Langmuir 1996, 12, 6443-6445
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Phase Behavior of Lecithin at the Oil/Water Interface Yu. A. Shchipunov* Institute of Chemistry, Far East Department, Russian Academy of Sciences, 690022 Vladivostok, Russia
P. Schmiedel Lehrstuhl Fu¨ r Physikalische Chemie I, Universita¨ t Bayreuth, Bayreuth, Germany Received January 25, 1996. In Final Form: June 20, 1996X In order to elucidate the mechanism for the formation of an interfacial film providing enhanced emulsion stability by lecithin, we performed dynamic contacting experiments in which water was brought into contact with a solution of the phospholipid in n-decane. Two main phenomena were observed in the vicinity of the interfacial boundary on the side of the nonaqueous phase. The first to be noted was the formation of a thick interfacial film, visible even to the naked eye. The second was the separation of a liquid phase. On the basis of the combined data from our current and previous experiments, we suggest the mechanism for interfacial processes of which the basis is the lecithin organogel formation. Water transferring from aqueous solution into the nonpolar phase through a hydration of the adsorbed phospholipid promotes a sequence of phase or pseudophase transitions near the phase boundary between immiscible liquids: spherical reverse micelles f three-dimensional network from entangled wormlike micelles f organogel separation into a diluted solution and a compact gel or solid mass precipitating on the interfacial boundary.
Introduction Lecithin is widely used as a natural emulsifying agent in the food industry, cosmetics, medicine, and biotechnology. Its effectiveness as an emulsion stabilizer is attributed to phospholipid self-assembly at the interfacial boundary between oil and water into thick films possessing liquid-crystalline organization. This structural arrangement has been concluded from studies on the emulsion droplet shells,1-7 phospholipid adsorption at the oil/water interface,8,9 and thinning of nonaqueous films into the bimolecular state.9-11 These works have mainly evidenced the presence of interfacial structures between oil and * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, November 1, 1996. (1) (a) Desnuelle, P.; Molines, J.; Dervichian, D. G. Bull. Soc. Chim. Fr. 1951, 18, 197. (b) Dervichian, D. G. Prog. Biophys. Mol. Biol. 1964, 14, 263. (2) (a) Friberg, S.; Mandell, L.; Larsson, M. J. Colloid Interface Sci. 1969, 29, 155. (b) Friberg, S.; Rydhag, L. Kolloid Z. Z. Polym. 1971, 244, 233. (c) Friberg, S.; Larsson, K. Adv. Liq. Cryst. 1976, 2, 173. (d) Rydhag, L.; Wilton, L. J. Am. Oil Chem. Soc. 1981, 58, 830. (e) Larsson, K. In Organized Solutions. Surfactants in Science and Technology; Friberg, S. E., Lindman, B., Eds.; Surfactant Science Series; Marcel Dekker: New York, 1992; Vol. 44, p 249. (3) (a) Ogino, K. J. Dispersion Sci. 1981, 2, 103. (b) Ogino, K.; Onishi, M. J. Colloid Interface Sci. 1981, 83, 18. (4) Davies, S. S.; Hansrani, P. J. Colloid Interface Sci. 1985, 108, 285. (5) Groves, M. J.; Wineberg, M.; Brain, A. R. P. Colloids Surf. 1985, 4, 237. (6) Pilpel, M. P.; Rabbani, M. E. J. Colloid Interface Sci. 1988, 112, 266. (7) (a) Bergenstahl, B. A.; Claesson, P. M. In Food Emulsions; Larrson, K., Friberg, S. E., Eds.; Marcel Dekker: New York, 1990; p 41. (b) Bergenstahl, B. A. In Food Polymers, Gels, and Colloids. Special Publication No. 82; Dickinson, E., Ed.; The Royal Society of Chemistry: London, 1991; p 123. (8) Shchipunov, Yu. A.; Kolpakov, A. F. Iz. Akad. Nauk SSSR, Ser. Fiz. 1991, 55, 1849. (9) (a) Shchipunov, Yu. A.; Kolpakov, A. F. Adv. Colloid Interface Sci. 1991, 35, 31. (b) Shchipunov, Yu. A. In Liquid-Liquid Interfaces: Theory and Methods; Deamer, D., Volkov, G., Eds.; CRC Press: Boca Raton, FL, 1996; Chapter 13. (10) Shchipunov, Yu. A.; Kolpakov, A. F. Kolloidn. Zh. 1988, 74, 1219. (11) (a) Kruglijakov, P. M.; Rovin, Yu. A.; Koretskii, L. F. In Surface Forces in Thin Films and Stability of Colloids; Derjaguin, B. V., Ed.; Nauka: Moscow, 1974; p 147. (b) Kruglijakov, P. M.; Rovin, Yu. A. Physico-Chemistry of Black Hydrocarbon Films; Nauka: Moscow, 1978.
