- Email: [email protected]
W emulsion liquid membranes under agitation conditions
Journal of Membrane Science 213 (2003) 1–12
Isotonic swelling behavior of W/O/W emulsion liquid membranes under agitation conditions Jun Yan, Rajinder Pal∗ Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 Received 9 July 2002; received in revised form 23 September 2002; accepted 11 October 2002
Abstract A new on-line technique is developed to continuously monitor the isotonic swelling behavior of water-in-oil-in-water (W/O/W) emulsion liquid membranes (ELMs) under agitation condition. The technique is based on electrical conductivity measurement. The effects of surfactant concentration, initial volume fraction of dispersed globules, and stirring speed on the swelling profiles of W/O/W ELMs are determined using the new on-line technique. With the increases in the stirring speed and initial volume fraction of dispersed-phase, the swelling rate increases. With the increase in the surfactant concentration, a more complicated behavior is observed—the swelling rate increases initially, reaches a maximum value and then falls off with further increase in the surfactant concentration. Under certain conditions, phase inversion of multiple W/O/W emulsion to simple W/O emulsion is observed. © 2002 Published by Elsevier Science B.V. Keywords: Liquid membranes; Swelling; Emulsion liquid membranes; Multiple emulsion
1. Introduction Emulsion liquid membrane (ELM) separation process constitutes an emerging technology with a wide variety of applications, such as the removal, recovery, and purification of many organic and inorganic compounds from dilute solutions of industrial interest. Examples of chemicals which can be removed or recovered from industrial streams using ELMs are: organic acids, phenols, cresols, amines, and metallic ions such as copper, cadmium and mercury [1]. ELMs are ideally suited for the treatment of wastewater as they have the potential of removing toxic substances down to very low levels. Because of the stability problems associated with the emulsion liquid membranes, ∗ Corresponding author. Fax: +1-519-746-4979. E-mail address: [email protected] (R. Pal).
ELMs have not yet found widespread usage in industrial applications. One serious problem associated with the ELMs is their tendency to undergo swelling. For example, the globules of water-in-oil-in-water (W/O/W) emulsion liquid membranes swell due to transfer of water from the external aqueous phase to the internal water droplets. Swelling often causes rapid increase in the volume of the internal phase. Furthermore, the swelling of membrane globules can trigger the breakdown of the globules. As a result, the process of enrichment of the solute is retarded and the efficiency of separation of the solute from the external aqueous phase becomes lower than the expected value. The swelling of the membrane globules also adversely affects the final demulsification step where the solute is recovered. The problems encountered in the demulsification step are mainly due to a sharp increase in the viscosity of the emulsion, caused by swelling
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J. Yan, R. Pal / Journal of Membrane Science 213 (2003) 1–12
of internal droplets [2–4].In our previous work [5], we studied the osmotic swelling behavior of globules of W/O/W ELMs under static condition. The osmotic pressure gradient between the internal and external aqueous phases was induced by creating a concentration difference of d-glucose between the two aqueous phases. The effects of surfactant concentration and initial volume fraction of internal aqueous phase (ϕ W/O (0)) on the swelling behavior of W/O/W globules were determined by monitoring the changes in the size of the globules over a long period. When dealing with unsupported liquid membrane separation, understanding the swelling behavior of globules under agitation condition is also important. While the osmotic swelling behavior of ELM globules under static condition has been systematically studied by many investigators [5], little attention has been given to isotonic swelling of ELM globules under agitation condition. Isotonic swelling of ELMs depends on the capability of the dispersed globules to entrap the continuous phase fluid. This capability of isotonic emulsification is strongly affected by the presence of surfactant in the dispersed globules. Several mechanisms have been proposed by researchers [6–12] to explain isotonic emulsification. The proposed mechanisms are: (1) Multiple-body collision mechanism [6,7]—according to this mechanism, the entrapment of the continuous phase fluid occurs when at least four dispersed droplets (globules) collide with each other. It is unlikely that multiple-body collision can cause isotonic emulsification at low concentrations of dispersed globules; only at high concentration of dispersed globules, multiple-body collision is likely to take place; (2) two-body collision mechanism [8,9]—this mechanism suggests that during collision of two droplets, a film of the continuous phase fluid gets entrapped between the colliding globules, leading to the formation of internal droplets; and (3) drop deformation mechanism [10–12]—according to this mechanism, the interface of the dispersed globules tends to develop a concave surface due to the presence of surfactant at the interface. Any fluctuation in the pressure or shear force acting on the globule interface extends the concave surface and this results in the entrapment of the continuous phase fluid within the globule. In this work, a new on-line technique is first developed to continuously monitor the isotonic swelling process of W/O/W ELMs under agitation conditions.
