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Langmuir 2006, 22, 1544-1550
Rheology of High Internal Phase Emulsions Cynthia F. Welch,† Gene D. Rose,* David Malotky, and Sarah T. Eckersley The Dow Chemical Company, Midland, Michigan 48674 ReceiVed August 12, 2005. In Final Form: NoVember 14, 2005 The mechanical dispersion technology used in this study employs rotor-stator mixers that produce water-continuous high internal phase emulsions (HIPEs) with narrow drop size distributions and small drop sizes, even when the internal phase (oil) viscosity is quite high. Analysis of these HIPEs reveals trends that are consistent with formation by a capillary instability mechanism in which a shear deformation produces highly elongated drops that rupture to form uniform, small droplets. In the search for a predictive tool to aid in the manufacture and use of HIPEs, rheology data for these shear-thinning HIPEs have been compared to data for models in the literature. Existing models do not correctly account for the effect of a high internal phase viscosity on the rheological properties of the HIPE. Another shortcoming is failure to correctly address the shear-thinning exponent. Whereas internal phase viscosity does not seem to affect the shear-thinning exponent, the surfactant apparently plays an important role, possibly through its modification of the interfacial tension and continuous phase rheology.
Introduction Many of the desirable properties of water-continuous high internal phase emulsions (HIPEs) result from their rheological properties and the small size of the internal phase (oil) droplets. A large body of literature, which has been recently reviewed,1,2 reveals a few overriding trends. For example, during HIPE formation, interfacial tension (σ) and internal phase volume fraction (φ) can strongly influence the final drop size (R). All three parameters, σ, φ, and R, work together to dominate the rheology of the final HIPE. However, additional factors, such as internal phase viscosity, have not been investigated to any appreciable extent, and these may also play a significant role in HIPE rheology. Also, we lack a clear understanding of how variables such as the rheological properties of the individual phases, the interfacial tension, and the internal phase volume fraction interrelate with the process used for the emulsification to influence both the formation of the HIPE and its final rheological properties. HIPEs are concentrated emulsions with internal phase volumes > ∼70% of the total emulsion volume; as a result, the mobility of the internal phase droplets is restricted. As the internal phase concentration increases, the droplets deform into polyhedrons because the more stable spherical shape cannot be maintained. Whereas a dilute emulsion flows with a viscosity proportional to that of the continuous phase, a HIPE responds like an elastic solid at low shear strains. At stresses above the yield stress, HIPEs exhibit a shear-thinning viscosity. In this paper, we provide examples of water-continuous HIPEs with small, uniform droplets of high-viscosity (300 000 cSt) silicone oil. We prepared our HIPEs using mechanical dispersion technology employing a rotor-stator mixing device. The complex, nonuniform flow field in these mixers3 yields HIPEs with narrow drop size distributions and small drop sizes, even when the internal * Corresponding author (e-mail
[email protected]). † Present address: Los Alamos National Laboratory, MS H805, Los Alamos, NM 87545. (1) Babak, V. G.; Stebe, M.-J. J. Dispersion Sci. Technol. 2002, 23 (1-3), 1 and references cited therein. (2) Princen, H. M. Food Sci. Technol. (New York) 2004, 132 [Food Emulsions (4th ed.)], 413-483 and references cited therein. (3) Atiemo-Obeng, V. A.; Calabrese, R. V. In Handbook of Industrial Mixing Science and Practice; Paul, E. L., Atiemo-Obeng, V. A., Kresta, S. M., Eds.; Wiley-Interscience: New York, 2004; Chapter 8.
phase viscosity is high.4 We are not aware of any work in the literature that examines the formation and rheology of HIPEs with such a high internal phase viscosity, especially with small, uniform droplets of the high viscosity fluid. We investigate the rheological properties of these HIPEs and of the ingredients used to prepare them to evaluate possible mechanisms that allow the formation of small, uniform droplets when the internal phase viscosity is high. Rheology data obtained for HIPE samples prepared with these mixers are compared to other data and models in the literature. Our data show that a high viscosity of the internal phase can significantly influence the rheological properties of the HIPE. A model that accurately describes the relationships between the variables important to HIPE formation and the rheology of the resulting HIPEs would provide a valuable key to controlling the rheological properties of the final HIPEs and would serve as a beneficial tool for their manufacture and use in various applications. Although no suitable model was found, the data and trends contained herein should serve to refine existing models or stimulate new ones.
