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Langmuir 1996, 12, 2362-2366
Ultrafiltration of Reverse Micelles in the Ternary System AOT/Isooctane/Water Reinhard Schoma¨cker* and Gerhard A. Braun† Bayer AG, Central Research and Development Division, ZF-TPT 4, Geb. E 41, D-51368 Leverkusen, Germany, and Department of Physical Chemistry, University of Cologne, Luxemburgerstrasse 116, D-50 939 Ko¨ ln, Germany Received July 17, 1995. In Final Form: January 16, 1996X Ultrafiltration is a separation method at the nanometer scale and is applicable for the rejection of self-assembled surfactants at membrane surfaces. We investigated the filtration of reverse micelles in the ternary system AOT/isooctane/water. The dependence of flux velocity and rejection on pressure, size, and concentration of the reverse micelles is described. The changing composition of the ternary system during the filtration process was examined and is represented in a Gibbs phase triangle. The concentration of the reverse micelles at the membrane wall is discussed with a model established for aqueous polymer solutions.
Introduction Micellar-enhanced ultrafiltration (MEUF) is a new method in separation technology which enables the filtration of molecules and metal salts from aqueous solution with a good retention and high flow rates.1,2 The concept of this method is to fix the target substance at surfactant aggregates and to reject them with an ultrafiltration membrane. In contrast to the widespread investigations on the MEUF of micelles in aqueous solutions, little is published3-5 about the application of ultrafiltration membranes to separate so-called reverse micelles. Reverse micelles are surfactant aggregates in nonpolar solvents where the hydrophilic headgroups of the amphiphilic molecules are orientated into the interior of spherical aggregates.6 These spheres are able to incorporate significant amounts of polar liquids, in general, water, but also glycerol or formamide. In this respect it is helpful to imagine reverse micelles as colloidal water drops dispersed in an organic solvent and stabilized by a surfactant monolayer. Reverse micellar solutions are used in biotechnology because the aqueous interior of the aggregates can serve as a host for water soluble enzymes and act in this way as a microreactor for substrates which are preferentially soluble in organic solvents.7 One of the main problems of this application in biotechnology is the regeneration of the enzymes after reaction. Membrane processes may be able to restore biocatalysts enclosed in reverse micelles in an inexpensive way. Luthi and Luisi3 as well as Luisi and Hatton4 investigated enzymecatalyzed reactions in membrane reactors with ultrafiltration membranes. They used the formation of a peptide bond catalyzed by R-chymotrypsin as a model reaction. The product is separated from the enzyme by dialysis. * To whom correspondence should be addressed at Bayer AG. † University of Cologne. Current address: Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45466 Mu¨lheim/ Ruhr, Germany. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Dunn, R. O.; Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1985, 20, 763. (2) Scamehorn, J. F.; Christian, S. D.; El-Sayed, D. A.; Uchiyama, H.; Younis, S. S. Sep. Sci. Technol. 1994, 29, 809. (3) Luisi, P. L.; Luthi, P. J. Am. Chem. Soc. 1984, 106, 7285. (4) Luisi, P. L.; Hatton, T. A. Bioseparation 1991, 2, 5. (5) Prazeres, D. M. F.; Garcia, F. A. P.; Cabral, J. M. S. Bioprocess Eng. 1994, 10, 21. (6) Eicke, H. F. Top. Curr. Chem. 1980, 87, 85. (7) Schoma¨cker, R.; Robinson, H. B.; Fletcher, P. D. I. J. Chem. Soc., Faraday Trans. 1988, 84, 4203.
