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Formation of fine emulsions by emulsification at high viscosity or low interfacial tension; A comparative study
Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 73–78
Formation of fine emulsions by emulsification at high viscosity or low interfacial tension; A comparative study Shahriar Sajjadi ∗ Division of Engineering, ECLAT, King’s College London, London, WC2R 2LS, United Kingdom Received 11 May 2006; received in revised form 11 November 2006; accepted 14 November 2006 Available online 18 November 2006
Abstract This article deals with preparation of a model oil-in-water emulsion in a conventional stirred vessel using two energy efficient emulsification paths; emulsification at high viscosity (via concentrated emulsions) and emulsification at low interfacial tension. The first type of emulsification was carried out by addition of the oil (dispersed phase) to a small fraction of the water (continuous phase), while being stirred, followed by its dilution with the remainder of the water. The second type was carried out via phase inversion route by addition of the water to the oil. Drop rupturing was assisted by the high viscosity of the emulsion in the first method, but by the low interfacial tension of the emulsion in the second one. For the system under study, emulsification at high viscosity was found to be more efficient in producing fine droplets only if a low concentration of surfactant was used. Whereas with a high surfactant concentration, where an ultra-low interfacial tension was achievable, phase inversion emulsification produced finer droplets. The superiority of the methods, however, may change with the extent of shear. Both emulsification techniques revealed a sharp drop in their characteristic features including drop size, viscosity and interfacial tension below a certain concentration of the surfactant. © 2006 Elsevier B.V. All rights reserved. Keywords: Emulsification; Phase inversion; Concentrated emulsion; Shear; Interfacial tension; Viscosity
1. Introduction In turbulent agitated liquid–liquid dispersions, drop break up occurs by inertia and viscous forces. Kolmogorov defined a length scale by η = (ν3 /ε)1/4 which can be used to define the eddy size and, thus, the conditions of flow [1]. ν and ε are kinematic viscosity and the local rate of energy dissipation per unit mass of fluid. If the drops diameter, d, are larger than η, then their size will be determined by velocity or pressure fluctuation [2]. However, when the diameter of drops is less than η, drop breakage results from viscous shear. The latter generally occurs in emulsification processes where surfactant reduces the interfacial tension and allows small droplets to be formed by moderate mechanical energy. The theory of drop deformation and break up by viscous shear was first formulated by Taylor long ago [3] and further developed by others [4–7]. For drop deformation and break up to occur, the Capillary number defined by the ratio of viscous
∗
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stress of the continuous phase, μc γ, over the Laplace pressure, 2σ/d, must exceed a critical value Ca =
2μc γd σ
(1)
where μc is the viscosity of the continuous phase, γ is the shear rate, σ is the interfacial tension and d is the drop diameter. The higher the viscosity of the continuous phase, the greater is the ease of breakup of liquid drop [8] and a smaller is the size of drops [9]. The variations in emulsion viscosity with the viscosity of the continuous phase, as well as with dispersed phase ratio (φ), is quite complex [10–12]. At low φ, the viscosity of the continuous phase determines the viscosity of emulsion as well as the emulsion properties. With increasing φ, the structure of emulsions shifts from simple liquid-in-liquid dispersions to emulsions consist of closed-packed dispersed droplets separated by a thin layer of continuous phase. These concentrated emulsions are also called gel emulsions. As a result of a high internal phase ratio of concentrated emulsions, their viscosities are enormously increased. In such a case the viscous emulsion itself acts as the continuous phase for rupturing
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S. Sajjadi / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 73–78
the drops. As a result, the rupturing of drops in the concentrated emulsions is boosted. This leads to formation of fine drops. Concentrated emulsions are widely used in food, cosmetic, and pharmaceutical formulations as well as in drug delivery and reaction media for making colloids [13]. However, the objective of this article is not to study the formation of concentrated emulsions, but is to show how fine emulsions can be produced via rupturing concentrated emulsions. The preparation of fine emulsions by fractionation of concentrated emulsions has been reported in the literature [14–19]. Excellent contribution to the field was provided by Mason and Bibette [20,21] who showed that if the shear rate is tightly controlled in the shear device, monodisperse concentrated emulsions can be made. The main objective of this communication is to evaluate how concentrated emulsification stands in comparison to the well-established phase inversion emulsification technique using a conventional stirred vessel [22]. Before proceeding, a brief description of phase inversion emulsification is presented here. The phase inversion emulsification is a process by which water-in-oil (W/O) emulsion can be inverted to water-in-oil (W/O) emulsion or vice versa. There are at least two types of phase inversions. Transitional phase inversion (TPI) occurs when the affinity of the surfactant for the water phase equals its affinity for the oil phase. The variation in the affinity of the surfactant can be conducted by alteration in the surfactant HLB, for example. The transitional phase inversion is associated with formation of three phase microemulsions and an ultra-low value of interfacial tension [23–27]. This technique has been widely used for the preparation of nanoemulsions. The catastrophic phase inversion occurs when a large increase in the rate of drop coalescence occurs so that the balance between the rate of drop coalescence and drop break up cannot be maintained any further. This can be caused by addition of the dispersed phase or alteration in any formulation parameter that promotes coalescence. Note both emulsification paths selected for this study are quite energy efficient, but in different ways. The phase inversion emulsification method via transitional route is extremely energy efficient as it uses a combination of interfacial tension lowering and formation of surfactant crystals to form and stabilise fine droplets. In fact the size of droplets from transitional phase inversion emulsification is mainly determined by the physicochemical properties of the surfactant system rather than the intensity of mechanical stirring. Concentrated emulsion path may also require less total energy than the conventional one despite the fact that emulsification occurs at a high viscosity. This is because a large fraction of the mechanical energy (>0.99) supplied to a dispersion contained in stirred vessel, is lost to fluid recirculation. The smaller the proportion of the continuous phase, the lesser amount of energy is wasted to fluid recirculation and more is directly applied to the dispersed phase to form droplets. Fig. 1 shows the road map for emulsifications via concentrated emulsion (C) and phase inversion (P) on a simplified dynamic phase map in terms of the water volume fraction (fw ) and the surfactant HLB. Change in HLB was made by mixing
Fig. 1. Schematic presentation of the emulsification paths.