Using the new on-line technique, complete isotonic swelling profiles of W/O/W ELMs are obtained. The effects of surfactant concentration, initial volume fraction of dispersed globules (ϕ W/O/W (0)), and stirring speed on the isotonic swelling behavior of W/O/W ELMs are determined. 2. New on-line technique for continuous monitoring of isotonic swelling of W/O/W ELMs under agitation conditions In the existing literature, the swelling of W/O/W ELMs is measured by periodically withdrawing samples from the mixing vessel. The primary emulsion is separated from the multiple emulsion (ELM) sample using gravity separator or centrifuge. The swelling ratio is then calculated by measuring the volume or density of the primary emulsion. The drawbacks of such off-line methods are (1) only a limited amount of information is obtained on the swelling behavior of ELMs; (2) errors can occur during manual sampling; (3) errors can also occur during separation of primary emulsion from the multiple emulsion; and (4) the collected sample may undergo further swelling during the separation process, especially if the system is non-isotonic. In order to overcome all these limitations associated with off-line manual approach, an on-line method for continuous monitoring of the swelling behavior of ELMs, under agitation conditions, is needed. The on-line technique developed in this work provides more accurate and detailed information on the swelling behavior of ELMs. The proposed technique is based on electrical conductivity measurement. The electrical conductivity of a dispersion (κ) is related to the volume fraction of the dispersed-phase (ϕ) as follows: 1 + 2βϕ κ = κe (1) 1 − βϕ where κ e is the electrical conductivity of the external phase and β = (α − 1)/(α + 2). Note that α = κd /κe , where κ d is the electrical conductivity of the dispersed-phase. Eq. (1) was originally derived by Maxwell [13]. When applied to W/O/W emulsion liquid membranes, Eq. (1) gives: 1 + 2βϕW/O/W κW/O/W = κe (2) 1 − βϕW/O/W
J. Yan, R. Pal / Journal of Membrane Science 213 (2003) 1–12
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Fig. 1. Comparison of experimental data with the predictions of Eq. (4).
where ϕ W/O/W is the volume fraction of the dispersedphase, that is, the volume fraction of the primary water-in-oil (W/O) emulsion in the overall multiple emulsion (emulsion liquid membrane). For W/O/W multiple emulsion (ELM),  is approximately equal to −0.5 as α = κW/O /κe ≈ 0. Therefore, Eq. (2) can be simplified to 1 − ϕW/O/W κW/O/W = κe (3) 1 + 0.5ϕW/O/W
From Eq. (3) ϕ W/O/W can be obtained as follows: ϕW/O/W =
1−χ 1 + 0.5χ
where
χ=
κW/O/W κe
(4)
Thus, the volume fraction of the water-in-oil emulsion (ϕ W/O/W ) present in the overall multiple emulsion can be calculated if χ is known. A conductivity probe (model U-19500-20, dipstyle, Pt electrode, 1 cm−1 cell constant, supplied by
Fig. 2. Mixing vessel (units in mm).
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J. Yan, R. Pal / Journal of Membrane Science 213 (2003) 1–12
Cole-Palmer) was installed in the mixing vessel. The probe was connected to a conductivity meter (Okaton model WD-35100-20). The signal from the conductivity meter was recorded by a Personal Computer through a data acquisition board (CIODAS08JR, 8 Channel A/D, 12 bit, supplied by ComputerBoard Inc.). The signal was recorded for a desired period of time at a data sampling rate of 5 s. From the knowledge of the real-time ELM conductivity κ W/O/W (t)
and the external aqueous-phase conductivity κ e , the volume fraction ϕ W/O/W (t) was calculated using Eq. (4). A simple emulsion of oil-in-water (O/W) type was used to verify Eq. (4). The oil-phase was kerosene and the aqueous phase was dilute sulfuric acid solution with pH = 3.10 and conductivity = 410 S/cm. The experimental data are compared with the predictions of Eq. (4) in Fig. 1. Clearly, Eq. (4) gives good
Fig. 3. Typical photomicrographs of W/O/W globules with surfactant (EMSORB 2500) concentration varying from 0.05% to 20 wt.% based on the membrane (oil) phase.
J. Yan, R. Pal / Journal of Membrane Science 213 (2003) 1–12
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Fig. 3. (Continued ).
estimate of experimental data over a wide range of dispersed-phase volume fraction.
3. Experimental work on isotonic swelling behavior of W/O/W emulsion liquid membranes 3.1. Materials The W/O/W ELMs were prepared using kerosene (supplied by Fisher Scientific) as the membrane phase.
The surfactant, EMSORB 2500 (sorbitan monooleate), was added to the membrane phase. EMSORB 2500 is a commercially available surfactant manufactured by Henkel Corporation. The hydrophile–lipophile balance (HLB) value of this surfactant is 4.6. The aqueous phase (internal as well as external) of the W/O/W ELMs consisted of a dilute sulfuric acid solution (pH = 3.10, conductivity κe = 410 S/cm). Polyethylene oxide (MW = 106 g/mol, supplied by Polysciences Inc.) was employed as a globule freezing agent for microscopy.
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Fig. 4. (a) The electrical conductivity of W/O/W emulsion liquid membranes, κ W/O/W (t), at different surfactant (EMSORB 2500) concentrations. (b) The dispersed-phase volume fraction of W/O/W emulsion liquid membranes, ϕ W/O/W (t), at different surfactant (EMSORB 2500) concentrations. (c) The swelling ratio, S(t)(%), of W/O/W emulsion liquid membranes at different surfactant (EMSORB 2500) concentrations. (d) The swelling rate, R(t)(%/h), of W/O/W emulsion liquid membranes at different surfactant (EMSORB 2500) concentrations.