The enzyme inside the surfactant aggregate is too bulky to pass through the membrane. The driving force for the separation process is the concentration gradient of the product. The disadvantage of this method is the low flow rate and therefore the need for large membrane areas. Prazeres and Garcia5 described the continuous lipolysis of olive oil in a membrane bioreactor using a lipase incorporated in reverse micelles. The separation of catalyst and product is pressure-driven, but the results were discussed only in respect to the substrate conversion. To our knowledge no fundamental investigation of the ultrafiltration of revese micelles was published. The aim of this paper is to describe the influence of pressure, concentration, and molar ratio of water to surfactant on the rejection and the flux during filtration of reverse micelles in ternary mixtures of ionic surfactant, oil, and water. The change of composition in the retentate is demonstrated in a Gibbs phase triangle. The results of the investigations are discussed with respect to the gel layer model, a well-known model of the ultrafiltration process.8 Experimental Section Materials. The ionic surfactant utilized in this work was sodium bis(2-ethylhexyl)sulfosuccinate, also known as Aerosol OT or AOT, obtained from Sigma Chemicals. The moisture was determined with Karl Fischer titration and stated to be 1 wt %, which means a molar ratio of water to surfactant of 0.25. Isooctane was obtained from Merck in synthesis quality, and they both were used as received. The water was deionized with Lewatit. The filtration cell used in the investigations was a highpressure batch cell with a magnetic stirrer bar, a capacity of 400 mL, and a membrane diameter of 78 mm from Berghof/Eningen. The ultrafiltration membrane was the asymmetric polyamide membrane BM 100 from Berghof with a molecular weight cutoff of 10 000 Da. Methods. The experiments were performed in the following manner: Surfactant and water were mixed in the required ratio using an analytical balance ((0.1 mg); then this mixture was added to the appropriate amount of isooctane and stirred until a clear solution resulted. The membrane was rinsed with 2 L of deionized water, and it was assured that a constant flux was reached. Then mixtures of water and acetone (50 mL each) with the ratios 2/1, 1/1, and 1/2 were used to adjust the membrane to apolar solvents. Afterward the membrane was rinsed with pure acetone, followed by mixtures of acetone and isooctane (50 mL each; 2/1, 1/1, 1/2, and 1/9) and finally 200 mL of pure isooctane. It was proved by UV measurements that no acetone was in the (8) Mulder, M. Basic Principles of Membrane Technology; Kluwer Academic Publisher: Dordrecht, 1991.
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permeate and a constant flux was assured. Before the experiment the filtration cell was dried with compressed air while the membrane was stored in pure isooctane. The flux velocity was determined with 50 g of the initial solution by weight measurements of the permeate in adequate time intervals. The rejection for different molar ratios of water to surfactant was investigated by starting with 50 g of reverse micellar solution. Two grams of the permeate was analyzed gravimetrically and by Karl Fischer titration after 5 g passed the membrane; then again 5 g passed and 2 g were analyzed. In that way three samples were taken from every batch. For gravimetric determination of the surfactant concentration the isooctane and water were evaporated with a heating lamp and the residue was weighed when mass constancy was reached. The change of concentration of water and surfactant in the permeate and the retentate during the filtration was investigated by starting with 160 g of microemulsion and by analyzing samples of 2 g in 10 g intervals of permeate. All experiments were performed at room temperature (22-25 °C) without thermostatic control. After the experiment the remaining microemulsion was removed from the filtration cell with a pipet and the surfactant concentration was determined to check whether the mass balance of the process was fullfilled. The membrane was rinsed with pure isooctane, and the flux velocity was determined. In this way it was examined whether irreversible fouling of the membrane took place. In all cases reported here the mass balance was correct and no irreversible fouling was found. Significant absorption of surfactant on the membrane was excluded by the following experiment: A binary mixture of AOT and isooctane (50 g) with 5 wt % surfactant (0.08 M) was filtered. The first 2 g of the permeate was taken for gravimetrical analysis, and then 5 g was passed; after this another 2 g was analyzed. Four samples were taken by repeating this procedure. The rejection of the first sample calculated with eq 1 was 29.4% and was within the standard deviation of the averaged rejection over all fractions, which was 29 ( 1%. If there would be a significant absorption of surfactant on the membrane or at the filtration cell below it, there would have to be a significant lower concentration of surfactant in the first fraction of filtrate. The change in the viscosities of the solutions with increasing reverse micelle concentration was measured with an Ubbelohde viscosimeter at 25 °C, applying the following relation:
Figure 1. Flux velocity as function of the transmembrane pressure drop and weight fraction of surfactant: (9) γ ) 0.02; (0) γ ) 0.026; ([) γ ) 0.035; (]) γ ) 0.062; (4) γ ) 0.093; (4) γ ) 0.15; (b) γ ) 0.18. The conditions are w0 ) 10, stirrer speed ) 1000 rpm, 50 g initial solution. m°i ) mass of isooctane in the initial solution, ∑mpi ) total amount of isooctane passing the membrane during the filtration process, m°w ) mass of water in the initial solution, and ∑mpw ) total amount of water passing the membrane during the filtration process. The weight fraction of surfactant in the retentate was calculated and not measured, because the volume of the retentate was not changed in that way. The assumption of fullfilled mass balance made for the calculation was checked experimentally. The size of the reverse micelles in the AOT/isooctane/water system is given by the following relation:9
r ) 1.8w0 [Å]
(5)
The radius r of the reverse micelle is given in angstroms; w0 represents the molar ratio of water to surfactant.
