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Influence of Ostwald Ripening on Rheology of Oil-in-Water Emulsions Containing Electrostatically Stabilized Droplets J. Weiss† and D. J. McClements*,‡ Department of Food Science and Technology, University of Tennessee, Knoxville, Tennessee 37091, and Biopolymer and Colloids Laboratory, Department of Food Science, University of Massachusetts, Amherst, Massachusetts 01003 Received July 14, 1999. In Final Form: November 15, 1999 The influence of droplet size and Ostwald ripening on the dynamic shear rheology of 25 wt % hydrocarbon oil-in-water emulsions containing small droplets (75 to 100 nm) stabilized by sodium dodecyl sulfate (SDS) was measured. Emulsions underwent a liquid-to-solid transition when the mean droplet radius was decreased below ∼85 nm because of overlap of the electrical double layers surrounding the droplets. Below this critical radius the emulsions had a yield stress and shear modulus that increased with decreasing droplet radius. Ostwald ripening studies were carried out using emulsions that initially contained small n-hexadecane (rinitial ) 72 nm) or n-octadecane droplets (rinitial ) 65 nm) and therefore had solidlike characteristics. There was appreciable growth in droplet radius of the n-hexadecane emulsions due to Ostwald ripening, which led to a solid-to-liquid phase transition when the double layer thickness became less than the surface-to-surface droplet separation. On the other hand, the n-octadecane emulsions remained solidlike during the same period because n-octadecane has a much lower water solubility than n-hexadecane and therefore Ostwald ripening was much slower. Our data has important implications for the formulation of emulsion-based products that must have specific textural properties.
Introduction The rheological properties of emulsion-based materials, such as foods, cosmetics, petrochemicals and pharmaceuticals, play a major role in determining their suitability for particular applications.1-6 Depending on its application an emulsion-based material may have rheological characteristics that are liquid, solid, plastic, or viscoelastic. The rheological constants of the material, such as viscosity, shear modulus, and yield stress, may also have to lie within a predefined range. It may also be desirable for the rheological properties of a material to remain constant throughout its use or for them to change in response to some specific alteration in the environment, such as pH, ionic strength, temperature, mechanical agitation, or dilution. To design and manufacture emulsion-based materials that are capable of exhibiting this wide range of rheological characteristics it is necessary to understand the factors that determine emulsion rheology. The rheology of suspensions of hard spheres has been studied extensively and is governed mainly by particle concentration, particle size, and shear stress.7-10 Theoretical and semiempirical equations have been developed that give good agreement with experimental measurements on this type of system. These equations have to be modified for emulsions because of the fluidity of the droplets and because of the existence of various kinds of attractive and repulsive interactions between the droplets, * To whom correspondence should be addressed. † University of Tennessee. ‡ University of Massachusetts. (1) Sherman, P. Industrial Rheology With Particular Reference To Foods, Pharmaceuticals and Cosmetics; Academic Press: London, 1970. (2) Barnes, H. A. Colloids Surf. 1994, 91, 89. (3) Pal, R.; Yan, Y.; Masliyah, J. H. In Emulsions: Fundamentals and Applications in the Petroleum Industry; Schramm, L. L., Ed.; American Chemical Society: Washington, D. C., 1992; Chapter 4. (4) Tadros, T. F. Colloids Surf. 1994, 91, 30. (5) Tadros, T. F. Adv. Colloids Int. Sci. 1996, 68, 97. (6) McClements, D. J. Food Emulsions: Principles, Practice and Techniques; CRC Press: Boca Raton, FL, 1998.