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water, but the mechanisms of their formation have not been revealed. A clear case of the formation and transformation of interfacial structures is usually examined by means of dynamic contacting experiments in which two immiscible liquids containing mutually soluble substance(s) are brought into contact. In early studies, solvent mixtures were used. The redistribution of a solvent between contacting phases was observed.12 More recently, in place of one of the liquids, a micellar solution, a dispersion of the lamellar liquid crystals in water or a microemulsion, was used.13,14 The solubilization or, in other words, the extraction of a solvent with the contacting disperse phase brings about phase transition(s) in regions adjacent to the liquid/liquid interface. The transformation of interfacial structures correlates rather well with the phase diagrams and, in the event of complicated phase behavior, proceeds via intermediate phase(s).13,14 We have met a hidden case of the redistribution of mutually insoluble constituents between contacting solutions when embarking on a study of the mechanism of lecithin self-organization at the alkane/water interface. After contact of water with a micellar solution of lecithin, the formation of an interfacial film has been observed to be concomitant with the separation of a liquid in the nonaqueous phase. Examination of the phenomenon has led us to conclude that separation processes in the vicinity of the oil/water interface are promoted by transferring trace amounts of water into the nonpolar solution. In our opinion, a similar phenomenon with lecithin in the immiscible liquids system has not been so far described in the literature. Here we report the conditions at which the phase formation and transition take place at the alkane/water (12) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York and London, 1963; Chapter 7. (13) Miller, C. A.; Raney, K. H. Colloid Surf., A 1993, 74, 169 and references therein. (14) (a) Friberg, S.; Mortensen, M.; Neogi, P. Sep. Sci. Technol. 1985, 20, 285. (b) Neogi, P.; Kim, M.; Friberg, S. Sep. Sci. Technol. 1985, 20, 613. (c) Ma, Z.; Friberg, S.; Neogi, P. Colloid Surf. 1988, 33, 249.
© 1996 American Chemical Society
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Figure 1. Schematic diagram showing a setup used for the observation of interfacial processes in the oil/water system.
interface in the presence of lecithin, describe the phenomenon features, and suggest a mechanism for interfacial processes. Experimental Section Soybean lecithin Epicuron 200 from Lucas Meyer (Germany) was used without additional purification. The double-distilled water was obtained in a common manner. n-Decane was purum grade (Fluka). Nonaqueous solutions contained 1% w/v lecithin, and aqueous solutions contained 0.1 M sodium nitrate or potassium chloride (both pro analysis grade, Merck). A nonaqueous solution equal in volume to the aqueous solution was carefully layered on top of the latter placed before in a rectangular spectrophotometer cell of 10 mm thickness. Figure 1 shows a diagram of the experimental setup. The region of contact between oil and water was viewed with transmitted light from a halogen lamp through crossed or partially crossed polarizers positioned on each of opposite sides of the cell. A water heat filter was placed before the light source. Interfacial processes were observed using a video camera equipped with a long focal length lens and recorded on a videotape. The densities (ρ) of solutions were determined by means of a densimeter DMA-40 (Anton Paar K.G., Austria). The difference in measured ρ values between various sets of solutions did not exceed 0.0003 g/cm3. To minimize the error in the density determination, we measured the temperature dependence of the density from ambient temperature to 60 °C.
