Langmuir 2002, 18, 1485-1489
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Ostwald Ripening of Alkane in Water Emulsions Stabilized by Polyoxyethylene (20) Sorbitan Monolaurate Thi Kieu Nguyen Hoang,† Van Binh La,‡ Luc Deriemaeker,† and Robert Finsy*,† Department of Physical and Colloid Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium, and Faculty of Chemical Technology, Hanoi University of Technology, Vietnam Received July 23, 2001. In Final Form: November 19, 2001 The Ostwald ripening of alkane in water emulsions stabilized by the surfactant polyoxyethylene (20) sorbitan monolaurate (Tween 20) was investigated. For emulsions prepared in the presence of excess micellar surfactant the Ostwald ripening rate enhancements with surfactant concentrations over the theoretical predictions of Lifshitz, Slyozov, and Wagner (LSW-theory) are less than a factor 3. For emulsions prepared with low surfactant concentration, followed by surfactant addition to the already-formed emulsion, the enhancements are up to a factor of 20, with increasing surfactant concentration.
1. Introduction The physical degradation of emulsions is due to the spontaneous trend toward a minimal interfacial area between the dispersed phase and the dispersion medium. Minimizing the interfacial area is mainly achieved by two mechanisms: first coagulation possibly followed by coalescence, and second Ostwald ripening (OR). The former is the most studied (see, e.g., ref 1). However, if properly stabilized against the coagulation/coalescence process, the latter can cause a substantial breakdown of the emulsion. Ostwald ripening is the process by which larger particles (or, for emulsions, droplets) grow at the expense of smaller ones due to the higher solubility of the smaller particles (Gibbs-Thomson or Kelvin effect) and to molecular diffusion through the continuous phase. A theoretical description of Ostwald ripening in twophase systems has been developed independently by Lifshitz, Slyozov, and Wagner (LSW-theory).2-5 One of their major results is that in the long time limit a stationary regime is reached for which the ripening rate v is given by
daN3 4 v) ) RDmC(∞) dt 9
(1)
In eq 1, aN denotes the number average particle radius and Dm is the dispersed phase molecular diffusion coefficient; R is a material dependent constant called the capillary length, defined by
R)
2γVm RT
(2)
In eq 2, Vm stands for the molar volume of the dispersed phase, γ is the interfacial tension, and R and T have their usual meaning of gas constant and absolute temperature. This result predicts that the aging or average droplet size * To whom correspondence should be sent. Tel +32 2 629 3485. Fax +32 2 629 3320. E-mail:
[email protected]. † Department of Physical and Colloid Chemistry. ‡ Faculty of Chemical Technology. (1) Tadros, T.; Vincent, B. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1983; p 131. (2) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35. (3) Wagner, C. Z. Electrochem. 1961, 65, 581. (4) Kabalnov, A. S.; Shchukin, E. D. Adv. Colloid Interface Sci. 1992, 38, 69. (5) Taylor, P. Adv. Colloid Interface Sci. 1998, 75, 107.
increase is mainly determined by the bulk solubility C(∞) of the dispersed phase in the continuous one. This feature of Ostwald ripening has been verified in several experimental studies6-17 of alkane in water emulsions stabilized against coagulation by surfactants. If added in sufficiently large amounts and after complete coverage of the oil-water interface, surfactants spontaneously form micelles in the continuous aqueous phase. The presence of micelles drastically increases the solubility of the oil phase. Therefore an effect of micelles on the Ostwald ripening may be anticipated. It might be simplistically expected that replacing the bulk oil solubility C(∞) in eq 1 by the concentration of oil solubilized by the micelles and using the micellar diffusion coefficient instead of the molecular one, would yield the Ostwald ripening rate in the presence of micelles. This approach predicts an increase of the rates by a factor of about 200-1000.5 Experimental studies report hardly any increase. Several, sometimes conflicting, results and explanations are given.6-17 Some authors8,9 observe ripening rates that increase with increasing surfactant concentration above the critical micellar concentration (CMC), which is with increasing number of micelles. The ripening rates relating to the ones at low surfactant concentration (below or at about the CMC) increases in both studies by a factor of 2 to 3. On the other hand, Kabalnov7 did not observe any significant increase with increasing surfactant (micellar) concentration above CMC and Desmet et al.13 observed (6) Kabalnov, A. S.; Markarov, K. N.; Pertzov, A. V.; Shchukin, E. D. J. Colloid Interface Sci. 1990, 138, 98. (7) Kabalnov, A. S. Langmuir 1994, 10, 680. (8) Taylor, P. Colloids Surf. A 1995, 99, 175. (9) Soma, J.; Papadopoulos, K. D. J. Colloid Interface Sci. 1996, 181, 225. (10) Bremer, L.; De Nijs, B.; Deriemaeker, L.; Finsy, R.; Gelade´, E.; Joosten, J. Part. Part. Syst. Charact. 