Influence of Semibatch Emulsification Process Conditions on The

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Ind. Eng. Chem. Res. 2010, 49, 6042–6046

Influence of Semibatch Emulsification Process Conditions on The Physical Characteristics of Highly Concentrated Water-in-Oil Emulsions Oscar A. Alvarez,*,†,‡ Lionel Choplin,† Ve´ronique Sadtler,† Philippe Marchal,† Marie-Jose´ Ste´be´,§ Julien Mougel,| and Christophe Baravian| GEMICO, ENSIC-INPL, 1 Rue GrandVille, BP 20451, F- 54001 Nancy Cedex, France, Grupo de Disen˜o de Productos y Procesos (GDPP), Departamento de Ingenierı´a Quı´mica, UniVersidad de los Andes, Carrera 1 E, No. 19A-40, Bogota´, Colombia, E´quipe Physico-chimie des Colloı¨des, UMR-CNRS No. 7565, UniVersite´ Henri Poincare´, F-54506 VandœuVre-le`s-Nancy Cedex, France, and Laboratoire d’e´nerge´tique et de me´canique the´orique et applique´e (LEMTA), UMR-CNRS No. 7563, 2 aVenue de la Foreˆt de Haye, BP 160, F-54501 VandœuVre-le`s-Nancy Cedex, France

We studied the energy consumption per unit volume during preparation of highly concentrated water-in-oil emulsions in a two-step semibatch process. In particular, we studied the effect of two process variables, the water addition flow rate (Qw) and the agitation speed (N). The oil used for emulsion preparation was n-dodecane, the surfactant was sorbitan monooleate (Span 80) and deionized water was used for the dispersed phase. The results obtained showed that two steps were required to get complete incorporation of the dispersed phase and a homogeneous and stable gel-emulsion. With the help of independent physical characteristics measurement performed at the end of the preparation process, we established two functional relationships, relating storage modulus (G′) with energy consumption (Ev), (G′ ∝ Ev0.6), and average liquid cell size (Rm) of the dispersed phase with energy consumption (Ev), (Rm ∝ EV-0.3). A structural scaling law can be deduced that relates the elastic modulus to the reciprocal of the square of the average liquid’s cell size of the gel-emulsion, corroborated by independent measurements and predicted by recent published models. 1. Introduction 1,2

Highly concentrated emulsions, also called gel-emulsions or biliquid foams, have many interesting industrial applications. Polymeric materials can be easily prepared from these emulsions, provided one or both of the phases contain monomer species.3-5Gel-emulsions can also be used as new drug delivery systems in pharmaceutical formulations,6liquid explosives,7,8and creams as cosmetic products. These gel-emulsions have an extremely high content of dispersed phase volume fraction (φv), for instance, higher than 0.90. As a consequence, the dispersed droplets resemble the bubbles of a foam; in other words, they are strongly deformed and form a polydispersed polyhedron. Their rheological behavior is similar to that of foams. They can be characterized by two essential mechanical (or rheological) properties: an elastic modulus, G, and a yield stress, τc, above which the gel-emulsion behaves as a shear-thinning liquid (viscoplastic behavior). The elastic modulus is essentially the storage modulus, G′, that can be obtained in a small amplitude (linear viscoelastic domain) oscillatory shear experiment, provided the loss (or viscous) modulus, G′′, is at least 1 order of magnitude lower. From a structure viewpoint, the G′ modulus, which is directly related to the (average) liquid cell size, is enough for characterizing the gel-emulsions. Two main emulsification processes are used for their preparation: a batch process, in which all the components are mixed together, 9 and a semibatch (two-step) process, by far the most widely used, in which the dispersed phase is progressively * To whom correspondence should be addressed. E-mail: oalvarez@ uniandes.edu.co. Tel.: (571) 3 39 49 49 ext. 3935. Fax: (571) 3 32 43 34. † GEMICO. ‡ Universidad de los Andes. § Universite´ Henri Poincare´. | UMR-CNRS No. 7563.