3.2. Procedures The W/O/W ELMs were prepared in a 65 mm diameter vessel equipped with two vertical baffles (Fig. 2). The mixing was provided with a magnetic stirrer using a stirring bar of length 50 mm. The mixing vessel was filled with well-defined external aqueous phase in advance. Then a known amount of the membrane phase was slowly poured into the vessel without any agitation. After the addition of the membrane phase, the agitation was started at a known stirrer speed. The surfactant concentration was varied from 0.05 to 20% by weight based on membrane phase (oil phase). The
initial volume fraction ϕ W/O/W (0) was varied from 0.05 to 0.39. Note that in this work, the membrane phase does not contain any internal aqueous phase initially, that is, ϕW/O (0) = 0 and therefore, ϕ W/O/W (0) is the volume fraction of membrane-phase (oil) in the initial oil-in-water (O/W) emulsion. Using the real-time conductivity data of W/O/W ELM, κ W/O/W (t), the volume fraction ϕ W/O/W (t) was calculated from Eq. (4). The swelling ratio S(t)(%), defined as S(t)(%) =
ϕW/O/W (t)V × 100 VM
(5)
J. Yan, R. Pal / Journal of Membrane Science 213 (2003) 1–12
was then evaluated. Note that in Eq. (5) VM is the volume of the membrane phase at initial time, and V is the total volume of the multiple emulsion (ELM). A Zeiss optical microscope equipped with a camera was used to observe the changes in the ELM globules. A sample of W/O/W multiple emulsion (about 0.1 ml) was withdrawn from the vessel every 20 min using a pipette. The sample was transferred into a viscous 1 wt.% polyethylene oxide solution so as to freeze the globules. A drop of the polymeric solution containing
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the globules was placed on the depression of a microscope glass slide. The sample was covered with a thin cover glass and the photomicrgraphs were taken under the microscope.
4. Results and discussion Fig. 3 shows the typical photomicrographs of the globules at different values of the surfactant
Fig. 5. Typical photomicrographs of W/O/W globules with different initial volume fraction of the dispersed-phase, ϕ W/O/W (0).
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Fig. 5. (Continued ).
concentration in the membrane (oil) phase. The initial volume fraction ϕ W/O/W (0) was kept at 0.19 and the stirring speed was set at 350 rpm. When the surfactant concentration is less than 0.5 wt.%, only a small number of internal water droplets appear in the W/O/W ELM globules. With the increase in the surfactant concentration, the number of internal water droplet increases. The profiles of electrical conductivity of W/O/W multiple emulsion (ELM), κ W/O/W (t), are shown in Fig. 4(a). From the electrical conductivity data, the profiles of ϕ W/O/W (t) were
calculated using Eq. (4). The volume fraction profiles ϕ W/O/W (t) are given in Fig. 4(b). The volume fraction, ϕ W/O/W (t), and hence swelling of W/O/W ELM, increases with time at any given surfactant concentration. The swelling ratio S(t)(%), defined in Eq. (5), is shown in Fig. 4(c). As Fig. 4(c) indicates, the swelling ratio increases with time at any given surfactant concentration. The swelling rate R(t)(%/h), defined as R(t)(%/h) =
dS(t)(%) dt
(6)
J. Yan, R. Pal / Journal of Membrane Science 213 (2003) 1–12
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Fig. 6. (a) The electrical conductivity of W/O/W emulsion liquid membranes, κ W/O/W (t), for different values of the initial volume fraction ϕ W/O/W (0). (b) The dispersed-phase volume fraction of W/O/W emulsion liquid membranes, ϕ W/O/W (t), for different values of the initial volume fraction ϕ W/O/W (0). (c) The swelling ratio, S(t)(%), of W/O/W emulsion liquid membranes for different values of the initial volume fraction ϕ W/O/W (0). (d) The swelling rate, R(t) (%/h), of W/O/W emulsion liquid membranes for different values of the initial volume fraction ϕ W/O/W (0).
is calculated from the initial linear portion of S(t)(%) versus time plots. Fig. 4(d) shows the plot of swelling rate, R(t) (%/h) as a function of the surfactant concentration. The swelling rate increases initially with the increase in the surfactant concentration; it reaches a maximum value at about 5 wt.% surfactant concentration and then falls off with further increase in the surfactant concentration. While the exact mechanism causing the observed swelling rate behavior, as shown in Fig. 4(d)), is not known at present, one possible explanation is as follows: with the increase in the surfactant concentration, the interfacial tension between the membrane phase and the external
aqueous phase decreases whereas the viscosity of the membrane phase increases. These are two opposing effects as far as swelling is concerned. The decrease in the interfacial tension is expected to enhance isotonic emulsification whereas an increase in the membrane viscosity is expected to retard secondary emulsification. It appears that at low concentrations of surfactant, the interfacial tension effect dominates whereas at high concentrations of surfactant, the viscosity effect dominates. However, further studies are needed to uncover the exact mechanisms. Fig. 5 shows the typical photomicrographs of W/O/W globules for different values of the initial
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Fig. 7. Typical photomicrographs of W/O/W globules with different stirring speeds.