1996, 13, 350. (11) De Smet, Y.; Malfait, J.; De Vos, C.; Deriemaeker, L.; Finsy, R. Bull. Soc. Chim. Belg. 1996, 105, 789. (12) De Smet, Y.; Malfait, J.; DeVos, C.; Deriemaeker, L.; Finsy, R. Prog. Colloid Polym. Sci. 1997, 105, 252. (13) De Smet, Y.; Deriemaeker, L.; Finsy, R. Langmuir 1999, 15, 6745. (14) Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S.; Lubetkin, S. D.; Mulqueen, P. J. Langmuir 1999, 15, 4495. (15) Weiss, J.; Herrmann, N.; McClements, D. J. Langmuir 1999, 15, 6652. (16) Weiss, J.; Cancelier, C.; McClements, D. J. Langmuir 2000, 16, 6833. (17) Hoang, N., K., T.; La, V. B.;Deriemaeker, L.; Finsy, R. Langmuir 2001, 17, 5166.
10.1021/la011151y CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002
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even a slight decrease. In these studies anionic surfactants (sodium dodecyl sulfate SDS and sodium dodecyl benzene sulfonate SDBS) were used. Since electrostatic repulsive interactions are important in such systems, it may well be that the observations and conclusions of these studies do not apply to systems with nonionic surfactants. Recently some authors14-16 reported that in the presence of nonionic surfactant micelles the ripening rate is 100-1000-fold larger than the Ostwald ripening rates predicted by eq 1. In the study of Weiss et al.,16 it is mentioned that although the observed ripening rates are a factor 30-130 larger than the ones predicted by eq 1, only a 4-fold increase between the lowest and the highest rates was measured. In a previous study17 the ripening rates of undecane and decane in water emulsions stabilized by poly(ethylene glycol) monolaurate (PEM) were determined at several surfactant concentrations. Surprisingly the measured ripening rates were significantly below the rate that can be predicted by the LSW-theory and decreased in the presence of surfactant micelles. In the present study the ripening rates of undecane, decane, and dodecane in water emulsions stabilized by polyoxythylene (20) sorbitan monolaurate (Tween 20) are presented at several surfactant concentrations. Two different ways of addition of the surfactant were investigated. 2. Experimental Section 2.1. Material and Emulsion Preparation. Several emulsions of alkanes in water stabilized by the surfactant Tween 20 (Riedel de Hahn, Fluka-Riedel de Hahn catalog number 63158) were prepared. The alkanes used were dodecane, undecane, and decane (Aldrich, purity 99+%). The first preparation method is the following. The oil component was added to the aqueous surfactant solution. After 10 min of premixing with an Ultra-Turrax T25 with rotor S25186, the coarse emulsion was further homogenized under high shear conditions during 10 min using a Y-110 microfluidizer. In this way emulsions with oil (alkane) volume fraction of 0.01 and surfactant concentrations in the range 2.0 × 10-4-1.0 × 10-1 M were prepared. In the second method, mother emulsions with alkane volume fraction 0.005 and surfactant concentration 2 × 10-2 M were prepared according to the previous procedure. To 2 mL samples of this mother emulsions were added typically 1 mL of surfactant solution with concentrations in the range 5 × 10-3-1 × 10-1 M. 2.2. Particle Size Measurement. Droplet sizes were determined with dynamic light scattering (DLS). The experimental setup consists of an Ar+ laser (wavelength λ ) 488 nm), a thermostated sample holder allowing to control the temperature (within (1 °C), and a photomultiplier (EMI 9863A) mounted at a detection angle of 90°. The signal of the photomultiplier was fed to a Brookhaven BI9000 correlator. The measurement temperature was 25 °C. Just before each particle size measurement a sample was prepared by diluting the emulsion about 1000 times with distilled water, to rule out interactions and multiple scattering effects. In a previous study12 it was shown that this measuring procedure did not affect the droplet size. Intensity averaged radii were computed from the intensity autocorrelation data with the cumulants method.18 2.3. Characteristic of the Surfactant Tween 20: Critical Micellar Concentration (cmc) and Specific Surface Area (As). These were determined in a previous study.19 The cmc was 7.1 × 10-5 M, and the specific area was 2.7 nm2.
