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Langmuir 1999, 15, 2246-2249
Surfactant Phase Transition Inducing Coalescence in Dense Emulsions B. Deminiere, T. Stora, A. Colin,* F. Leal-Calderon, and J. Bibette Centre de Recherche Paul PascalsCNRS, Av. Dr. Schweitzer, 33600 Pessac, France Received September 17, 1998. In Final Form: January 13, 1999 We study the behavior of concentrated monodisperse oil-in-water emulsions stabilized by nonionic surfactants. With an increase of temperature, the surfactant undergoes a diphasic phase transition and simultaneously coalescence occurs. Surprisingly, the coalescence transforms the initial monodisperse emulsion into a new monodisperse one made of bigger droplets, indicating that this instability disappears after a short period of time. We discuss the origin of this process in terms of dewetting phenomena induced by the surfactant phase transition, and we present a model that accounts for the number of coalescence events.
Metastable cellular material made out of two different phases, one being dispersed into the other, such as dense emulsion, biliquid or air-liquid foam, off-critical decomposing mixture have a natural tendency to rearrange or coarsen in time. With appropriate surface-active species, these dispersions can be sufficiently stable to make them very attractive for many applications ranging from cosmetics to coatings, and from food to medicines. In all these domains, the applicability of emulsion requires the control of the destabilizing phenomena. The destruction of these metastable materials may proceed through two distinct mechanisms: Ostwald ripening and coalescence. Ostwald ripening is a well understood phenomenon.1-4 Because of the difference in Laplace pressure between small and big droplets, the dispersed phase diffuses from the small to the big droplets through the continuous phase. The other mechanism, coalescence, involves the breaking of the thin liquid film that separates two droplets.5-8 The thin film that forms when two droplets are contacting is a metastable molecular assembly, and its lifetime will be a key factor in determining the lifetime of the bulk dispersion. Understanding the thin film metastability and breaking at a microscopic level should significantly consolidate the fundamental knowledge of emulsions and further allow explanation of the persistence of such metastable materials including foams and biological bilayers. Usually, coalescence is viewed as a thermally activated process governed by the nucleation of a tiny molecular sized hole in the films that further grows under the action of surface tension. In some particular cases, coalescence may be induced by mechanical instabilities.9 As an example, antifoams are usually droplets dispersed in foaming solutions that emerge into the air-water interfaces. The soap films become unstable and break when their thickness is comparable to the size of the particles and when the contact angle between the liquid * Corresponding author. (1) Ostwald, W. Z. Phys. Chem. 1901, 37, 385. (2) Lifshitz, I. M.; Slyozov, V. V. Soviet Physics JETP 1959, 35, 331. (3) Kabalnov, A. S.; Pertzov, A. V.; Shchukin, E. D. J. Colloid Interface Sci. 1987, 118, 590. Durian, D. J.; Weitz, D. A.; Pine, D. J. Science 1991, 252, 686. (4) Sagui, C.; Desai, R. C. Phys. Rev. Lett. 1995, 74, 1119. (5) Chernomordik, L. V.; Kozlov, M. M.; Melikyan, G. B.; Abidor, I. G.; Markin, V. S.; Chizmadzhev, Y. A. Biochim. Biophys. Acta 1985, 812, 643. (6) De Gennes, private communication. (7) Bergeron, V. Langmuir 1997, 13, 3474. (8) Kabalnov, A.; Wennerstro¨m, H. Langmuir 1996, 12, 1931. (9) Garrett, P. R. J. Colloid Interface Sci. 1979, 69, 107.
and the droplets measured through the liquid phase is larger than 90°.10 This condition corresponds to a positive value of the so-called bridging coefficient defined as: B ) γaw2 + γpw2 - γpa2 where γaw, γpw, and γpa are the surface tensions between air and water, antifoam particles and water, and antifoam particles and air, respectively. In this Letter, we produce concentrated monodisperse oil-in-water emulsions stabilized by nonionic surfactants and we observe their evolution under the effect of coalescence which occurs when temperature is increased. Surprisingly, this coalescence transforms the initial monodisperse emulsion into a new monodisperse one indicating that coalescence is arrested after a short period of time. We discuss the origin of this process in terms of dewetting phenomena induced by the surfactant phase transition, and we present a model that accounts for the droplet growth. A crude silicone-in-water emulsion was prepared by slowly adding 90 g of silicone oil to an emulsifier solution (5 g of Lauropal 205/5 g of water) and by gently shearing the mixture. Silicone oil (Rhodorsil 47 V10000) was obtained from Rhoˆne-Poulenc Co., and Lauropal 205 (a mixture of ethoxylated nonionic surfactants C10E5C12E5) was obtained from Witco. This first step leads to an emulsion with a large size distribution. We then prepare monodisperse emulsions by using a fractionated crystallization method.11 Following this method, we are able to produce monodisperse samples with a characteristic diameter that may vary from 0.2 to 2 µm. We then wash these emulsions to set the surfactant concentration in the continuous phase to 0.06% by weight and the oil volume fraction φ between 70% and 95%. These monodisperse samples do not exhibit any coarsening at room temperature for more than two years but become unstable as soon as the temperature is sufficiently raised. The phase diagram of Lauropal 205 in water as a function of the temperature is presented in Figure 1. When temperature is increased, we observe a succession of different diphasic equilibria. The interfacial surface tensions were measured between the coexisting surfactant phases and between silicone oil and each of these phases by the drop weight method using Fagan’s corrections.12 (10) Garrett, P. R. Defoaming: Theory and industrial application; Surfactant Sciences Series 45; Marcel Dekker: New York, 1993; Chapter 1. (11) Bibette, J. J. Colloid Interface Sci. 1991, 147, 474. (12) Tate, T. Philos. Mag. 1864, 27, 176. Harkins, W. D.; Brown F. E. J. Am. Chem. Soc. 1919, 41, 499. Fagan, M. E. Thesis Berkeley 1988.
10.1021/la9812774 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/09/1999
Letters
Langmuir, Vol. 15, No. 7, 1999 2247
Figure 1. Phase diagram of Lauropal 205 in water as a function of temperature: L1, dilute direct micellar phase; L1′, concentrated direct micellar phase; L2, inverted micellar phase; L3, sponge phase; LR, Lamellar phase. Table 1 temp (°C)
γo1 (mN/m)
γ12 (mN/m)
γo2 (mN/m)
B
65 75
4 ( 0.2 5.7 ( 0.2
0.1 ( 0.1 0.1 ( 0.1
4.5 ( 0.2 4 ( 0.2
