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Emulsion Stability in the Presence of Nonionic Surfactant Micelles: Role of Micellar Ordering and Ostwald Ripening† Youngsun Kong, Alex Nikolov, and Darsh Wasan* Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, Illinois 60616
The phenomenon of surfactant micelle ordering (i.e., stratification) in emulsion films was investigated using the reflected-light microinterferometric technique. In thinning films formed from a nonionic micellar solution of ethoxylated alcohol (1 wt %), it was found that the small droplets (i.e., less than 5 µm) are separated by thick (>0.1 µm) stable films containing surfactant micelles in multilayers that prevent droplet flocculation and coalescence. The Ostwald ripening process governs emulsion stability over a long-term. The direct microscopic observations of the evolution of the drop size distribution over time of hexadecane drops (∼25 vol %) dispersed in an aqueous micellar solution (≈ 120 times critical micellar concentration) was compared with that calculated from the Ostwald ripening model. Introduction Concentrated emulsions are prepared using micellar solutions encountered in many processes: food, pharmaceutical, cosmetic, agricultural, and consumer products. Emulsions are thermodynamically unstable systems. Emulsion stabilizers such as surfactant micelles, proteins, macromolecules, and colloidal particles are used to control emulsion stability and enhance shelf life. In a concentrated emulsion system, the drops of the dispersed phase are separated by liquid films or lamellae. The interaction between the droplets in the presence of common emulsifiers (surfactant micelles) in the continuous phase (the solvent) plays an important role in controlling the kinetic stability of the emulsion. Video 1 (in the Supporting Information) shows animation of the interaction between two droplets in the presence of nanoparticles such as surfactant micelles in the continuous phase. Because of Brownian motion, the micelles interact with the droplet surface. The droplet surface induces micelle ordering around the droplet and this ordering (layering) propagates over several diameters. This long-range structure results in a repulsive structural barrier that prevents the droplets from approaching each other. In general, the effective pair interaction potential induced by surfactant micelles or nanocolloids layering between two droplets in the presence of other droplets is oscillatory, including both the attractive depletion (i.e., so that no micelles can fit in the gap between the droplets) and the repulsive structural barrier.1-11 Our theoretical calculations based on the statistical mechanics approach have clearly indicated that the structural energy barrier is large enough to prevent the droplets from approaching each other when the effective micelle concentration (including the double layer and hydration layer) is 20 vol % or higher.12,13 Emulsion stability based on the number of drops and their size distribution over time is defined in different ways: flocculation, coalescence, and Ostwald ripening. Flocculation leads to emulsion phase separation: droplets tend to cream (phase † This brief paper on emulsion stability is dedicated to Dimitri Gidaspow, an outstanding chemical engineer. Dimitri has been recognized with many honors and awards for his contributions to the subject of multiphase flows, especially in fluid-particle systems. He is our friend and a long-time colleague; Darsh has also had the pleasure of working with Dimitri in cosupervising many graduate students. * To whom correspondence should be addressed. E-mail: wasan@ iit.edu.