a low HLB surfactant and a high HLB surfactant. The locus of optimum HLB value (HLBop ) or the transitional inversion line is shown by a slanted solid line. The transitional line divides the domains of O/W and W/O emulsions; O/W emulsions below the transitional line (HLB > HLBop ) and W/O emulsions above the transitional line. The vertical lines in Fig. 1 indicate the maximum dispersed phase that can be accommodated before catastrophic phase inversion to opposite emulsion occurs. Arrow P in Fig. 1 represents a phase inversion emulsification route in which water containing the water-soluble surfactant is added to oil containing the oil-soluble surfactant. The broken arrow C represents an emulsification route via concentrated emulsion. Because the initial O/W emulsion formed across route C is normal, the phase inversion does not occur unless a high dispersed phase ratio is reached. In the emulsification process C, the addition of oil (concentration stage) stops before the catastrophic inversion locus is crossed, followed by dilution with the continuous phase. 2. Experimental 2.1. Chemical Cyclohexane was used as the oil phase. Two grades of Igepal (polyoxyethylene nonylphenylether, NPE), supplied by Aldrich, were used as surfactants. They are Igepal co520, and Igepal co720 with nonyl phenol ethoxylate chain length of 5 (NPE5) and 12 (NPE12), respectively. NPE5 has the HLB of 10 and is an oil soluble grade. NPE12 has a HLB of 14.2 and is a watersoluble grade. 2.2. Apparatus Emulsifications were carried out in a 1-l jacketed glass reactor with a diameter of 10 cm equipped with 4 baffles, with the width of 1.0 cm, equally spaced at 90◦ interval, a 4-bladed turbine impeller with the diameter of 5.0 cm. The stirring speed was controlled at the constant value of 500 rpm during emulsifications. The emulsifications were carried out at room temperature (22 ◦ C). Emulsion inversions were determined
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75
from measurements of emulsion conductivities and where a large change in electrical conductivity occurred. 2.3. Target emulsion The target emulsion was defined using a model cyclohexanein-water (O/W) emulsion using a combination of NPE5 and NPE12. The HLB of the target emulsions was set at 12.30 (HLBop ≈ 10.4–11.2 for fw = 0.10–0.90). This is the HLB value that can provide a good stability for the cyclohexane/water (O/W) emulsions. Weight averages were used for making a surfactant mixture with the predetermined HLB. Volumes of 200 ml of cyclohexane and 200 ml of de-ionised water were used (fw = 0.50). The interfacial tension of the target emulsion was measured to be 1.20 mN/m for 3.0 wt% surfactant concentration, which is sufficiently low to deliver a stable emulsion as well as stable concentrated emulsion [28]. The water-soluble and oil-soluble surfactants were always placed in water and oil, respectively. The overall concentration of surfactant mixture was based on the total mass of oil and water phase.
Fig. 2. Variations in d32 with fw for (a) concentrated emulsification and (b) transitional phase inversion emulsification with surfactant concentration of 3.0 wt%. The data for route P is only for the post-inversion region where O/W emulsion formed.
from the impeller region. The viscosity of emulsions at the shear rate of 102 s−1 was used for the discussion.
2.4. Emulsification procedures 3. Results and discussion Two types of emulsification processes were used to make the target emulsion. In phase inversion emulsification, route P, water containing the water-soluble surfactant was added to the oil containing the oil-soluble surfactant. Both phases contained a constant concentration of the appropriate surfactant. The concentrated emulsification path, route C, comprises of two stages; concentration stage followed by a dilution stage. In this route, the whole water-soluble surfactant in the recipe was placed in the 10 vol.% of the total water. Then the oil containing the oil-soluble surfactant was added to the water to make concentrated O/W emulsion with φ ≈ 0.90. The resulting emulsion was then diluted with addition of the remaining water (90 vol.% of the total water), which contained no surfactant. The rate of addition of the second phase to the emulsification vessel was 20 ml/10 min for both methods. 2.5. Measurements The average drop diameter and the size distribution of drops were obtained by a laser diffraction method (Coulter LS130). Samples for drop measurements were taken from one cm below the surface of the emulsions. Measurements of average sizes were checked by processing images obtained from emulsions by a video-camera connected to an optical microscope and computer. The interfacial tension measurements were carried out with Du Nouy tensiometer at room temperature. A volume of 10 ml of water dissolving a certain concentration of NPE12 was poured into a glass vial, with a diameter of 5 cm, and then 10 ml of oil containing the same concentration of NPE5 was gently placed on the water. The samples were sealed and left for 4 days to allow for the partitioning of the surfactant molecules in the solutions. Several readings were made and an average value was used. Viscosities of emulsions were measured using a viscometer (Scientific). Samples for viscosity measurements were taken