introduced into the continuous one under relatively gentle stirring conditions. This “incorporation step” is followed by a “homogenization step” in the same equipment.1,2,6,10-22 In both processes, the emulsification protocol as well as detailed information is rarely described with the consequence that it is extremely difficult and even impossible to reproduce the same gel-emulsion. Even if it is well-known and/or accepted that the gel-emulsion properties are highly dependent upon their preparation mode, the protocol description remains qualitative. The influence of the process conditions has not been yet quantitatively reported. This study will aim at partly fulfilling this drawback in the particular case of the semibatch two-step process for the preparation of gel-emulsions, as well as at trying to establish a relationship between the mechanical energy supply needed to prepare them and their characteristic physical properties. 2. Materials Water was deionized by a Milli-Q system (Millipore). The oil was n-dodecane (Labosi, 99% Purum). The nonionic surfactant was a sorbitan mono-oleate (Span 80, HLB ) 4.3) obtained from Aldrich and previously dispersed in the oil. The refractive indices of water and oil phases are 1.33 and 1.443, respectively. In all experiments, the concentration of surfactant in oil was much larger than the critical micellar concentration (CMC), resulting in a constant interfacial oil/water tension (σ ) 2.8 mN/m measured using pendant drop technique at 27 °C). For the prepared emulsions the dispersed phase (aqueous phase) volume fraction (φv) was 0.91 or 0.93. 3. Methods 3.1. Semibatch Two-Step Emulsification Process. The gelemulsions were prepared at 27 °C by progressive incorporation of the water into the oil phase, using a peristaltic pump

10.1021/ie9020073  2010 American Chemical Society Published on Web 05/14/2010

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Figure 1. Schematic of agitation systems: (1) rheomixer and (2) jacketed agitated vessel.

Figure 2. Typical stress sweep for gel-emulsion.

Table 1. Geometrical Dimensions Used in the Agitation Systemsa

confirm the viscoplastic behavior of the gel-emulsions and to determine the flow curves, also at 27 °C. 3.4. Average Liquid Cell (Droplet) Size Determination. The measurement device is a steady light transport apparatus,24 placed on a AR 1000 rheometer (TA Instruments, USA), allowing us to simultaneously determine a light transport length (L*) and rheological functions. All determinations were realized with gel-emulsions freshly prepared, in other words, between 2 and 3 h after the end of the homogenization step. The transport length is related to the average droplet size (Rm) through the Mie theory, according to the following equation:

agitation system

Dv (mm)

Rav ) Da/Dv

system 1 system 2

50 70

0.9 0.7

a Dv is the vessel diameter and Da/Dv is the ratio of impeller diameter to vessel diameter.

(Bioblock Scientific). The emulsification process was divided in two steps: (1) addition of water into the oil phase at a given flow rate (Qw) and a given agitation speed (N); (2) homogenization of the gel-emulsion for a given homogenization time (th) at the same agitation speed than in the previous step. 3.2. Agitation Systems. We used two different agitation systems, illustrated in Figure 1.23 System 1. This system incorporates a “rheomixer”, which consists of a jacketed mixing vessel and an impeller both being installed on a RFS II rheometer (Rheometric Scientific, now TA Instruments, USA). The impeller is connected to the torquemeter of the rheometer, allowing a precise measurement of the torque during the emulsification process. The impeller consists of a 45° pitched 4-blade turbine and is positioned at the oil phase surface at the beginning of the process. The ratecontrolled rheometer allows us to maintain the agitation speed (the impeller is in relative motion with respect to the vessel) constant, whatever the viscosity evolution of the fluid contained in the vessel during the whole emulsification process. The dimensions of this system are given in Table 1. To avoid sample ejection from the mixing vessel, as well as overload of the torquemeter, we limited the agitation speed (N) to 525 rpm. System 2. This system consists in a jacketed mixing vessel equipped with the same kind of impeller, identically positioned at the oil phase surface at the beginning of the process. The impeller is connected to a torquemeter (Turbo test 33/750 P, Rayneri Groupe VMI), equipped with a speed control ensuring a constant rotational speed (N), from 200 up to 3000 rpm. The incorporation of water uses the same procedure as in the case of system 1. The dimensions of system 2 are also given in Table 1. 3.3. Rheological Characterization of Gel-Emulsions. Rheological behavior of gel-emulsions was evaluated at 27 °C with a controlled stress rheometer AR2000 (TA Instruments, USA) immediately after emulsion preparation. Rheological analysis was performed with parallel plate geometry using 40 mmdiameter plate dimensions and a 1 mm-gap. To determine the storage (G′) and loss (G′′) modulus, dynamic stress sweep experiments were carried out at a frequency of 1 rad/s in which the stresses ranged from 0.1 to 1000 Pa. A typical experiment is reported in Figure 2. Steady shear flow experiments were also performed, with shear rates ranging from 1 to 1000 s-1, to

