higher the chemical shifts of the aromatic and methyl protons of wavenumbers suggesting a more polar environment and the tosilate counterions correspond to those of a n aqueous a transition into a more fluid phase (Figure 7C,D). envir~nment.~~ CTAT forms micelles at low concentrations (cmc= 0.011 Rheology also provides information about the phase wt %; Figure 3) and above TK.At higher concentrations behavior. Below TK, no linear viscoelastic region (i.e., the (ct = 0.04 wt % at 25 "C),a transition to cylindrical micelles region where the dynamic moduli are strain-independent) is observed (Figure 3). The concentration a t which was found for any concentration examined (Figure 9). cylindrical micelles form (ct) depends strongly on temMoreover, samples show a predominantly viscous reperature, in contrast to the cmc which is quite insensitive sponse, i.e., G < G at all amplitude deformations at T to temperature (Figure 3). This is probably due to the < TK.This behavior is typical of concentrated dispersions cylindrical nature of the micelles, whose size is known to depend strongly on concentration and t e m p e r a t ~ r e . ~ ~ of rigid particle^.'^ Above TK,linear viscoelastic regions are detected (Figure 9)whose extension diminishes as Similar results were found by Ohlendorfet al. for a series concentration increases (Figure 10). It is noteworthy, that of alkyltrimethylammonium salicylates and for CTAT.2 G and G" are strain-independent in the whole strain range At low concentrations, CTAT micellar solutions are examined (0.1-200%) only for micellar solutions. Once transparent and have viscosities similar to that of water. that the hexagonal phase forms, and though the response At around 0.2 wt %, viscosity starts to rise dramatically, continues to be elastic (for T < T,), the extent of the strainalthough no elastic effects are observed. At concentrations independent region decreases with concentration up to around 0.9 wt %, a transparent, viscoelastic solution forms. the upper phase boundary (ca. 55%a t 25 "C),where, again, Evidently, as the concentration of surfactant is increased no linear viscoelastic regime is observed (Figure 10). above ct, the cylindrical micelles start to grow with a In oscillatory linear viscoelastic measurements, the concomitant increase in viscosity, and when the length of frequency a t which G and G crosses over is equal to the the micelles is long enough for entanglements, a gel-like reciprocal of the main relaxation time (or disentanglement viscoelastic material forms. time) of the system (t). Here, the temperature sweeps Above 1 wt % and up to 27 wt %, samples are were done at 10 rad/s in the linear viscoelastic regime. transparent, nonbirefringent, and viscoelastic at room Hence a t the temperature a t which G and G crosses over temperature-both the lower and the higher boundaries (T,) the main relaxation time (or disentanglement time) move to higher concentrations as temperatures is inof the system should be 0.1 s. The temperature T, shifts creased. These micellar solutions show strong streaming to lower values as CTAT concentration increases, i.e., the birefringence, which persists for a few seconds after the temperature interval (from TKto T,) where G dominates shear is stopped. Also the Weissenberg effect, which is becomes smaller (Figure 11).From the time-temperature produced by the presence of normal shear stresses,lg is superposition principle,lg one can deduce that the main detected. These phenomena suggest the formation of relaxation time at a given temperature should decrease shear-induced structures with a relatively slow relaxation as CTAT concentration increases. In fact, frequency time. However, a smaller homologue of CTAT, tetradecyltrimethylammonium salicylate, only forms shearsweeps a t constant temperatures presented elsewhere13J8 induced structures a t quite low concentrations (1- 10 show that within the micellar region, the main relaxation mM).35 Hence, it is expected that CTAT in water must time decreases steadily with concentration; also, in the also yield this kind of structure but a t concentrations hexagonal phase the main relaxation time is smaller than smaller than 1wt %. So, the streaming birefringence and those detected in micelles and stays fairly constant within the Weissenberg effect observed are probably the result the hexagonal one-phase region. This can explain why of orientation and alignment of the micelles under shear the linear viscoelastic region of the hexagonal phase is flow. smaller than those of the micellar solutions (Figure 10). lH-NMR spectra of these solutions (spectrum at 25 "C The latter, being quite labile, can re-form faster than the in Figure 6) are similar to those of viscoelastic micellar hexagonal phase when subjected to oscillatory strain solutions.22 The elastic nature of this isotropic micellar deformations. Hence, it can stand large oscillatory solution is evident by oscillatoryrheological measurements amplitude strainsand relaxwithin the experimental time. (Figure 11). In fact, at room temperature and up to 30On the other hand, the long-range force hexagonal phase can be disrupted with relatively smaller strains, and it (34) Israelachvili, J. N. Intermolecular & Surface Forces; Academic Press: New York, 1992. (35)Hoffmann, H.; Hofmann, S.;Rauscher, A.; Kalus, J. Prog. Colloid Polym. Sci. 1991,84, 24.
(36) Doraiswamy, D.; Mujundar,A. N.; Tsao, I.; Beris, A. N.; Danforth, S. C.; Metzer, A. B. J.Rheol. 1991,35,647.
Soltero et al.
3346 Langmuir, Vol. 11, No. 9, 1995 cannot re-form fast enough, within the scale of the experiment, to give a linear response.
Conclusions In summary, the phase behavior of the CTAT/water system was reported here as a function of temperature and the structures and structural changes were related with the rheological response of the system. At low concentrations and above the Kraf€t temperature (TK= 23 f 1"C), low viscosity Newtonian solutions of spherical micelles are found. Above ct, cylindrical micelles form, and the viscosity of the solutions increases 2 to 3 orders of magnitude as micelles grow but no elastic effects are observed until the elongated micelles are long enough to form entanglements. At this concentration (*0.9wt % a t 25 "C) and up to 27 wt %, transparent, viscoelastic gellike micellar solutions are present. These solutions are not birefringent but they exhibit streaming birefringence and the Weissenberg effect when subjected to shear. At higher concentrations and up to 47 wt %, a hexagonal phase appears at room temperature. This phase is also viscoelastic and demonstrates yield stress. The linear viscoelastic region spans up to 100% strain amplitude from the viscoelastic micellar solutions and diminishes once the hexagonal phase forms. In this region, the linear viscoelasticregime decreases rapidly with CTAT concentration, and it vanishes completely when a solid paste forms a t higher concentrations (ca. 60 wt % CTAT).
All phase boundaries move to higher CTAT concentrations with increasing temperature. Samples within the cylindrical micellar solution and the hexagonal phase region depict three regimes of viscoelasticity with temperture variations. In the first one, at temperatures below TK, no linearviscoelastic region was found and G < G . Above TK and up to T, (the temperature a t which the main relaxation time is equal to 0.1 s in this paper), elasticity dominates a t all concentrations. Finally, above T,, viscous behavior is observed again ( G> G I . T,shifts to lower temperatures as the concentration of CTAT increases up to 60 wt %, where the hexagonal phase disappears and a solid dispersion forms. These solid pastes are viscous and show almost no elasticity. The shift of T,to lower temperatures implies that the relaxation time in the micellar solutions decreases upon increasing CTAT concentration. This issue will be addressed in a following publication where the linear viscoelastic behavior of CTAT/water mixtures is examined.13
Acknowledgment. This work was supported by a Grant from the Consejo Nacional de Ciencia y Tecnologia de MBxico (Grant 0161-E). J.F.A.S.M. acknowledges the support of CONACyT. Technical support from Isabel ChAvez and Raul Montiel is acknowledged. LA940770M