3.1. Drop size average and distribution Fig. 2a shows a typical variation in the Sauter mean diameter (d32 ) of oil droplets during the emulsification route C for the system with 3.0 wt% surfactant. In the first stage, a concentrated O/W dispersion was formed by continuous addition of the oil to a fraction of water. The droplet size remained almost constant during the initial stage. The stagnation of drop size in this region could be due to several opposing factors such as decreasing interfacial tension [22], increasing coalescence frequency, and turbulence damping with addition of the oil phase. The first factor could reduce the size of drops, whereas the last two factors could contribute to drop enlargement. After sometime, when the viscosity of emulsion increased sufficiently, drop rupturing by viscosity force became predominant and droplets reduced in size with further addition of oil (decreasing fw ). In the second stage, the remaining water was added to the emulsion to adjust the final formulation. Only a small increase in the droplet size was observed during dilution due to presence of a large quantity of surfactant. Route P in Fig. 2b represents transitional phase inversion emulsification for the surfactant concentration of 3.0 wt%. Emulsification started with the oil phase and as more water was added, the resulting W/O emulsion inverted to an O/W emulsion at fw ∼ = 0.10. The interfacial tension approached a value smaller than 10−1 mN/m at the inversion point at which nanodroplets were formed. Further addition of water phase did not significantly affect the size of drops. The size distributions of final droplets obtained from the two routes are shown in Fig. 3. The drop size distribution obtained with the surfactant concentration of 3.0 wt% was sharp for route P, but rather broad for route C. At the low surfactant concentration of 0.50 wt%, size distributions of drops from both routes
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Fig. 5. The evolution of interfacial tension and emulsion viscosity with water volume fraction (fw ) for routes P and C, respectively, for the surfactant concentration of 3.0 wt%. The closed and open symbols represent the concentration stage (1) and dilution stage (2) for concentrated emulsification (route C), and pre-inversion (1) and post-inversion (2) for the phase inversion emulsification (P), respectively. The symbols for the data points corresponding to the final emulsions are enlarged.
sification at high viscosity generated smaller droplets below the critical value. 3.2. Interfacial tension and viscosity and their significance Fig. 3. Size distribution of final drops obtained from routes P and C at surfactant concentration of (a) 3.0 wt% and (b) 0.50 wt%.
were rather broad, but emulsification path C produced narrower distribution. Fig. 4 shows the variations in d32 with surfactant concentrations for both paths. Both processes revealed a deflection around the surfactant concentration of 3.0 wt% below which drops significantly enlarged. Mason and Bibette reported the same pattern for variations in the size of drops versus surfactant concentration [21]. Above the surfactant concentration of 3.0 wt%, phase inversion emulsification produced finer droplets, whereas emul-
Fig. 4. Variations in d32 with surfactant concentration for the two paths.
The concentrated emulsions were non-Newtonian and showed shear thinning behaviour; viscosity decreased with increasing shear rate. Shear rates in agitated tanks have been measured directly and indirectly. In the vicinity of the impeller, shear rate has been found proportional to agitator speed; γ = kN where the constant of proportionality, k ≈ 10 [29]. N is the impeller speed. The experiments were carried out at N = 500 rpm. From this correlation, shear rate in the vicinity of the impeller is estimated to be around 102 s−1 . Therefore, the viscosity data are reported for this shear rate. Fig. 5 depicts the variations in interfacial tension and viscosity across routes P and C, respectively. The data shown are for the surfactant concentration of 3.0 wt%. Across route P, the minimum interfacial tension was achieved at the locus of transitional inversion where water-in-oil emulsion inverted to oil-in-water emulsion. Further addition of water increased the interfacial tension to higher values [22]. Across route C and during the concentration stage, the viscosity of emulsion continuously and exponentially increased with the oil phase. The dilution stage started when the dispersed phase ratio of around 90%, at which the viscosity was maximum, was reached. The dilution of the resulting concentrated emulsion led to a decrease in the viscosity of emulsion with a rate faster than that of the concentration stage. This is because dilution was carried out with pure water leading to concomitant decrease in the surfactant concentration in the water phase and a small increase in the size of drops. The viscosity of the emulsions in the phase inversion emulsification path appeared to increase after phase inversion particularly when a high concentration of the surfactant was used. This is because inversion of a dilute W/O emulsion resulted in formation of concentrated O/W emulsion. For the surfactant concentration of 3.0 wt%, phase inversion occurred at fw = 0.10
S. Sajjadi / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 73–78
Fig. 6. The minimum interfacial tension and maximum viscosity encountered across routes P and C, respectively, as a function of surfactant concentration.
