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Langmuir 1999, 15, 6652-6657
Ostwald Ripening of Hydrocarbon Emulsion Droplets in Surfactant Solutions J. Weiss, N. Herrmann, and D. J. McClements* Biopolymer and Colloids Laboratory, Department of Food Science, University of Massachusetts, Amherst, Massachusetts 01003 Received December 17, 1998. In Final Form: May 28, 1999 Ultrasonic attenuation spectra (1-150 MHz) of a series of 5 wt % oil-in-water emulsions containing hydrocarbon droplets (n-decane, n-dodecane, n-tetradecane, n-hexadecane, n-octadecane) stabilized by various surfactants (Brij 35, Tween 20, SDS, Triton X-100) were measured as a function of time. Droplet size distributions were calculated from attenuation spectra using ultrasonic scattering theory. Changes in droplet size distribution were also measured by static light scattering on the same emulsions after dilution ( Tween 20, Triton X-100 > SDS. The addition of excess surfactant (0.5-5 wt % Tween 20) to n-tetradecane emulsions stabilized by Tween 20 had little influence on ripening rates.
Introduction Many natural and manufactured products exist in the form of emulsions, e.g., foods, cosmetics, pharmaceuticals, paints, petrochemicals, explosives, and agrochemicals.1-4 Macroemulsions are thermodynamically unstable systems that break down over time due to a variety of physicochemical processes, e.g., gravitational separation, flocculation, coalescence, and Ostwald ripening.3-6 One of the principal concerns of emulsion scientists is to better understand the factors that influence these processes, so that they can control the rate at which they proceed in a more systematic fashion. In this study, we are primarily concerned with the phenomenon of transcondensational ripening or Ostwald ripening in oil-in-water emulsions. Ostwald ripening is the process whereby larger droplets grow at the expense of smaller ones, because of the transport of dispersed phase molecules from the smaller to the larger droplets through the intervening continuous phase.7-9 The driving force for this process is the increase in the solubility of the dispersed phase in the continuous phase that occurs when the droplet curvature increases; i.e., the droplet size decreases.10 In a polydisperse system, there is a higher concentration of dissolved dispersed phase molecules surrounding the smaller droplets than surrounding the larger droplets. As a result of this concentration gradient, there is a net movement of dispersed phase molecules from the smaller to the larger droplets. * To whom correspondence should be addressed. (1) Becher, P. Encyclopedia of Emulsion Technology; Marcel Dekker: New York, 1985; Vol. 2. (2) Becher, P. Encyclopedia of Emulsion Technology; Marcel Dekker: New York, 1988; Vol. 3. (3) McClements, D. J. Food Emulsions: Principles, Practice and Techniques; CRC Press: Boca Raton, FL, 1998. (4) Friberg, S. E.; Karsson, K. Food Emulsions, 3rd ed.; Marcel Dekker: New York, 1997. (5) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997. (6) Hunter, R. J. Introduction to Modern Colloid Science; Oxford Science: Oxford, U.K., 1993. (7) Wagner, C. Z. Elektochem. 1961, 65, 581. (8) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35. (9) Kabalnov, A. S. Langmuir 1994, 10, 680. (10) Skinner, L. M.; Sambles, J. R. Aerosol Sci. 1972, 3, 199.