3. Results and Discussion 3.1. Ostwald Ripening or Coalescence? In a first series of experiments, it was investigated whether the (18) Koppel, D. J. Chem. Phys. 1972, 57, 4814. (19) De Smet, Y.; Deriemaeker, L.; Parloo, E.; Finsy, R. Langmuir 1999, 15, 2327.
Nguyen Hoang et al.
Figure 1. Evolution of a3 with time for an undecane emulsion (crosses) and a dodecane emulsion (squares).
increase in particle size could be ascribed to Ostwald ripening or to coalescence. The Ostwald ripening rate is proportional to the solubility of the alkane (the factor C(∞) in eq 1). Since the solubilities of alkanes in water vary significantly with the alkane chain length, a considerable variation of the ripening rate is to be expected on changing the alkane chain length. On the other hand, the coalescence rate depends, for a given system, on the initial particle size and concentration. Hence, changing only the alkane chain length would not alter the coalescence rate. In Figure 1, the cube of the droplet radius a is plotted as a function of time for the undecane and dodecane mother emulsions. The surfactant (Tween 20) concentration was 2 × 10-2 M for both emulsions; the alkane volume fraction was 0.005. In all the experiments no visible traces of creaming or flocculation were observed during the monitoring of the ripening process by DLS. Clearly replacing undecane by dodecane reduces the ripening rates. Assuming a linear increase of a3 with time, the ripening rates were estimated from the slope of the linear fits shown in Figure 1. The ratio 3.7 of these rates is in good agreement with the ratio 3.8 for the bulk solubility of the two alkanes (2.0 × 10-8 mL of undecane and 5.2 × 10-9 mL of dodecane in 1 mL of water6). In a similar way, a ratio of 3.5 for ripening rates of decane to undecane emulsions was found, again in good agreement with the ratio of 3.55 of the solubilities (7.1 × 10-8 mL of decane in 1 mL of water); see section 3.2 for the details. Therefore, the main aging process is Ostwald ripening and not coalescence. This implies that the main aging mechanism results from the transport of the alkane by molecular diffusion through the continuous water phase. 3.2. Ostwald Ripening as a Function of Surfactant Concentration. To investigate the effect of an increasing number of surfactant (Tween 20) micelles, ripening experiments were carried out as a function of the surfactant concentration. In a first step, seven undecane and four decane emulsions, all with volume fraction 0.01 but with different Tween 20 concentrations in the range 2 × 10-4-1.0 × 10-1 M, were prepared in the first way as described in section 2.1. The surfactant concentrations and initial particle sizes are reported in Table 1. The number of moles ns surfactant needed to cover the initial water/oil interface was estimated by the procedure explained in section 4.2.1 of ref 13. In this procedure the total interfacial area is estimated from a model (lognormal) droplet size distribution. This leads to surfactant concentrations in the range 4 × 10-4-1.2 × 10-3 M. The results of the monitoring of the droplet size as a function of time are displayed in Figures 2 and 3 and Table 1. The experimental Ostwald ripening rates were determined following the procedure explained in ref 19. This
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Table 1. Ostwald Ripening (OR) Rates as a Function of Surfactant Concentration for Several Undecane and Decane Emulsions Prepared under High Shear Conditions Tween 20 concn needed to cover the initial oil-water interface
Tween 20 concn in the continuous phase (M)
emulsion and alkane
initial radius a0 (nm)
Tween 20 concn (M)
UA1 UA2 UA3 UA4 UA5 UA6 UA7
106 67 50 41 31 40 34
2 × 10-4 5 × 10-4 3.8 × 10-3 7.8 × 10-3 2.0 × 10-2 5.0 × 10-2 1.0 × 10-1
Undecane 4 × 10-4 6 × 10-4 8 × 10-4 1 × 10-3 1.2 × 10-3 1 × 10-3 1.2 × 10-3
3.0 × 6.8 × 10-3 1.9 × 10-2 4.9 × 10-2 9.9 × 10-2
58 57 45 34
3.8 × 10-3 7.8 × 10-3 5.0 × 10-2 1.0 × 10-1
Decane 6 × 10-4 6 × 10-4 8 × 10-4 1.2 × 10-3
3.2 × 10-3 7.2 × 10-3 4.9 × 10-2 9.9 × 10-2
D3 D4 D6 D7
Figure 2. Ostwald ripening of the undecane emulsions (UA1 bottom to UA7 top) with increasing surfactant concentration and prepared under high shear conditions. For clarity the different series have been translated along the ordinate over 30 × 105 nm3.