and a concentrated O/W emulsion with φ = 0.90 was formed. This value of φ was coincidently equal to the phase ratio at which the concentrated emulsification was carried out. One may consider that viscosity effects have played an important role in phase inversion emulsification route as well. The viscosity of the inverted emulsion could not be measured at the inversion point off-line because of its extreme instability. However, it is known that viscosity of emulsions is usually decreased at the transitional inversion point due to separation and formation of the surfactant phase [30]. Furthermore, for the surfactant concentration of 5.0 wt%, with which phase inversion occurred at fw = 0.23 and as a result a more diluted emulsion was formed (φ = 0.77 for the inverted emulsion), the resulting drops were even smaller than those obtained with the surfactant concentration of 3.0 wt%. It can be concluded that drop rupturing in the phase inversion emulsification route can not be attributed to viscosity effect. In Fig. 6, the minimum interfacial tension and the maximum viscosity of emulsions encountered across routes P and C, respectively, are plotted against the surfactant concentration. Similar to the plot of drop size variations with surfactant concentration, as shown in Fig. 4, both curves show deflection within the range of [S] = 2.0–3.0 wt%. The drop sizes in fact reflect these minima/maxima values. The maximum viscosity of emulsions made via route C showed a mild increase with surfactant concentration above the deflection point but an abrupt decease below it. The presence of sufficient amount of surfactant is vital to stability of concentrated emulsions. Surfactant protects drops against coalescence, thus reducing the drop size as well as increasing the viscosity. Below the surfactant concentration of 0.25 wt% (not shown) the resulting concentrated emulsions were quite unstable. On the other hand, a large increase in surfactant concentration leads to an increase in the micelle concentration in the continuous phase and thus to destabilisation of concentrated emulsions [28]. This may slow down the increase in emulsion viscosity with surfactant concentration above a certain range. For route P, with a high surfactant concentration, the interfacial tension associated with transitional phase inversion was sufficiently low to reduce the drop size significantly. Note we were not able to measure the interfacial tension below 10−1 mN/m, so zero was arbitrary used for presentations in
77
Fig. 6, where σ < 10−1 mN/m. The interfacial tension falls to 10−4 mN/m in the inversion region [23–27]. Route P did not cross the transitional inversion line for the surfactant concentrations lower than 3.0 wt%. This indicates that for this range of surfactant concentration, route P represents a catastrophic type of phase inversion in which the cause of phase inversion is mainly due to increased rate of drop coalescence with dispersed phase ratio [31]. This led to formation of large drops as shown in Fig. 4. Catastrophic phase inversion occurred at fw = 0.22 for surfactant concentration of 2.0 wt%. So the interfacial tension at this point, where the W/O emulsion inverted to the O/W emulsion, was used in Fig. 6. For the surfactant concentrations of 1.0, and 0.50 wt%, catastrophic phase inversion did not occur before the composition of the final emulsion was reached; i.e., fw = 0.50. For these cases, the resulting W/O emulsions were maintained under agitation until phase inversion to O/W emulsions occurred by time as a result of further inclusion of the continuous phase into the dispersed drops [32]. The interfacial tension of the final emulsions at fw = 0.5 was thus used for these two runs. The reason or one of the reasons for a shift in the superiority of the routes, within the range of surfactant concentration used, can be inferred from Fig. 6. According to Eq. (1), the two important parameters controlling the size of drops are interfacial tension and viscosity. A high viscosity as well as a low interfacial tension can enhance formation of droplets in a shear-induced flow. The increase in interfacial tension below the critical surfactant concentration seems to be steeper than the fall in viscosity within the same range. Therefore, emulsification via concentrated emulsion path appears to be more efficient in forming fine droplets with low surfactant concentrations, as shown in Fig. 4, in the context of this study. There is a point worthy of attention here. This article concerns drop formation in conventional stirred vessels and by no means was aimed at an exhaustive study of potentials of emulsification at high internal phase ratio. Emulsification was carried out at φ = 0.90, while it could be theoretically done at a higher value of φ to boost formation of smaller droplets. To enhance shear rate, any value of φ smaller than 0.95–0.97 at which catastrophic phase inversion to W/O emulsion may occur, could be used. Furthermore, it was the objective of this research to generate the shear required for emulsification in a conventional vessel stirred by a turbine impeller. With increasing viscosity of the emulsions, the stirring became quite difficult. Stagnant zones were formed and drops were not equally and sufficiently exposed to shear. More efficient homogenisation device with a smaller mixing chamber and a proper design of impeller are required, if an extensive shear is desired. Applying a more extensive shear during emulsification, may lead to superiority of the emulsification path C within a broader range of surfactant concentration. Meleson et al. [18] reported formation of nanosize emulsions by using an extreme shear. 4. Conclusion Two emulsification paths, phase inversion emulsification and concentrated emulsification, were compared using a conven-
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tional stirred vessel. Concentrated emulsification path was found to be more efficient in producing fine droplets than catastrophic phase inversion emulsification. However, above the critical surfactant concentration where transitional phase inversion was possible, phase inversion emulsification proved to be more efficient. The priority of the methods, however, may change with the extent of shear. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
A.N. Kolmogoroff, C.R. Acad. Sci. USSR 30 (1941) 301. R. Shinnar, J. Fluid Mech. 10 (1961) 259. G.I. Taylor, Proc. R. Soc. Lond., Ser. A 146 (1934) 501. D.V. Khakar, J.M. Ottino, Int. J. Multiphase Flow 13 (1987) 71. H.A. Stone, Annu. Rev. Fluid Mech. 26 (1994) 65. B.J. Bentley, L.G. Leal, J. Fluid Mech. 167 (1986) 241. B.J. Briscoe, C.J. Lawrence, W.G.P. Mietus, Adv. Colloid Interface Sci. 81 (1999) 1. H.J. Karam, J.C. Bellinger, Ind. Eng. Chem. Fund. 7 (1968) 576. P. DeRoussel, D.V. Khakhar, J.M. Ottino, Chem. Eng. Sci. 56 (2001) 5511. T.G. Mason, Curr. Opin. Colloid Interface Sci. 4 (1999) 231. R. Pal, J. Colloid Interface Sci. 263 (2003) 296. K.M.B. Jansen, W.G.M. Agterof, J. Mellema, J. Rheol. 45 (2001) 227. C. Solans, J. Esquena, N. Azemar, Curr. Opin. Colloid Interface Sci. 8 (2003) 156.
