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Langmuir 2004, 20, 46-52
Coarsening of Alkane-in-Water Emulsions Stabilized by Nonionic Poly(oxyethylene) Surfactants: The Role of Molecular Permeation and Coalescence V. Schmitt,* C. Cattelet, and F. Leal-Calderon† Centre de Recherche Paul Pascal, Av. Schweitzer, 33600 Pessac, France, and Laboratoire des Milieux Disperse´ s Alimentaires, ISTAB, Av. des Faculte´ s, 33405 Talence, France Received May 2, 2003. In Final Form: October 8, 2003 We produce different alkane-in-water concentrated emulsions stabilized by the same nonionic surfactant, and we follow their kinetic evolution by granulometry. The size distribution becomes remarkably narrow during the first stages of coarsening and progressively turns to a wide function as time passes. We get evidence that the size evolution occurs under the effect of molecular permeation and coalescence. A second hydrophobic species of large molecular size is dissolved in the dispersed phase. This latter is expected to inhibit the permeation mechanism, and coalescence should act alone. Surprisingly, coalescence is also suppressed, even at a very low concentration of the second component (∼1% w/w). We vary the chemical nature and concentration of the second species, and we propose a simple mechanism to explain the stabilizing effect with respect to coalescence.
I. Introduction Emulsions are metastable colloids made out of two immiscible fluids, one being dispersed in the other in the presence of surface-active agents. From diluted to highly concentrated, emulsions exhibit very different internal dynamics and mechanical properties for various commercial applications.1 The lifetime of emulsions may vary considerably from one system to another; it can change from minutes to many years, depending on the nature of the surfactants, the nature of both phases, and their volume ratio. Despite the large amount of work devoted to this issue, predicting the destruction scenario and the emulsion lifetime still remain challenging questions, especially in concentrated emulsions. Irreversible coarsening of emulsions may proceed through two distinct mechanisms. The first mechanism, known as Ostwald ripening, is driven by the difference in Laplace pressure between droplets having different radii: the dispersed phase is transferred from the smaller to the larger droplets. The rate of droplet growth may be determined by the molecular diffusion across the continuous phase or by the permeation across the surfactant films. Diffusion-controlled ripening has been recognized in submicrometer diluted emulsions stabilized by ionic surfactants;2-5 so far, permeationcontrolled ripening has been proposed to account for the coarsening of concentrated air foams.6,7 The second mechanism, known as coalescence, consists of the rupture of the thin film that forms between droplets, leading them * Corresponding author. Centre de Recherche Paul Pascal, Av. Schweitzer, 33600 Pessac, France. Tel.: 33 5 56 84 56 46. Fax: 33 5 56 84 56 00. E-mail:
[email protected]. † ISTAB. (1) Becher, P. Encyclopedia of Emulsion Technology; Marcel Dekker: New York, 1985. (2) Lifshitz, I. M.; Slyozov, V. V. Sov. Phys. JETP 1959, 35, 331 (3) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35 (4) Kabalnov, A. S.; Pertzov, A. V.; Shchukin, E. D. J. Colloid Interface Sci. 1987, 118, 590. (5) Kabalnov, A. S.; Makarov, K. N.; Pertzov, A. V.; Shchukin, E. D. J. Colloid Interface Sci. 1990, 138, 98. (6) Durian, D. J.; Weitz, D. A.; Pine, D. J. Science 1991, 252, 686. (7) Durian, D. J.; Weitz, D. A.; Pine, D. J. Phys. Rev. A 1991, 44, R7902.