4 πR 3 3 m φvL* ) Csca(x, m)[1 - g(x, m)] Where x ) kRm, in which k is the wavenumber and m is the ratio of the “particle” to medium refractive indices. Csca and g are the scattering cross-section and the anisotropy factor, respectively. 4. Results and Discussion Figure 3 presents the evolution of power consumption during the preparation of a gel-emulsion in the system 1 (rheo-mixer) as a function of the agitation speed, N (Figure 3a), and as a function of the added water flow rate, Qw (Figure 3b). For all experiments, the occurrence of the end of water addition is indicated on the figures. If we can observe a monotonous increase in power during the water addition step, the second step is clearly divided into two subperiods: a fast kinetics followed by a slower one. We have observed that the first kinetics was associated with the process of effective final incorporation of water (provided not all the water was “really” incorporated into the emulsion medium at the end of the addition period), whereas the second kinetics was associated with a real homogenization of the gelemulsion. For the sake of simplicity, we will call as the first step, the step corresponding to the water addition, and as the second step, the step corresponding to both the final incorporation of water and the homogenization period. From all these curves, it is possible to calculate the energy consumption (the surface below these curves), distinguishing between the energy consumption during the first and the second steps. For the calculation during the second step, it is necessary to define the homogenization time. We have taken the time needed to reach a constant value in power. In all our experiments, this homogenization time was essentially constant and equal to 600 s. The results are reported in Figure 4. Clearly, the energy consumption during the water addition step is negligible as compared to the energy consumption during the

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Figure 5. Power curve for two different Rav (ratio of impeller diameter to vessel diameter).

Figure 3. Power consumption during the emulsification: (a) power consumption vs agitation speed (N); (b) power consumption vs water flow rate (QW).

Figure 4. Energy consumption (Ev) for two different agitation speeds (N), with a volume fraction of dispersed phase of 0.91.

homogenization step. Nevertheless, the first step with a progressive addition of water was revealed essential for the gel-emulsion preparation with the agitation systems and agitation speeds used. It is clear that in our experimental window, the highest is the proportion of the energy consumed during the homogenization step, at a given agitation speed. The power (torque) measurement appears to be independent of the water addition flow rate at a

Figure 6. Viscous flow curve for a volume fraction of dispersed phase of 0.93.

given agitation speed and is an increasing function of the agitation speed for a given water addition flow rate. Because the total energy consumption can be considered as equivalent to the energy consumption during the homogenization step, and because during this homogenization step the emulsion volume is constant, we will use the energy consumption per unit volume, EV (J/m3). In the case of system 2, the measurement of the torque, therefore the evaluation of the power is not as precise as in the case of system 1. As a consequence, the evaluation of energy consumption per unit volume during the preparation of gelemulsions in such a system requires the knowledge of the corresponding power curve,25 and the use of the Metzner-Otto concept.26 The power curve of the agitated vessel equipped with the impeller used in both systems 1 and 2 was first determined for two different Rav (ratio of impeller diameter to vessel diameter) with the help of the rheomixer for allowing precise measurement of torque, and consequently of power. The volume of the vessel was limited to 50 mL. A Newtonian viscous silicone oil (PDMS 47 V1000, from Rhodia Silicones) was used for the determination of the power curve in the laminar regime. Figure 5 shows that for the two different sizes of the same impeller, the power curve is essentially unchanged. The same power curve (dimensionless power versus Reynolds number) can be used for the case of system 2, provided the vessel-impeller configuration is homothetic to the configuration used in the rheomixer (the dimensions are just twice). With the help of the flow curve (viscosity as a function of the shear rate) determined independently, at the end of the gelemulsion preparation in system 2, after sampling and off-line measurement with a AR2000 rheometer, and shown in Figure 6, and considering that during the whole homogenization step, where the torque or power measurement does not show any significant variation (cf. Figure 3), the rheology of the gel-

