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Mass Transport Phenomena in Oil-in-Water Emulsions Containing Surfactant Micelles: Ostwald Ripening Jochen Weiss Department of Food Science and Technology, University of Tennessee, Knoxville, Tennessee 37901-1071
Carlotta Canceliere and David Julian McClements* Department of Food Science, University of Massachusetts, Amherst, Massachusetts 01003 Received November 10, 1999. In Final Form: May 17, 2000 The influence of nonionic surfactant type and concentration on the kinetics of Ostwald ripening (OR) of hydrocarbon emulsion droplets suspended in aqueous micellar solutions was investigated. OR rates were determined by measuring changes in the droplet size distribution of 5 wt % n-tetradecane oil-in-water emulsions containing different surfactant types (Tween 20, Tween 40, Tween 60, Tween 80, Triton SP 190, and Triton SP 175) and concentrations (0.5-10 wt %). At low surfactant concentrations, the OR rate increased with surfactant concentration probably because micelles incorporated oil molecules and transported them across the aqueous phase. At high surfactant concentrations, the OR rate decreased with surfactant concentration for some of the surfactants. The OR rate also depended on the molecular structure of the surfactants: Tween 80 > Tween 20 > Tween 40 > Tween 60 > Triton SP 190 > Triton SP 175 (at 2 wt % surfactant). No clear correlation was found between the OR rate and the solubilization kinetics or solubilization capacity of the micelles.
1. Introduction Ostwald ripening (OR) is the process whereby larger droplets grow at the expense of smaller ones because the solubility of a material within a droplet increases as the interfacial curvature increases.1 In an oil-in-water emulsion, the kinetics of OR depend on the solubility and diffusion of the oil molecules in the aqueous phase separating the droplets. Currently, there is considerable debate about the influence of surfactant micelles on the OR rate in oil-in-water emulsions.2-5 In the absence of micelles, oil is transported through the aqueous phase by the diffusion of individual molecules. Previous studies have shown that surfactant micelles may either increase or decrease the OR rate, but the origin of these effects is unknown. Some researchers have proposed that mass transport is facilitated by the presence of surfactant micelles because they are able to solubilize oil molecules in their hydrophobic interior and carry them across the aqueous phase separating the droplets.3-4,6 Nevertheless, it is still unclear whether micelles directly take up oil from the aqueous phase immediately surrounding the droplets2 or take up oil through a fusion/fission mechanism with the droplet surface.3-4 Recently, an alternative mechanism has proposed that micelles enhance OR through their ability to increase the solubility of individual oil molecules in water, rather than acting as carriers.5 Several experimental studies of emulsions containing surfactant micelles have found that the OR rate is only a fewfold higher than that expected for molecular diffusion (1) Kabalnov, A. S.; Shchukin, E. D. Adv. Colloid Interface Sci. 1992, 38, 69-97. (2) Kabalnov, A. S. Langmuir 1994, 10, 680-684. (3) Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S. Langmuir 1998, 14, 5402-5411. (4) Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S. Langmuir 1999, 15, 4495-4501. (5) De Smet, Y.; Deriemaeker, L.; Finsy, R. Langmuir 1999, 15, 67456754. (6) McClements, D. J.; Dungan, S. R. J Phys. Chem. 1993, 97, 73047308.
of oil molecules through the aqueous phase.5,7,8 These studies suggest that surfactant micelles only have a minor effect on OR kinetics. On the other hand, some recent studies have shown that the presence of surfactant micelles enhances the OR rate 100- to 1000-fold greater than would be expected for molecular diffusion.3,4,9 It is clear that further experimental studies are required to enable us to better understand the role of surfactant micelles on Ostwald ripening. Recently, we studied the influence of surfactant type and concentration on the solubilization of n-tetradecane in micellar solutions.10 In the current study, we examine the influence of the same surfactants on OR of ntetradecane oil-in-water emulsions. By comparing the data from the two studies we hope to gain a better insight into the role of surfactant micelles on mass transport processes in emulsions. 2. Experimental Section 2.1. Materials. Polyoxyethylene (20) sorbitan monolaurate (Tween 20), polyoxyethylene (20) sorbitan monopalmitate (Tween 40), polyoxyethylene (20) sorbitan monostearate (Tween 60), polyoxyethylene (20) monooleate (Tween 80), and n-tetradecane (>99% pure) were purchased from Sigma Chemical Co. (St. Louis, MO). Two surfactants from the Triton SP family (polyoxyethylene isooctylphenyl ethers), Triton SP-175 (degree of ethoxylation per mole: 7.5) and Triton SP-190 (degree of ethoxylation per mole: 9.0), were obtained from Union Carbide (Danbury, CT). Distilled and deionized water was used in the preparation of all solutions and emulsions. 2.2. Emulsion Preparation. A surfactant solution was prepared by dissolving 2 wt % of surfactant in 98 wt % water. n-Tetradecane (20 wt %) and 80 wt % of surfactant solution were then homogenized in a high-speed blender (Waring Product (7) Kabalnov, A.; Weers, J. Langmuir 1996, 12, 3442-3448. (8) Taylor, P. Colloids Surf., A 1995, 99, 175-185. (9) Weiss, J.; Herrmann, N.; McClements, D. J. Langmuir 1999, 15, 6652-6657. (10) Weiss, J.; McClements, D. J. Langmuir, in press.