to fuse into a single one. At a microscopic scale, a coalescence event proceeds through the nucleation of a thermally activated hole, which reaches a critical size above which it becomes unstable and grows. The energy cost Ea for reaching the critical hole radius will be termed as the hole activation energy. In principle, understanding the metastability of emulsions should require two types of information that account for two distinct phenomena. The first one concerns the microscopic mechanism of the instabilities. The second one concerns the scenario of destruction, that is, the time and space distribution of the coarsening events. Generally, the destruction scenario of emulsions results from the interplay between coalescence and Ostwald ripening. For the sake of simplicity, most of the studies have been performed in conditions such that one type of instability is dominating the other one, enabling one to monitor its progress quite precisely. In this limit, theoretical models as well as experiments have revealed that Ostwald ripening generates emulsions with narrow size distributions in the asymptotic regime.2-8 In contrast, coalescence favors the diverging growth of large droplets at the expense of the smaller ones.9 In concentrated systems, neighboring cells are in permanent contact and larger cells will grow faster because they exhibit a larger surface contact area with their neighbors.10 As a result of their different consequences on the droplet evolution and distribution, coalescence and Ostwald ripening can be easily identified: a system evolving under the effect of Ostwald ripening alone exhibits a narrow size distribution with a droplet growth rate that progressively vanishes; instead, coalescence events have the effect of increasing the polydispersity and accelerating the rate of coarsening because of the divergent growth of large nuclei. In this paper, we examine the destruction of emulsions in the limit of high droplet volume fraction (φ > 75%) and large diameters (>1 µm). That way, we expect to observe (8) Wagner, C. Z. Elektrochem. 1961, 65, 581. (9) Bibette, J.; Morse, D. C.; Witten, T. A.; Weitz, D. A. Phys. Rev. Lett. 1992, 69, 2439. (10) Hasmy, A.; Paredes, R.; Sonnneville-Aubrun, O.; Cabane, B.; Botet, R. Phys. Rev. Lett. 1999, 82, 3368.
10.1021/la034747p CCC: $27.50 © 2004 American Chemical Society Published on Web 12/10/2003
Coarsening of Alkane-in-Water Emulsions
both the instabilities and to determine the conditions (in terms of droplet size) for the predominance of one type of instability over the other one. We get evidence that the coarsening kinetics is controlled by Ostwald ripening, followed by coalescence, as the droplets become larger. The rate of Ostwald ripening is large compared to the theoretical predictions, and we propose that the transient hole formation in the films increases the overall mass flux/ripening rate. Finally, we study the impact of a highmolecular-weight component dissolved in the dispersed phase. II. Experimental Section 1. Emulsion Preparation. For the oil phase, we used different alkanes: heptane, octane, nonane, decane, dodecane, and hexadecane, all of them purchased from ACROS (puriss grade). They were used as received without any further purification. To stabilize the emulsions, the employed surfactant was Ifralan D 0205 from IFraChem. It consists of a mixture of pentaethylene glycol mono n-dodecyl ether, C12E5, and pentaethylene glycol mono n-decyl ether, C10E5. The interfacial tension between the alkanes and water was measured by means of a drop spinning tensiometer (Kru¨ss apparatus) at 20 °C, in the presence of Ifralan D 0205 initially dissolved in the water phase (5% w/w). The emulsions were fabricated according to the following procedure. The initial step consists of preparing a crude polydisperse emulsion, the so-called premixed emulsion. It is obtained by incorporating the dispersed phase under gentle manual stirring. A typical premixed emulsion is comprised of 90% oil, 2% surfactant, and 8% water in weight. A Couette-type mixer consisting of two concentric cylinders was used to fragment the premixed emulsion to reduce the average diameter and to obtain a lower degree of polydispersity.11,12 After emulsification, the emulsions are diluted with pure water to fix the oil volume fraction at 78%. In the first set of experiments, we fabricate emulsions with approximately the same initial average diameter and different pure alkane oils from heptane, denoted C7, to hexadecane, denoted C16, all of them stabilized by Ifralan D 0205. They are stored at room temperature (20 °C), and a small amount of them is taken at regular time intervals to measure the droplet size distribution. 2. Emulsion Characterization. We used a static light scattering device (Malvern Mastersizer granulometer) to measure the size distribution of the emulsions. The collected scattered intensity as a function of the angle is transformed into the size distribution using the Mie theory (the refractive index of each alkane was measured by means of an Abbe´ refractometer). The mean droplet size in volume D[4,3] is defined as
∑N D
4
∑N D
3
i
i
i
D[4,3] )
i
i
i
Where Ni is the total number of droplets with diameter Di. The polydispersity of the emulsion is characterized by a parameter termed as “uniformity” and defined as
U)
1 D h
∑N D i
3
i
|D h - Di|
i
∑N D i
3
i
i
Where D h is the median diameter, that is, the diameter for which the cumulative undersized volume fraction is equal to 50%. The granulometric measurements were qualitatively checked by (11) Mabille, C.; Schmitt, V.; Gorria, P.; Leal Calderon, F.; Faye, V.; Deminie`re, B. Langmuir 2000, 16, 422. (12) Mabille, C.; Leal Calderon, F.; Bibette, J.; Schmitt, V. Europhys. Lett 2003, 61, 708.