Figure 3. Ostwald ripening of the decane emulsions (D3 bottom to D7 top) with increasing surfactant concentration and prepared under high shear conditions. For clarity the different series have been translated along the ordinate over 30 × 105 nm3.
procedure accounts for the initial nonstationary growth regime.20 Thereby the time evolution of a model of the droplet size distribution is computed during the ripening (20) De Smet, Y.; Danino, D.; Deriemaeker, L.; Talmon, Y.; Finsy, R. Langmuir 2000, 16, 961.
10-3
experimental aging rate (nm3/s)
rate enhancement factor
13 10 9.4 15 19 24 26
2.0 1.5 1.4 2.3 2.9 3.7 4.1
60 79 116 92
2.7 3.6 5.3 4.2
process.21 At every step of the ripening, the intensity and number weighted droplet sizes are computed from the actual size distribution. With such tables of corresponding intensity and number averages, the intensity weighted average sizes determined experimentally by DLS are converted into number average sizes. The ratio of the experimental rates to the ones predicted by eq 1,13 i.e., 6.5 nm3/s for the undecane emulsions and 22 nm3/s for the decane emulsions, are reported in Table 1 as rate enhancement factors. Note that the oil volume fraction of the studied emulsions (0.01) is small enough to exclude a volume fraction dependent term in eq 1.22 Assuming that the first action of the surfactant is to cover the oil-water interface, all emulsions, except UA1 and UA2, contain enough surfactant in the continuous phase to form micelles. Given the initial particle size, the amount of surfactant present in undecane emulsion UA1 is not enough to cover the water-oil interface. Hence, for this emulsion, the interfacial tension is higher than the one for a completely covered interface (about 10-2 N m-1).6 Since the Ostwald ripening rates are proportional to the interfacial tension (see eq 1) this may explain the 2-fold higher ripening rate compared to the one predicted by eq 1. The amount of surfactant in emulsion UA2 is initially slightly less than the estimated amount needed to cover the initial oil-water interface. However, during the ripening process, the smaller oil droplets disappear and hence less surfactant is needed to cover the interface. Therefore, the oil-water interface is completely covered during the major part of the ripening process. Rate enhancement factors, shown in Figure 4, are computed as the ratio of the experimental rates to the one predicted by eq 1. For undecane they increase from 1.4 for the emulsion UA3 with small amounts of micelles to about 4 for the emulsion with the largest amount of micelles (UA7). Note that the enhancement factor is 1.4 for emulsion UA3. For this emulsion the oil-water interface is covered completely. This is in agreement with the model whereby the continuous phase consists of a dispersion of small oil droplets with average radius typical the size of a micelle.13 Since this emulsion is prepared under the high shear conditions of the microfluidizer, one can assume that the alkane and the surfactant are distributed in a quasi-equilibrium state between the micelles (swollen by alkane) and the larger emulsion droplets. The equilibrium (alkane) solubility in a continuous phase of small droplets (21) De Smet, Y.; Deriemaeker, L.; Finsy, R. Langmuir 1997, 13, 6884. (22) Voorhees, P. J. Stat. Phys. 1985, 38, 231.