separate) without any change in the drop size distribution (the micellar film between the approaching drops is stable). Coalescence results from the film rupture between the flocculated droplets; the drop size tends to increase and the number of drops decreases over time. When the emulsion stability is governed by the Ostwald ripening process, both the drop size distribution and the number of drops in a polydisperse system change over time. The larger droplets increase in size, while the small droplets decrease in size and dissolve. In this case, the driving force is the difference in the capillary pressure between the small (with a greater Laplace pressure) and large droplets. Therefore, there is a chemical potential gradient that drives the dispersed phase to change sizesfrom small droplets into large onessthereby coarsening the emulsion.14-24,37,38 In this paper, we will focus on an oil-in-water emulsion with a polydispersed drop size distribution stabilized by nonionic surfactant micelles in the aqueous phase with an effective micellar concentration of 20 vol %. In this case, the micelle ordering (i.e., layering) phenomenon prevents the droplets from flocculating and coalescing, but the Ostwald ripening process governs their emulsion stability. A direct comparison is made between the experimental data on the number and size evolution of the droplets and the theoretical predictions using LifshitzSlyozov-Wagner’s (LSW) Ostwald ripening model.25-31 Experimental Section The oil-in-water emulsion was prepared in two steps, coarse and fine. The coarse emulsion was prepared using 20 vol % n-hexadecane (Sigma-Aldrich, 99%), dispersed in an aqueous phase of the nonionic surfactant, Neodol 45-13 (carbon number ) 14-15, contains 13 ethoxylated groups, and MW: 793; product of Shell Co., TX) with a concentration of 1 wt % (120 times the critical micellar concentration, CMC). The oil phase was added into the aqueous micellar solutions and mildly stirred (∼200 rpm) for 24 h at room temperature. The fine emulsion was prepared by the homogenization at 800/rpm for 5 min of the coarse emulsion. Then the fine emulsion was placed in the 10 cm glass vial (60 mL of capacity, 28 mm in diameter) for 12 h, and the emulsion’s top layer was collected. The collected emulsion was diluted 10 times with 1 wt % micellar solution and mildly mixed. A 0.02 mL sample of the emulsion was collected and placed in the specially designed optical cell shown in Figure 1. The cell contains two vertical concentric cylinders
10.1021/ie901825c 2010 American Chemical Society Published on Web 04/27/2010
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Figure 1. Reflected light microinterferometric set up to monitor emulsion film thickness (a) and micelle layering phenomenon (b).
with their bottoms sealed to a horizontal, optical glass plate. The outer cylinder (with a diameter of 3 cm) is placed 0.3 cm above the upper part of the inner cylinder (with a diameter of 1 cm). The inner cylinder is filled with the oil-in-water emulsion above the rim to form a concave meniscus. The concave meniscus helps the oil droplets accumulate at the upper part of the meniscus. To avoid the evaporation of the aqueous phase, the top of the cell was filled with the pre-equilibrated oil phase. The cell containing the coarse emulsion was placed under the reflected-light interference microscope and the entire setup was placed on an antivibration table. Special precautions were taken to eliminate light reflection and evaporation from the air-oil surface. The high refractive index of the oil phase helps in obtaining a high quality image of the emulsion. The immersion-type objective was used to observe in the reflected light the film size, film thickness, and droplet distribution over time as shown in the micrograph in Figure 1. The concentric interference patterns around the emulsion film separating the oil droplets from the oil phase were used to evaluate the film thickness. The microscopic arrangement as depicted in Figure 1a was used to monitor the oil drop size distribution over time. The oil drop size images were regularly captured (after 10 days), and the drop size distribution was analyzed using Image Pro software (Media Cybernetics, MD). More than 1000 droplets were measured for statistical analysis. The drop size distribution was monitored initially and then again after 6 days. We did not observe any change in the number of droplets, indicating that no coalescence had occurred. However, there were changes in the drop size distribution over time due to the Ostwald ripening effect. The reflected light apparatus (Figure 1b) was also used to study the ordering (layering) of the surfactant micelles inside the thinning liquid film. In this case, a different glass cell was used. Microscopic thin films were formed in a cylindrical capillary with hydrophilic inner walls inside a temperature controlled glass cell. Liquid was slowly sucked out of the drop through a capillary orifice in the tube wall to create a horizontal flat film with a biconcave liquid meniscus (sketch in Figure 1b). The biconcave meniscus mimics the film separating the surface of two droplets. The details of this cell are available elsewhere.9,32,33 The equilibrium interfacial tension of the pre-equilibrated hexadecane/surfactant aqueous solution was obtained using the Laplace equation to fit the sessile drop shape profile. The interfacial tension of the hexadecane/aqueous micellar solution was 2.9 mN/m.