e.g., van der Waals, electrostatic, steric, depletion, and hydrophobic.5-7,11,12 When attractive interactions predominate the droplets tend to aggregate, which increases their effective size and volume fraction, thereby altering emulsion rheology.5,13 In addition, the structural integrity of aggregates is often disrupted by applied shear forces,14 which means that aggregated colloidal dispersions often have strong shear-thinning characteristics.4,5 When repulsive forces predominate, the effective volume fraction and size of the droplets is also increased, but this time because they are not able to approach as closely together as hard spheres.15,16 When the distance of closest approach is significant compared to the particle radius, these systems behave as though they have a particle concentration that is much greater than the actual droplet concentration. As a result they may be much more viscous than expected or even exhibit elastic behavior.5 The creation of emulsions that can be stored for long periods before use is important for many commercial products. Unfortunately, emulsions are thermodynamically unstable systems that break down over time through a variety of physicochemical instability mechanisms, e.g., gravitational separation, flocculation, coalescence, and (7) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: Oxford, 1986; Vol. 1. (8) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: Oxford, 1989; Vol. 2. (9) Liu, S.; Masliyah, J. H. In Suspensions: Fundamentals and Applications in the Petroleum Industry; Schramm, L. L., Ed.; American Chemical Society: Washington, D. C., 1996, Chapter 3. (10) Macosko, C. W. Rheology: Principles, Measurements and Applications; VCH Publishers: New York, 1994. (11) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York. (12) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1992. (13) de Vries, A. J. In Rheology of Emulsions; Sherman, P., Ed.; Pergamon Press: New York, 1963. (14) Serra, T.; Casamitjana, X. AIChE J. 1998, 44, 1724. (15) Goodwin, J. W.; Rhider, A. M. In Colloid and Interface Science; Kerker, M., Ed.; Academic Press: New York, 1976; Vol. 4, p 529. (16) Buscall, R.; Goodwin, J. W.; Hawkins, M. W.; Ottewill, R. H. J. Chem. Soc., Faraday Trans. 1982, 78, 2889.
10.1021/la9909392 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/05/2000
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Ostwald ripening.6,7,11 These instability mechanisms cause changes in the spatial distribution and/or size of the droplets, which may cause significant alterations in emulsion rheology. In this study we focus on the influence of Ostwald ripening on the rheology of hydrocarbon oilin-water emulsions containing small electrostatically stabilized droplets. 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.17-20 Materials and Methods Materials. The n-hexadecane (>99% pure), n-octadecane (>99% pure), sodium chloride, and SDS were obtained from Sigma Chemical Company (St. Louis, MO). Double distilled and deionized water was used to prepare all solutions and emulsions. Emulsion Preparation. An aqueous surfactant solution was prepared by dissolving 50 mM SDS in distilled water. 25 wt % n-hydrocarbon and 75 wt % surfactant solution were initially homogenized in a high-speed blender (Waring Product Division, New Hartford, CT) to form a coarse premix. Emulsion premixes were further homogenized using a high-pressure valve homogenizer (APV Gaulin, Limited; West Sussex, UK) and/or a sonicator (B. Braun Biotech., Melsungen, Germany) to obtain a range of droplet radii between 75 and 100 nm. All emulsions were prepared and stored at 25 °C ((2 °C) for a period of up to 140 h and the droplet size and rheological properties were analyzed at regular intervals. Droplet Size Characterization. A laser diffraction instrument (Horiba LA-900, Horiba Instruments Incorporated, Irving, CA) was used to measure the droplet size of emulsions containing relatively large particles (r > 90 nm). A relative refractive index of 1.08 (equal to the ratio of the refractive index of the oil/refractive index of the aqueous phase) was used by the instrument to calculate the droplet size distribution. Droplet size measurements are reported as the mean diameter: r )Σniri/Σni, where ni. is the number of droplets of diameter ri. A number of emulsions contained droplets that were too small to analyze using the laser diffraction instrument. The mean droplet size of these emulsions was determined using a turbidimetric technique.21 Emulsions were diluted with distilled water, placed in a quartz cuvette, and their turbidity measured at 800 nm using a UV-visible spectrophotometer (UV-2101PC, Shimadzu Scientific Instruments, Columbia, MD). The mean droplet size was then estimated using the following equation21