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Figure 4. Rate enhancement factors as a function of overall surfactant concentration (M). Undecane (diamonds) and decane (crosses) emulsions prepared under high shear conditions.
is given by Kelvin’s equation
Cm/C(∞) ) exp(R/a)
(3)
In eq 3, a is the droplet radius. For a ratio of Cm/C(∞) equal to the observed enhancement factor of 1.4, this yields an average droplet radius of 5 nm. Note also that similar results were obtained for undecane emulsions stabilized by the anionic surfactant sodium dodecyl benzenesulfonate (SDBS) and prepared in the same way as emulsions UA1UA7.13 The ratio of the highest to the lowest observed ripening rate is about 3. This compares well to the ratio of about 4 between lowest and highest rate reported by Weiss et al.16 for tetradecane emulsions stabilized by the same surfactant (Tween 20). On an absolute scale there are, however, large differences between our rates and the aging rates reported by Weiss et al.16 The latter report rates that are between 1 and 2 orders of magnitude larger than the ones estimated with eq 1, whereas ours are no more than 5 times larger. The main difference is that Weiss et al.16 used tetradecane instead of decane and undecane. The aging of tetradecane was determined from 8 measurements spread over about 25 days. In this study the aging of the emulsions was monitored almost continuous during typically 1 day for decane emulsions and 3 days for the undecane emulsions. Possible contributions of other aging mechanism are therefore less probable in our study. The fact that the ripening rates are proportional to the solubility of the different alkanes used in our study also confirms the absence of other aging mechanisms in our study. Nevertheless, compared to our studies of emulsions stabilized by the anionic surfactant SDBS13 and to those of Kabalnov of emulsions stabilized by the anionic surfactant SDS,7 there is now a small but significant effect of the presence of the micelles on the ripening rate. On the other hand the observed slight enhancement is similar to the ones observed in the studies of emulsions stabilized by anionic surfactants (mainly SDS) reported by Taylor8 and Soma and Papadopoulos.9 Our results of the ripening rates for the emulsions stabilized with the nonionic Tween 20 are also quite different with those of the undecane and decane emulsions stabilized by the nonionic Polythylene glycol monolaurate (PEM).17 In the latter case the ripening rates were significantly below the rate predicted by eq 1 and decreased in the presence of surfactant micelles. This all illustrates that the control and the understanding of the aging mechanism are still in development. The initial sizes of the decane emulsions, with the same overall amounts of surfactant as the corresponding undecane emulsions, are higher compared to the undecane
emulsions (compare D3 to UA3, D4 to UA4, D6 to UA6, and D7 to UA7). However, for the ones with the highest surfactant concentration (UA7 and D7), the initial sizes are the same. The rate enhancement factors for decane are somewhat higher for emulsions D3, D4, and D6 compared to the corresponding undecane emulsions (UA3, UA4, and UA6). For the emulsions with the highest surfactant concentration and with the same initial size the rate enhancement factors are also the same (compare D7 to UA7). The ratio of the ripening rates of these two emulsions amounts 3.5 which is again in good agreement with the ratio of 3.4 of the rates estimated with eq 1. This confirms again that, for these emulsions, Ostwald ripening is the main aging process. The relative higher initial sizes together with the relative higher aging rates for the other three decane emulsions suggest that the decane emulsions are (a little) less well stabilized against other aging mechanisms than OR, than the undecane emulsions. Nevertheless the rate enhancement factors are hardly higher compared to those of the undecane emulsion. The ratio of the highest to lowest is only about a factor 2. In a second step, seven new undecane and six dodecane emulsions were prepared in the second way. Two mother emulsions (undecane and dodecane) with 2 × 10-2 M Tween 20 were prepared. Samples of 2 mL of these emulsions were monitored until the average radius was 50 nm. At that moment typically 1 mL surfactant solution of different concentrations were added. The details are reported in Table 2. The results of the monitoring of the droplet size as a function of time are displayed in Figures 5-7 and Table 2. With the same assumption as before all the emulsions contain enough surfactant in the continuous phase to form micelles. Figure 7 clearly shows that the rate enhancement factor increases with increasing surfactant concentration. Compared to the emulsions prepared in the first way the rate enhancement factors (or the aging rates) are significantly higher, by a factor up to 5. Note that the concentration range of the emulsions prepared in the second way is much smaller than