Figure 2. Micrograph depicting stepwise thinning (layer-by-layer) of micellar film formed from an aqueous solution of Neodol 45-13 (concentration 120 CMC).
To obtain information on the CMC, the surface tension of Neodol 45-13 was measured by varying the surfactant concentrations. The Wilhelmy plate method apparatus designed by KRUSS (Germany) was used to measure the surface tension. The CMC of Neodol 45-13 was about 1 × 10-4 mol/L, and its 1 wt % solution corresponded to 120 times the CMC. Results and Discussion Micellar Film Thinning and Stability. The reflected-light microinterferometric setup shown in Figure 1 in conjunction with the cell was used to study the film thinning process, film thickness stability, and the stratification (micelle ordering or layering) phenomenon. It was observed that the film (∼150 µm in diameter), formed from an aqueous solution of a nonionic surfactant, Neodol 45-13, at 1 wt % (120 CMC) concentration, thinned in a stepwise manner layer by layer. The photomicrograph in Figure 2 clearly shows that the surfactant micelles (of approximately 11 nm in diameter) form layered structures induced by the confined boundaries of the film. The amplitudes of the thickness transitions remained constant at approximately 10 ( 1 nm for the 1 wt % sample, which corresponds to the effective micellar diameter. The film with a diameter of 150 µm had two equal stepwise thickness transitions and then the film remained at an equilibrium thickness of 18 nm without any micelle layers inside it (Figure 2); when the film size decreased to 40 µm in diameter, only one stepwise thickness transition occurred and the film remained at an equilibrium thickness with only one micellar layer inside it. Upon a further
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Figure 4. Droplet size evolution over a period of 10 days: (]) 4, (9) 3, (() 2, (2) 1.2, (O) 1.0, (/) 0.8 µm.
Figure 3. Plot of the cube of the average emulsion oil drop size versus time in minutes.
decrease in film size (e.g., below 20 µm in diameter), the film remained at an equilibrium thickness with two micellar layers inside it. The phenomenon of particle layering offers a novel stabilization mechanism for emulsions. The lifetime of emulsions with stratifying films is much longer than that of the emulsions without the micelles. We also used the reflected light microinterferometric method (Figure 1) to determine the thickness and the size of the emulsion film between the oil droplets and the oil homophase. The droplet with a diameter of 55 µm had a film size of 8 µm and a film thickness of 65 ( 4 nm. When the droplet size was reduced to 20 µm, the film size decreased to 3 µm and the film thickness increased to 85 ( 7 nm. The small droplets (less than 5 µm), which were accounted for by the Ostwald ripening process, were separated by a film with a thickness of about 100 nm or more. Therefore, the aqueous emulsion film containing micelles was thick, stable, and prevented droplet flocculation. Ostwald Ripening. Ostwald ripening is a kinetic process, and at a steady-state, the theory predicts that the process becomes independent of the drop size distribution and is characterized by a linear growth of the cube of the average number droplet radius (aj) over time: 8σDceqVmγ(φ) daj3avg ) dt 9RT
(1)
