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Phase Diagrams of the Ternary Systems Sodium Dodecyl Sulfate/Poly(propylene glycol) (425 and 1000 Molecular Weight)/H2O. Study of the Physical Properties of the L Phase M. L. Sierra and E. Rodenas* Departamento de Quı´mica Fı´sica, Universidad de Alcala´ de Henares, 28871 Alcala´ de Henares, Madrid, Spain Received April 18, 1995. In Final Form: July 28, 1995X The phase diagrams for the SDS/PPG (MW ) 425 and 1000)/H2O ternary systems have been studied. The systems show an L phase and two liquid crystalline regions characterized as hexagonal and lamellar, respectively. The low viscous L phase, which is clear and isotropic, extends from the water-rich corner to the polymer-rich corner in both phase diagrams and is similar to the L phase in the CTAB/PPG (MW ) 425 and 1000)/H2O systems. Conductivity, steady-state quenching fluorescence, and rheology have been used to study the physical properties of the L phase; its influence on the basic hydrolysis of the crystal violet has been analyzed in order to elucidate the presence of a reverse micelles region at high PPG concentration.
Introduction The effect of neutral water-soluble polymers on the physical properties of micelles in aqueous solution has been widely studied,1 but few studies examine the effect of these polymers in ternary systems at high surfactant and polymer concentrations. Nagarajan has postulated a theoretical model to predict the effect of the polymer on a known surfactant structure, spherical and cylindrical micelles, vesicles, liquid crystals, and so on,2 and attempts have been made to study the location of the polymer chain in the crystalline structure.3 More attention has been paid to the biphasic regions which appear at low surfactant concentrations in the ternary surfactant/polymer/water systems, in order to model the interaction between the surfactant and the polymer.4,5 Most of the studies using poly(propylene glycol) (PPG) as a polymer have been carried out in aqueous solutions, at low surfactant and polymer concentrations,6,7 but from our results, obtained in the ternary system CTAB/PPG (MW ) 425 and 1000)/H2O at high CTAB and PPG concentrations,8 and in the region of the micelles in aqueous solutions,9 we conclude that PPG (MW ) 425 and 1000) behaves as a medium-chain alcohol.10 The
ternary system formed with the cationic surfactant CTAB and the low molecular weight polymer, PPG (MW ) 425 and 1000), showed an isotropic clear phase extending from the water-rich corner to the polymer-rich corner in the phase diagram at 25 ( 0.1 °C,8 that was similar to the phase obtained in the ternary systems CTAB/1-butanol/ H2O29 and CTAB/benzyl alcohol/H2O.12 No liquid crystal regions were found to the right of the L phase, because all the samples prepared with those mixtures crystallized at 25 ( 0.1 °C. This paper shows the phase diagrams of the ternary systems SDS/PPG/H2O, using two different molecular weights of PPG, MW ) 425 and 1000. Both phase diagrams show an isotropic clear phase with very low viscosity, the L phase, extending from the water-rich corner to the polymer-rich corner, and two liquid crystalline phases, characterized as hexagonal and lamellar regions. The physical properties of the L phase have been studied by conductivity, steady-state quenching fluorescence, and rheology, and the influence of the samples on the basic hydrolysis of the crystal violet has been analyzed in order to elucidate the existence of a reverse micelles region, L2, at high PPG contents. Experimental Section
* To whom correspondence should be addressed: fax, 34 1 885 47 63. X Abstract published in Advance ACS Abstracts, December 15, 1995. (1) Goddard, E. D. Colloids Surf. 1986, 19, 255-300. Nagarajan, R. Colloids Surf. 1985, 13, 1-17 and references therein. (2) Nagarajan, R. J. Phys. Chem. 1989, 90, 1980-1994. (3) Kekicheff, P.; Cabane, B.; Rawiso, M. J. Colloid Interface Sci. 1984, 102, 51-70. (4) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149-178. (5) Zhang, K.; Karlstro¨m, G.; Lindman, B. Prog. Colloid Polym. Sci. 1992, 88, 1-7. Zhang, K.; Karlstro¨m, G.; Lindman, B. J. Phys. Chem. 1994, 98, 4411-4421. (6) Breuer, M. M.; Robb, I. D. Chem. Ind. 1972, 530-535. Schwuger, M. J. J. Colloid Interface Sci. 1973, 43, 491-498. Witte, F. M.; Engberts, J. B. F. N. Colloids Surf. 1989, 36, 417-426. Brackman, J. C.; van Os, N. M.; Engberts, J. B. F. N. Langmuir 1988, 4, 1266-1269. Brackman, J. C.; Engberts, J. B. F. N. J. Colloid Interface Sci. 1989, 132, 250-255; J. Am. Chem. Soc. 1990, 112, 872-873; Langmuir 1991, 7, 2097-2102; Langmuir 1992, 8, 424-428. Anthony, O.; Zana, R. Langmuir 1994, 10, 4048-4052. (7) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 85-92. (8) Sierra, M. L.; Rodenas, E. Langmuir 1994, 10, 4440-4445. (9) Sierra, M. L.; Rodenas, E. J. Phys. Chem. 1993, 97, 12387-12392.
0743-7463/96/2412-0573$12.00/0
Sodium dodecyl sulfate (SDS) (Sigma), poly(propylene glycol) (PPG) (MW ) 425, 1000) (Aldrich), crystal violet (Merck), and 4-(1-pyrenyl)butyric acid (PBA) (Aldrich) were of the best grade available and were used without further purification. The quencher, N-cetylpyridinium chloride (CPyCl) (Merck), was recrystallized from methanol/diethyl ether mixtures. PBA is insoluble in water and an aqueous solution with [NaOH] ) 0.01 M was used. To delimit the two-phase region, the clear isotropic region, and the liquid crystal regions in the phase diagrams, samples with the different component contents were homogenized and left in a bath at 25 ( 0.1 °C, before being analyzed by simple visual observation. The samples in the liquid crystal regions were optically anisotropic as they presented optical birrefringence when observed under crossed polar lenses. All the samples were prepared with a difference of 5% (w/w) of PPG in both phase (10) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208-223. (11) Valiente, M.; Rodenas, E. Colloid Polym. Sci. 1993, 271, 494498. (12) Montalvo, G.; Valiente, M.; Rodenas, E. J. Colloid Interface Sci. 1995, 172, 494-501.
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diagrams. Once the liquid crystal regions were almost established, and in order to know their limits, new samples were prepared with a difference of ≈3% (w/w) in the SDS or PPG content, depending on the cases, at both PPG molecular weights. The optically anisotropic samples were observed with a Leitz Laborlux S microscope under crossed polarizers, coupled to a Yashica 108 Multiprogram camera with a Leitz Periplan 10×/18 TL 160 mm lens. The samples for the study of the L phase physical properties had a constant SDS content of 10% (w/w) and different PPG (MW ) 425 and 1000) and water contents. The specific conductivities of the isotropic solutions were measured with a Crison 525 conductometer. The solution flask containing the conductivity cell (cell constant 1.000 cm-1) was immersed in a water bath at 25 ( 0.1 °C. All the kinetics were run at 25 ( 0.1 °C in thermostated cuvettes in a Hewlett-Packard 8452-A diode array spectrophotometer. The reaction was followed at 590 nm and the initial crystal violet concentration was 2.67 × 10-5 M in all the experiments. The hydroxide concentration was always kept in a large excess with respect to the substrate concentration. Under these conditions the pseudo-first-order rate constants were determined with a program incorporated in the spectrophotometer using a Marquardt algorithm, with a standard deviation of