volume fraction ϕ W/O/W (0). The surfactant concentration and stirring speed were fixed at 5 wt.% and 350 rpm, respectively. As expected, the W/O/W globules consist of a large number of small aqueous droplets entrapped within the membrane (oil) phase. The complete profiles of κ W/O/W (t), ϕ W/O/W (t), and S(t)(%) are shown in Fig. 6(a)–(c). At low values of the initial volume fraction ϕ W/O/W (0), the volume fraction ϕ W/O/W (t) as well as the swelling ratio S(t)(%) increase nearly linearly with the agitation time. Interestingly, at high values of the initial volume fraction
ϕ W/O/W (0), the plots of ϕ W/O/W (t) and S(t)(%) show a sharp rise after a certain period of time. The sharp increase in ϕ W/O/W (t) or S(t)(%) is indicative of phase inversion. The multiple emulsion of W/O/W type undergoes phase inversion to a simple water-in-oil (W/O) emulsion. The phase inversion is found to occur around ϕ W/O/W values of 0.55–0.60; these are quite realistic values in light of the fact that the random close packing volume fraction of uniform spheres is 0.64. Fig. 6(d) shows the plot of swelling rate R(t) (%/h) as a function of the initial volume fraction
J. Yan, R. Pal / Journal of Membrane Science 213 (2003) 1–12
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Fig. 8. (a) The electrical conductivity of W/O/W emulsion liquid membranes, κ W/O/W (t), at different stirring speeds. (b) The dispersed-phase volume fraction of W/O/W emulsion liquid membranes, ϕ W/O/W (t), at different stirring speeds. (c) The swelling ratio, S(t)(%), of W/O/W emulsion liquid membranes at different stirring speeds. (d) The swelling rate, R(t)(%/h), of W/O/W emulsion liquid membranes at different stirring speeds.
ϕ W/O/W (0). As mentioned earlier, the swelling rate is calculated from the initial linear portion of S(t)(%) versus time plot. The swelling rate increases with the increase in the initial volume fraction ϕ W/O/W (0). This is an expected behavior as multiple-body collision, and hence entrapment of the continuous phase fluid within the colliding globules, increases with the increase in the initial volume fraction ϕ W/O/W (0). Fig. 7 shows the typical photomicrographs of W/O/W globules for different stirring speeds. The surfactant concentration and ϕ W/O/W (0) are fixed at 5 wt.% and 0.19, respectively. The photomicrographs indicate that with the increase in the stirring speed,
more internal water droplets appear in the globules indicating an increase in swelling/secondary emulsification. Fig. 8(a)–(c) show the complete profiles of κ W/O/W (t), ϕ W/O/W (t), S(t)(%) at different stirring speeds. The phase inversion of multiple W/O/W emulsion to simple W/O emulsion occurs only for the highest speed of 450 rpm. At lower speeds, no phase inversion occurred during the 2 h observation period. The phase inversion at 450 rpm occurred at a volume fraction ϕ W/O/W of about 0.65 which is close to the random packing of uniform spheres. Fig. 8(d) shows the plot of swelling rate R(t)(%/h) as a function of stirring speed. The swelling rate increases with the
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increase in the stirring speed because of the increase in the collision frequency of globules. 5. Concluding remarks A new on-line technique based on electrical conductivity measurement was developed to continuously monitor the isotonic swelling process of W/O/W ELMs under agitation conditions. Using the new on-line technique, complete isotonic swelling profiles of W/O/W ELMs are obtained. The swelling rate of W/O/W ELMs increases initially with the increase in the concentration of the surfactant present in the membrane phase; the swelling rate reaches a maximum value at about 5 wt.% surfactant concentration and then falls off with further increase in the surfactant concentration. The swelling rate increases with the increase in the initial volume fraction ϕ W/O/W (0); for ϕW/O/W (0) > 0.19, phase inversion of multiple W/O/W emulsion to simple W/O emulsion is observed after a certain period of agitation. With the increase in the stirring speed, the rate of swelling of W/O/W ELMs increases. At a high stirrer speed of 450 rpm, phase inversion of multiple emulsion to simple W/O emulsion takes place after a certain period of agitation. The phase inversion phenomenon generally occurs at a ϕ W/O/W (t) value close to random packing of spheres. Acknowledgements Financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada is appreciated.
