Langmuir 2000, 16, 5879-5883
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Mass Transport Phenomena in Oil-in-Water Emulsions Containing Surfactant Micelles: Solubilization Jochen Weiss† and D. Julian McClements*,‡ Department of Food Science and Technology, University of Tennessee, Knoxville, Tennessee 37901-1071, and Department of Food Science, University of Massachusetts, Amherst, Massachusetts 01003 Received November 10, 1999. In Final Form: April 4, 2000 The influence of surfactant type and concentration on the solubilization of n-hydrocarbon emulsion droplets in aqueous solutions of nonionic surfactant micelles was investigated. Solubilization was monitored by measuring the decrease in droplet concentration with time after n-tetradecane droplets were suspended in surfactant solutions (Tween 20, Tween 40, Tween 60, Tween 80, Triton X-100, Triton SP-190, and Triton SP-175). The kinetics and extent of solubilization increased with surfactant concentration. The solubilization characteristics also depended on surfactant structure, with the interfacial mass transport coefficient and specific solubilization capacity of the micelles increasing as the HLB number moved from 17 to 12. This study shows that surfactant type and concentration have a pronounced influence on the kinetics and extent of solubilization in micellar solutions.
Introduction Emulsions consist of one liquid dispersed in the form of small, spherical droplets in another immiscible liquid.1 A wide variety of commercially important materials exist in an emulsified state, including agrochemicals, cosmetics, petrochemicals, pharmaceuticals, and foods.2-7 Emulsions are thermodynamically unstable systems that tend to break down over time due to a variety of physicochemical mechanisms, e.g., gravitational separation, flocculation, coalescence, and Ostwald ripening. The development of effective strategies to retard these changes relies on a thorough understanding of the origin and nature of the various instability mechanisms. Small molecule surfactants are usually added to emulsions to enhance their formation and stability.1,8 These molecules adsorb to the surface of emulsion droplets during homogenization and provide a protective membrane that prevents the droplets from flocculating or coalescing. Under certain circumstances, surfactants may have a negative impact on emulsion stability because of their ability to form micelles that enhance mass transport processes, such as solubilization, Ostwald ripening, and compositional ripening.9 These mass transport processes * Corresponding author. † University of Tennessee. ‡ University of Massachusetts. (1) Hunter, R. J. Foundations of Colloid Science; Oxford Press: Oxford, U.K., 1986; Vol. 1. (2) Smith, A. L. Theory and Practice of Emulsion Technology; Academic Press: New York, 1976. (3) Becher, P. Encyclopedia of Emulsion Science. Volume II: Applications; Marcel Dekker: New York, 1986. (4) Becher, P. Encyclopedia of Emulsions Science. Volume III: Basic Theory; Marcel Dekker: New York, 1988. (5) Schramm, L. L. Suspensions: Fundamentals and Applications in the Petroleum Industry; American Chemical Society: Washington, D.C., 1996. (6) Schramm, L. L. Emulsions: Fundamentals and Applications in the Petroleum Industry; American Chemical Society: Washington, D.C., 1992. (7) McClements, D. J. Food Emulsions - Principles, Practice and Techniques; CRC Press: Boca Raton, FL, 1999. (8) Hiemenz, P. C.; Rajagalopan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997. (9) Dickinson, E.; McClements, D. J. Advances in Food Colloids; Blackie Academic & Professional: Glasgow, U.K., 1996.
can cause significant changes in droplet concentration, composition, and size distribution and may therefore adversely influence the bulk physiochemical properties of an emulsion, such as appearance, rheology, and stability.7,10-12 Solubilization involves the movement of oil molecules from emulsion droplets to the surrounding aqueous medium.13-21 Ostwald ripening involves the movement of oil molecules from smaller droplets to larger droplets.10,22-25 Compositional ripening involves the exchange of oil molecules between emulsion droplets with different compositions.10,11,26,27 The thermodynamic driving force for each of these mechanisms is the difference in chemical potential of the oil molecules between the different environments. There is considerable debate about the molecular basis of the influence of surfactant micelles on mass transport processes in emulsions. In the absence of micelles, mass transport occurs by the movement of individual oil molecules into and through the aqueous phase separating (10) Pertzov, A. V.; Kabalnov, A. S.; Kumacheva, E. E.; Amelina, E. A. Colloid J. 1988, 50, 616. (11) McClements, D. J.; Dungan, S. R. J. Phys. Chem. 1993, 97, 7304. (12) McClements, D. J.; Dungan, S. R.; German, J. B.; Kinsella, J. E. Food Hydrocolloids 1992, 6, 415. (13) McBain, M. E. L.; Hutchinson, E. Solubilization and Related Phenomena; Academic Press: New York, 1955. (14) MacKay, R. A. In Solubilization; Schick, M. J., Ed.; Marcel Dekker: New York, 1987. (15) Weiss, J.; Coupland, J. N.; McClements, D. J. J. Phys. Chem. 1996, 100, 1066. (16) Weiss, J.; Coupland, J. N.; Brathwaite, D.; McClements, D. J. Colloids Surf., A 1997, 121, 53. (17) Miller, C. A.; Raney, K. H. Colloids Surf., A 1993, 74, 169. (18) Chen, B. H.; Miller, C. A.; Garrett, P. R. Colloids Surf., A 1997, 128, 129. (19) Aveyard, R.; Binks, B. P.; Clark, S.; Fletcher, P. D. I. Chem. Technol. Biotechnol. 1990, 48, 161. (20) Carroll, B. J. J. Colloid Interface Sci. 1981, 79, 126. (21) Kabalnov, A. Curr. Opin. Colloid Interface Sci. 1998, 3, 270. (22) Wagner, C. Z. Elektrochem. 1961, 65, 581. (23) Lifshitz, I. M.; Slyozov, R. J. Phys. Chem. Solids 1961, 19, 35. (24) Kabalnov, A. S. Langmuir 1994, 10, 680. (25) De Smet, Y.; Deriemaeker, L.; Finsy, R. Langmuir 1999, 15, 6745. (26) Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S. Langmuir 1998, 14, 5402. (27) Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S. Langmuir 1999, 15, 4495.
