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Self-Depletion Flocculation of Tetralin Oil-in-Water Emulsions Alex M. Djerdjev, Robert J. Hunter, and James K. Beattie* School of Chemistry, UniVersity of Sydney, NSW 2006, Australia ReceiVed July 6, 2005. In Final Form: September 19, 2005 Oil-in-water emulsions of slightly soluble oils such as tetralin prepared by high-pressure homogenization and stabilized by sodium dodecyl sulfate undergo depletion flocculation induced by an initially polydisperse droplet size distribution. The smaller droplets flocculate the larger ones; the flocculation can be reversed by gentle sonication. After aging, the flocs disappear because the smaller droplets dissolve through Ostwald ripening. These effects were observed by electroacoustic measurements, supplemented by light scattering.
Introduction Emulsions are thermodynamically unstable. The droplets can grow by coalescence or by Ostwald ripening of a polydisperse population through the slight solubility of the dispersed phase in the continuous medium. The spatial distribution of an emulsion can alter by creaming, sedimentation, or flocculation. The latter can be effected by a bridging agent, such as a polymer, or by a depletion mechanism due to nonadsorbing polymers, particles, or micelles.1 Depletion flocculation is an entropic effect that occurs because the center of mass of the depleting agent cannot approach the emulsion droplet more closely than its effective radius. Hence, there is a shell of volume around the droplet that is not accessible to the depleting agent, reducing its entropy. If the total depletion volume of the droplets is reduced by the overlap of their depletion zones, the excluded volume is decreased, and the volume accessible to the depleting agent is increased, increasing its entropy. Hence, there appears to be an attractive force between the droplets, leading to their flocculation and, ultimately, to phase separation. The force holding the floc together is weak, however, and the system can be readily redispersed. Depletion flocculation of oil-in-water emulsions has been effected by SDS-micelles,2,3 nonionic micelles,4-8 biopolymers such as hydroxyethyl cellulose,9,10 xanthan,11 and proteins.12-16 None of these depletion agents is present in the system under study here, yet we still observe what we believe to be depletion flocculation. In this case, it is caused by the smaller droplets of * Corresponding author. E-mail:
[email protected]. (1) McClements, D. J. Food Emulsions; CRC Press: Boca Raton, FL, 1999. (2) Bibette, J. J. Colloid Interface Sci. 1991, 147, 474. (3) Bibette, J.; Roux, D.; Nallet, F. Phys. ReV. Lett. 1990, 65, 2470. (4) Fairhurst, D.; Aronson, M. P.; Gum, M. L.; Goddard, E. D. Colloids Surf. 1983, 7, 153. (5) Aronson, M. P. Langmuir 1989, 5, 494. (6) Aronson, M. P. Colloids Surf. 1991, 58, 195. (7) McClements, D. J. Colloids Surf. A 1994, 90, 25. (8) Shields, M.; Ellis, R.; Saunders, B. R. Colloids Surf. A 2001, 178, 265. (9) Manoj, P.; Fillery-Travis, A. J.; Watson, A. D.; Hibberd, D. J.; Robins, M. M. J. Colloid Interface Sci. 1998, 207, 283. (10) Manoj, P.; Watson, A. D.; Hibberd, D. J.; Fillery-Travis, A. J.; Robins, M. M. J. Colloid Interface Sci. 1998, 207, 294. (11) Chanamai, R.; Herrmann, N.; McClements, D. J. J. Colloid Interface Sci. 1998, 204, 268. (12) Hemar, Y.; Pinder, D. N.; Hunter, R. J.; Singh, H.; He´braud, P.; Horne, D. S. J. Colloid Interface Sci. 2003, 264, 502. (13) Dickinson, E.; Radford, S. J.; Golding, M. Food Hydrocolloids 2003, 17, 211. (14) Berli, C. L. A.; Quemada, D.; Parker, A. Colloids Surf. A 2002, 203, 11. (15) Radford, S. J.; Dickinson, E. Colloids Surf. A 2004, 238, 71. (16) Blijdenstein, T. B. J.; Veerman, C.; van der Linden, E. Langmuir 2004, 20, 4881.