References [1] J.W. Frankfeld, N.N. Li, Handbook of Separation Process Technology, Wiley, New York, 1987 (Chapter 19). [2] K. Fujinawa, M. Morishita, M. Hozawa, Demulsification of W/O emulsion by use of high voltage of ac fields, J. Chem. Eng. Jpn. 17 (1984) 632–636. [3] T. Hano, T. Ohtake, K. Takagi, Demulsification kinetics of W/O emulsion in an electric field, J. Chem. Eng. Jpn. 21 (1988) 345–351. [4] M. Goto, J. Irie, K. Kondo, Electrical demulsification of W/O emulsion by continuous tubular coalescer, J. Chem. Eng. Jpn. 22 (1989) 401–407. [5] J. Yan, R. Pal, Osmotic swelling behavior of globules of W/O/W emulsion liquid membranes, J. Membr. Sci. 190 (2000) 79–91. [6] B.W. Brooks, H.N. Richmond, Phase inversion in non-ionic surfactant-oil-water systems-II. Drop size studies in catastrophic inversion with turbulent mixing, Chem. Eng. Sci. 49 (1994) 1065–1175. [7] S. Kumar, On phase inversion characteristic of stirred dispersions, Chem. Eng. Sci. 51 (1996) 831–834. [8] G.A. Davies, G.V. Jeffreys, D.V. Smith, F.A. Ali, The formation of secondary droplets in a dispersion at a phase boundary, Can. J. Chem. Eng. 48 (1970) 328–329. [9] F. Groeneweg, W.G.M. Agterof, P. Jaeger, J.J.M. Janssen, J.A. Wieringa, J.K. Klahn, On the mechanism of the inversion of emulsions, Trans. Inst. Chem. Engrs. 76 (1998) 55– 62. [10] T. Ohtake, T. Hano, K. Takagi, F. Nakashio, Effects of viscosity on drop diameter of W/O emulsion dispersion in a stirred tank, J. Chem. Eng. Jpn. 20 (1987) 443– 447. [11] T. Ohtake, T. Hano, K. Takagi, F. Nakashio, Analysis of water entrainment into dispersed W/O emulsion drops, J. Chem. Eng. Jpn. 21 (1988) 272–276. [12] X.C. Ding, F.Q. Xie, Study of the swelling phenomena of liquid surfactant membrane, J. Membr. Sci. 59 (1991) 183– 188. [13] J.C. Maxwell, A Treatise on Electricity and Magnetism, second ed., vol. I, Clarendon Press, Oxford, 1881.
Isotonic swelling behavior of W/O/W emulsion liquid membranes under agitation conditions Jun Yan, Rajinder Pal∗ Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 Received 9 July 2002; received in revised form 23 September 2002; accepted 11 October 2002
Abstract A new on-line technique is developed to continuously monitor the isotonic swelling behavior of water-in-oil-in-water (W/O/W) emulsion liquid membranes (ELMs) under agitation condition. The technique is based on electrical conductivity measurement. The effects of surfactant concentration, initial volume fraction of dispersed globules, and stirring speed on the swelling profiles of W/O/W ELMs are determined using the new on-line technique. With the increases in the stirring speed and initial volume fraction of dispersed-phase, the swelling rate increases. With the increase in the surfactant concentration, a more complicated behavior is observed—the swelling rate increases initially, reaches a maximum value and then falls off with further increase in the surfactant concentration. Under certain conditions, phase inversion of multiple W/O/W emulsion to simple W/O emulsion is observed. © 2002 Published by Elsevier Science B.V. Keywords: Liquid membranes; Swelling; Emulsion liquid membranes; Multiple emulsion
1. Introduction Emulsion liquid membrane (ELM) separation process constitutes an emerging technology with a wide variety of applications, such as the removal, recovery, and purification of many organic and inorganic compounds from dilute solutions of industrial interest. Examples of chemicals which can be removed or recovered from industrial streams using ELMs are: organic acids, phenols, cresols, amines, and metallic ions such as copper, cadmium and mercury [1]. ELMs are ideally suited for the treatment of wastewater as they have the potential of removing toxic substances down to very low levels. Because of the stability problems associated with the emulsion liquid membranes, ∗ Corresponding author. Fax: +1-519-746-4979. E-mail address: [email protected] (R. Pal).
ELMs have not yet found widespread usage in industrial applications. One serious problem associated with the ELMs is their tendency to undergo swelling. For example, the globules of water-in-oil-in-water (W/O/W) emulsion liquid membranes swell due to transfer of water from the external aqueous phase to the internal water droplets. Swelling often causes rapid increase in the volume of the internal phase. Furthermore, the swelling of membrane globules can trigger the breakdown of the globules. As a result, the process of enrichment of the solute is retarded and the efficiency of separation of the solute from the external aqueous phase becomes lower than the expected value. The swelling of the membrane globules also adversely affects the final demulsification step where the solute is recovered. The problems encountered in the demulsification step are mainly due to a sharp increase in the viscosity of the emulsion, caused by swelling
0376-7388/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 3 7 6 - 7 3 8 8 ( 0 2 ) 0 0 5 0 1 - X
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J. Yan, R. Pal / Journal of Membrane Science 213 (2003) 1–12
of internal droplets [2–4].In our previous work [5], we studied the osmotic swelling behavior of globules of W/O/W ELMs under static condition. The osmotic pressure gradient between the internal and external aqueous phases was induced by creating a concentration difference of d-glucose between the two aqueous phases. The effects of surfactant concentration and initial volume fraction of internal aqueous phase (ϕ W/O (0)) on the swelling behavior of W/O/W globules were determined by monitoring the changes in the size of the globules over a long period. When dealing with unsupported liquid membrane separation, understanding the swelling behavior of globules under agitation condition is also important. While the osmotic swelling behavior of ELM globules under static condition has been systematically studied by many investigators [5], little attention has been given to isotonic swelling of ELM globules under agitation condition. Isotonic swelling of ELMs depends on the capability of the dispersed globules to entrap the continuous phase fluid. This capability of isotonic emulsification is strongly affected by the presence of surfactant in the dispersed globules. Several mechanisms have been proposed by researchers [6–12] to explain isotonic emulsification. The proposed mechanisms are: (1) Multiple-body collision mechanism [6,7]—according to this mechanism, the entrapment of the continuous phase fluid occurs when at least four dispersed droplets (globules) collide with each other. It is unlikely that multiple-body collision can cause isotonic emulsification at low concentrations of dispersed globules; only at high concentration of dispersed globules, multiple-body collision is likely to take place; (2) two-body collision mechanism [8,9]—this mechanism suggests that during collision of two droplets, a film of the continuous phase fluid gets entrapped between the colliding globules, leading to the formation of internal droplets; and (3) drop deformation mechanism [10–12]—according to this mechanism, the interface of the dispersed globules tends to develop a concave surface due to the presence of surfactant at the interface. Any fluctuation in the pressure or shear force acting on the globule interface extends the concave surface and this results in the entrapment of the continuous phase fluid within the globule. In this work, a new on-line technique is first developed to continuously monitor the isotonic swelling process of W/O/W ELMs under agitation conditions.