Background Mechanism of HIPE Formation. The process of forming concentrated or high internal phase emulsions is a complex one, and the mechanism is not well understood. Theoretical treatments of emulsification are generally limited to the rupture of isolated droplets as first derived by Taylor.5,6 Although important extensions7-9 have been developed, the underlying physics remains the same. When an isolated, spherical droplet of radius R0 and relatively low viscosity ηi is sheared in a fluid of viscosity ηe, the droplet will deform into an ellipsoid or elongated cylinder only when the shear stress ηeγ˘ surpasses the interfacial stress (4) Pate, J. E.; Peters, J.; Lutenske, N. E.; Pelletier, R. R. Process for Preparing High Internal Phase Ratio Emulsions and Latexes Derived Thereof. U.S. Patent 5,539,021A, 1996; U.S. Patent 5,688,842, 1997. (5) Taylor, G. I. Proc. R. Soc. A 1932, 138, 41. (6) Taylor, G. I. Proc. R. Soc. A 1934, 146, 501. (7) Janssen, J. M. H.; Meijer, H. E. H. J. Rheol. 1993, 37, 597. (8) Grace, H. P. Dispersion Phenomena in High Viscosity Immiscible Fluids Systems and Application of Static Mixers as Dispersions Devices in Such Systems. Presented at Third Engineering Foundation of Research Conference on Mixing, Aug 9-11, 1971, Andover, NH, AN-15937; republished as Chem. Eng. Commun. 1982, 14, 225. (9) Rallison, J. M. Annu. ReV. Fluid Mech. 1984, 16, 45.
10.1021/la052207h CCC: $33.50 © 2006 American Chemical Society Published on Web 01/17/2006
Rheology of High Internal Phase Emulsions
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Figure 1. Schematic of an “isolated” droplet deforming in an external phase composed of either a concentrated surfactant solution or a concentrated emulsion.
σ/R0, where γ˘ is the shear rate and σ is the interfacial tension. When the capillary number (Ca ) ηeγ˘ R0/σ, the ratio of the two competing stresses) exceeds a critical value, Cacrit, the elongated droplet will rupture to give smaller droplets of average radius R:
R ∝ Cacrit
σ ηeγ˘
(1)
Cacrit depends on the viscosity ratio (ηi/ηe) and the type of flow.6,8 Taylor’s analysis assumes a Newtonian continuous, or external, phase such that ηe is independent of γ˘ , a condition that holds for dilute emulsions with low concentrations of surfactant. Nevertheless, this equation has also proven to be effective in describing the average drop radius for some systems with a shear-thinning continuous phase or a high internal phase volume fraction (φ), if eq 1 is modified by replacing ηe with an effective external phase viscosity, ηeff.10,11 Here, ηeff is a function of γ˘ (i.e., ηeff ∝ γ˘ -x) and represents the viscosity of either a concentrated surfactant solution or the highly viscous, shear-thinning concentrated emulsion itself. In both cases, we need only be concerned with how a single, “isolated” droplet will deform and rupture. Here, the shear stress acting on the droplet is ηeffγ˘ . Figure 1 illustrates this concept schematically. As with the idealized Taylortype system, we can imagine that our “isolated” internal phase droplet will deform into a cylindrical one under shear. Mason and Bibette11 show that if the “external” phase exhibits a yield stress, the cylindrical droplet will continue to elongate until its interfacial stress (σ/r; r ) cylinder radius) exceeds the yield stress of the external phase; at this point, the cylinder undergoes a capillary instability to form many smaller droplets of a uniform size. The yield stress of the external phase produces smaller droplets than would be obtained with a Newtonian external phase. Additionally, a shear-thinning external phase actually promotes a higher degree of monodispersity