that of those prepared in the first way. This implies that the enhancement effect is about an order of magnitude larger than for the emulsions prepared in the first way. Clearly for those emulsions there must be an additional aging mechanism. A possible explanation is the following. Before adding additional surfactant, there is in the already formed emulsion a quasi-equilibrium state whereby the already present surfactant and the alkane molecules are distributed between micelles and the larger emulsion droplets. The main factor disturbing this quasi-equilibrium is the Ostwald ripening process. On average, the alkane is transported slowly from smaller droplets to larger ones by molecular diffusion through the continuous phase. The rate of this process is mainly determined by the alkane solubility in the continuous phase (the factor C(∞) in eq 1). However, this quasiequilibrium state is a dynamic one whereby micelles and droplets are continuously broken down and rebuild. Addition of extra surfactant disturbs this dynamic equilibrium: more alkane droplets and micelles are broken down. During the evolution toward a new quasi-equilibrium distribution of alkane and surfactant there are now on average more alkane molecules present in the continuous phase. Hence the factor C(∞) in eq 1 is increased, resulting in an increase in the OR rate. An alternative explanation is a micelle mediated transport mechanism.14 The argument in favor is the fact that the rate enhancement increases with increasing surfactant (micelle) concentration. The argument against
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Table 2. Ostwald Ripening Rates as a Function of Added Surfactant for Several Undecane and Dodecane Emulsions exptl rate concn and volume of added Tween 20 emulsion and overall surfatant ripening rate ehancement alkane (M) (mL) concn after addition (M) (nm3/s) factor Undecane UB1 UB2 UB3 UB4 UB5 UB6 UB7
5 × × 10-3 5 × 10-3 2.5 × 10-2 5 × 10-2 7.5 × 10-2 1.0 × 10-1
0.8 2 1 1 1 1 Dodecane
Do1 Do2 Do4 Do5 Do6 Do7
1.0 × 10-2 2.5 × 10-2 5 × 10-2 7.5 × 10-2 1.0 × 10-1
1 1 1 1 1
Figure 5. Aging of the undecane emulsions (UB1, bottom, to UB7, top) prepared under high shear conditions and to which extra surfactant was added at different concentrations. For clarity the measurements of UB2 have been translated along the ordinate over 5 × 105 nm3; the measurements of UB3-UB7 have been translated by 10 × 105 nm3.
2.0 × 10-2 1.6 × 10-2 1.3 × 10-2 2.2 × 10-2 3.0 × 10-2 3.8 × 10-2 4.7 × 10-2 2.0 × 10-2 1.7 × 10-2 2.2 × 10-2 3.0 × 10-2 3.8 × 10 -2 4.7 × 10-2
18 16 16 26 43 55 113 4.8 5.4 6.5 14 17 34
2.8 2.4 2.5 4.0 6.6 8.5 17.4 2.7 3.0 3.6 7.7 9.2 19
Figure 7. Rate enhancement factors as a function of overall surfactant concentration (M) for undecane (diamonds) and dodecane (crosses) emulsions prepared under high shear conditions and to which extra surfactant was added at different concentrations.
the molecular mechanism of micelle mediated transport is different from previous one: breakdown and rebuilding of emulsion droplets and micelles saturated with the alkane by micelles without alkane leaving on average more alkane in the continuous phase. Whatever the exact mechanism is, the experimental results are consistent with the hypothesis that the surfactant micelles are not in local equilibrium with oil molecules as proposed by Kabalnov7 and confirmed experimentally in another study.13 4. Conclusions
Figure 6. Aging of the dodecane emulsions (Do1, bottom, to Do7, top) prepared under high shear conditions and to which extra surfactant was added at different concentrations. For clarity the measurements of Do2 have been translated along the ordinate over 5 × 105 nm3; the measurements of Do4-Do7 have been translated by 10 × 105 nm3.
it is that for emulsions prepared in the first way, that is emulsions for which the alkane is already distributed between micelles and larger emulsion droplets, the rate enhancement is much smaller even at higher surfactant concentration. Furthermore, one can wonder if
The rate of Ostwald ripening in alkane emulsions stabilized by the nonionic surfactant Tween 20 was determined. When prepared under high shear conditions using a Microfluidizer the ripening rates are only slightly enhanced by the presence of surfactant micelles. The ratio of the highest rate, observed at the highest surfactant concentrations, to the lowest ones is no more than a factor 3. On an absolute scale the enhancement is no more than a factor of about 5. The surfactant and the alkanes molecules are distributed in a quasi-equilibrium state between emulsion droplets and the micelles. Addition of extra surfactant to already formed emulsions increases the aging rates by almost an order of magnitude. This enhancement may be due to a redistribution mechanism of the alkanes and the surfactant between the emulsion droplets and the micelles in the continuous phase. It also indicates that in this case the surfactant micelles are not in local equilibrium with the oil molecules. LA011151Y