The growth of the droplets in an emulsion has pronounced adverse effects on its bulk physicochemical properties; e.g., it can reduce the stability of droplets to flocculation, coalescence, or gravitational separation or it can change emulsion appearance and rheology.3 Consequently, it is important to improve our understanding of the factors that influence Ostwald ripening. Ostwald ripening is particularly important in emulsions in which the dispersed phase has an appreciable solubility in the continuous phase, e.g., low molecular weight hydrocarbon oils.11 The rate of Ostwald ripening is usually assumed to be limited by the diffusion of dispersed phase molecules across the continuous phase. Nevertheless, there are a number of other physicochemical processes that may also influence its rate, such as the movement of molecules across the interfacial membranes surrounding the droplets12 and the presence of surfactant micelles in the continuous phase.13 Micelles are able to incorporate appreciable amounts of nonpolar molecules in their hydrophobic interior, through a process known as solubilization.14 Whether or not empty and/or swollen micelles participate in the mass transport process between droplets is still a matter of debate.9,15 Investigation of the factors that influence mass transport processes in emulsions relies on the availability of analytical techniques to characterize changes in their droplet size distribution. Traditionally, light scattering and electrical pulse counting techniques have been used to monitor these processes, but they are only applicable to dilute emulsions.3,5 Instruments based on ultrasonic attenuation spectroscopy have recently been developed to analyze droplet size and concentration.16 These instruments have major advantages over traditional methods (11) Kabalnov, A. S.; Shchukin, E. D. Adv. Colloids Interface Sci. 1992, 38, 69. (12) Yao, J. H.; Laradji, M. Phys. Rev. E 1993, 47, 2695. (13) Soma, J.; Papadopoulos, K. D. J. Colloid Interface Sci. 1996, 181, 225. (14) Weiss, J.; Coupland, J.; McClements, D. J. J. Phys. Chem. 1996, 100, 1066. (15) Taylor, P. Colloids Surf. A 1995, 99, 175. (16) McClements, D. J. Langmuir 1996, 12, 3454.
10.1021/la981739d CCC: $15.00 © 1999 American Chemical Society Published on Web 08/05/1999
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because they can be used to analyze concentrated and optically opaque emulsions without any sample preparation. The two objectives of this study are to show that ultrasonic attenuation spectroscopy can be used to monitor Ostwald ripening in concentrated emulsions and to investigate the influence of hydrocarbon type, surfactant type, and surfactant concentration on the aging of emulsion droplets. Ostwald Ripening Theory The change in droplet size distribution of an emulsion with time due to Ostwald ripening is described by the following equations:7,8
[
]
∂q0(r,t) R ∂ q0(r,t)(r - rk) )0 + ∂t rk∂r r2 R)
2Dγcrf∞Vm2 RT
(1)
(2)
where q0(r,t) is the droplet size distribution function at time t, r is the droplet radius, rk is a critical droplet radius, γ is the interfacial tension at the oil-water interface, D is the diffusion coefficient of the oil through the aqueous phase, crf∞ is the solubility of the oil (when contained in an infinitely large droplet) in the aqueous phase, Vm is the molar volume of the oil, R is the gas constant and T is the absolute temperature. If the transport of oil is purely diffusion controlled, the critical radius is equal to the mean radius:
∫0∞q0(r,t)r dr rk ) ∞ ∫0 q0(r,t) dr
(3)
In general, eq 1 has to be solved numerically;17 however it is possible to obtain analytical solutions by making certain simplifying assumptions. If it is assumed that the emulsion is dilute and the system has reached steady state, then the droplet size distribution as a function of a normalized radius F ) r/rk will follow the expression
q0(r,t) ) f(t)F2h(F) ) const 3 F2 t 4/3 3 + F 1+ τ′D
(
) (3 -3 2F)
(
)
7/3
11/3
(4) (3-2F - 2F)
exp
for F < 3/2, and q0(r,t) ) 0 for F g 3/2. It should be noted that the change in droplet size distribution with time is the product of a function that depends only on time f(t) and a function that depends only on the normalized radius h(F). The time constant τ′D is given as
τ′D )
9(rjt)0)3RT
(5)
8γDcrf∞Vm2
Equation 4 is also known as the Lifshitz-SlyozovWagner (LSW) model.7,8 The change in mean droplet radius with time is given by
rj3t
-
3 rjt)0
(
8γDcrf∞Vm2t ) 9RT
(17) Marder, M. Phys. Rev. A 1987, 36, 858.