Results and Discussion ηp Fptp ηr ) ) η0 F0t0
(1)
(4)
Effect of Pressure and Surfactant Concentration. The change of flux velocity with pressure and concentration was examined in the following manner: Six solutions with different surfactant concentrations and a constant molar ratio of water to surfactant were filtered at increasing pressure. Figure 1 shows the flux plotted versus the transmembrane pressure drop. At low pressure (0-2 bar) and low concentration the flux velocity increases nearly linearly with the pressure; then the slope of the curves becomes less steep and, finally, at high pressure (>8 bar) it is almost zero. The maximum value of the flux decreases with increasing concentration of the solution. This dependence of flux velocity on concentration and pressure is known from the ultrafiltration of macromolecules in aqueous solutions. The maximum value of the flux velocity is called the limiting flux JV∞. The dependence of rejection on the transmembrane pressure drop is shown in Figure 2. The rejection decreases with increasing pressure; remarkably there is still a rejection of more than 40% even at 10 bar. Effect of Water-to-Surfactant Ratio. The flux velocity declines significantly with increasing w0. This result is in good agreement with theory because the flux is inversely proportional to the radius of the reverse micelle, as shown in eq 6.
γr ) weight fraction of surfactant in the retentate, m°s ) mass of surfactant in the initial solution, ∑mps ) total amount of surfactant passing the membrane during the filtration process,
(9) Luisi, P. L.; Giomini, M.; Pileni, M. P.; Robinson, B. H. Biochim. Biophys. Acta 1988, 947, 209. (10) Tamamushi, B.; Watanabe, N. Colloid Polym. Sci. 1980, 258, 174.
with ηr ) relative dynamic viscosity, ηp ) viscosity of the probe, η0 ) viscosity of the pure solvent, Fp ) density of the probe, F0 ) density of the pur solvent, tp ) flow time of the probe, and t0 ) flow time of the pure solvent. Calculations. The rejection R was calculated using
R)1-
γp γr
(2)
with γp ) weight fraction of surfactant in the permeate and γr ) weight fraction of surfactant in the retentate. The weight fraction of surfactant γ is defined by
γ)
ms ms + mw + mi
(3)
where ms, mi, and ms and the weight of surfactant, of isooctane, and of water, respectively. The surfactant concentration in the retentate was calculated from the mass balance and the concentration of the permeate using eq 4. The dead volume of the filtration cell was taken into account.
γr )
∑m m° - ∑m + m° - ∑m s
p s
m°s -
p s
i
p i
+ m°w +
∑m
p w
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Figure 2. Dependence of rejection on the transmembrane pressure drop. The conditions are w0 ) 10, stirrer speed ) 1000 rpm, γ ) 0.1, 50 g initial solution.