Using the new on-line technique, complete isotonic swelling profiles of W/O/W ELMs are obtained. The effects of surfactant concentration, initial volume fraction of dispersed globules (ϕ W/O/W (0)), and stirring speed on the isotonic swelling behavior of W/O/W ELMs are determined. 2. New on-line technique for continuous monitoring of isotonic swelling of W/O/W ELMs under agitation conditions In the existing literature, the swelling of W/O/W ELMs is measured by periodically withdrawing samples from the mixing vessel. The primary emulsion is separated from the multiple emulsion (ELM) sample using gravity separator or centrifuge. The swelling ratio is then calculated by measuring the volume or density of the primary emulsion. The drawbacks of such off-line methods are (1) only a limited amount of information is obtained on the swelling behavior of ELMs; (2) errors can occur during manual sampling; (3) errors can also occur during separation of primary emulsion from the multiple emulsion; and (4) the collected sample may undergo further swelling during the separation process, especially if the system is non-isotonic. In order to overcome all these limitations associated with off-line manual approach, an on-line method for continuous monitoring of the swelling behavior of ELMs, under agitation conditions, is needed. The on-line technique developed in this work provides more accurate and detailed information on the swelling behavior of ELMs. The proposed technique is based on electrical conductivity measurement. The electrical conductivity of a dispersion (κ) is related to the volume fraction of the dispersed-phase (ϕ) as follows: 1 + 2βϕ κ = κe (1) 1 − βϕ where κ e is the electrical conductivity of the external phase and β = (α − 1)/(α + 2). Note that α = κd /κe , where κ d is the electrical conductivity of the dispersed-phase. Eq. (1) was originally derived by Maxwell [13]. When applied to W/O/W emulsion liquid membranes, Eq. (1) gives: 1 + 2βϕW/O/W κW/O/W = κe (2) 1 − βϕW/O/W
J. Yan, R. Pal / Journal of Membrane Science 213 (2003) 1–12
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Fig. 1. Comparison of experimental data with the predictions of Eq. (4).
where ϕ W/O/W is the volume fraction of the dispersedphase, that is, the volume fraction of the primary water-in-oil (W/O) emulsion in the overall multiple emulsion (emulsion liquid membrane). For W/O/W multiple emulsion (ELM),  is approximately equal to −0.5 as α = κW/O /κe ≈ 0. Therefore, Eq. (2) can be simplified to 1 − ϕW/O/W κW/O/W = κe (3) 1 + 0.5ϕW/O/W
From Eq. (3) ϕ W/O/W can be obtained as follows: ϕW/O/W =
1−χ 1 + 0.5χ
where
χ=
κW/O/W κe
(4)
Thus, the volume fraction of the water-in-oil emulsion (ϕ W/O/W ) present in the overall multiple emulsion can be calculated if χ is known. A conductivity probe (model U-19500-20, dipstyle, Pt electrode, 1 cm−1 cell constant, supplied by
Fig. 2. Mixing vessel (units in mm).
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Cole-Palmer) was installed in the mixing vessel. The probe was connected to a conductivity meter (Okaton model WD-35100-20). The signal from the conductivity meter was recorded by a Personal Computer through a data acquisition board (CIODAS08JR, 8 Channel A/D, 12 bit, supplied by ComputerBoard Inc.). The signal was recorded for a desired period of time at a data sampling rate of 5 s. From the knowledge of the real-time ELM conductivity κ W/O/W (t)
and the external aqueous-phase conductivity κ e , the volume fraction ϕ W/O/W (t) was calculated using Eq. (4). A simple emulsion of oil-in-water (O/W) type was used to verify Eq. (4). The oil-phase was kerosene and the aqueous phase was dilute sulfuric acid solution with pH = 3.10 and conductivity = 410 S/cm. The experimental data are compared with the predictions of Eq. (4) in Fig. 1. Clearly, Eq. (4) gives good
Fig. 3. Typical photomicrographs of W/O/W globules with surfactant (EMSORB 2500) concentration varying from 0.05% to 20 wt.% based on the membrane (oil) phase.
J. Yan, R. Pal / Journal of Membrane Science 213 (2003) 1–12
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Fig. 3. (Continued ).
estimate of experimental data over a wide range of dispersed-phase volume fraction.