10.1021/la9914763 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/08/2000
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Langmuir, Vol. 16, No. 14, 2000
Weiss and McClements
Figure 1. Initial droplet size distributions of n-tetradecane emulsions stabilized by different surfactant types.
different environments.24 Many studies have shown that surfactant micelles enhance the rates of Ostwald and compositional ripening in emulsions, although the origin of this effect is still unclear.24-27 Some researchers have proposed that mass transport in the presence of micelles still involves the movement of individual oil molecules through the aqueous phase but that the micelles increase the water solubility of the oil, which enhances the transport rate.25 Other researchers have proposed that mass transport is facilitated because surfactant micelles solubilize oil molecules in their hydrophobic interior and transport them across the aqueous phase separating the droplets.11,26,27 Whether the oil molecules are taken up by the micelles directly from the aqueous phase surrounding the droplets24 or by the fusion/fission of a micelle with a droplet surface26,27 is still uncertain. The overall objective of this study is to carry out experiments that will provide further insight into the effect of surfactant micelles on mass transport processes in emulsions. In this paper, we examine the effect of surfactant type and concentration on the solubilization of n-hydrocarbon emulsion droplets in aqueous micellar solutions. In a subsequent paper, we intend to use the surfactant solubilization characteristics to understand the role of surfactant micelles in Ostwald ripening. Materials and Methods Materials. Polyoxyethylene (20) sorbitan monolaureate (Tween 20), polyoxyethylene (20) sorbitan monopalmitate (Tween 40), polyoxyethylene (20) sorbitan monostearate (Tween 60), polyoxyethylene (20) monooleate (Tween 80), polyoxyethylene (10) isooctylphenyl ether (Triton X-100), and n-tetradecane (>99% pure) were purchased from Sigma Chemical Company (St. Louis, MO). Triton SP-175 (degree of ethoxylation per mole, 7.5) and Triton SP-190 (degree of ethoxylation per mole, 9.0) were obtained from Union Carbide Corporation (Danbury, CT). Distilled and deionized water was used in the preparation of all solutions and emulsions. These surfactants were studied because they had a range of different HLB numbers. Sample Preparation. A surfactant solution was prepared by dissolving 2 wt % surfactant in 98% distilled and deionized water. A mixture of 10 wt % n-tetradecane and 90 wt % surfactant solution was then homogenized in a high-speed blender (Waring Product Division, New Hartford, CT) to form a coarse emulsion premix. This premix was further homogenized using a singlevalve high-pressure laboratory homogenizer (APV Limited, West Sussex, U.K.) at 100 MPa until a mean droplet diameter of ∼250300 nm was achieved. The droplet size distribution of all of the emulsions used in the study is shown in Figure 1. Each emulsion was diluted into micellar surfactant solutions (csurf ) 0.2-5 wt %) to give a range of droplet concentrations
Figure 2. Transmittance (at wavelength of 632 nm) of n-tetradecane emulsions with different droplet concentrations and mean droplet radii. (c0 ) 0.025-0.5 wt %). All solutions were stored at 25 °C ((2 °C), and samples were withdrawn at regular intervals for analyses of droplet concentration and size. Droplet Size Measurements. A static light-scattering technique (Horiba LA-900, Horiba Instruments, Inc., Irving, CA) was used to measure the droplet size distribution of diluted emulsions. The technique uses Mie theory to calculate the droplet size distribution from measured light intensity at various scattering angles.28 A relative refractive index of 1.08 (refractive index of the droplets/refractive index of the surrounding phase) was used in the calculations. Each emulsion was diluted with distilled water prior to measurement to give a concentration of
