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Ostwald Ripening of Alkane Emulsions Stabilized by Polyethylene Glycol 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 February 27, 2001. In Final Form: May 18, 2001 The Ostwald ripening of alkane in water emulsions stabilized by the surfactant poly(ethylene glycol) monolaurate (PEM) was investigated. Surprisingly the measured ripening rates were significantly below the rate which can be predicted by the Lifshitz-Slyozov-Wagner (LSW) theory and decreased in the presence of surfactant micelles. A possible explanation of the effect of these micelles is that they withdraw oil from the continuous phase and hence slow the transport of oil from smaller to larger droplets.
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. 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
v)
daN3 4 ) 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)
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 * To whom correspondence should be sent. Telephone: +32 2 629 3485. Fax: +32 2 629 3320. E-mail:
[email protected]. † Vrije Universiteit Brussel. ‡ Hanoi University of 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.
result predicts that the aging or average droplet size 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 studies6-16 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.6-16 Some authors8,9 observe ripening rates which increase with increasing surfactant concentration above the critical micellar concentration (cmc), that 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 2-3. On the other hand, Kabalnov7 did not observe any significant increase with increasing surfactant (micellar) concentration above cmc and Desmet et al.13 observed even a slight (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.; DeVos, 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.
10.1021/la010306z CCC: $20.00 © 2001 American Chemical Society Published on Web 07/26/2001
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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- to 1000-fold greater 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. Therefore, further experimental work on the effect of nonionic surfactant micelles on the Ostwald ripening rate is required. In this study the ripening rates of undecane and decane in water emulsions stabilized by poly(ethylene glycol) monolaurate (PEM) were determined at several surfactant concentrations.
Figure 1. Surface tension of aqueous PEM solution versus PEM concentration.
2. Experimental Section 2.1. Material and Emulsion Preparation. Several emulsions of alkanes in water stabilized by the surfactant PEM (poly(ethylene glycol) monolaurate, average molecular weight of 600, Aldrich) were prepared. This surfactant was chosen at random between the nonionic surfactants reported to stabilize emulsions.17 The alkanes used were undecane and decane (Aldrich, purity 99+%). The oil component was added to the aqueous surfactant solution. After 10 min of premixing with an Ultra-Turrax T25 with rotor S25-18G, the coarse emulsion was further homogenized during 5 min using an Y-110 microfluidizer. 2.2. Particle Size Measurement. Droplet sizes were determined with dynamic light scattering. The experimental setup consists of an Ar+ laser (wavelength λ ) 488 nm), a thermostated sample holder allowing temperature control (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 PEM: Critical Micellar Concentration and Specific Surface Area. The critical micellar concentration is determined as the concentration at which the surface tension of a surfactant solution reaches a limiting constant value. The specific surface area As is calculated from the Gibbs adsorption equation:
Γ)
-1 dγ RT d(ln Cs)
(3)
where Γ ) 1 /(NAAs) is the adsorption of surfactant molecules at the droplet surface, γ is the surface tension, NA is Avogadro’s number and Cs is the surfactant concentration. The surface tension of several PEM solutions was measured with a Lauda TVT1 tensiometer. The results are reported in Figure 1. From these measurements the values of 2 x 10-4 M for the cmc and 0.39 nm2 for As were estimated.
3. Results and Discussion 3.1. Oswald Ripening or Coalescence? In the first series of experiments, it was investigated whether the (17) Surfactants Europa; Hollis, L., Ed.; The Royal Society of Chemistry: Letchworth, UK, 1995. (18) Koppel, D. J. Chem. Phys. 1972, 57, 4814.
Figure 2. Increase in a3 with time for an undecane emulsion (crosses) and a decane emulsion (circles).