[14] N. Fujii, M. Hamano, K. Yuasa, Biosci. BioTech. BioChem. 59 (1995) 667. [15] M. Perez, N. Zambrano, M. Ramirez, E. Tyrode, J.L. Salager, J. Dispersion Sci. Technol. 23 (2002) 55. [16] M. Zerfa, S. Sajjadi, B.W. Brooks, Colloids Surf. A 178 (2001) 41. [17] S. Sajjadi, M. Zerfa, B.W. Brooks, Colloids Surf. A 218 (2003) 241. [18] K. Meleson, S. Graves, T.G. Mason, Soft Mat. 2 (2004) 109. [19] L.A. Roberts, F. Xie, B.W. Brooks, Colloids Surf. A 274 (2006) 179. [20] T.G. Mason, J. Bibette, Phys. Rev. Lett. 77 (1996) 3481. [21] T.G. Mason, J. Bibette, Langmuir 13 (1997) 4600. [22] S. Sajjadi, Chem. Eng. Sci. 61 (2006) 3009. [23] J.L. Salager, J. Morgan, R.S. Schechter, W.H. Wade, E. Vasquez, Soc. Pet. Eng. J. 19 (1979) 107. [24] K. Shinoda, S. Friberg, Emulsion and Solubilization, John Wiley, New York, 1986. [25] T. Sottmann, R. Strey, J. Chem. Phys. 106 (1997) 8606. [26] C.A. Miller, R.N. Hwan, W. Benton, T. Fort Jr., J. Colloid Interface Sci. 61 (1997) 554. [27] P.B. Binks, J. Meunier, O. Abillon, D. Langevin, Langmuir 5 (1989) 415. [28] V.G. Babak, M.J. Stebe, J. Dispersion Sci. Technol. 23 (2002) 1. [29] A.B. Metzner, R.E. Otto, AICHE J. 3 (1957) 3. [30] J.L. Salager, M. Minanaperez, J.M. Anderez, J.L. Grosso, C.I. Rojas, I. Layrisse, J. Dispersion Sci. Technol. 4 (1983) 161. [31] S. Sajjadi, Langmuir 22 (2006) 5597. [32] S. Sajjadi, M. Zerfa, B.W. Brooks, Chem. Eng. Sci. 57 (2002) 663.
Formation of fine emulsions by emulsification at high viscosity or low interfacial tension; A comparative study Shahriar Sajjadi ∗ Division of Engineering, ECLAT, King’s College London, London, WC2R 2LS, United Kingdom Received 11 May 2006; received in revised form 11 November 2006; accepted 14 November 2006 Available online 18 November 2006
Abstract This article deals with preparation of a model oil-in-water emulsion in a conventional stirred vessel using two energy efficient emulsification paths; emulsification at high viscosity (via concentrated emulsions) and emulsification at low interfacial tension. The first type of emulsification was carried out by addition of the oil (dispersed phase) to a small fraction of the water (continuous phase), while being stirred, followed by its dilution with the remainder of the water. The second type was carried out via phase inversion route by addition of the water to the oil. Drop rupturing was assisted by the high viscosity of the emulsion in the first method, but by the low interfacial tension of the emulsion in the second one. For the system under study, emulsification at high viscosity was found to be more efficient in producing fine droplets only if a low concentration of surfactant was used. Whereas with a high surfactant concentration, where an ultra-low interfacial tension was achievable, phase inversion emulsification produced finer droplets. The superiority of the methods, however, may change with the extent of shear. Both emulsification techniques revealed a sharp drop in their characteristic features including drop size, viscosity and interfacial tension below a certain concentration of the surfactant. © 2006 Elsevier B.V. All rights reserved. Keywords: Emulsification; Phase inversion; Concentrated emulsion; Shear; Interfacial tension; Viscosity
1. Introduction In turbulent agitated liquid–liquid dispersions, drop break up occurs by inertia and viscous forces. Kolmogorov defined a length scale by η = (ν3 /ε)1/4 which can be used to define the eddy size and, thus, the conditions of flow [1]. ν and ε are kinematic viscosity and the local rate of energy dissipation per unit mass of fluid. If the drops diameter, d, are larger than η, then their size will be determined by velocity or pressure fluctuation [2]. However, when the diameter of drops is less than η, drop breakage results from viscous shear. The latter generally occurs in emulsification processes where surfactant reduces the interfacial tension and allows small droplets to be formed by moderate mechanical energy. The theory of drop deformation and break up by viscous shear was first formulated by Taylor long ago [3] and further developed by others [4–7]. For drop deformation and break up to occur, the Capillary number defined by the ratio of viscous
∗
Tel.: +44 20 78482322; fax: +44 20 78482932. E-mail address: [email protected].