Results and Discussion Phase-forming processes developing after contact of a nonaqueous solution of lecithin with water are obvious from a succession of photographs in Figure 2. Two main phenomena should be recognized. (i) Formation of an Interfacial Film between the Immiscible Liquids. With time a film grows in thickness and becomes visible even to the naked eye (Figure 2A). It possesses a complex composition; there are nontranspar-
Shchipunov and Schmiedel
ent solid particles and isotropic and anisotropic areas. A visible film forms at a high rate. Thus, at a lecithin concentration of 10 g/L, it revealed itself within minutes after the immiscible liquids were brought into contact. (ii) Separation of the Liquid Phase in the Nonaqueous Solution. In the initial stage, there is a thin layer that forms over the above-considered interfacial film on the side of the nonaqueous solution. The film and layer increase concurrently in thickness, but the latter increases more quickly. With time the layer surface facing the nonaqueous solution develops a wavelike appearance. This can be seen in Figure 2B. At a certain moment, one of the crests bursts at the upper point, which causes a flow of the separated liquid phase to the oil surface (Figure 2C). Near the air/oil interface the flow spreads along it, taking on a form which looks like a “mushroom” (Figure 2D and E). The experimental observation illustrated by the sequence of pictures in Figure 2 suggests that the separated phase has a density less than that of the initial nonaqueous solution. To provide support for this conclusion, we measured the densities of various solution samples that were taken from the upper, middle, and lower layers by using a cell designed especially for this purpose. The “lower” sample was isolated from a region as close as possible to the interfacial boundary between oil and water. Since it could include solid particles from the interfacial film, the sample was passed through a filter (Spartan 30/B; Schleicher & Schuell) with a pore diameter of 0.45 µm. A preliminary test showed that the filtration did not have an effect on the density of the initial phospholipid solution. The temperature dependencies of the densities of various solution samples are represented in Figure 3. A point to be noted is that no difference between the initial nonaqueous solution and a solution isolated from the middle of the nonaqueous phase is observed. On the other hand, samples from the upper and lower layers differ in density from them. They both are less dense than the initial solution. Their densities are approximately the same as that of the n-decane used for dissolving the lecithin. The results presented in Figures 2 and 3 might be interpreted as a separation of a diluted solution due to rearrangement of phospholipid structures near the oil/ water interface. In support of this interpretation we can argue that the separated liquid phase disappears gradu-
Figure 2. Photographs showing a minor part of an aqueous solution at the bottom and a nonaqueous solution above. (A) Taken with completely crossed polarizers within 5 min after contact of a 1% w/v lecithin solution with a 0.1 M KCl aqueous solution. Light colored spots at the bottom are anizotropic and light-scattering regions of the interfacial film located between water and the nonpolar phase. (B-E) Taken in succession withibn 3.0, 3.5, 3.6, and 3.8 min, respectively. A distinct liquid separating in the nonaqueous phase could be observed only with partially crossed polarizers. Such an illumination made the interfacial film hidden by a dark band at the bottom. The upper edge of the pictures corresponds to the position that is just below the surface of the nonaqueous solution. The bar indicates 1 mm.
Behavior of Lecithin at the Oil/Water Interface
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Figure 4. Schematic drawings representing the phase and pseudophase transitions in a 1% w/v lecithin solution after addition of water. The threshold concentrations at which the corresponding transitions occur are shown on the axis in accordance with ref 18. Figure 3. Temperature dependence of the density of n-decane, the initial nonaqueous solution, and various samples of the nonaqueous solution taken within 5 min after contact of the 1% w/v lecithin solution with a 0.1 M KCl aqueous solution.