where D is the oil diffusivity in water, σ is the equilibrium interfacial tension between the oil and the aqueous phase, ceq is the equilibrium dimension’s solubility of the dispersed oil phase in the water phase, Vm is the molar volume of the oil phase, R is the universal gas constant, T is the dissolute temperature, and γ(φ) is the Ostwald rate coefficient (which depends on the dispersed phase volume fraction, φ18). We plotted the cube of the number average drop size (ajavg) versus time in Figure 3 to evaluate whether our data follow the Ostwald ripening model. The data show a linear dependence over time, verifying qualitatively the model predictions. The rate of droplet growth calculated from the slope of the curve in Figure 3 is determined to be 7.1 × 10-25 m3/sec which is much higher than the value of 5.6 × 10-31 m3/sec calculated on the basis of the LSW model (eq 1). In the calculations, we used the diffusion coefficient of 3.8 × 10-6 cm2/sec and the equilibrium dimensions solvability of 4 × 10-12 g/g.36 The discrepancy between the experimentally determined value and the value calculated from the model is attributed to the fact that the LSW model is valid only for a very dilute emulsion system, whereas we used a very concentrated emulsion of 25 vol %. Furthermore, our
observations in the evolution of the drop size distribution revealed that the droplets smaller than the critical size (0.8 (m) reduce their size not by continuous dissolution into the aqueous phase but by dissolving spontaneously. As a result the local solubility of the oil droplet is greater than the equilibrium oil solubility. As a result of the supersaturation of oil in the aqueous phase, the droplet growth rate of larger droplets is faster than the LSW predictions even if we have corrected the LSW predictions for the volume fractions effect (N). We used direct microscopic observations to measure the droplet size distribution over time in our concentrated (30 vol % dispersed oil phase) emulsion system. Therefore, we were limited by the optical resolution for a droplet size measurement to 0.5 µm. Figure 4 displays our measurements of the droplet size distribution over a period of 10 days. These observations clearly show that the small droplets decrease in size and dissolve while the large droplets tend to increase in size; the mediumsized droplet (1.2 µm) fraction remained unchanged. The droplet size (i.e., below which the small droplets dissolve and the large droplets increase in size) was 0.6 ( 0.2 µm. Equation 2 is a modified version of eq 1 and relates the rate of drop size change with time:
(
DceqR 1 da 1 ) dt a ac a
)
(2)
where a is the drop size, ac is the critical radius, and R is characteristic length (2σVm((N)/RT). We used eq 2 to predict the drop size distribution versus time. We used 345.45 cm3/mol for the molar volume (the diffusivity of hexadecane in the aqueous solution calculated based on the Stokes-Einstein equation at 25 °C was 3.8 × 10-6 cm2/sec) and the value of 5.5 × 10-6 g/g for the solubility of hexadecane in water at 25 °C. As we have stated earlier, our observations revealed that the hexadecane droplet below 0.8 µm in diameter dissolved spontaneously. Therefore, we assumed that the amount of hexadecane dissolved (i.e., solubility) is governed by the spontaneously dissolved droplet size rather than the equilibrium solubility between two planar oil-water surfaces. The solubility value (cr*) corresponding to this drop size (r*) was calculated using the Gibbs-Thomson equation,Cr* ) Ceq exp(2σVm/ (r*kT)). Figure 5 shows a plot of the calculated droplet number frequency distribution based on the Ostwald ripening model (eq 2). For N we used the value of 2.5. This plot also shows both the experimentally measured (at 6 days) and initial number frequency distribution. The agreement between the Ostwald model predictions and the experimental data corrected for the oil solubility was satisfactory. Therefore, we concluded that the Ostwald ripening process is the governing destabilizing mechanism for our particular emulsion system comprising hexadecane
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Figure 5. Plot of the emulsion droplet number frequency distribution vs droplet diameter: initial (2) and after six days (b). The dashed line represents droplet size distribution predicted by the Ostwald ripening model.