JV ∝
kBT δ6πηrm
(6)
with JV ) flux, kB ) Boltzmann’s constant, T ) temperature, η ) viscosity of the solvent, δ ) thickness of the boundary layer near the membrane, where the concentration of the reverse micelles is higher than in the bulk of the solution, and rm ) radius of the reverse micelle. This relation describes the dependency of the flux on the increasing concentration of reverse micelles in the area of the membrane wall and their diffusion back into the bulk of the solution occurring during the filtration process.8 This phenomenon is denoted concentration polarization. The flux should depend linearly on 1/w0; experiment proves that expectation, as shown in Figure 3. The change of rejection of surfactant with increasing ratio of water to surfactant (w0) was investigated and plotted in Figure 4. The molar ratio of water to AOT (w0) determines the size of the reverse micelles, as described by eq 4. The rejection increases with increasing w0, whereby it approaches 92%. The shape of the curve is reminiscent of the curves known from sieving processes. Obviously the main difference is that there is also a significant rejection at w0 ) 0. This derives from the fact that the utilized surfactant was not completely dry and so the AOT molecules formed aggregates which were hindered in passing the membrane. This was checked experimentally by drying the surfactant in vacuum over P4O10 at 50 °C; the isooctane was dried with P4O10 and then distilled. The binary mixture of the dried components had a rejection of 9.3%. Total loss of rejection is not achieved because the small amount of water present from the pretreatment of the membrane initiates self-aggregation.3 Ultrafiltration Performance of AOT Reversed Micelles. The change in the compositions of the retentate and the permeate and thereby the change of rejection are of substantial interest for application. Figure 5 shows this change during the ultrafiltration process. Until 50% of the initial solution has passed the membrane, the rejection remains nearly constant at 90%. If more than half of the reverse micellar solution is filtered, the rejection decreases significantly, a phenomenon that is well-known from the ultrafiltration of aqueous macromolecular solutions and discussed elsewhere.11 Figure 5 shows also the portion of water and surfactant in the permeate and the (11) Nguyen, Q. T.; Neel, J. J. Membr. Sci. 1983, 14, 111.
Figure 3. Flux as a function of the reciprocal of the molar ratio of water to surfactant w0. The conditions are 1 bar transmembrane pressure drop, γ ) 0.15, stirrer speed ) 1000 rpm, 50 g initial solution.
Figure 4. Rejection as a function of the molar ratio of water to surfactant. The conditions are 1 bar transmembrane pressure drop, γ ) 0.1, stirrer speed ) 1000 rpm.
Figure 5. Crucial parameters in the filtration process: (0) surfactant in the permeate; (]) surfactant in the retentate; ([) water in the permeate; (9) rejection of surfactant during the filtration process. The conditions are 1 bar transmembrane pressure drop, w0 ) 10, stirrer speed ) 1000 rpm, 160 g initial solution. The numbers above the values of the filtrate give the molar ratio of water to surfactant w0 in the filtrate.
w0-values. The molar ratio of water to surfactant in the stripping solution is on average the same as in the initial solution. This is not surprising because the AOT reverse micelles are quite monodispersed.12 From the values in the permeate we can calculate the critical micelle concentration of the system. We assume that water can only
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Figure 6. Change of composition in the ternary mixture AOT/ isooctane/water in the retentate plotted in the oil-rich corner of the Gibbs phase triangle. In the area labeled 1Φ a single phase of water dispersed in isooctane exists; in the 2Φ area the oil-rich phase coexists with a water-rich phase; LC denotes a liquid cristalline phase within the two-phase region.
exist in the system when incorporated in reverse micelles and all surfactant is located at the interface of water and isooctane. The surfactant concentration is given by
c*s )
cw w0
(7)
with cw ) measured concentration of water in the permeate, w0 ) molar ratio of water to surfactant (10 in the actual case), and c* s ) surfactant concentration in the case that no free surfactant exists beside the reverse micelles. The difference between the measured surfactant concentration (cs) and the calculated value is the cmc:
cmc ) cs - c*s
(8)
The cmc measured in this way is 1.88 × 10-3 M, which is in reasonable agreement with the value 4.9 × 10-4 M published by Eicke et al.6 Figure 6 shows the composition of the retentate during the filtration process plotted in the oil-rich corner of the Gibbs phase triangle for the ternary mixture AOT/isooctane/water. The phase triangle is taken from ref 10. Modeling of Reversed Micelles’ Ultrafiltration. For applications in biotechnology, with enzymes incorporated in the interior of reversed micelles it is necessary to examine whether there is a phase transition from liquid isotropic to liquid cristalline phase during the filtration process. There is a much higher concentration of reversed micelles near the membrane surface than in the bulk solution, because the solvent can pass the membrane while the reversed micelles are rejected. This phenomenon is called concentration polarization and is well-described.8 The concentration at the membrane surface in the case of 100% rejection is given by eq 9:
{ }
cm ) cb exp
δJV D
(9)
with cm ) concentration at the membrane, cb ) concentration in the bulk of the solution, δ ) thickness of the boundary layer near the membrane, D ) diffusion coefficient, and JV ) flux velocity. The ratio D/δ is called the mass transfer coefficient, abbreviated k. The determination of k and the calculation of cm are possible using the gel layer model which is established for ultrafiltration (12) Ricka, J.; Borkovec, M.; Hofmeier, U. J. Chem. Phys. 1991, 94, 8503.