ally. This means that the solvent dissolves into its own solution at a slow rate. The most simple experiment to test the rate of miscibility is to place an alkane drop on the bottom of the cell filled with a nonaqueous phospholipid solution alone. On doing so, we have observed that n-decane has not actually been dissolved fast. Moreover, there has been a flow of the solvent from the bottom to the surface, resembling the flow pattern in Figure 2B-E. Thus, the experimental observations suggest the separated liquid phase to be a diluted solution. This raises the question as to which phase transition(s) near the interfacial boundary is/are responsible for the established phenomenon. An essential feature of lecithin is its high hygroscopicity, which is retained after dissolving in nonpolar solvents.15 Water has the tendency of transferring into a nonaqueous phospholipid solution as they are brought into contact with each other.9,16 In this connection it is good to bear in mind that even trace amounts of polar solvents induce phase or pseudophase transitions. Initially there is a sphere-to-rod transformation of reverse lecithin micelles and then their extensive one-dimensional growth resulting in the formation of a jelly-like phase made up of a threedimensional network of entangled wormlike micellar aggregates.17,18 With further increasing water content, the role of water is changed from that of a thickening agent into that of a coagulator. At a particular threshold concentration of water, there is a separation of the homogeneous organogel into a highly viscous mass and a diluted nonviscous solution.17,18 Lecithin micelles and phase or pseudophase transitions induced by water (15) Elworthy, P. H.; McIntosh, D. S. J. Phys. Chem. 1964, 68, 3448. (16) Sokolova, A. E.; Schagina, A. V.; Malev, V. V.; Gracheva, D. A. Bioorgan. Khim. 1976, 2, 611. (17) (a) Scartazzini, R.; Luisi, P. L. J. Phys. Chem. 1988, 92, 829. (b) Schurtenberger, P.; Scartazzini, R.; Magid, L. J.; Leser, M. B.; Luisi, P. L. J. Phys. Chem. 1990, 94, 3695. (c) Schurtenberger, P.; Scartazzini, R.; Luisi, P. L. Rheol. Acta 1989, 28, 372. (d) Schurtenberger, P. Chimia 1994, 48, 72. (18) (a) Shchipunov, Yu. A. Colloid J. 1995, 57, 556. (b) Shchipunov, Yu. A.; Shumilina, E. V. Colloid J. 1996, 58, 123. (c) Shchipunov, Yu. A.; Shumilina, E. V. Mater. Sci. Eng., C 1995, 3, 43.
addition into a nonaqueous solution are shown in the schematic drawings in Figure 4. More comprehensive data on the lecithin organogel formation and the properties depending on the nature of the polar additives can be found in our previous publications.18 It is worthy of mention that the phase or pseudophase transitions occur in a narrow range of the water concentration (see Figure 4). The organogel begins forming at about one H2O molecule per phospholipid molecule. The lecithin jelly-like phase incorporates up to 90% of the organic solvent that is trapped between entangled wormlike micelles self-organizing into a three-dimensional network. Even at a concentration that somewhat exceeds two water molecules per lecithin molecule, most of the solvent is released, owing to the organogel coagulation. We can say with reasonable confidence that the micelle shape change and attendant processes should take place as soon as the lecithin solution and water are brought into contact. The present study has provided reliable evidence for the formation of corresponding products, that is, a compact gel mass and solid precipitate on the oil/ water interface and a diluted nonaqueous solution in the nonpolar phase (Figure 2). On the basis of the combined data from the current and previous work, we may suggest the following mechanism for interfacial processes. The schematic drawings in Figure 4 can serve as an illustration. When brought into contact, water transfers into the lecithin solution through the hydration of adsorbed phospholipid. If hydrated lecithin molecules are included in aggregates, they trigger the micelle shape change. The micelle transformation should be accompanied by jellification of nonaqueous solution layers adjacent to the interfacial boundary wherein water can penetrate. When the critical concentration of polar solvent is attained, the organogel begins separating into a compact gel or solid precipitate and a diluted nonaqueous solution. The phase separation terminates the processes. A precipitated phase every so often tends to slow down the velocity of the interfacial processes (see, e.g., ref 14). Acknowledgment. We thank Prof. H. Hoffmann for interest in this study and for useful discussions. Financial support from the Sonderforschungsbereich 213 “Topomac” for the stay of one of us (Y.A.S.) at Bayreuth is gratefully acknowledged. LA960082Y