Langmuir, Vol. 20, No. 1, 2004 47
Figure 1. Time evolution of the volume-averaged D[4,3] diameter for emulsions made of 78 vol % of different alkanes dispersed in an aqueous phase. The surfactant concentration is about 6.5 wt % with respect to the aqueous phase. 9, Heptane; 0, octane; 2, nonane; 4, decane; b, dodecane; and O, hexadecane.
Figure 2. Size distribution of a hexadecane-in-water emulsion right after preparation (0) and after 5 (+) and 11 (4) months of storage. observing the emulsions with a phase contrast optical microscope (Zeiss, Axiovert ×100).
III. Results and Discussion 1. Pure alkanes. In Figure 1, the temporal evolution of D[4,3] is reported for the different systems. The hexadecane (C16)-in-water emulsion does not exhibit any appreciable evolution over a period of several months. The average diameter is not changing, and the size distribution remains the same over the observation period (Figure 2). In contrast, the heptane (C7)-in-water emulsion is being destroyed very rapidly: after a few hours, the average diameter becomes 2 times larger than the initial one, and after 1 week of storage, a macroscopic oil layer appears at the top of the sample. The values that are reported in Figure 1 were all obtained well before the appearance of the macroscopic layer. Between the two previously mentioned emulsions, the rate of evolution is clearly correlated to the number of carbon atoms in the hydrocarbon chain: the shorter the alkane, the faster the evolution. In other words, the kinetic stability of alkanein-water emulsions increases with the molecular length of the hydrocarbon chain. For all the alkanes, the size distributions follow the same qualitative evolution. Figure 3a corresponds to the distribution measured right after the fragmentation in the Couette cell for the C8-in-water emulsion. The initial uniformity is on the order of 40%. As time passes, U decreases down to around 15%, a low value indicating the formation of an emulsion with a high degree of monodispersity (Figure 3b). At the later time point, the distribution becomes broad again and we can observe a new mode at large diameters (around 10 µm, Figure 3c). Both the intensity and width of this mode regularly increase during the storage period until the occurrence of the oil layer. The tendency revealed by Figure 3 is in perfect agreement with the observations under the microscope. One can notice in the image of Figure 4 that droplets much larger than the average are present after 24 h of storage, therefore confirming the existence of the mode
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Figure 3. Size distributions of an octane-in-water emulsion (a) just after preparation and (b) after 4, (c) 6, and (d) 23.5 h of storage.
that was detected by means of our commercial granulometer (Figure 3c,d). In Figure 5, we report the evolutions of D[4,3] and U as a function of time for each alkane separately. The time interval where the uniformity index U is lower than 20% has been shaded. For all the alkanes under study (except C16), the shaded zone roughly covers the same size interval, from D1 ≈ 1.5 µm to D2 ≈ 2.5 µm. The growth rate is continuously decreasing, which suggests that Ostwald ripening is the rate-determining mechanism in that interval. Note that theoretical calculations predict that the uniformity is lower than 20% for both diffusionand permeation-controlled ripening, in the asymptotic regime.2,3,8 According to the definition given in section II-2, we find U ) 10.4% for diffusion-controlled ripening and U ) 15.7% for permeation-controlled ripening. It is probable that the same mechanism is operative when the average diameter is smaller than D1, but the asymptotic regime is not yet achieved because of the large polydispersity of our initial emulsions. For D[4,3] > D2, the uniformity index continuously increases and the diameter growth becomes divergent: this evolution is consistent
Schmitt et al.