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Figure 7. Elastic modulus (G′) vs energy consumption per unit volume (Ev).

emulsion is not significantly changed, the determination of the energy consumption per unit volume can be calculated using the Metzner-Otto concept. This concept states that, in laminar regime and for shear-thinning liquids, there appears to be an average of effective deformation (γ) for a mixer, which adequately characterizes the power consumption, and which is directly proportional to the agitation speed: γ˙ ) KS.N In which γ˙ is the effective shear rate and KS is the Metzner-Otto constant (equal to 14 for the impeller geometry used). Therefore, for a given agitation speed, N, we calculate the corresponding effective shear rate. The viscous flow curve gives us the corresponding apparent viscosity, which is then used for calculating the corresponding Reynolds number. Using the power curve, the power is determined, and then using the homogenization time and the volume of the gel-emulsion prepared, we obtain the energy consumption per unit volume, EV. 4.1. Relationships (Scaling Laws) between Mechanical Energy Supply and Physical Characteristics of the Gel-Emulsions. Several gel-emulsions were prepared in both system 1 and system 2, according to the specifications given in Table 1, for different agitation speeds (N), and different water addition flow rates (QW). In all cases, the process parameters were selected in order to ensure the system being in the laminar regime, principally during the homogenization period. In addition, we have used values of the water addition flow rate higher than 14 g/min, to calculate the energy consumption per unit volume according to the method described in the previous section. At the end of the preparation process, samples of gelemulsions were analyzed. The elastic modulus, G′, and the average liquid cell size, Rm, were obtained independently with small amplitude oscillatory and steady light transport experiments, respectively. These measurements of the physical characteristics of the gelemulsions were realized within a maximum of 3 hours after the end of the preparation process. In a previous paper, we reported that during such a period of time, the evolution of these physical characteristics can be considered as almost negligible.27 Figure 7 shows the elastic modulus as a function of the energy consumption per unit volume for all experiments. The experimental data can be represented by the following functional relationship: G′ ∝ EV0.6

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Figure 8. Average liquid cell size (Rm) vs energy consumption per unit volume (Ev).

Figure 8 shows the average liquid cell size as a function of the energy consumption per unit volume for experiments corresponding to gel-emulsions in which the volume fraction of the dispersed phase is 0.93. The experimental data can be represented by the following functional relationship: Rm ∝ Ev-0.3 As a consequence, we can deduce the following functional dependency between the two structural parameters, the elastic modulus and the average liquid cell size: G′ ∝ Rm-2 This last relationship has been shown to remain valid for much longer periods of time,27 and is in fact a structural relationship which is not in agreement with the up-to-now accepted model proposed for such highly concentrated emulsions.28 This model based on geometrical arguments predicts that the elastic modulus is inversely proportional to the average droplet size. We have however proposed in a previous work another interpretation based on physicochemical arguments, in other words based on the hypothesis that, for our formulations with nonionic surfactants, the van der Waals forces dominate. From dimensional considerations, we can express the elasticity as follows:27 G′ ∝

W V

where W has the dimension of an energy and V the dimension of a volume. If we consider V as the continuous phase volume by unit droplet Vf, then Vf ∝ Rm3

(1 - φv) φv

where Rm is the average liquid cell size and φv is the dispersed phase volume fraction. If W is due to van der Waals forces in the case of nonionic surfactant, and the amplitude of van der Waals energy between two identical droplets in a different surrounding medium can be expressed as29 W)

AHRm 12D

where D is the distance between the surfaces of the two droplets and AH is the Hamaker constant, G′ can be expressed by G′ ∝

AHφv 12DRm2(1 - φV)