)
(6)
Equation 6 indicates that the cube of the mean droplet size of the emulsions will increase linearly with time, with a slope given by ω ) 4R/9. The parameter ω is therefore a useful mean of describing the Ostwald ripening rate of emulsions. Materials and Methods Materials. Polyoxyethylene (20) sorbitan monolaureate (Tween 20), Polyoxyethylene (23) lauryl ether (Brij 35), polyoxyethylene (10) isooctylphenyl ether (Triton X-100), sodium dodecyl sulfate (SDS), n-decane, n-dodecane, n-tetradecane, n-hexadecane, and n-octadecane were purchased from Sigma Chemical Co. (St. Louis, MO). Distilled and deionized water was used in the preparation of all solutions and emulsions. Selection of surfactants was based on differences in interfacial properties, e.g., structure of the interfacial layer and surface charge density. Selection of hydrocarbons was based on differences in molar volume (200-350 mol/L). Sample Preparation. Surfactant solutions were prepared by dissolving 20 mM of surfactant in distilled water. The 5 wt % n-hydrocarbon and 95 wt % of surfactant solution were then homogenized in a high-speed blender (Waring Product Division, New Hartford, CT) to form a coarse premix. Emulsion premixes stabilized by nonionic surfactants were further homogenized using a single valve high-pressure laboratory homogenizer (APVGaulin, model 8.30R, Willmington, MA) at 100 MPa until a mean droplet radius of ≈100 nm was achieved. Emulsion premixes stabilized by SDS were further homogenized using a sonicator (B. Braun Biotech., Melsungen, Germany) at 350 W until a mean droplet radius of ≈40 nm was achieved. All emulsions were prepared and stored at 25 °C (( 2 °C) for a period of up to 6 months, and the droplet size distribution was analyzed at regular intervals. Static Light Scattering Technique. A static light scattering technique (Horiba LA-900, Horiba Instruments, Inc., Irving, CA) was employed 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.5 A relative refractive index of 1.08 (ratio of the refractive index of the droplets to that of the surrounding phase) was used in the calculations. To prevent multiple scattering effects, each emulsion was diluted with distilled water prior to measurement in order to obtain a droplet concentration 12) is actually greater than expected from this commonly used extrapolation technique. Influence of Surfactant Concentration on Ostwald Ripening Rate. A series of 5 wt % emulsions containing n-tetradecane droplets stabilized by Tween 20 were prepared, and different concentrations of surfactant were added to them (0, 1.5, 2.5, and 5 wt % Tween 20). The Ostwald ripening rates of the emulsions, as determined by ultrasonic spectroscopy, initially decreased with surfactant concentration and then increased slightly (Figure 8). This type of behavior has also been observed for other types of surfactants13 and is due to changes in the location (22) McAuliffe, C. Science 1969, 163, 478.
3φΓ r32FE
(7)
Here, csat is the surfactant concentration per unit mass of emulsion. For the emulsion used in this study, the concentration of surfactant required to saturate the droplet surface has been calculated as 0.05 wt % (φ ) 0.05, Γ ) 4.6 × 10-7 kg m-2, r32 ) 150 × 10-9 m, FE ) 1000 kg m-3). In reality, the surfactant will be in equilibrium between the droplet surface and the bulk aqueous phase, and therefore, a greater amount of surfactant will have to be added before the droplet surface becomes fully saturated. We expect that the emulsion containing no additional surfactant would be below the amount required for saturation, and therefore, there would be no surfactant micelles present in the continuous phase. Once the droplet surfaces become saturated, any additional surfactant will form micelles in the aqueous phase, which increases the amount of oil that can be solubilized in it and therefore increases the rate (eq 4). Nevertheless, the increase in solubilization rate is relatively small, compared to the increase in the amount of material that can be solubilized in the micelles. The presence of 1 wt % micelles in the aqueous phase increases the amount of oil solubilized in the aqueous phase almost 106-fold, i.e., from ≈3 × 10-8 wt % for pure water to ≈0.02 wt % for a 1 wt % surfactant solution. On the other hand, the Ostwald ripening rate only increases by about 25%, and therefore, we can conclude that the increase in solubility of the hydrocarbons in the aqueous phase cannot account for the observed behavior. This is in agreement with several studies that have also shown that the presence of surfactants has only marginal influence on ripening rates.9 Influence of Surfactant Type on Ostwald Ripening Rate. Ostwald ripening of a series of 5 wt % emulsions containing n-tetradecane droplets stabilized by different surfactant types was measured using ultrasonic spectroscopy (Figure 9). A constant surfactant concentration of 20 mM was used in all the emulsions, which was above their CMC’s. The ripening rate was strongly dependent on the surfactant type, decreasing in the following order: Brij 35 > Tween 20, Triton X-100 > SDS. Surfactant type could influence ripening rate in a number of ways. First, the interfacial tension at the droplet surface is dependent on surfactant type (Table 3). Overall, there was no clear correlation between the interfacial tension of the different surfactants and the ripening rate; i.e., SDS has the second highest interfacial tension but the lowest ripening rate.