Figure 4. Microscopic images of an octane-in-water emulsion obtained after (top) 4 h of storage (same sample as that in Figure 3b) and (bottom) 23.5 h of storage (same sample as that in Figure 3d).
with a coalescence-driven mechanism. In the particular case of C16, the Ostwald ripening is so slow that the asymptotic regime was never reached within the time scale of the observation period (11 months). On the basis of the previous data, it can be argued that, for all the systems under study, the coarsening process is determined by both Ostwald ripening and coalescence. It is only the time scale of these two limiting mechanisms that continuously grows as the hydrocarbon chain length increases from C7 to C16. a. Analysis of the First Regime. The so-called “first regime” refers to the homogeneous growth observed in the shaded zones of Figure 5. We discuss here its possible origins and its dependence with respect to the hydrocarbon chain length. The impact of alkane molecular weight on Ostwald ripening has been extensively explored in the limit of highly diluted emulsions.5,13,14 The experimental data were most often interpreted within the frame of the diffusion-controlled model developed by Lifshitz and (13) Weiss, J.; Coupland, J. N.; Brathwaite, D.; McClements, D. J. Colloids Surf., A 1997, 121, 53. (14) Weiss, J.; Herrmann, N.; McClements, D. J. Langmuir 1999, 15, 6652.
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Figure 5. Time evolution of (9) the mean diameter D[4,3] and (0) the uniformity (see text for details) for the different alkanes: (a) heptane, (b) octane, (c) nonane, (d) decane, (e) dodecane, and (f) hexadecane. Table 1. Comparison of the Theoretical Ωth and Experimental Ωexp Rates of Ostwald Ripeninga heptane octane nonane decane dodecane hexadecane
Ωth (m3‚s-1)
Ωexp (m3‚s-1)
Λ (m2‚s-1)
2.00 × 10-24 5.17 × 10-25 1.48 × 10-25 3.58 × 10-26 2.56 × 10-27 1.12 × 10-29
3.5 × 10-22 7.6 × 10-23 2.3 × 10-23 1.2 × 10-23 8.9 × 10-25
1.0 × 10-16 2.1 × 10-17 6.9 × 10-18 4.0 × 10-18 2.8 × 10-19
a The permeation rate Λ has also been reported (see text for more details).
Slyozov.2,3 Such a model predicts that, in the asymptotic regime, the rate of ripening Ω is given by
Ω)
64γDiffSVm d (Dc)3 ) dt 9RT
(1)
where γ is the interfacial tension between the dispersed and continuous phases and R is the molar gas constant. S, Vm, and Diff are respectively the bulk solubility, the molar volume, and the molecular diffusion coefficient of the dispersed phase. Dc is the critical diameter, which will be assumed to be equal to the volume averaged diameter D[4,3]. In Table 1 and Figure 7a, we report the experimental values of Ωexp obtained in the regime where the droplet uniformity is lower than 20%. For the measurement of Ωexp, (D[4,3])3 was plotted as a function of time (Figure 6) in the asymptotic regime (shaded zone of Figure 5). For the sake of comparison, the theoretical rate Ωth has been reported in the same table. Ωth was calculated considering the solubility of the alkanes in pure water (the contribution of micelles to the total solubilization is not considered here). From relation 1, a dramatic dependence of the growth rates on the hydrocarbon chain length is expected, essentially reflecting the critical water solubility dependence. Molecular solubilities of hydro-