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We obtain that the elastic modulus is inversely proportional to the square of the average liquid cell size, in agreement with our experimental results.27 Furthermore, a recent model,30 based on either geometrical or dimensional arguments, and other works31,32 predict the same functional dependency. 5. Conclusions A semibatch emulsification process can be divided into two steps, an incorporation step and a homogenization step. The homogenization step has been revealed to be essential for obtaining gel-emulsions, stable over a period of at least 3 hours after the end of the preparation process. It has been shown that, under specific process conditions (agitation speed and water addition flow rate), the total energy consumption per unit volume was essentially the one corresponding to the homogenization period. Physical characteristics (elastic modulus and average liquid cell size) were obtained at the end of the preparation process. Two coherent scaling laws were obtained independently at the end of the emulsification process: a relationship between the elastic modulus and the energy consumption per unit volume (G′ ∝ Ev-0.6) and a relationship between an average liquid cell size and the energy consumption per unit volume (Rm ∝ EV-0.3). A structural scaling law (G′ ∝ Rm-2) has been deduced, corroborated by independent measurements, and supported by a model developed on the basis of physicochemical arguments. Acknowledgment Financial support provided by the sponsoring companies of the GEMICO-ENSIC-INPL laboratory is gratefully acknowledged. Financial support provided to Oscar Alvarez by the Universidad de los Andes (Bogota, Colombia) is also fully acknowledged. Literature Cited (1) Pons, R.; Erra, P.; Solans, C.; Ravey, J. C.; Ste´be´, M. J. Viscoelastic Properties of Gel Emulsions: Their Relationship with Structure and Equilibrium Properties. J. Phys. Chem. 1993, 97, 12320. (2) Ravey, J. C.; Ste´be´, M. J.; Sauvage, S. Gelification d′e´mulsions Fluore´es. J. Chim. Phys. 1994, 91, 259. (3) Cameron, N. R.; Sherrington, D. C. High Internal Phase Emulsions (HIPE’s)-Structure, Properties and Use in Polymer Preparation. AdV. Polym. Sci. 1996, 126, 163. (4) Solans, C.; Pinazo, A.; Caldero, G.; Infante, M. R. Highly Concentrated Emulsions as Novel Reaction Media. Colloids Surf. A 2001, 176, 101. (5) Solans, C.; Esquena, J.; Azemar, N. Highly Concentrated (Gel) Emulsions, Versa´til Reaction Media. Curr. Opin. Colloid. Interface Sci. 2003, 8, 156. (6) Rocca, S.; Muller, S.; Ste´be´, M. J. Release of a Model Molecule from Highly Concentrated Fluorinated Reverse Emulsions Influence of Composition Variables and Temperature. J. Controlled Release 1999, 61, 251. (7) Malkin, A. Y.; Masalova, I.; Slatter, P.; Wilson, K. Effect of Droplet Size on the Rheological Properties Highly Concentrated w/o Emulsions. Rheol. Acta 2003, 43, 584–591. (8) Masalova, I.; Malkin, A. Y. Master Curves for Elastic and Plastic Properties of Highly Concentrated Emulsions. Colloid J. 2008, 73, 327– 336. (9) Pons, R.; Carrera, I.; Erra, P.; Kunieda, H.; Solans, C. Novel Preparation Methods for Highly Concentrated Water-in-Oil Emulsions. Colloids. Surf. A 1994, 91, 259.