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.11.023
stress of the continuous phase, μc γ, over the Laplace pressure, 2σ/d, must exceed a critical value Ca =
2μc γd σ
(1)
where μc is the viscosity of the continuous phase, γ is the shear rate, σ is the interfacial tension and d is the drop diameter. The higher the viscosity of the continuous phase, the greater is the ease of breakup of liquid drop [8] and a smaller is the size of drops [9]. The variations in emulsion viscosity with the viscosity of the continuous phase, as well as with dispersed phase ratio (φ), is quite complex [10–12]. At low φ, the viscosity of the continuous phase determines the viscosity of emulsion as well as the emulsion properties. With increasing φ, the structure of emulsions shifts from simple liquid-in-liquid dispersions to emulsions consist of closed-packed dispersed droplets separated by a thin layer of continuous phase. These concentrated emulsions are also called gel emulsions. As a result of a high internal phase ratio of concentrated emulsions, their viscosities are enormously increased. In such a case the viscous emulsion itself acts as the continuous phase for rupturing
74
S. Sajjadi / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 73–78
the drops. As a result, the rupturing of drops in the concentrated emulsions is boosted. This leads to formation of fine drops. Concentrated emulsions are widely used in food, cosmetic, and pharmaceutical formulations as well as in drug delivery and reaction media for making colloids [13]. However, the objective of this article is not to study the formation of concentrated emulsions, but is to show how fine emulsions can be produced via rupturing concentrated emulsions. The preparation of fine emulsions by fractionation of concentrated emulsions has been reported in the literature [14–19]. Excellent contribution to the field was provided by Mason and Bibette [20,21] who showed that if the shear rate is tightly controlled in the shear device, monodisperse concentrated emulsions can be made. The main objective of this communication is to evaluate how concentrated emulsification stands in comparison to the well-established phase inversion emulsification technique using a conventional stirred vessel [22]. Before proceeding, a brief description of phase inversion emulsification is presented here. The phase inversion emulsification is a process by which water-in-oil (W/O) emulsion can be inverted to water-in-oil (W/O) emulsion or vice versa. There are at least two types of phase inversions. Transitional phase inversion (TPI) occurs when the affinity of the surfactant for the water phase equals its affinity for the oil phase. The variation in the affinity of the surfactant can be conducted by alteration in the surfactant HLB, for example. The transitional phase inversion is associated with formation of three phase microemulsions and an ultra-low value of interfacial tension [23–27]. This technique has been widely used for the preparation of nanoemulsions. The catastrophic phase inversion occurs when a large increase in the rate of drop coalescence occurs so that the balance between the rate of drop coalescence and drop break up cannot be maintained any further. This can be caused by addition of the dispersed phase or alteration in any formulation parameter that promotes coalescence. Note both emulsification paths selected for this study are quite energy efficient, but in different ways. The phase inversion emulsification method via transitional route is extremely energy efficient as it uses a combination of interfacial tension lowering and formation of surfactant crystals to form and stabilise fine droplets. In fact the size of droplets from transitional phase inversion emulsification is mainly determined by the physicochemical properties of the surfactant system rather than the intensity of mechanical stirring. Concentrated emulsion path may also require less total energy than the conventional one despite the fact that emulsification occurs at a high viscosity. This is because a large fraction of the mechanical energy (>0.99) supplied to a dispersion contained in stirred vessel, is lost to fluid recirculation. The smaller the proportion of the continuous phase, the lesser amount of energy is wasted to fluid recirculation and more is directly applied to the dispersed phase to form droplets. Fig. 1 shows the road map for emulsifications via concentrated emulsion (C) and phase inversion (P) on a simplified dynamic phase map in terms of the water volume fraction (fw ) and the surfactant HLB. Change in HLB was made by mixing
Fig. 1. Schematic presentation of the emulsification paths.
a low HLB surfactant and a high HLB surfactant. The locus of optimum HLB value (HLBop ) or the transitional inversion line is shown by a slanted solid line. The transitional line divides the domains of O/W and W/O emulsions; O/W emulsions below the transitional line (HLB > HLBop ) and W/O emulsions above the transitional line. The vertical lines in Fig. 1 indicate the maximum dispersed phase that can be accommodated before catastrophic phase inversion to opposite emulsion occurs. Arrow P in Fig. 1 represents a phase inversion emulsification route in which water containing the water-soluble surfactant is added to oil containing the oil-soluble surfactant. The broken arrow C represents an emulsification route via concentrated emulsion. Because the initial O/W emulsion formed across route C is normal, the phase inversion does not occur unless a high dispersed phase ratio is reached. In the emulsification process C, the addition of oil (concentration stage) stops before the catastrophic inversion locus is crossed, followed by dilution with the continuous phase. 2. Experimental 2.1. Chemical Cyclohexane was used as the oil phase. Two grades of Igepal (polyoxyethylene nonylphenylether, NPE), supplied by Aldrich, were used as surfactants. They are Igepal co520, and Igepal co720 with nonyl phenol ethoxylate chain length of 5 (NPE5) and 12 (NPE12), respectively. NPE5 has the HLB of 10 and is an oil soluble grade. NPE12 has a HLB of 14.2 and is a watersoluble grade. 2.2. Apparatus Emulsifications were carried out in a 1-l jacketed glass reactor with a diameter of 10 cm equipped with 4 baffles, with the width of 1.0 cm, equally spaced at 90◦ interval, a 4-bladed turbine impeller with the diameter of 5.0 cm. The stirring speed was controlled at the constant value of 500 rpm during emulsifications. The emulsifications were carried out at room temperature (22 ◦ C). Emulsion inversions were determined
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from measurements of emulsion conductivities and where a large change in electrical conductivity occurred. 2.3. Target emulsion The target emulsion was defined using a model cyclohexanein-water (O/W) emulsion using a combination of NPE5 and NPE12. The HLB of the target emulsions was set at 12.30 (HLBop ≈ 10.4–11.2 for fw = 0.10–0.90). This is the HLB value that can provide a good stability for the cyclohexane/water (O/W) emulsions. Weight averages were used for making a surfactant mixture with the predetermined HLB. Volumes of 200 ml of cyclohexane and 200 ml of de-ionised water were used (fw = 0.50). The interfacial tension of the target emulsion was measured to be 1.20 mN/m for 3.0 wt% surfactant concentration, which is sufficiently low to deliver a stable emulsion as well as stable concentrated emulsion [28]. The water-soluble and oil-soluble surfactants were always placed in water and oil, respectively. The overall concentration of surfactant mixture was based on the total mass of oil and water phase.
Fig. 2. Variations in d32 with fw for (a) concentrated emulsification and (b) transitional phase inversion emulsification with surfactant concentration of 3.0 wt%. The data for route P is only for the post-inversion region where O/W emulsion formed.
from the impeller region. The viscosity of emulsions at the shear rate of 102 s−1 was used for the discussion.