carbons in water were taken from ref 15. The diffusion coefficients were estimated according to the HaydukLaudie equation.16 It can be noticed that, even though Ωth and Ωexp follow the same qualitative evolution, the measured values are more than 100 times larger than the theoretical ones. We propose here some possible explanations to justify the origin of the discrepancy. (i) Equation 1 is in principle valid in the limit of very dilute emulsions. In general, it is expected that emulsions with higher volume fractions of the disperse phase will have broader particle size distributions and faster absolute growth rates than those predicted by the Lifshitz and Slyozov model.17-20 The theoretical rate of ripening should, therefore, be corrected by a factor f(φ) that reflects the dependence of the coarsening rate on the dispersed phase volume fraction φ. Following the mean field argument from Lemlich,19 we assume that the diffusion occurs over a characteristic distance on the order of the film thickness h, which can be evaluated to be h ) Dc(1 - φ)/6φ. The function f(φ) in the concentrated regime (φ > 60%) should, therefore, vary as 4.7φ/(1 - φ). For our systems with φ ) 78%, we find f(φ) ≈ 17, which brings a considerable but insufficient correction to account for the discrepancy. (ii) The transport across the continuous phase could be mediated by nonionic surfactant micelles that are swollen by the oil molecules. Micellar transport should involve at least two different steps: (I) the transfer of oil between the micelles and the emulsion drops and (II) the diffusion of the micelles across the continuous phase Step I can in principle be ruled out because the swollen micelles are stable objects at their equilibrium spontane(15) McAuliffe, C. J. Phys. Chem. 1966, 70, 1267. (16) Hayduk, W.; Laudie, H. AIChE J. 1974, 20, 611. (17) Enamoto, Y.; Kawasaki, K.; Tokuyama, M. Acta Metall. 1987, 35, 907. (18) Tokuyama, M.; Kawasaki, K. Physica A 1984, 123, 386. (19) Lemlich, R. Ind. Eng. Chem. Fundam. 1978, 17, 89. (20) Patzek, T. W. AIChE J. 1993, 39, 1697.
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Schmitt et al.
Figure 7. Experimental rates of (a) diffusion-controlled Ωexp and (b) permeation-controlled Λ Ostwald ripening as a function of the alkane chain length (deduced from Figure 6).
Figure 6. Time evolution of (a) the cube and (b) the square of D[4,3] corresponding to the shaded zone of Figure 5 for different alkanes: 9, heptane; 0, octane; 2, nonane; and 4, decane. Because of the very different time scale, the evolution of the cube ([) and the square (]) of D[4,3] for dodecane has been reported in part c. For each curve, the correlation coefficient r corresponding to the linear fit has been reported.
ous curvature.21 However, we could admit that some volume fluctuations are authorized in the micelles, especially if their elastic modulus of average curvature is small. If oil is exchanged between micelles and drops, the rate of Ostwald ripening should be proportional to the amount of micelles present in the continuous phase. However, we observe in Figure 8 that the evolution rate is roughly the same for three different surfactant concentrations (namely, 6.5, 10, and 20%). To see whether step II could be rate-determining, we measured the effective solubilities S of C8 and C16 in a water phase containing 6.5 wt % Ifralan D 0205. S essentially reflects the solubilizing capacity of the micelles and was determined by turbidity: if the alkane concentration exceeds S, the water phase becomes highly turbid because of the formation of large alkane droplets in water. We found S8 ) (3.5 ( 0.2) × 10-2 mol‚L-1 for C8 and S16 ) (5.4 ( 0.2) × 10-2 mol‚L-1 for C16. The effective solubilities are quite large, and the theoretical ripening rates calculated with these values (eq 1) are 103-108 higher than those observed experimentally. Moreover, C16 exhibits a negligible coarsening rate, even though it is more soluble within the surfactant micelles than C8. Again, we get convincing evidence that the transport of matter across the continuous phase mediated by micelles is not the rate-determining mechanism in our system.
Figure 8. Time evolution of the volume-averaged diameter D[4,3] as a function of time for an octane-in-water emulsion with varied surfactant concentrations: (0) 6.5, (O) 10, and (4) 20 wt % with respect to the aqueous phase.