droplets in an aqueous solution of nonionic micellar solution at a surfactant concentration much above the critical micelle concentration (that is, 120 times the CMC). It has been suggested in the literature that surfactant micelles can contribute to the oil interdroplet mass transfer process through an interfacially dominated mechanism and other related micelle transport mechanisms.34,35 However, our results show that the critical size of the droplets which dissolved spontaneously contribute to the main mechanism of oil exchange among droplets through molecular diffusion in the continuous aqueous phase containing micelles. Conclusions We used reflected-light microinterferometry to study the micellar layering (i.e., stratification) phenomenon inside liquid films containing micelles and to examine micellar film thinning and stability. We also used this technique to determine the emulsion film thickness and the droplet sizes in hexadecane in an aqueous solution containing nonionic micelles (120 times above the CMC). We found that the emulsion droplets were separated by micellar films of greater than 100 nm in thickness, rendering the emulsion stable enough to coalesce. We used direct microscopic observations to determine the evolution of the droplet size distribution over time. We observed that the small droplets diminished in size and dissolved spontaneously in the aqueous phase below a critical size, while the large droplets increased in size with time. We compared the experimentally measured droplet number frequency distribution with the calculated one using the classical Ostwald ripening model but using the oil solubility based on our observations of spontaneous dissolution of droplets below the critical size. The agreement was reasonable. In conclusion, despite many excellent studies published so far in the literature on the Ostwald ripening process for emulsion destabilization, many questions still remain to be answered, especially for concentrated emulsion systems comprising a highly dispersed phase volume in concentrated surfactant micellar solutions. Supporting Information Available: A video clip showing the interaction between two droplets in the presence of nanocolloids such as surfactant micelles in the continuous phase. This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Aronson, M. P. The Role of Free Surfactant in Destabilizing Oilin-Water Emulsions. Langmuir 1989, 5, 494.
(2) Bibette, J.; Rowe, D.; Wallet, F. Depletion Interactions and FluidSolid Equilibrium in Emulsions. Phys. ReV. Lett. 1990, 65, 2470. (3) Aronson, M. P. Flocculation of Emulsions by Free Surfactant in Purified Systems. Colloids Surf. 1991, 58, 195. (4) Wasan, D. T.; Nikolov, A. D.; Aimetti, F. Texture and Stability of Emulsions and Suspensions: Role of Oscillatory Structural Forces. AdV. Colloid Interface Sci. 2004, 108, 187. (5) Kumar, K.; Nikolov, A. D.; Wasan, D. T. Mechanisms of Stabilization of Water-in-Oil Crude Oil Emulsions. Ind. Eng. Chem. Res. 2001, 40, 3009. (6) Wasan, D. T.; Nikolov, A. D.; Henderson, D. New Vistas in Dispersion Science and Engineering. AIChE J. 2003, 49, 550. (7) Nikolov, A. D.; Wasan, D. T. Encyclopedia of Surface and Colloid Science; Somasundaran, P., Ed.; Taylor & Francis: New York, 2008; p 1. (8) Wasan, D.; Nikolov, A. Foams and Emulsions: The Importance of Structural Forces. Aust. J. Chem. 2007, 60, 633. (9) Wasan, D. T.; Nikolov, A. D. Thin Liquid Films Containing Micelles or Nanoparticles. Curr. Opin. Colloid Interface Sci. 2008, 13, 128. (10) Kong, Y.; Nikolov, A.; Wasan, D.; Ogawa, A. Emulsion Texture and Stability: Role of Micellar Interactions. Ind. Eng. Chem. Res. 2008, 47, 9108. (11) Dherdjev, A. M.; Beattie, J. K. Enhancement of Ostwald Ripening by Depletion Flocculation. Langmuir 2008, 24, 