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 2, the cube of the droplet radius a is plotted as a function of time for an undecane and decane emulsion. The surfactant (PEM) concentration was 2 x 10-2 M for both emulsions; the alkane volume fraction was 0.05. Clearly, the ripening rate is reduced on replacing the decane by undecane. Assuming a linear increase of a3 with time, the ripening rates were estimated from the slope of the linear fits shown in Figure 2. The ratio 3.6 of these rates is in good agreement with the ratio 3.55 for the bulk solubilities of the two alkanes (2.0 x 10-8 mL of undecane and 7.1 x 10-8 mL of decane in 1 mL of water6). 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 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 (PEM) micelles, ripening experiments were carried out as a function of the surfactant concentration. Five undecane emulsions all with volume fraction 0.01 but with different PEM concentrations in the range (3.8 x 10-3)-(2 x 10-2) M, were prepared as described in section 2.1. The surfactant concentrations and initial particle sizes are reported in Table 1. The
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Figure 3. Ostwald ripening of undecane emulsion (volume fraction of 0.01) at different surfactant concentrations. Diamonds, 3.8 x 10-3 M; squares, 4.6 x 10-3 M; triangles, 7.8 x 10-3 M; crosses, 1.02 x 10-2 M; circles, 2 x 10-2 M. Table 1. Ostwald Ripening (OR) Rates as a Function of Surfactant Concentration for Several Undecane Emulsions emulsion
a0, nm
PEM concn, M
exptl OR rate, nm3/s
1 2 3 4 5
35 37 33 22 14
3.80 x 10-3 4.60 x 10-3 7.80 x 10-3 1.02 x 10-2 2.00 x 10-2
1.27 1.14 0.88 0.06 0.17
number of moles ns of surfactant needed to cover the water/ oil interface of an oil volume V is estimated by
ns )
3V aoAsNA
(4)
where a0 is the initial particle radius. This implies a minimum surfactant concentration of about 5 x 10-3 M for emulsions 1-3 and about 1 x 10-2 M for emulsions 4 and 5. The results of the monitoring of the droplet size as a function of time are displayed in Figure 3 and Table 1. The initial particle radius and the aging rate allow us to distinguish between the emulsions with relative low surfactant concentration (1-3) and relative high surfactant concentration (4 and 5). For emulsions 1-3, the average initial particle radius (about 35 nm) is larger compared to emulsions 4 and 5. Assuming that the first action of the surfactant is to cover the oil-water interface, all of it is used to cover this interface and no surfactant is left in the continuous phase. Hence for these emulsions there are no micelles which
can affect the ripening behavior. The observed ripening rates are about 1 nm3/s, which is significantly below the rate of 6.5 nm3/s which can be predicted by eq 1.13 Possible explanations are effects of impurities of the surfactant and increase of the surface elasticity with increasing droplet size. Impurities may act as cosurfactants, reducing the interfacial tension γ. This will result in a decreased ripening rate: eq 1 predicts a linear dependency of the ripening rate on γ. Since the manufacturer does not provide detailed information about the purity of the surfactant and the nature of possible impurities, this hypothesis cannot be explored further. The increase of surface elasticity with increasing droplet size may also slow the increase of the average droplet size. At present quantitative data are, however, not available. For emulsions 4 and 5, the average initial particle radius is smaller. This indicates that, especially for emulsion 5, there must be enough surfactant left after complete coverage of the oil/water interface to ensure the presence of surfactant micelles. Surprisingly the presence of these micelles decreases the Ostwald ripening rate instead of increasing it. Note that the scattering power of a 2 x 10-2 M surfactant solution, containing even more micelles than the emulsion with the same surfactant concentration, is at least an order of magnitude smaller than that of the mentioned emulsion. Hence the intensity weighted average sizes of the emulsion droplets are hardly affected by the presence of the micelles. A possible explanation for the reduced ripening rate may be that the micelles withdraw oil from the continuous phase; that is, they reduce the factor C(∞) in eq 1 and hence slow the transport of oil from smaller to larger droplets.13 4. Conclusions The Ostwald ripening in alkane emulsions stabilized by the nonionic surfactant PEM was determined. The Ostwald ripening behavior is different from that of alkane emulsions stabilized by ionic surfactants such as SDS and SDBS6-13 and even different from that of emulsions stabilized by other nonionic surfactants.14-16,19 In particular at lower surfactant concentration and in the absence of surfactant micelles the ripening rate is surprisingly smaller that predicted by the LSW theory. At higher surfactant concentration and in the presence of surfactant micelles the ripening rate is even more reduced. This effect may be due to the fact that the micelles withdraw oil from the ripening process. LA010306Z (19) De Smet, Y.; Deriemaeker, L.; Parloo, E.; Finsy, R. Langmuir 1999, 15, 2327.