in drop size.11 Although ηe plays a large role in HIPE formation, the effect of internal phase viscosity ηi on the process is usually assumed to be negligible. Another common assumption is that highly viscous oils cannot be emulsified in water because of the large viscosity mismatch. Grace reported that when the viscosity ratio (ηi/ηe) exceeds 3.5, the energy required to form the emulsion increases drastically, making the inverse emulsion much more likely to form.8 As we have just seen, the shear stress responsible for droplet breakup is ηeffγ˘ , and in HIPEs, ηeff can be quite large.11,12 Therefore, the viscosity ratio appropriate for determining the energy required to form the emulsion is really ηi/ηeff, and formation of water-continuous HIPEs of high-viscosity oils is (10) Aronson, M. P. Langmuir 1989, 5, 494. (11) Mason, T. G.; Bibette, J. Langmuir 1997, 13, 4600. (12) Bricen˜o, M.; Salager, J. L.; Bertrand, J. Trans IChemE Part A 2001, 79, 943.
not impossible. However, one question remains: is there an upper limit for ηi beyond which monodisperse HIPEs cannot be formed? Rheology of Concentrated Emulsions or HIPEs. Because the rheological properties of HIPEs are so important to their application, much literature can be found on the subject. However, the understanding of how variables such as R, σ, ηe, ηi, and φ contribute to the shear-dependent viscosity is limited. Much of it is based on Princen’s geometry-based model that relates the microstructure of an idealized HIPE to its macroscopic rheological properties.13-16 In two dimensions, the droplets are represented as hexagons in the unstrained state. As shear is applied, the hexagons distort until they transform into parallelograms, which have a higher total film area. When the parallelogram becomes unstable, a new film is generated and the droplets revert to the hexagonal configuration, although the position of the droplets in one layer has shifted with respect to the adjacent layers. By extending this model to three dimensions, Princen proposed equations for the viscosity (ηHIPE) of HIPEs16
ηHIPE )
(
τ0 σ + C(φ)ηe γ˘ ηeR32γ˘
)
1/3
(2)
where R32 is the surface-volume (Sauter) mean drop radius and C(φ) is a numerical factor that may depend on φ. To arrive at these equations, Princen made the following assumptions: (1) ηi can be neglected, (2) the film surfaces are rendered immobile by the presence of surfactant, (3) all of the continuous phase is contained in the Plateau borders at the intersections of films, and (4) the films separating the droplets are negligibly small. Although many investigators have found Princen’s model to appropriately describe modulus and yield stress data,17-21 the functional form of eq 2 has been successful in fitting experimental data only by changing the exponent in the last term to 1/2 and by using τ0 and C(φ) as fit parameters.16 The use of eq 2 in a predictive sense is questionable. As an alternative to Princen’s model, Otsubo and Prud’homme proposed a simple scaling-type analysis to superimpose viscosityshear rate data for various HIPEs with a given φ.22 They found that a master curve could be created if the data were plotted in the form of (2R32ηHIPE)/(σηe) versus ηeγ˘ . This analysis assumes a simpler dependence of ηHIPE on σ, ηe, and R than Princen’s model. The approach works well for the HIPEs studied in ref 22 but has not been extensively tested for a wide range of HIPEs. Princen and Kiss,16 Otsubo and Prud’homme,22 and many others23-25 have shown through experimental studies that ηHIPE is proportional to σ, ηe, and φ, but inversely proportional to R. No study has shown a dependence of ηHIPE on ηi, but all published