7711. (12) Chu, X.; Nikolov, A. D.; Wasan, D. T. Effect of Particle Size and Polydispersity on the Depletion and Structural Forces in Colloidal Dispersions. Langmuir 1996, 12, 5004. (13) Wasan, D. T.; Nikolov, A. D.; Trokhymchuk, A.; Henderson, D. Confinement-Induced Structural Forces in Colloidal Systems. In Encyclopedia of Surface and Colloid Science; Hubbard, A., Ed.; Marcel Dekker: New York, 2002; p 1181. (14) Pena, A. A.; Miller, C. A. Kinetics of Compositional Ripening in Emulsions Stabilized with Nonionic Surfactants. J. Colloid Interface Sci. 2001, 244, 154. (15) Soma, J.; Papadopoulos, K. D. Ostwald Ripening in Sodium Dodecyl Sulfate-Stabilized Decane-in-Water Emulsions. J. Colloid Interface Sci. 1996, 181, 225. (16) De Smet, Y.; Deriemaeker, L.; Finsy, R. Ostwald Ripening of Alkane Emulsions in the Presence of Surfactant Micelles. Langmuir 1999, 15, 6745. (17) Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippen, S. Kinetics of Swelling of Oil-in-Water Emulsions. Langmuir 1998, 14, 5402. (18) Kabalnov, A. S. Can Micelles Mediate a Mass Transfer between Oil Droplets. Langmuir 1994, 10, 680. (19) Weiss, J.; Herrmann, N.; McClements, D. J. Ostwald Ripening of Hydrocarbon Emulsion Droplets in Surfactant Solutions. Langmuir 1999, 15, 6652. (20) Weiss, J.; McClements, D. J. Influence of Ostwald Ripening on Rheology of Oil-in-Water Emulsions Containing Electrostatically Stabilized Droplets. Langmuir 2000, 16, 2145. (21) Taylor, P. Ostwald Ripening in Emulsions. Colloids Surf., A 1995, 99, 175. (22) Kabalnov, A. S.; Pertzov, A. V.; Shchukin, E. B. Ostwald Ripening in Emulsions. I. Direct Observations of Ostwald Ripening in Emulsions. J. Colloid Interface Sci. 1987, 118, 590. (23) Kabalnov, A. S.; Makarov, K. N.; Pertzov, A. V.; Shchukin, E. B. Ostwald Ripening in Emulsions. II. Ostwald Ripening in Hydrocarbon Emulsions: Experimental Verification of Equation for Absolute Rates. J. Colloid Interface Sci. 1990, 138, 98. (24) Weiss, J.; Canceliere, C.; McClements, D. J. Mass Transport in Oil-in-Water Emulsions Containing Surfactant Micelles: Ostwald Ripening. Langmuir 2000, 16, 6833. (25) Lifshitz, I. M.; Slyozov, V. V. The Kinetics of Precipitation from Supersaturated Solid Solutions. J. Phys. Chem. Solids 1961, 19, 35. (26) Lifshitz, I. M.; Pitaevskii, L. P. Physical Kinetics in Landau and Lifshitz Course of Theoretical Physics; Butterworth-Heinemann: Oxford, 1995; Vol 10. (27) Wagner, C. Theorie Dealtering von Niederschlagen durch Umlosen. Ber. Bunsen. Phys. Chem. 1961, 65, 581. (28) Yarranton, H. W.; Masliyah, J. H. Numerical Simulation of Ostwald Ripening in Emulsions. J. Colloid Interface Sci. 1997, 196, 157. (29) Kabalnov, A. S.; Schukin, E. Ostwald Ripening Theory: Applications to Flurocarbon Emulsion Stability. AdV. Colloid Interface Sci. 1992, 38, 69. (30) Voorhees, P. W. The Theory of Ostwald Ripening. J. Stat. Phys. 1985, 38, 231. (31) Kawasaki, K. Ordering Kinetics in Phase Transitions. In Progress in Statistical Mechanics; Hu, C., Ed.; World Scientific: Singapore, 1988; p 171.
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(37) (a) Baldan, A. Progress in Ostwald Ripening Theories and Their Applications to Nickel-Base Superalloys Part I: Ostwald Ripening Theories. J. Mater. Sci. 2002, 37, 2171. (b) Baldan, A. Progress in Ostwald Ripening Theories and Their Applications to the γ′-Precipitates in Nickel-Base Superalloys Part II: Nickel-Base Superalloys. J. Mater. Sci. 2002, 37, 2379. (38) Schmitt, V.; Cattelet, C.; Leal-Calderon, F. Coarsening of Alkane-inWater Emulsions Stabilized by Nonionic Poly(oxyethylene) Surfactants: The Role of Molecular Permeation and Coalescence. Langmuir 2004, 20, 46.
ReceiVed for reView November 17, 2009 ReVised manuscript receiVed April 9, 2010 Accepted April 12, 2010 IE901825C