10.1021/la991477v CCC: $19.00 © 2000 American Chemical Society Published on Web 07/12/2000
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Figure 1. Time dependence of droplet size distribution of a 5 wt % n-tetradecane emulsion suspended in a 5 wt % Tween 20 solution. Division, New Hartford, CT) to form a coarse premix. Emulsion premixes were further homogenized using a single-valve highpressure laboratory homogenizer (APV, West Sussex, U.K.) at 100 MPa until a mean droplet radius of ≈140 nm for all emulsions was achieved. Each emulsion was diluted into micellar surfactant solutions to give a final droplet concentration of 5 wt % and a surfactant concentration between 0.5 and 10 wt %. Emulsions were then stored at 25 °C (( 2 °C) and samples were withdrawn at regular intervals for analysis of droplet size distribution. 2.3. Droplet Size Measurements. A static light scattering technique (Horiba LA-900, Horiba Instruments, Irving, CA) was used to measure the droplet size distribution of diluted emulsions. The technique uses Mie theory to calculate the droplet size distribution from measured light intensity at various scattering angles.11 A relative refractive index of 1.08 (ratio of the refractive index of the droplets to refractive index of surrounding phase) was used in the calculations. Each emulsion was diluted with distilled water before measurement to give a droplet concentration of 0.5 wt %. These results suggest that droplet ripening was mainly driven by molecular diffusion at low surfactant concentrations (28 h) was more consistent with OR than with coalescence.5,12-16 In addition, previous studies of droplet ripening in oil-in-water emulsions containing hydrocarbons with different molecular volumes also suggest that the dominant droplet growth mechanism is OR.16 In the remainder of the manuscript we therefore use the micelle-transport ripening rate (ω2) to characterize the OR process because it gave better agreement with our experimental data than the molecular-diffusion ripening rate (ω3). 3.3. Influence of Tween Type and Concentration on OR. The Tweens used in this study were esters of polyoxyethylene sorbitols and fatty acids. The main difference between the surfactants was the length and degree of unsaturation of the fatty acid chains: Tween 20, 12 carbons, saturated; Tween 40, 16 carbons, saturated; Tween 60, 18 carbons, saturated; Tween 80, 18 carbons, monounsaturated. These differences would be expected to affect the ability of the surfactant micelles to incorporate oil into their hydrophobic interior and transport them across an aqueous phase. Ripening rates of 5 wt % n-tetradecane emulsions with Tween 20 surfactant concentrations between 0.5 wt % and 10 wt % were evaluated from plots of rj2 versus time (Figure 3). There was an increase in ripening rate with surfactant concentration up to concentrations of about 8 wt % Tween 20, after which the ripening rate decreased (Figure 4). These results suggest that Tween 20 micelles are capable of accelerating ripening at relatively low concentrations, but that they retard it at high concentrations. Other workers have reported an approximately linear increase in OR rate with surfactant concentration,17 but they did not use the relatively high concentrations studied here. It is informative to compare the measured ripening rate with that expected in the absence of micelles, that is, if oil exchange occurred only by molecular diffusion of oil molecules through the aqueous phase. Using literature values for the physical characteristics of the hydrocarbon oil stabilized by Tween 20, we calculated a moleculardiffusion ripening rate (ω3) of 220 (nm)3 h-1. The measured (15) Kabalnov, A.; Weers, J. Langmuir 1996, 12, 3442-3448. (16) Bremer, L.; DeNijs, B.; Deriemaeker, L.; Finsy, R.; Gelade, E.; Joosten, J. Part. Part. Syst. Charact. 1996, 13, 350-353. (17) Soma, J.; Papadopoulos J. Colloid Interface Sci. 1996, 181, 225231.
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rates were between 1 and 2 orders of magnitude greater than the calculated rate for molecular diffusion alone (Table 1), which suggests that micelles were capable of significantly enhancing the diffusion rate. However, it should be pointed out that the difference between the lowest and highest ripening rates measured in our study was only about 4-fold. These results highlight the problem in determining the magnitude of the enhancement of OR by nonionic surfactant micelles that have been reported by other researchers.3,4,18 The 1 or 2 orders of magnitude increase above the expected ripening rate is more consistent with the 100- to 1000-fold increase reported in some studies,3,4 whereas the 4-fold increase between the lowest and highest rates is closer to the smaller enhancement rates reported in other studies.18 If it is assumed that oil can be transported between droplets via surfactant micelles, then the enhancement factor (φ) in the molecular-diffusion ripening rate (ω3) over that in the absence of micelles is given by:2
φ)1+
Ccj∞,mDm c∞,mDm )1+ c∞D c∞D
(3)
where c∞,m is the equilibrium concentration of oil solubilized within the micellar solution, Dm is the diffusion coefficient of the micelles, C is the surfactant concentration (grams of surfactant per cubic meter of aqueous phase), and cj∞,m is the specific solubilization capacity of the micelles (moles of oil per gram of surfactant). The specific solubilization capacity of Tween 20 was measured in a previous paper to be 0.023 g of tetradecane per gram of surfactant,10 which is equivalent to 0.12 mM oil per gram of surfactant. The diffusion coefficient of a Tween 20 micelle is approximately 4.2 × 10-11 m2 s-1.6 Inserting these values into eq 3 gives the following expression for the expected enhancement of the OR rate by the presence of surfactant micelles: φ ) 1 + 7.0 × 104 C (when C is expressed in wt %). The value of φ should therefore increase linearly from 1 to 7.0 × 105 as the surfactant micelle concentration increases from 0 to 10 wt %. The observed enhancement factor increased linearly with surfactant concentration at low Tween 20 concentrations (