work has been limited to HIPEs with relatively low ηi. Although no theoretical models have been successful in describing the relationship among all of these variables in a quantitative sense, a fairly good qualitative description has been established. In summary, the rheological properties of HIPEs are governed by (13) Princen, H. M. J. Colloid Interface Sci. 1983, 91, 160. (14) Princen, H. M. J. Colloid Interface Sci. 1985, 105, 150. (15) Princen, H. M.; Kiss, A. D. J. Colloid Interface Sci. 1986, 112, 427. (16) Princen, H. M.; Kiss, A. D. J. Colloid Interface Sci. 1989, 128, 176. (17) Pal, R. Colloid Polym. Sci. 1999, 277, 583. (18) Ponton, A.; Cle´ment, P.; Grossiord, J. L. J. Rheol. 2001, 45, 521. (19) Hemar, Y.; Horne, D. S. Langmuir 2000, 16, 3050. (20) Pons, R.; Solans, C.; Tadros, Th. F. Langmuir 1995, 11, 1966. (21) Jager-Le´zer, N.; Tranchant, J.-F.; Alard, V.; Vu, C.; Tchoreloff, P. C.; Grossiord, J.-L. Rheol. Acta 1998, 37, 129. (22) Otsubo, Y.; Prud’homme, R. K. Rheol. Acta 1994, 33, 29. (23) Kawaguchi, M.; Tsujino, T.; Kato, T. Langmuir 2000, 16, 5568. (24) Das, A. K.; Mukesh, D.; Swayambunathan, V.; Kotkar, D. D.; Ghosh, P. K. Langmuir 1992, 8, 2427. (25) Hayakawa, K.; Kawaguchi, M.; Kato, T. Langmuir 1997, 13, 6069.
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Table 1. HIPE Samples Evaluated sample
wt % oil
wt % surfactant
wt % water
R32 (µm)
300000 cSt DC200 silicone oil with sodium laureth sulfate surfactant SLS-300k-75.3 75.3 5.3 19.5 0.50 SLS-300k-78.6 78.6 5.5 16.0 0.43 SLS-300k-83.8 83.8 5.8 10.4 0.33 SLS-300k-85.8 85.8 6.9 7.4 0.36 100 cSt DC200 silicone oil with sodium laureth sulfate surfactant SLS-100-68.2 68.2 4.3 27.5 0.75 SLS-100-75.4 75.4 4.7 19.9 0.60 SLS-100-79.8 79.8 4.7 15.5 0.49 SLS-100-80.4 80.4 4.5 15.1 0.42 SLS-100-80.0 80.0 5.3 14.7 0.21 SLS-100-86.1 86.1 6.0 7.9 0.10 SLS-100-89.1 89.1 5.6 5.2 0.15 100 cSt DC200 silicone oil with 50:50 Brij 30/Brij 56 surfactants Brij-100-86.0 86.0 4.6 9.4 0.33 Brij-100-84.0 84.0 4.4 11.6 0.46
the three-dimensional interconnected structure of the continuous phase. When small shear deformations are applied, the internal phase droplets do not coalesce; rather, they deform as the thin liquid films that separate them flow. If the viscosity of the internal phase is low, it has no direct influence on the viscosity of the emulsion. In this case, the rheological properties of the emulsion are controlled by R, σ, ηe, and φ, as confirmed by many literature studies. However, if a HIPE with extremely high ηi could be formed, would the viscous droplets resist deformation under shear and thereby contribute to the viscosity of the emulsion? In this paper, we explore this possibility for the first time by comparing experimental data for our unique HIPEs with extremely high ηi to the models of Princen and Kiss16 and Otsubo and Prud’homme.22 Other theoretical studies26,27 and experimental studies28 have presented thought-provoking ideas regarding the rheological behavior of HIPEs. However, unlike the works of Princen and Kiss16 and Otsubo and Prud’homme,22 none of these have provided a model that can be used in a predictive sense to unambiguously relate emulsion properties to their rheological behavior. Experimental Section Materials. The HIPE samples evaluated in this study are listed in Table 1. These samples were prepared using a laboratory-scale mechanical dispersion process described in detail elsewhere.4 It is a continuous emulsification process in which we merge the water with a surfactant/oil mixture in a toothed, rotor-stator mixer that imposes complex, nonuniform flow conditions on the fluids.3 The relative feed rates to the mixers are the same as