3. Experimental work on isotonic swelling behavior of W/O/W emulsion liquid membranes 3.1. Materials The W/O/W ELMs were prepared using kerosene (supplied by Fisher Scientific) as the membrane phase.
The surfactant, EMSORB 2500 (sorbitan monooleate), was added to the membrane phase. EMSORB 2500 is a commercially available surfactant manufactured by Henkel Corporation. The hydrophile–lipophile balance (HLB) value of this surfactant is 4.6. The aqueous phase (internal as well as external) of the W/O/W ELMs consisted of a dilute sulfuric acid solution (pH = 3.10, conductivity κe = 410 S/cm). Polyethylene oxide (MW = 106 g/mol, supplied by Polysciences Inc.) was employed as a globule freezing agent for microscopy.
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Fig. 4. (a) The electrical conductivity of W/O/W emulsion liquid membranes, κ W/O/W (t), at different surfactant (EMSORB 2500) concentrations. (b) The dispersed-phase volume fraction of W/O/W emulsion liquid membranes, ϕ W/O/W (t), at different surfactant (EMSORB 2500) concentrations. (c) The swelling ratio, S(t)(%), of W/O/W emulsion liquid membranes at different surfactant (EMSORB 2500) concentrations. (d) The swelling rate, R(t)(%/h), of W/O/W emulsion liquid membranes at different surfactant (EMSORB 2500) concentrations.
3.2. Procedures The W/O/W ELMs were prepared in a 65 mm diameter vessel equipped with two vertical baffles (Fig. 2). The mixing was provided with a magnetic stirrer using a stirring bar of length 50 mm. The mixing vessel was filled with well-defined external aqueous phase in advance. Then a known amount of the membrane phase was slowly poured into the vessel without any agitation. After the addition of the membrane phase, the agitation was started at a known stirrer speed. The surfactant concentration was varied from 0.05 to 20% by weight based on membrane phase (oil phase). The
initial volume fraction ϕ W/O/W (0) was varied from 0.05 to 0.39. Note that in this work, the membrane phase does not contain any internal aqueous phase initially, that is, ϕW/O (0) = 0 and therefore, ϕ W/O/W (0) is the volume fraction of membrane-phase (oil) in the initial oil-in-water (O/W) emulsion. Using the real-time conductivity data of W/O/W ELM, κ W/O/W (t), the volume fraction ϕ W/O/W (t) was calculated from Eq. (4). The swelling ratio S(t)(%), defined as S(t)(%) =
ϕW/O/W (t)V × 100 VM
(5)
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was then evaluated. Note that in Eq. (5) VM is the volume of the membrane phase at initial time, and V is the total volume of the multiple emulsion (ELM). A Zeiss optical microscope equipped with a camera was used to observe the changes in the ELM globules. A sample of W/O/W multiple emulsion (about 0.1 ml) was withdrawn from the vessel every 20 min using a pipette. The sample was transferred into a viscous 1 wt.% polyethylene oxide solution so as to freeze the globules. A drop of the polymeric solution containing
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the globules was placed on the depression of a microscope glass slide. The sample was covered with a thin cover glass and the photomicrgraphs were taken under the microscope.
4. Results and discussion Fig. 3 shows the typical photomicrographs of the globules at different values of the surfactant
Fig. 5. Typical photomicrographs of W/O/W globules with different initial volume fraction of the dispersed-phase, ϕ W/O/W (0).
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Fig. 5. (Continued ).
concentration in the membrane (oil) phase. The initial volume fraction ϕ W/O/W (0) was kept at 0.19 and the stirring speed was set at 350 rpm. When the surfactant concentration is less than 0.5 wt.%, only a small number of internal water droplets appear in the W/O/W ELM globules. With the increase in the surfactant concentration, the number of internal water droplet increases. The profiles of electrical conductivity of W/O/W multiple emulsion (ELM), κ W/O/W (t), are shown in Fig. 4(a). From the electrical conductivity data, the profiles of ϕ W/O/W (t) were
calculated using Eq. (4). The volume fraction profiles ϕ W/O/W (t) are given in Fig. 4(b). The volume fraction, ϕ W/O/W (t), and hence swelling of W/O/W ELM, increases with time at any given surfactant concentration. The swelling ratio S(t)(%), defined in Eq. (5), is shown in Fig. 4(c). As Fig. 4(c) indicates, the swelling ratio increases with time at any given surfactant concentration. The swelling rate R(t)(%/h), defined as R(t)(%/h) =
dS(t)(%) dt
(6)
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Fig. 6. (a) The electrical conductivity of W/O/W emulsion liquid membranes, κ W/O/W (t), for different values of the initial volume fraction ϕ W/O/W (0). (b) The dispersed-phase volume fraction of W/O/W emulsion liquid membranes, ϕ W/O/W (t), for different values of the initial volume fraction ϕ W/O/W (0). (c) The swelling ratio, S(t)(%), of W/O/W emulsion liquid membranes for different values of the initial volume fraction ϕ W/O/W (0). (d) The swelling rate, R(t) (%/h), of W/O/W emulsion liquid membranes for different values of the initial volume fraction ϕ W/O/W (0).