From the previous considerations, we deduce that the transfer of oil from smaller drops to the larger ones must occur through the direct contact of the droplets: the initial homogeneous growth is most probably because of the permeation of oil molecules across the surfactant films. Because coalescence occurs in our emulsions, it is likely that reversible, thermally activated holes are continuously formed in the thin films separating the drops. The characteristic size of the reversible holes may allow the passage of oil molecules from smaller to larger droplets, and this could explain the origin of the permeation process. Molecular permeation was proposed by Taisne and Cabane21 to account for the growth of alkane droplets stabilized by the nonionic surfactant C12E5. This mechanism was identically suggested by Pays et al.22 to explain the leakage of encapsulated species in water-oil-water double emulsions. In this limit, we expect
d(Dc)2 )Λ dt
(2)
Λ being a phenomenological constant that accounts for all the microscopic parameters involved in the permeation (21) Taisne, L.; Cabane, B. Langmuir 1998, 14, 4744. (22) Pays, K.; Giermanska-Kahn, J.; Pouligny, P.; Bibette, J.; LealCalderon, F. Phys. Rev. Lett. 2001, 87, 178304.
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Figure 9. Scheme of the hole nucleation mechanism.
process.6-8,23 Within the experimental uncertainty, eq 2 correctly accounts for the evolution of the diameter as a function of time (see Figure 6b). In Table 1 and in Figure 7b, we report the experimental slopes Λ deduced from the quadratic scaling. It is clear that larger alkanes reduce permeation because Λ decreases over three decades between C7 and C12. b. Analysis of the Second Regime. The second regime is the one following the shaded zones in Figure 5 (righthanded side). In presence of short alkanes (C7, C8, C9, and C10), the mode appearing at large diameters and the formation of macroscopic oil layers give convincing evidence that coalescence takes place and strongly influences the evolution rate (at least at long times; Figures 3 and 5). The impact of the alkane molecular weight on coalescence is qualitatively the same as that for Ostwald ripening: smaller alkanes coalesce more rapidly. Recently, a mechanistic explanation for the coalescence mechanism was proposed, according to which the surfactant packing type (spontaneous curvature) affects both the phase behavior of microemulsions and the stability of emulsions. Kabalnov and Wennerstro¨m24 argue that the effect of the spontaneous coalescence energy curvature on emulsion stability comes from the kinetics of the hole nucleation in emulsion films. Consider an oil film separating two oil droplets (Figure 9). The hydrophilic surfactant covering the droplets forms a monolayer that can be regarded as an elastic membrane with a positive spontaneous curvature (oil-in-water shape). The propagation of a hole is damped, because the monolayer at the edge of the nucleation hole is frustrated because it is curved against the direction favored by the spontaneous curvature. Because of this, for the film rupture to occur, the system must pass through an energy barrier Ea, after which the growth becomes spontaneous. This state can be reached only by a thermal fluctuation and has a low probability because of the unfavorable spontaneous curvature. From our experiments, we learn that the rate of coalescence depends on the hydrocarbon chain length of the dispersed phase. We believe that this dependence is at least partially related to the spontaneous curvature of the surfactant monolayers. We suggest that the longer chains, like hexadecane, can hardly penetrate the surfactant brush, covering the surfaces, and, therefore, the natural spontaneous curvature is quite elevated, thus stabilizing the direct films against hole nucleation. Instead, shorter oil chains such as octane can more easily penetrate and swell the surfactant brush, providing a less positive average curvature, which allows rapid formation of holes in the oil-water-oil films. The same mechanism can be proposed to explain the qualitative evolution of the coalescence rate and of the permeation parameter Λ with the alkane molecular weight. 2. Influence of a Solute with Large Molecular Size. a. Experimental Results. With the aim of suppressing one source of instability, we dissolved a second hydrophobic (23) Marworth, A. J. J. Colloid Interface Sci. 1985, 107, 569. (24) Kabalnov, A. S.; Wennerstro¨m, H. Langmuir 1996, 12, 276.