2.4. Emulsification procedures 3. Results and discussion Two types of emulsification processes were used to make the target emulsion. In phase inversion emulsification, route P, water containing the water-soluble surfactant was added to the oil containing the oil-soluble surfactant. Both phases contained a constant concentration of the appropriate surfactant. The concentrated emulsification path, route C, comprises of two stages; concentration stage followed by a dilution stage. In this route, the whole water-soluble surfactant in the recipe was placed in the 10 vol.% of the total water. Then the oil containing the oil-soluble surfactant was added to the water to make concentrated O/W emulsion with φ ≈ 0.90. The resulting emulsion was then diluted with addition of the remaining water (90 vol.% of the total water), which contained no surfactant. The rate of addition of the second phase to the emulsification vessel was 20 ml/10 min for both methods. 2.5. Measurements The average drop diameter and the size distribution of drops were obtained by a laser diffraction method (Coulter LS130). Samples for drop measurements were taken from one cm below the surface of the emulsions. Measurements of average sizes were checked by processing images obtained from emulsions by a video-camera connected to an optical microscope and computer. The interfacial tension measurements were carried out with Du Nouy tensiometer at room temperature. A volume of 10 ml of water dissolving a certain concentration of NPE12 was poured into a glass vial, with a diameter of 5 cm, and then 10 ml of oil containing the same concentration of NPE5 was gently placed on the water. The samples were sealed and left for 4 days to allow for the partitioning of the surfactant molecules in the solutions. Several readings were made and an average value was used. Viscosities of emulsions were measured using a viscometer (Scientific). Samples for viscosity measurements were taken
3.1. Drop size average and distribution Fig. 2a shows a typical variation in the Sauter mean diameter (d32 ) of oil droplets during the emulsification route C for the system with 3.0 wt% surfactant. In the first stage, a concentrated O/W dispersion was formed by continuous addition of the oil to a fraction of water. The droplet size remained almost constant during the initial stage. The stagnation of drop size in this region could be due to several opposing factors such as decreasing interfacial tension [22], increasing coalescence frequency, and turbulence damping with addition of the oil phase. The first factor could reduce the size of drops, whereas the last two factors could contribute to drop enlargement. After sometime, when the viscosity of emulsion increased sufficiently, drop rupturing by viscosity force became predominant and droplets reduced in size with further addition of oil (decreasing fw ). In the second stage, the remaining water was added to the emulsion to adjust the final formulation. Only a small increase in the droplet size was observed during dilution due to presence of a large quantity of surfactant. Route P in Fig. 2b represents transitional phase inversion emulsification for the surfactant concentration of 3.0 wt%. Emulsification started with the oil phase and as more water was added, the resulting W/O emulsion inverted to an O/W emulsion at fw ∼ = 0.10. The interfacial tension approached a value smaller than 10−1 mN/m at the inversion point at which nanodroplets were formed. Further addition of water phase did not significantly affect the size of drops. The size distributions of final droplets obtained from the two routes are shown in Fig. 3. The drop size distribution obtained with the surfactant concentration of 3.0 wt% was sharp for route P, but rather broad for route C. At the low surfactant concentration of 0.50 wt%, size distributions of drops from both routes
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Fig. 5. The evolution of interfacial tension and emulsion viscosity with water volume fraction (fw ) for routes P and C, respectively, for the surfactant concentration of 3.0 wt%. The closed and open symbols represent the concentration stage (1) and dilution stage (2) for concentrated emulsification (route C), and pre-inversion (1) and post-inversion (2) for the phase inversion emulsification (P), respectively. The symbols for the data points corresponding to the final emulsions are enlarged.
sification at high viscosity generated smaller droplets below the critical value. 3.2. Interfacial tension and viscosity and their significance Fig. 3. Size distribution of final drops obtained from routes P and C at surfactant concentration of (a) 3.0 wt% and (b) 0.50 wt%.
were rather broad, but emulsification path C produced narrower distribution. Fig. 4 shows the variations in d32 with surfactant concentrations for both paths. Both processes revealed a deflection around the surfactant concentration of 3.0 wt% below which drops significantly enlarged. Mason and Bibette reported the same pattern for variations in the size of drops versus surfactant concentration [21]. Above the surfactant concentration of 3.0 wt%, phase inversion emulsification produced finer droplets, whereas emul-
Fig. 4. Variations in d32 with surfactant concentration for the two paths.
The concentrated emulsions were non-Newtonian and showed shear thinning behaviour; viscosity decreased with increasing shear rate. Shear rates in agitated tanks have been measured directly and indirectly. In the vicinity of the impeller, shear rate has been found proportional to agitator speed; γ = kN where the constant of proportionality, k ≈ 10 [29]. N is the impeller speed. The experiments were carried out at N = 500 rpm. From this correlation, shear rate in the vicinity of the impeller is estimated to be around 102 s−1 . Therefore, the viscosity data are reported for this shear rate. Fig. 5 depicts the variations in interfacial tension and viscosity across routes P and C, respectively. The data shown are for the surfactant concentration of 3.0 wt%. Across route P, the minimum interfacial tension was achieved at the locus of transitional inversion where water-in-oil emulsion inverted to oil-in-water emulsion. Further addition of water increased the interfacial tension to higher values [22]. Across route C and during the concentration stage, the viscosity of emulsion continuously and exponentially increased with the oil phase. The dilution stage started when the dispersed phase ratio of around 90%, at which the viscosity was maximum, was reached. The dilution of the resulting concentrated emulsion led to a decrease in the viscosity of emulsion with a rate faster than that of the concentration stage. This is because dilution was carried out with pure water leading to concomitant decrease in the surfactant concentration in the water phase and a small increase in the size of drops. The viscosity of the emulsions in the phase inversion emulsification path appeared to increase after phase inversion particularly when a high concentration of the surfactant was used. This is because inversion of a dilute W/O emulsion resulted in formation of concentrated O/W emulsion. For the surfactant concentration of 3.0 wt%, phase inversion occurred at fw = 0.10
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Fig. 6. The minimum interfacial tension and maximum viscosity encountered across routes P and C, respectively, as a function of surfactant concentration.
