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Aqueous-Phase Behavior and Cubic Phase-Containing Emulsions in the C12E2-Water System Matthew L. Lynch,* Kelly A. Kochvar, Janet L. Burns, and Robert G. Laughlin The Procter & Gamble Company, Corporate Research Division, Miami Valley Laboratories, 11810 East Miami River Road, Route 27, Ross, Ohio 45061 Received October 15, 1999. In Final Form: January 3, 2000 The aqueous-phase behavior of C12E2 was examined using the diffusive interfacial transport-near infrared method (DIT-NIR), a new isothermal swelling method for assembling accurate and precise phase diagrams. The system exhibits a large number of liquid-crystal phases over a remarkably small temperature and composition window including a sponge phase, two bicontinuous cubic phases, and a lamellar phase. The phase behavior of the system is similar to Class II polar lipids, such as monoglycerides, excluding the liquid-liquid miscibility gap and the sponge phase associated with ethoxylated surfactants. The data collected by the DIT-NIR method provide a marked improvement to the currently accepted phase diagram. Finally, temperature steps can create temperature-induced cubic-phase-containing emulsions in which pyramidal-shaped L1 droplets are dispersed within the V2(1) bicontinuous cubic-phase continuum, reflecting the epitaxy of the cubic-phase lattice.
Introduction Ethoxylated-alcohol surfactants are widely used for emulsification and cleaning.1 Unlike ionic surfactants, they are generally compatible with other surfactants and insensitive to electrolyte, and their solubility decreases with increasing temperature.2 These surfactants can exhibit a large variety of lyotropic liquid-crystal phases including sponge, bicontinuous cubic, hexagonal, and lamellar phases. They can also exhibit a liquid-liquid miscibility gap at low surfactant concentration characterized in part by the lower consolute boundary, or the cloud point.3 C12E2 has recently been used to modify the structure of model biological membranes.4,5 The conformation and hydration of the ethylene oxide (EO) chain in C12E2 has been studied by computational methods.6,7 In general, the behavior of a particular C12EX surfactant depends on the hydrophilic-hydrophobic balance of the molecule which is governed by the ethylene oxide and the aliphatic chain lengths, respectively. Long EO chain length compounds (e.g., C12E6, C12E8) exhibit traditional surfactant-like behavior.8 There are lamellar and hexagonal liquid-crystal phases which occupy large temperature-composition regions, and the liquid-liquid miscibility gap is clearly visible at relatively high temperatures. At the other extreme, short EO chain length compounds (e.g., C12E1) have very low solubility in water and behave as amphiphilic oils exhibiting neither liquid-crystal formation nor the liquid-liquid miscibility gap.9 With an intermedi* To whom correspondence should be addressed. Tel.: (513)627-0392. Fax: (513)527-1233. E-mail:
[email protected]. (1) Laughlin, R. G. The Aqueous Behavior of Surfactants; Academic Press: London, 1994. (2) Johsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: New York, 1998. (3) Nonionic Surfactants Physical Chemistry; Shick, M. J., Ed.; Marcel Dekker: New York, 1987. (4) Funari, S. S. Eur Biophys J. Biophy. 1998, 27 (6), 590-594. (5) Gutberlet T.; Dietrich, U.; Klose G.; Rapp G. J. Colloid. Interface Sci. 1998, 203 (2), 317-327. (6) Banhyopadhyay, S.; Tarek, M.; Lynch, M. L.; Klein, M. L. Langmuir 2000, 16, 942-946. (7) Kong, Y. C.; Nicholson, D.; Parsonage, N. G.; Thompson, L. J. Chem. Soc., Faraday Trans. 1994, 90 (16), 2375-2380. (8) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975.
ate EO chain length (e.g., C12E2, C12E3), the liquid-liquid miscibility gap and the lamellar liquid crystal region “interfere”, creating a rich variety of liquid crystals that exist within a remarkably small temperature-composition region, including the formation of the sponge phase. The diffusive interfacial transport (DIT) method was first reported by Laughlin et al.10 with the NIR version of this experiment reported later11 and recently validated.12 The method is a controlled swelling experiment in which surfactant and water are brought into contact and allowed to swell within a small-dimensioned capillary. A continuum of composition is created along the length of the capillary, which is interrupted by compositional discontinuities. The discontinuities are interfaces which separate the phase bands. Although the composition within the phase bands and the position of the interfaces change with time, the composition at the interface is invariant and equal to the thermodynamic compositional limit of the phase.13 Near-infrared microscopy is used to measure the water composition at the interface to define the phase boundaries at each temperature. Similar lyotropic swelling experiments have been done with X-ray diffraction14,15 but these experiments do not offer quantitative information. Experimental Section C12E2 Preparation and Storage. Because C12E2 readily undergoes autoxidation, special purification and storage procedures were developed and utilized. Reasonable quantities of “unrefined” C12E2 were purchased from Fluka (catalog no. 32268) or were fabricated by internal synthesis (reaction of 1-bromododecane with diethylene glycol under basic conditions). The assay of commercially supplied C12E2 was found by gas chro(9) Laughlin, R. G. In Micelles, Microemulsions and Monolayers Science and Technology; Mercel Dekkar, Inc.: New York, 1998; pp 73-99. (10) Laughlin, R. G.; Munyon, R. L. J. Phys. Chem. 1987, 91, 32993305. (11) Marcott, C.; Laughlin, R. G.; Sommer, A. J.; Katon, J. E. In FTIR Spectroscopy in Colloid and Interface Science; Scheuing, D. R., Ed.: ACS Symposium Series; The American Chemical Society: Washington, D.C., 1991; pp 71-86. (12) Laughlin, R. G.; Marcott, C.; Lynch, M. L.; Kochvar, K. A. J. Phys. Chem. B., submitted. (13) Laughlin, R. G. In The Aqueous Behavior of Surfactants; Academic Press: London, 1994; pp 92-97. (14) Caffrey, M. Biophys. J. 1987, 51, 444a. (15) Kekicheff, P.; Cabane, B. J. Phys. 1987, 48, 1571-1583.
10.1021/la991366w CCC: $19.00 © 2000 American Chemical Society Published on Web 03/09/2000
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matographic analysis (silylated with BTSFA in methylene chloride) to be typically 96-97%. This level of impurities is unacceptable for definitive physical studies because impurities affect the phase behavior.16 Fortunately, most autoxidation products can be removed by recrystallization in pentane. A 15% C12E2-pentane solution is cooled to -10 °C in a jacketed closedpressure filter to precipitate the surfactant, filtered, washed with cold pentane to remove the dissolved impurities, dissolved in pentane at room temperature to recover the surfactant, and dried to constant weight in a vacuum in a rotary evaporator to remove the remaining pentane. After two recrystallizations, GC assay of the C12E2 was typically >99.9%. The purity of the C12E2 was confirmed before each phase study. Once purified, it is necessary to protect C12E2 because its high purity strongly enhances its reactivity to autoxidation. Samples were promptly subdivided into ≈100-mg aliquots which were transferred to 2-cm3 glass ampules. Rubber tubing was used to attach the ampules to a glass manifold, and the samples were degassed in a vacuum using several freeze(dry ice/acetone bath)/ thaw cycles. Afterward, the vials were sealed. Properly deoxygenated and sealed, C12E2 was found to be stable indefinitely at room temperature. Attempts to degas samples simply by evacuating the vials for a period of time and then sealing failed to remove the oxygen and significant oxidation occurred during subsequent storage. Unused portions of opened ampules were best stored in a vacuum desiccator at pressures