the polydisperse population flocculating the larger ones, until the smaller ones disappear by Ostwald ripening, leading to spontaneous redispersion of the floc. In this study, tetralin was chosen as a suitable oil for its relatively high solubility in water (40 ppm), making Ostwald ripening fast; its relatively high density (0.966), close to that of water, preventing significant creaming; and its high boiling point (208 °C), preventing significant evaporation. The effects described appear to be general, however, for oils of moderate solubility in water; we have observed similar behavior with emulsions of hexane (10 ppm), toluene (515 ppm), and p-cymene (isopropyltoluene) (350 ppm). In contrast, oils of low solubility in water (95%), sodium dodecyl sulfate (SDS) from Sigma (∼99% GC), AR-grade NaCl from Ajax Chemicals (99%), and ultrapure Milli-Q water from a Millipore system. Emulsion Preparation and Measurement. Emulsions were prepared at a concentration of 5 vol % in aqueous solutions of SDS/ NaCl (5.0 mM/1.0 mM). The oil was emulsified at 25 °C by passing the mixture through a homogenizer (Milko-tester Mark III F3140, A/S N. Foss Electric, Denmark) 15 times in rapid succession. ESA Measurements. Electroacoustic measurements were performed on the emulsion immediately after preparation by injecting the sample via a syringe into the ESA cell of a prototype of the AcoustoSizer-II (Colloidal Dynamics Inc., Warwick, RI). The dynamic mobility of the emulsions was measured at 25 °C and was observed as a function of time over the course of several hours. To examine the reversibility of the flocs, the flocculated emulsion was removed from the cell and sonicated (Unisonics Pty Ltd., Australia, 50 Hz) for 30 s and then returned to the cell and remeasured. After about 3 h, when there was little further change in the ESA signal, the sample was removed and stored for 1 day. The sample was then remeasured to observe any changes in the ESA signal. In another experiment, a tetralin emulsion was prepared and immediately diluted with SDS/NaCl, (5/1 mM) to 2 or 1 vol % and measured as a function of time. Dynamic Light Scattering. The droplet size distribution of a 5 vol % tetralin emulsion was measured by light backscattering with a Malvern High Performance Particle Sizer immediately after preparation and then as a function of time.
Results When a 5 vol % tetralin emulsion was prepared in 5 mM SDS and 1 mM NaCl and injected into the AcoustoSizer cell, the dynamic mobility spectra showed dramatic changes over the
10.1021/la0518041 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/23/2005
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Figure 1. Change in the dynamic mobility with time for a 5 vol % tetralin emulsion stabilized with SDS/NaCl (5/1 mM).
course of 1 h (Figure 1). The initial mobility, measured about 5 min after preparation by injection into the AcoustoSizer cell, showed the normal decrease in magnitude with frequency, with an argument that was progressively more negative with increasing frequency. Over 30 min, the magnitudes decreased and displayed a minimum with frequency, and the arguments decreased significantly at lower frequencies and showed large positive phase angles at higher frequencies. We now recognize that these are features of the dynamic mobility spectra of flocculated emulsions.17,18 When the sample was removed and then remeasured after gentle sonication, the unusual phase angles were removed, and the magnitudes were partly restored to the initial values (Figure 1). On aging once more, the same trend was repeated; the arguments became more positive at higher frequencies, until, after ∼2 h, a steady mobility was obtained. After being allowed to stand for 1 day, the sample was redispersed and remeasured. The mobility showed very little time dependence (Figure 2). The magnitudes had decreased somewhat, and the arguments were more negative, indicative of larger droplets. (The time dependence at higher frequencies might indicate some instability of the larger emulsion droplets, which tend to adsorb to the AcoustoSizer electrodes.) Dynamic light scattering measurements were also made on the 5 vol % emulsions. Initially, some minutes after preparation, three distinct peaks could be seen: a population of droplets