and a concentrated O/W emulsion with φ = 0.90 was formed. This value of φ was coincidently equal to the phase ratio at which the concentrated emulsification was carried out. One may consider that viscosity effects have played an important role in phase inversion emulsification route as well. The viscosity of the inverted emulsion could not be measured at the inversion point off-line because of its extreme instability. However, it is known that viscosity of emulsions is usually decreased at the transitional inversion point due to separation and formation of the surfactant phase [30]. Furthermore, for the surfactant concentration of 5.0 wt%, with which phase inversion occurred at fw = 0.23 and as a result a more diluted emulsion was formed (φ = 0.77 for the inverted emulsion), the resulting drops were even smaller than those obtained with the surfactant concentration of 3.0 wt%. It can be concluded that drop rupturing in the phase inversion emulsification route can not be attributed to viscosity effect. In Fig. 6, the minimum interfacial tension and the maximum viscosity of emulsions encountered across routes P and C, respectively, are plotted against the surfactant concentration. Similar to the plot of drop size variations with surfactant concentration, as shown in Fig. 4, both curves show deflection within the range of [S] = 2.0–3.0 wt%. The drop sizes in fact reflect these minima/maxima values. The maximum viscosity of emulsions made via route C showed a mild increase with surfactant concentration above the deflection point but an abrupt decease below it. The presence of sufficient amount of surfactant is vital to stability of concentrated emulsions. Surfactant protects drops against coalescence, thus reducing the drop size as well as increasing the viscosity. Below the surfactant concentration of 0.25 wt% (not shown) the resulting concentrated emulsions were quite unstable. On the other hand, a large increase in surfactant concentration leads to an increase in the micelle concentration in the continuous phase and thus to destabilisation of concentrated emulsions [28]. This may slow down the increase in emulsion viscosity with surfactant concentration above a certain range. For route P, with a high surfactant concentration, the interfacial tension associated with transitional phase inversion was sufficiently low to reduce the drop size significantly. Note we were not able to measure the interfacial tension below 10−1 mN/m, so zero was arbitrary used for presentations in
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Fig. 6, where σ < 10−1 mN/m. The interfacial tension falls to 10−4 mN/m in the inversion region [23–27]. Route P did not cross the transitional inversion line for the surfactant concentrations lower than 3.0 wt%. This indicates that for this range of surfactant concentration, route P represents a catastrophic type of phase inversion in which the cause of phase inversion is mainly due to increased rate of drop coalescence with dispersed phase ratio [31]. This led to formation of large drops as shown in Fig. 4. Catastrophic phase inversion occurred at fw = 0.22 for surfactant concentration of 2.0 wt%. So the interfacial tension at this point, where the W/O emulsion inverted to the O/W emulsion, was used in Fig. 6. For the surfactant concentrations of 1.0, and 0.50 wt%, catastrophic phase inversion did not occur before the composition of the final emulsion was reached; i.e., fw = 0.50. For these cases, the resulting W/O emulsions were maintained under agitation until phase inversion to O/W emulsions occurred by time as a result of further inclusion of the continuous phase into the dispersed drops [32]. The interfacial tension of the final emulsions at fw = 0.5 was thus used for these two runs. The reason or one of the reasons for a shift in the superiority of the routes, within the range of surfactant concentration used, can be inferred from Fig. 6. According to Eq. (1), the two important parameters controlling the size of drops are interfacial tension and viscosity. A high viscosity as well as a low interfacial tension can enhance formation of droplets in a shear-induced flow. The increase in interfacial tension below the critical surfactant concentration seems to be steeper than the fall in viscosity within the same range. Therefore, emulsification via concentrated emulsion path appears to be more efficient in forming fine droplets with low surfactant concentrations, as shown in Fig. 4, in the context of this study. There is a point worthy of attention here. This article concerns drop formation in conventional stirred vessels and by no means was aimed at an exhaustive study of potentials of emulsification at high internal phase ratio. Emulsification was carried out at φ = 0.90, while it could be theoretically done at a higher value of φ to boost formation of smaller droplets. To enhance shear rate, any value of φ smaller than 0.95–0.97 at which catastrophic phase inversion to W/O emulsion may occur, could be used. Furthermore, it was the objective of this research to generate the shear required for emulsification in a conventional vessel stirred by a turbine impeller. With increasing viscosity of the emulsions, the stirring became quite difficult. Stagnant zones were formed and drops were not equally and sufficiently exposed to shear. More efficient homogenisation device with a smaller mixing chamber and a proper design of impeller are required, if an extensive shear is desired. Applying a more extensive shear during emulsification, may lead to superiority of the emulsification path C within a broader range of surfactant concentration. Meleson et al. [18] reported formation of nanosize emulsions by using an extreme shear. 4. Conclusion Two emulsification paths, phase inversion emulsification and concentrated emulsification, were compared using a conven-
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tional stirred vessel. Concentrated emulsification path was found to be more efficient in producing fine droplets than catastrophic phase inversion emulsification. However, above the critical surfactant concentration where transitional phase inversion was possible, phase inversion emulsification proved to be more efficient. The priority of the methods, however, may change with the extent of shear. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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