Figure 7. Determination of the mass transfer coefficient k and the weight fraction of surfactant at the membrane surface using the gel layer model. For this the experimental data shown in Figure 1 are evaluated with eq 10. The values are taken at an 8 bar transmembrane pressure drop.
of macromolecules in aqueous solution. The idea of the gel layer model is that the higher concentration of the rejected substance near the membrane leads to increased viscosity in this area and results in the formation of a gel layer. This gel layer hinders the passage of the solvent through the membrane, thereby lowering the flux. With increasing pressure more solvent is pressed through the membrane and consequently more of the filtered particles are accumulated there; the gel layer becomes thicker and lowers the flux. These contradictory tendencies equilibrate, and an increase in pressure leads to no further increase in flux velocity; the region of limiting flux is reached.8 The limiting flux changes with the concentration in the bulk. With these assumptions eq 9 can be written as
ln
cg J∞V ) cb k
(10)
with cg ) concentration in the gel layer, cb ) concentration in the bulk of the solution, J∞V ) limiting flux, and k ) mass transfer coefficient. The concentrations in eq 10 can be replaced by weight fractions because both are linearly proportional at constant w0. If the limiting flux is plotted versus the logarithm of the weight fraction in the bulk solution, a straight line results where the slope is k and the intersection with the abscissa is the weight fraction in the gel layer. The experiments described above, where flux velocity was measured as a function of pressure (Figure 1), can be evaluated with respect to the gel layer model. The result is shown in Figure 7. The slope of the straight line, and thus -k, is -37.9 kg/(m2 h); the intersection with the abscissa is at γ ) 0.26. If the gel layer model describes the filtration of reverse micelles adequately, there is no phase transition at the membrane wall. This is as would be expected from Figure 6. Viscosity measurements at this γ gave a relative dynamic viscosity of 7.2 if that of pure isococtane is defined as 1; the dynamic viscosity is 3621.6 mPa s at 298 K. The gel layer model gives a theoretical description of ultrafiltration processes and is often applied for macromolecular aqueous solutions. In the case of reversed micelles it permits a qualitative interpretation of the experimental results analogous to these systems. However, it is questionable if a boundary layer with a dynamic viscosity of 3621.6 mPa s explains the slow rise of the flux velocity with increasing pressure.
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The ternary system with γ ) 0.26 and w0 ) 10 doesn’t behave like a gel but shows the quality of a viscous liquid. Although it is well-established, the literature referring to the gel layer model lacks reliable data for the viscosity that is necessary to change the flux in the described manner. Because there are no values available for comparison, it is impossible to determine if the gel layer model gives a quantitative description of the experimental data. The osmotic pressure model,8 another wellestablished model in filtration theory, was also tested, but the assumptions made in this theory were not consistent with our results. Conclusion The ultrafiltration of reversed micelles in the ternary system AOT/isooctane/water is possible with a rejection of more than 90% at a 1 bar transmembrane pressure drop using an ultrafiltration membrane with a molecular weight cutoff of 10 000 Da. Rejection and flux velocity
Schoma¨ cker and Braun
depend on the size and the concentration of the reverse micelles in a way that is known from the ultrafiltration of macromolecules in aqueous solution. At increasing pressure a limiting flux is reached. This can be interpreted with the gel layer model, and the weight fraction of surfactant at the membrane wall can thus be calculated. For application in biotechnology, it is necessary to improve the rejection at constant flux to minimize contamination of the product with surfactant and to assure that no phase transition near the membrane occurs during the filtration process which could inactivate enzymes. If these problems can be solved, the MEUF of reverse micelles could be a powerful method for the separation of product and catalyst in membrane reactors. Acknowledgment. This work was carried out in the laboratories of Bayer AG/Leverkusen. The authors are grateful for the generous material and financial support. LA9505935