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This may account for the extremely slow ripening rate for the SDS emulsions compared to the ones stabilized by nonionic surfactants. Third, the amount of hydrocarbon solubilized in the aqueous phase is dependent on the maximum solubilizing capacity (MSC) of the surfactant micelles. In a previous study, we measured the MSC of Tween 20 and Triton X-100 as 0.02 and 0.08 g of tetradecane/g of surfactant, respectively.23 Despite having significantly different MSC’s the Tween 20 and Triton X-100 had fairly similar ripening rates (Table 3).
Figure 9. Dependence of droplet growth on surfactant type in 5 wt % n-tetradecane oil-in-water emulsions stabilized by 20 mM SDS. Table 3. Physical Characteristics of Surfactants Used in Experimentsa surfactant
∼Mw (kg/mol)
10-6CMC (M)
10-3γ (N/m)
ωmeasd (m3/s)
Tween 20 Brij 35 Trition X-100 SDS
1.228 1.198 1.875 0.288
49 91 320 8200
3.5 10.2 1.6 8.4
2.0 × 10-25 2.1 × 10-24 2.5 × 10-25 5.5 × 10-26
a
The interfacial tensions and ripening rates were measured in our laboratory. All the other parameters were taken from the literature.28,29
Nevertheless, if we only considered nonionic surfactants, then the ripening rates were fairly well correlated with the interfacial tensions (Table 3); i.e., the higher the interfacial tension, the faster the ripening rate (as predicted by eq 6). Second, the ripening rate may depend on the interactions between a micelle and the droplet surface. In the emulsions containing SDS there will be a strong electrostatic repulsion between the droplets and micelles that prevents them from coming into close contact.
Conclusions Ultrasonic spectroscopy could be used to monitor the rate of Ostwald ripening in fairly concentrated oil-in-water emulsions without the need for sample dilution. It therefore has considerable advantages over alternative technologies, such as light scattering or electrical pulse counting. The rate of Ostwald ripening in emulsions depends on the type and concentration of surfactant present, as well as the chemical structure of the nonpolar phase. Our results suggest that Ostwald ripening is a complex process that can be influenced by many factors, such as solubilization, interfacial tension, and colloidal interactions. At present, there is still a fairly poor understanding of the factors that influence Ostwald ripening, and it is clear that more systematic research is needed in this area. Acknowledgment. The authors thank Malvern Instruments for supplying the ultrasonic spectrometer used in these experiments and the United States Department of Agriculture for partly supporting this project. LA981739D (23) Weiss, J.; Brathwaite, D.; McClements, D. J. Presented at the 89th AOCS Annual Meeting, Chicago, IL, 1998. (24) McClements, D. J. J. Am. Acoust. Soc. 1992, 91, 849. (25) Epstein, P. S.; Carhart, R. R. J. Am. Acoust. Soc. 1953, 25, 553. (26) Wilke, C. R.; Chang, P. AIChE J. 1955, 1, 264. (27) McAuliffe, C. J. Phys. Chem. 1966, 70, 1267. (28) Schick, M. J. Nonionic Surfactants; Marcel Dekker: New York, 1967. (29) Zhou, J. S.; Dupeyrat, M. J. Colloids Interface Sci. 1990, 134, 320.