the phase ratios given in Table 1, except for the Brij-100 samples, which are diluted after emulsification as described below. The stainless steel mixer has a double mechanical seal and is jacketed for temperature control. All of these samples are water-continuous emulsions of silicone oil [poly(dimethylsiloxane)]. We used two silicone oils of widely differing viscosities: 100 cSt (96.4 cP) and 300 000 cSt (293 100 cP) DC200 Fluids (Dow Corning Corp.). The surfactant used to emulsify these oils was either the anionic Empicol ESB (∼70 wt % sodium laureth sulfate in water) or the nonionic 50:50 mixture of Brij 30 [polyoxyethylene (4) lauryl ether] and Brij 56 [polyoxyethylene (10) cetyl ether]. We collected the Empicol ESB samples immediately after they exited the mixer; the two Brij-containing samples were further diluted by adding more aqueous phase to the emulsion in a second dilution mixer that was also jacketed for temperature control. (26) Cates, M. E.; Sollich, P. J. Rheol. 2004, 48, 193. (27) Sollich, P. Phys. ReV. E 1998, 58, 738. (28) Mason, T. G.; Bibette, J.; Weitz, D. A. J. Colloid Interface Sci. 1996, 179, 439.
Figure 2. Particle size distribution for a symmetric HIPE prepared with 300 000 cSt DC200 silicone fluid (sample SLS-300k-83.8 in Table 1). Emulsion composition was confirmed via moisture determination with the OHaus MB45. A moisture analysis of Empicol ESB revealed a surfactant concentration of 73 wt %; this value was used to calculate the actual concentration of surfactant (sodium laureth sulfate) in these samples, as listed in Table 1. Oil droplet size was measured with the Coulter LS230 Laser Light Scattering Particle Sizer; the data are reported as the Sauter mean radius, R32 () ΣniRi3/ΣniRi2, also known as the “surface-volume” mean due to its relationship with the surface area per unit volume of the dispersed phase: AV ) 3/R32). All of the samples have a fairly narrow drop size distribution. Figure 2 illustrates the narrow, symmetric, small particle size that can be achieved by our mechanical dispersion process. Rheology. Rheology experiments were performed with either a Rheometrics RFS II or RFS III Fluids Spectrometer. Concentrated emulsions often cause slip at solid boundaries in rheometers because the dispersed oil can adsorb and coalesce at these solid surfaces.29 Therefore, we conducted a series of experiments to determine if wall slip was a problem. In our system, a stainless steel Couette geometry was susceptible to wall slip problems, but no evidence of slip was observed when either a titanium cone and plate or a parallel plate geometry was used. Accordingly, all rheological data reported below were collected with the titanium cone and plate geometry. For the data of Figure 6 only, the titanium parallel plate geometry was used as a mixing device to shear a coarse emulsion and create smaller droplets.
Results and Discussion Rheology of the Continuous Phase. As discussed under Background, the viscosity of the external phase (ηe) can play an important role in both the emulsification process and the viscosity of the final emulsion. During emulsification, a higher ηe will produce a lower final drop size, as shown by eq 1. This equation assumes a low internal phase viscosity (ηi), and literature results indicate that small drop sizes become increasingly difficult to obtain as ηi increases past a certain critical viscosity.11 The ability to create aqueous HIPEs with small droplets of highly viscous oils is, therefore, a significant challenge. Yet, as demonstrated in Table 1 and Figure 2, our mechanical dispersion process can routinely create water-continuous HIPEs with small drop sizes (