x 3
r ) r0
τ0 τ
(1)
where r and τ are the radius and turbidity of the unknown emulsion, and r0 and τ0 are the radius and turbidity of an emulsion of known droplet size (the emulsion with the smallest droplets that could be measured using the laser diffraction technique). This equation assumes that the droplets are much smaller than the wavelength of light (r < λ/6), which was valid for the emulsions analyzed by this method because the largest radius used was about 90 nm and the wavelength of light used was 800 nm. For both laser diffraction and turbidity measurements the emulsions were diluted to 0.01 wt % with 50 mM SDS solution prior to analysis to avoid multiple scattering effects. Rheology Measurement. The rheological properties of emulsions were measured using a dynamic shear rheometer with a concentric cylinder measurement cell (Constant Stress Rheometer, CS-10, Bohlin Instruments, Cranbury, NJ). The diameter of the inner cylinder was 25 mm, and the diameter of the outer cylinder was 27.5 mm. Samples were placed in the temperature(17) Wagner, C. Z. Elektochem. 1961, 65, 581. (18) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35. (19) Kabalnov, A. S.; Shchukin, E. D. Adv. Coll. Int. Sci. 1992, 38, 69. (20) Taylor, P. Colloids Surf. A 1995, 99, 175. (21) Kerker, M. The Scattering of Light and Other Electromagnetic Radiation; Academic Press: New York, 1969.
Figure 1. Shear strain dependence of shear modulus and phase angle of 25 wt % n-octadecane oil-in-water emulsions stabilized by 50 mM SDS. Subsequent measurements were made in the linear viscoelastic region, where the rheology is independent of shear strain.
Figure 2. Dependence of shear stress on shear rate for 25 wt % n-octadecane oil-in-water emulsions with different mean droplet radii (see box). controlled measurement vessel and allowed to equilibrate to the required temperature (25 °C) for 5 min prior to making the measurements. The shear stress of the samples was measured as the shear rate was increased from 0 to 1000 s-1. The resulting curves were analyzed to determine the yield stress (τ0) of the emulsions. This was done by extrapolating the measurements in the linear region above the yield stress to the y-axis. An oscillation test was used to measure the magnitude of the shear modulus (G*) and phase angle (δ) of emulsions over a range of frequencies (0.01-10 Hz). The linear viscoelastic region of each sample was determined from measurements carried out over a range of shear strains (0.0001 to 1). The shear rheology of the samples was independent of shear strain below a critical level, but above this level the shear modulus decreased and the phase angle increased indicating sample disruption or slip (Figure 1). For this reason subsequent experiments were always carried out at strains below the critical level to ensure the measurements were in the linear viscoelastic region.
Results and Discussion Influence of Droplet Size on Emulsion Rheology. The dependence of shear stress on shear rate for 25 wt % n-octadecane oil-in-water emulsions with different droplet radii was measured (Figure 2). The n-octadecane oil was used in these studies to minimize changes in droplet size with time due to Ostwald ripening (see below). The emulsion containing the smallest droplets (r ) 76 nm) did not flow until the applied shear stress exceeded about 90
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Figure 3. Dependence of yield stress on droplet radius for 25 wt % n-octadecane oil-in-water emulsions.
Figure 5. Frequency dependence of phase angle for 25 wt % n-octadecane oil-in-water emulsions with different mean droplet radii (see box).
Figure 4. Frequency dependence of complex shear modulus for 25 wt % n-octadecane oil-in-water emulsions with different mean droplet radii (see box).
Figure 6. Dependence of complex shear modulus and phase angle (at 1 Hz) on droplet radius for 25 wt % n-octadecane oil-in-water emulsions.