Figure 10. Time evolution of the mean diameter D[4,3] of an emulsion made of octane + 2 wt % hexadecane with two different initial sizes (b and 9) and an emulsion made of octane + 3 wt % silicone oil (2).
component in the dispersed phase. The solute was chosen so that it possesses a large molecular size and is waterinsoluble: that way, we hope to inhibit any transfer of this molecule across the continuous phase or through any permeation mechanism. We can, thus, focus exclusively on the second mechanism of instability: coalescence. Higuchi and Misra25 were the first to show that, if one of the components of a dispersed phase is completely insoluble in the continuous one, then even small amounts of such a substance may stop diffusion-controlled Ostwald ripening in the system. The condition to be fulfilled is that the osmotic pressure of the trapped species overcomes the average Laplace pressure of the droplets.26 Assuming that the solvent-solute mixture in the droplets is ideal, the previous conditions can be written as
φ s g φ s* )
4γMs RTFsD[4,3]
(3)
where φs, Ms, and Fs are respectively the weight fraction, the molar mass, and the density of the solute. The same condition holds if, instead of diffusion, the coarsening is due to molecular permeation as long as the solute cannot cross the surfactant films separating the drops. The reason is that both mechanisms result from the same microscopic driving force: the difference in Laplace pressure between drops having different radii. The interfacial tension between octane and water in the presence of Ifralan D 0205 was found to be 0.75 × 10-3 N/m. In the following set of experiments, we systematically dissolved an amount of solute larger than that predicted by relation 3. That way, we can ensure that Ostwald ripening is totally halted right after the emulsion fragmentation (provided the solute is trapped in the droplets). We fabricated two different octane-in-water emulsions with the same initial average diameter around 1 µm, all of them stabilized by Ifralan D 0205. A water-insoluble and large species is dissolved in octane prior to the emulsification process. One emulsion contains 2% hexadecane (Ms ) 226 g‚mol-1, Fs ) 0.77 g‚cm-3, φs* ) 0.48%), and the other contains 3% silicone oil (Ms ∼ 1500 g‚mol-1, Fs ) 0.93 g‚cm-3, φs* ∼ 2.6%). In Figure 10, we report the values of D[4,3] measured at different time intervals after the emulsion preparation: the diameter absolutely does not vary in time even after 1 month of storage. The same conclusion holds after 6 months of storage (data not reported in Figure 10). We can conclude that the presence of the second oil component has the expected inhibiting effect with respect to permeation-controlled ripening. It is, however, surprising that coalescence also does not take place. To favor the occurrence of coalescence, we fabricated another emulsion containing 2% hexadecane and an initial diameter close (25) Higuchi, W. I.; Misra, J. J. Pharm. Sci. 1962, 51, 459. (26) Webster, A. J.; Cates, M. E. Langmuir 1998, 14, 2068.
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Figure 11. Time evolution of the averaged diameter of octanein-water emulsions with different amounts of hexadecane. The solid lines are only guides for the eyes.