(10) Lissant, K. The Geometry of High-Internal-Phase-Ratio Emulsions. J. Colloid Interface Sci. 1966, 22, 462. (11) Aronson, M1989. The Role of Free Surfactant in Destabilizing Oilin-Water Emulsions. Langmuir 1989, 5, 494. (12) Chen, H. H.; Ruckenstein, E. Effect of the Nature of Hydrophobic Oil Phase and Surfactant in the Formation of Concentrated Emulsions. J. Colloid Interface Sci. 1991, 145, 260. (13) Ruckenstein, E.; Ebert, G.; Platz, G. Phase Behavior and Stability of Concentrated Emulsions. J. Colloid Interface Sci. 1989, 133, 432. (14) Das, A. K.; Mukesh, D.; Swayambunathan, V.; Kotkar, D.; Ghosh, P. Concentrated Emulsions. 3. Studies on the Influence Continuous-Phase Viscosity, Volume Fraction, Droplet and Temperature on Emulsion Viscosity. Langmuir 1992, 8, 2421. (15) Aronson, M.; Petko, M. Highly Concentrated Water-in-Oil Emulsions: Influence of Electrolyte on their Properties and Stability. J. Colloid Interface Sci. 1993, 159, 134. (16) Rocca, S.; Ste´be´, M. J. Mixed Concentrated Water/Oil Emulsions (Fluorinated/Hydrogenated): Formulation, Properties and Structural Studies. J. Phys. Chem. B 2000, 104, 10490. (17) Jager-Le´zer, N.; Tranchant, J. F.; Alard, V.; Vu, C.; Tchoreloff, P. C.; Grossiord, J. L. Rheological Analysis of Highly Concentrated W/O Emulsions. Rheol. Acta 1998, 37, 129. (18) Langenfeld, A.; Schmitt, V.; Ste´be´, M. J. Rheological Behavior of Fluorinated Highly Concentrated Reverse Emulsions with Temperature. J. Colloid Interface Sci. 1999, 218, 522. (19) Pal, R. Yield Stress and Viscoelastic Properties of High Internal Phase Ratio Emulsions. Colloid Polym. Sci. 1999, 277, 583. (20) Terreros, A.; Galera Gomez, P.; Lopez-Cabarcos, E. Influence of the Surfactant Chain Length and the Molecular Weight of Poly(oxyethylene) on the Stability of Oil-in-Water Concentrated Emulsions. Prog. Colloid. Polym. Sci. 2000, 156, 21. (21) Peker, S.; Bora, K.; Over, Y. Effect of Interfacial Properties on the Drop Size of High Internal Phase Ratio Emulsions. Colloids. Surf. A 2001, 182, 43. (22) Langenfeld, A.; Ste´be´, M. J. Influence of Physico-Chemical Parameters on Rheological Properties of Concentrated Reverse Emulsions. Phys. Chem. Chem. Phys. 2002, 4, 322. (23) Alvarez O. A. Emulsions Inverses Tre`s Concentre´es: Influence du Proce´de´ et de la Formulation sur leurs proprie´te´s rhe´ologiques. Ph.D. Thesis. Institut National Polytechnique de Lorraine, Nancy, France, 2006. (24) Baravian, C.; Caton, F.; Dillet, J.; Mougel, J. Steady Light Transport Under Flow: Characterization of Evolving Dense Random Media. Phys. ReV. E 2005, 71, 0666031. (25) Harnby, N.; Edwards, M. F.; Nienow, A. W. Mixing in the Process Industries; Marcel Dekker: New York, 1992. (26) Metzner, A. B.; Otto, R. E. Agitation of Non-Newtonians Fluids. AIChE. J. 1957, 3, 3. (27) Mougel, J.; Alvarez, O.; Baravian, C.; Caton, F.; Marchal, P.; Ste´be´, M. J.; Choplin, L. Aging of a Unstable W/O Emulsion with a Nonionic Surfactant. Rheol. Acta 2006, 45, 555. (28) Princen, H.; Kiss, D. Rheology of Foams and Highly Concentrated Emulsions: III. Static Shear Modulus. J. Colloid Interface Sci. 1986, 112, 427. (29) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: New York, 1991. (30) Masalova, I.; Malkin, A. Y. Rheology of Highly Concentrated Emulsions: Concentration and Droplet Size Dependency. Appl. Rheol. 2007, 17:4, 422501. (31) Malkin, A. Y.; Masalova, I.; Slatter, P.; Wilson, K. Effect of Droplet Size on the Rheological Properties of Highly-Concentrated W/O Emulsions. Rheol. Acta 2004, 43, 584. (32) Masalova, I.; Malkin, A. Y. Peculiarities of Rheological Properties and Flow of Highly Concentrated EmulsionssThe Role of Concentration and Droplet Size. Colloid J. 2007, 69, 185.

ReceiVed for reView December 18, 2009 ReVised manuscript receiVed April 30, 2010 Accepted May 06, 2010 IE9020073