Pa. From a rheological standpoint this emulsion could therefore be classified as a plastic, which behaved as an elastic solid below the yield stress (τ0 < 90 Pa) and as a liquid above it (τ0 > 90 Pa). The magnitude of the yield stress decreased when the size of the droplets in the emulsions increased (Figure 3). When the droplet radius exceeded about 90 nm the emulsions did not exhibit a yield stress and behaved as fluids. Similar trends were observed in the oscillation experiments (Figures 4 to 6). The magnitude of the shear modulus decreased dramatically when the droplet radius increased above about 8090 nm (Figures 4 and 6). The phase angle of the emulsions was close to zero for the smallest emulsions, indicating that they behaved as elastic solids, but rapidly increased to close to 90° when the droplet radius increased above 80-90 nm, indicating that they behaved as viscous fluids (Figures 5 and 6). The shear modulus and phase angle of the small emulsions (r < 80 nm) was relatively independent of frequency, which is expected for rigid materials (Figures 4 and 5). On the other hand, the shear modulus of the fluidlike emulsions (r > 90 nm) increased and their phase angle decreased with frequency, which indicated that they became more rigid at high frequencies. It should be stressed that these dramatic changes in emulsion rheology were due entirely to changes in microstructure, as all emulsions had the same overall composition. Transitions from fluid-to-solid behavior with decreasing particle size have also been observed for polystyrene latex suspensions, where they were attributed
to electrostatic repulsion between particles.5 When the surface-to-surface distance (h) between droplets becomes less than twice the thickness of the electrical double layer (κ-1) there is a strong electrostatic repulsion between the droplets that causes an emulsion to gain “solidlike” characteristics.15,16 The thickness of the double layer is related to the ionic strength of the liquid between droplets dispersed in water at room temperature by the following relationship7
κ ) 3.2 × 109 xI
(2)
here I is the ionic strength of the electrolyte solution surrounding the charged surfaces, I ) 1/2ΣmiZi2, where zi and mi are the valancy and molarity of the electrolyte ions. We calculated a value of κ-1 ) 2.0 nm using the known concentration of Na+ counterions (50 mM) in the system. We assumed that the dodycl sulfate ions (C12H25SO4-) did not contribute to the ionic strength because they were either adsorbed to the surfaces of the emulsion droplets or they formed micelles in the aqueous phase. An emulsion should start to gain solidlike characteristics when the effective volume fraction of the droplets rises above that required for close packing (φ0). The effective volume fraction of the droplets in an electrostatically stabilized emulsion is given by the following equation5
(
φeff ) φ 1 +
δ r
)
3
(3)
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where δ is the distance of closest approach between the droplets, which is usually equated to the double layer thickness, κ-1. We calculated the value of δ required for the emulsions to become rigid (i.e., φeff ≈ φ0) using the above equation and the physical parameters of the system (φ ) 0.30, r ) 85 nm) to be 25 nm. This value is an order of magnitude greater than the thickness of the double layer calculated using eq 2. The most likely reason for this discrepancy is that many of the Na+ ions were associated with the negatively charged surfactant headgroups (C12H25-SO4-Na+), rather than being free in the aqueous phase, and therefore they did not contribute to electrostatic screening. The driving force for this association could be the close proximity of the surfactant headgroups at the oil-water interface. If they were all electrically charged there would be a strong electrostatic repulsion between them, which is energetically unfavorable, and therefore they bind counterions to reduce this effect. This hypothesis is supported by measurements of the ζ-potential of SDS stabilized emulsion droplets. If all of the dodecyl sulfate ions were completely ionized, then the surface charge density of the emulsion droplets should have been σd ) -e/a0, where e is the unit electrical charge and a0 is the surface area of an SDS molecule. This would give a surface charge density of 0.28 C m-2, using a0 ) 0.57 nm2.12 On the other hand, the surface charge density can also be determined from the ζ-potential of a droplet7