is calculated from the initial linear portion of S(t)(%) versus time plots. Fig. 4(d) shows the plot of swelling rate, R(t) (%/h) as a function of the surfactant concentration. The swelling rate increases initially with the increase in the surfactant concentration; it reaches a maximum value at about 5 wt.% surfactant concentration and then falls off with further increase in the surfactant concentration. While the exact mechanism causing the observed swelling rate behavior, as shown in Fig. 4(d)), is not known at present, one possible explanation is as follows: with the increase in the surfactant concentration, the interfacial tension between the membrane phase and the external
aqueous phase decreases whereas the viscosity of the membrane phase increases. These are two opposing effects as far as swelling is concerned. The decrease in the interfacial tension is expected to enhance isotonic emulsification whereas an increase in the membrane viscosity is expected to retard secondary emulsification. It appears that at low concentrations of surfactant, the interfacial tension effect dominates whereas at high concentrations of surfactant, the viscosity effect dominates. However, further studies are needed to uncover the exact mechanisms. Fig. 5 shows the typical photomicrographs of W/O/W globules for different values of the initial
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Fig. 7. Typical photomicrographs of W/O/W globules with different stirring speeds.
volume fraction ϕ W/O/W (0). The surfactant concentration and stirring speed were fixed at 5 wt.% and 350 rpm, respectively. As expected, the W/O/W globules consist of a large number of small aqueous droplets entrapped within the membrane (oil) phase. The complete profiles of κ W/O/W (t), ϕ W/O/W (t), and S(t)(%) are shown in Fig. 6(a)–(c). At low values of the initial volume fraction ϕ W/O/W (0), the volume fraction ϕ W/O/W (t) as well as the swelling ratio S(t)(%) increase nearly linearly with the agitation time. Interestingly, at high values of the initial volume fraction
ϕ W/O/W (0), the plots of ϕ W/O/W (t) and S(t)(%) show a sharp rise after a certain period of time. The sharp increase in ϕ W/O/W (t) or S(t)(%) is indicative of phase inversion. The multiple emulsion of W/O/W type undergoes phase inversion to a simple water-in-oil (W/O) emulsion. The phase inversion is found to occur around ϕ W/O/W values of 0.55–0.60; these are quite realistic values in light of the fact that the random close packing volume fraction of uniform spheres is 0.64. Fig. 6(d) shows the plot of swelling rate R(t) (%/h) as a function of the initial volume fraction
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Fig. 8. (a) The electrical conductivity of W/O/W emulsion liquid membranes, κ W/O/W (t), at different stirring speeds. (b) The dispersed-phase volume fraction of W/O/W emulsion liquid membranes, ϕ W/O/W (t), at different stirring speeds. (c) The swelling ratio, S(t)(%), of W/O/W emulsion liquid membranes at different stirring speeds. (d) The swelling rate, R(t)(%/h), of W/O/W emulsion liquid membranes at different stirring speeds.
ϕ W/O/W (0). As mentioned earlier, the swelling rate is calculated from the initial linear portion of S(t)(%) versus time plot. The swelling rate increases with the increase in the initial volume fraction ϕ W/O/W (0). This is an expected behavior as multiple-body collision, and hence entrapment of the continuous phase fluid within the colliding globules, increases with the increase in the initial volume fraction ϕ W/O/W (0). Fig. 7 shows the typical photomicrographs of W/O/W globules for different stirring speeds. The surfactant concentration and ϕ W/O/W (0) are fixed at 5 wt.% and 0.19, respectively. The photomicrographs indicate that with the increase in the stirring speed,
more internal water droplets appear in the globules indicating an increase in swelling/secondary emulsification. Fig. 8(a)–(c) show the complete profiles of κ W/O/W (t), ϕ W/O/W (t), S(t)(%) at different stirring speeds. The phase inversion of multiple W/O/W emulsion to simple W/O emulsion occurs only for the highest speed of 450 rpm. At lower speeds, no phase inversion occurred during the 2 h observation period. The phase inversion at 450 rpm occurred at a volume fraction ϕ W/O/W of about 0.65 which is close to the random packing of uniform spheres. Fig. 8(d) shows the plot of swelling rate R(t)(%/h) as a function of stirring speed. The swelling rate increases with the
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increase in the stirring speed because of the increase in the collision frequency of globules. 5. Concluding remarks A new on-line technique based on electrical conductivity measurement was developed to continuously monitor the isotonic swelling process of W/O/W ELMs under agitation conditions. Using the new on-line technique, complete isotonic swelling profiles of W/O/W ELMs are obtained. The swelling rate of W/O/W ELMs increases initially with the increase in the concentration of the surfactant present in the membrane phase; the swelling rate reaches a maximum value at about 5 wt.% surfactant concentration and then falls off with further increase in the surfactant concentration. The swelling rate increases with the increase in the initial volume fraction ϕ W/O/W (0); for ϕW/O/W (0) > 0.19, phase inversion of multiple W/O/W emulsion to simple W/O emulsion is observed after a certain period of agitation. With the increase in the stirring speed, the rate of swelling of W/O/W ELMs increases. At a high stirrer speed of 450 rpm, phase inversion of multiple emulsion to simple W/O emulsion takes place after a certain period of agitation. The phase inversion phenomenon generally occurs at a ϕ W/O/W (t) value close to random packing of spheres. Acknowledgements Financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada is appreciated.
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