to 3.5 µm. Even in the presence of larger drops, coalescence did not occur, as can be deduced from Figure 10. It is, therefore, clear that the presence of the solute stops the coalescence process and that the effect is independent of the chemical nature of the trapped species because we were able to reproduce it with two different solute molecules. To explore the transition between short- and long-term kinetic stability, we varied the concentration φs of the solute (hexadecane) between 0 and 2%. All the emulsions initially possess the same diameter, but their evolutions reveal significant differences (Figure 11). Above 0.5% hexadecane, the initial emulsion persists, with no apparent sign of destabilization. Below 0.5%, the average diameter progressively raises and the phenomenology is roughly the same as that already described for pure octane. As time passes, the distribution becomes narrower than the initial one. After a sufficient period of storage, the size histogram is broad again because of the presence of large drops. Ostwald ripening is certainly the rate-determining mechanism at short times. This is confirmed by the initial shrinkage of the distribution function and by the fact that the growth rate continuously decreases. It is worth noting that the rate of coarsening regularly decreases with φs. Coalescence negligibly perturbs the molecular permeation process during the first hours. However, the fast evolution (Figure 11) and the presence of large drops in the samples with low C16 content are due to coalescence. The diameter measurements were interrupted after 300 h, but the samples were kept in storage conditions for a longer period of time. We observed that, after 3 months of storage, all the emulsions containing less than 0.5% hexadecane exhibited a significant oil release. On the contrary, no sign of coalescence was detected after the same period for the emulsions containing more than 0.5% hexadecane. The hexadecane concentration of 0.5% can, therefore, be regarded as a threshold value to ensure long-term stability with respect to coalescence. b. Discussion: Origin of the Stabilizing Effect of the Solute. Our experimental results are quite similar to those reported by Aronson and Petko.27 These authors observed that the dissolution of electrolytes in the aqueous phase of inverted water-in-oil emulsions has a remarkable stabilizing effect. In the case of inverted emulsions, the interfacial films are comprised of surfactant molecules with their polar head immersed into the dispersed phase and the tails oriented toward the continuous phase. A shell of water molecules is strongly attached to the surfactant head as a result of hydrogen bonds or strong dipolar interactions. The effective head volume and, thus, the spontaneous curvature of the films are strongly connected to the hydration degree of the surfactant. To (27) Aronson, M. P.; Petko, M. F. J. Colloid Interface Sci. 1993, 159, 134.
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explain the stabilizing role of electrolytes, it is generally admitted that the salt ions play the role of an osmotic pump that dehydrates the surfactant (this is the so-called “salting out” effect): as the effective head size is reduced, the spontaneous curvature becomes more negative, resulting in enhanced stabilization against coalescence. Moreover, reducing the effective head size allows reaching a higher degree of compaction of the surfactant molecules at the droplet interface, which further stabilizes the films against coalescence. We believe that the same type of mechanism is operative in direct emulsions: the trapped species (hexadecane or silicone oil) is likely to produce a desolvation of the surfactant brushes covering the drops. Indeed, because the molecules that were probed are large compared to the characteristic size of the surfactant tail (10 methyl groups on average), they are at least partially excluded from the brush. Conversely, small alkanes such as heptane or octane can swell the brush, but the entropy of mixing favors their migration toward the oil phase in the presence of such large molecules. This entropy-driven desolvation reduces the effective volume of the hydrophobic tails, and at the same time, increases the positive spontaneous curvature of the surfactant monolayer. Following the Kabalnov and Wennerstro¨m theory,24 this should result in a reduction of the coalescence frequency in direct films, as observed experimentally. IV. Conclusion In this paper, we have explored the stability of oil-inwater emulsions stabilized by nonionic poly(oxyethylene) surfactants. The coarsening involves two different regimes that could have the same microscopic origin: hole nucleation in the thin liquid films. Among the many reversible holes that are continuously formed, a fraction may allow the passage of the dispersed molecules from smaller to larger drops. This mechanism could be at the origin of the first homogeneous regime. The second regime is controlled by the largest irreversible holes that produce coalescence events. Such a regime is characterized by the development of droplets larger than the average with a diverging growth rate. We have revealed that the presence of trapped species has a dramatic effect: in addition to the expected inhibition of molecular permeation, coalescence is also halted. The fact that the two types of instability disappear at the same time suggests that the two regimes have the same microscopic origin. It should be within the reach of future experiments to explore the generality and the limitations of the ideas presented here. Finally, we should like to stress that the conclusions raised by the present paper apply to concentrated emulsions (φ > 64%) comprised of large drops (D > 1 µm), with rather short-range repulsions in the thin liquid films. In the limit of highly dilute droplets (φ < 1%), with longrange repulsions and small diameters (,1 µm), the collision frequency and the approach between the droplets may become insufficient for the hole nucleation process to be efficient. Ostwald ripening is then preferentially controlled by the molecular diffusion of the dispersed molecules across the continuous phase, as observed in many previous experimental studies.4,5,21 Acknowledgment. The authors are very grateful to J. Giermanska-Kahn and S. Arditty for fruitful discussions and technical assistance. LA034747P