σd )
( )
2κkBT0R zeζ sinh ze 2kBT
(4)
where kB is the Boltzmann constant, T is the absolute temperature, 0 is the permittivity of free space, R is the relative permittivity of the material surrounding the droplets, z is the valance of the ions, and e is the unit of electrical charge. Using this approach the surface charge density was calculated as 0.07 C m-2 using the thickness of the double layer given by eq 2 and a ζ-potential of -105 mV, which was measured in a previous study using hydrocarbon droplets stabilized by SDS.22 If we assumed that the concentration of free Na+ ions was less than 50 mM because of some binding to the adsorbed dodecyl sulfate ions, then the thickness of the electrical double layer would have been larger, which would mean that the surface charge density calculated using the above approach would have been even lower. The experimentally determined value of σd is therefore at least 4 times lower (and possibly much more) than the calculated value, which suggests that many of the sodium ions were indeed bound to the surface of the droplets and therefore did not participate in electrostatic screening. This would account for the discrepancy between the predicted and measured double layer thickness mentioned earlier. It should also be noted that the Debye length is the distance for the magnitude of the electrostatic interactions to reduce to 1/e of their surface value. It is likely that electrostatic repulsive interactions between droplets are still significant at distances greater than this value (i.e., δ > κ-1). An expression has been derived for relating the shear modulus of electrostatically stabilized colloidal dispersions to particle characteristics15,16
(
G ) 4πRR0r2ζ2
Figure 7. Prediction of the shear modulus of 25 wt % n-octadecane oil-in-water emulsions with different droplet radius and double layer thickness (see box) using eq 5 in text.
)
κ2d2 + 2κd + 2 exp[-κ(d - 2r)] (5) d4
where R is a parameter that depends on the packing of the (22) Mei, L., Decker, E. A., McClements, D. J. J. Agric. Food Chem. 1998, 46, 5072.
Figure 8. Frequency dependence of complex shear modulus for 25 wt % n-octadecane oil-in-water emulsions (r ) 65 nm) with different NaCl concentrations (see box).
particles () 0.833), ζ is the zeta potential, and d is the center-to-center separation of the droplets () 2r + h). Calculations of the dependence of shear modulus on droplet radius for emulsions with different electrical double layer thickness are shown in Figure 7. These predictions indicate that the shear modulus should increase as the droplet radius decreased, as was observed in our experiments (Figure 6). They also showed that the shear modulus predicted using eq 2 (κ-1 ) 2.0 nm) was about 6 orders of magnitude less than the measured value (Figures 4 and 6), which also suggested that this double layer thickness was too small, probably because ion binding effects reduced the free Na+ concentration. A double layer thickness between about 10 and 20 nm is required to give reasonable agreement between the shear modulus predicted by eq 5 and the measured values shown in Figure 6. This suggested that the free Na+ ion concentration in the aqueous phase that contributed to electrostatic screening was between about 0.2 and 2 mM. The importance of electrostatic interactions in determining the rheology of the emulsions was further demonstrated by examining the influence of NaCl (Figures 8 and 9). There was a dramatic decrease in the shear modulus and increase in the phase angle when the NaCl concentration exceeded 2 mM, which can be attributed to electrostatic screening effects. At relatively high salt concentrations the double layer is compressed and there-
Ostwald Ripening in O/W with Electrostatically Stabilized Droplets
Figure 9. Frequency dependence of phase angle for 25 wt % n-octadecane oil-in-water emulsions (r ) 65 nm) with different NaCl concentrations (see box).
Figure 10. Growth of droplet radius with time for 25 wt % n-octadecane oil-in-water emulsions and 25 wt % n-hexadecane oil-in-water emulsions due to Ostwald ripening.
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Figure 12. Time dependence of complex shear modulus (at 1 Hz) for 25 wt % n-octadecane oil-in-water emulsions and 25 wt % n-hexadecane oil-in-water emulsions undergoing Ostwald ripening.
Figure 13. Time dependence of phase angle (at 1 Hz) for 25 wt % n-octadecane oil-in-water emulsions and 25 wt % n-hexadecane oil-in-water emulsions undergoing Ostwald ripening.
hexadecane emulsion increased significantly over time, whereas that of the n-octadecane emulsion remained fairly constant. The rate of Ostwald ripening in a dilute emulsion is given by the following expression17,18
jrt3 - rjt)03 )
Figure 11. Time dependence of yield stress for 25 wt % n-octadecane oil-in-water emulsions and 25 wt % n-hexadecane oil-in-water emulsions undergoing Ostwald ripening.
fore the effective volume fraction of the droplets is approximately equal to their actual volume fraction (eq 3). Influence of Ostwald Ripening on Droplet Size. The evolution of the mean droplet size of 25 wt % n-hexadecane and n-octadecane oil-in-water emulsions was measured using laser diffraction and turbidity measurements (Figure 10). The droplet size of the n-
8γDCrf∞V2mt 9RT
(6)
where rj is the mean droplet radius, t is the time, γ is the interfacial tension at the oil-water interface, D is the diffusion coefficient of the oil through the aqueous phase, crf∞ 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. The main reason that the Ostwald ripening rate of the n-hexadecane droplets is appreciably greater than the n-octadecane droplets is because the former has a greater water solubility.12 Equation 6 indicates that the cube of the mean droplet radius of the emulsions should increase linearly with time, with a slope that depends on the physical characteristics of the oil and aqueous phases. There was an approximately linear relationship between rj3 and time for the emulsions used in this study (Figure 10), with a slope of 3390 ( 150 nm3 h-1 (r2 ) 0.985, n ) 10) for the n-hexadecane droplets and 1680 ( 150 nm3 h-1 (r2 ) 0.941, n ) 10) for the n-octadecane droplets.
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Influence of Ostwald Ripening on Emulsion Rheology. The time-dependence of the yield stress (Figure 11), shear modulus (Figure 12) and phase angle (Figure 13) of 25 wt % n-hexadecane and n-octadecane oil-in-water emulsions was measured. These measurements clearly show that the rheological characteristics of the emulsions changed dramatically over time due to Ostwald ripening. The yield stress and shear modulus of the n-hexadecane emulsions decreased appreciably within the first 40 h, whereas that of the n-octadecane emulsions remained relatively constant. The phase angle measurements indicated that the n-hexadecane emulsions underwent a transition from solidlike to fluidlike behavior after 40 h, whereas the n-octadecane emulsions remained solidlike throughout the experiment. The difference in the time dependence of the rheology of the two emulsions can be attributed to differences in Ostwald ripening behavior. At the beginning of the experiment both emulsions have droplet sizes below the critical size required to give solidlike behavior, i.e., r < 80 nm. The radii of the droplets in the n-octadecane emulsions only increased slightly during the experiment and remained below the critical radius. On the other hand, the radii of the droplets in the n-hexadecane emulsions increased above the critical radius (∼80 nm) after about 40 h because of Ostwald ripening (Figure 10).
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Conclusions The rheological characteristics of oil-in-water emulsions containing electrostatically stabilized droplets is particularly sensitive to droplet radius when the thickness of the electrical double layer is of the order of the surface-tosurface droplet separation. Emulsion instability mechanisms that cause significant changes in droplet size, such as Ostwald ripening or coalescence, may therefore have a pronounced influence on emulsion rheology. This has important implications for the development of emulsionbased products that are required to have a long shelf life. To create a product that maintains its desirable rheological characteristics over a sufficiently long period it may be necessary to prevent or retard emulsion instability mechanisms. On the other hand, it may be possible to develop emulsion-based products that exhibit dramatic changes in rheological characteristics in response to small changes in environmental conditions. An emulsion with solidlike characteristics could be made to have liquidlike characteristics by the addition of small amounts of salt or by dilution. Acknowledgment. This material is based upon work supported by the Cooperative State Research, Education and Extension Service, U.S. Department of Agriculture, under Agreement Number 97-35503-4371. LA9909392
