Langmuir 1995,11, 4691-4694
4691
Kinetics of Mass Transfer of Tryptophan between an Aqueous Phase and a Water-in-OilMicroemulsion Pawel Plucinski" and Walter Nitsch Technische Universitat Miinchen, Institut fur Technische Chemie, 85748 Garching, BRD Received April 1, 1994. I n Final Form: August 7, 1995@ The kinetics of the mass transfer of tryptophan (Trp) between an aqueous phase and a water-in-oil microemulsion (isooctane,AOT, water), measured for variable convection in a stirred cell, is presented. The main result concerns the change of the limitation of the mass transfer resistance depending on the concentration of Trp in the aqueous phase. For higher tryptophan concentrations ([Trp+]o L 7 x km0Vm3) the characteristic influence of the convection on the mass transfer proves transport processes to be rate controlling(equilibriumat the interface). The mass transport resistance is locatedin the aqueous phase for a high AOT concentration or in the microemulsion phase for a low AOT concentration. For low km01/m3)the observed independence of the flux from the concentrations of tryptophan ([Trp+loI 7 x convection signifies a rate-determininginterfacial process (nonequilibrium at the interface). These results are discussed with respect to the bud mechanism, assuming that the Trp+ fraction in the shell of the bud determines the residence time of buds at the interface and therefore the degree of equilibration.
Introduction
In recent years extensive studies of amino acid extraction via solubilization in reverse micelles have been to elucidate the mechanism of the process of micellar extraction. This process seems to be an interesting alternative to classical methods used for recovery of bioproducts.lOJ1 The electrostatic interactions were found to be the most important regarding the mass transfer ~ i e l d ;however, ~-~ the role of hydrophobic interactions2as well as the external salinity3 is discussed too. In our previous works the results of kinetics studies of phenylalanine (Phe) extraction using a water-in-oil (w/o) microemulsion (isooctane, Aerosol OT, and water) as the extracting agent were presented together with a proposal of a mass transfer mechanism.12J3 Experiments performed in a stirred cell showed unequivocally that the process of mass transfer was always an interfacial one, and specific experiments led to the so-called bud mechanism consisting of three consecutive steps:14 (a) sticky collision of micelles with the interface, (b) mass transfer through the neck of a formed bud, and (c) fusion of the shell. The sticky collision means that the micelle attaches to the macroscopic interface, and a channel between the aqueous pool ofthe reverse micelle and the aqueous phase is formed (bud). Through the channel of such a bud the @Abstractpublished in Advance A C S Abstracts, November 1, 1995. (1)Leodidis, E. B.; Hatton, T. A. J . Phys. Chem. 1990,94,6400. (2)Leodidis, E.B.; Hatton, T. A. J.Phys. Chem. 1990,94,6411. (3)Leodidis, E. B.; Hatton, T. A. J.Phys. Chem. 1991,95,5957. (4)Leodidis, E. B.; Bommarius, A. S.; Hatton, T. A. J . Phys. Chem. 1991,95,5493. ( 5 ) Leodidis, E. B.; Hatton, T. A. J . CoZEoid Interface Sci. 1991,147, 163. (6)Leser, M. E.; Luisi, P. L. BiotechnoZ. Tech. 1989,3, 149. (7)Leser, M.E.;Luisi, P. L. Chimia 1990,44,270. (8)Furusaki, S.;Kishi, K. J . Chem. Eng. Jpn. 1990,23,91. (9)Adachi, M.;Harada, M.; Shioi, A,; Sato, Y. J.Phys. Chem. 1991, 95,7925. (10)Hatton, T. A. In Surfactant-Based Separation Processes;Scamehorn, J. F., Harwel, J. H., Eds.; Marcel Dekker Inc.: New York, 1989; pp 55-90. (11)Dekker, M.; Hilhorst, R.; Laane, C. Anal. Biochem.1989,178, 317. (12)Plucinski, P.; Nitsch, W. J . Phys. Chem. 1993,97,8983. (13)Plucinski,P.;Nitsch, W.; Reitmeir,J.;Solano-BauerJ. InSolvent Extraction in the Process Industries; Logsdail, D. H., Slater,M. J.,Eds.; Elsevier Applied Science: London, 1993;pp 1064-1071. (14)Plucinski, P.; Nitsch, W. Langmuir 1994,10,371. 0743-746319512411-4691$09.00/0
mass transfer (ion exchange) takes place. The degree of equilibration depends on the residence time of the bud at the interface and the rate of exchange. M e r successful fusion the micelles release themselves from the interface and diffuse back to the bulk of the organic phase. Preliminary results of Trp extraction via reverse micelles showed that the process of Trp solubilization was dependent on agitation speed in the stirred cell13in exciting contradiction to the behavior of phenylalanine.12J3 These results initiated the present work, which should clear up the prerequisites for the limiting kinetic cases in view of the bud mechanism.
Experimental Section The dynamics of the Trp mass transfer between an aqueous phase and a wlo microemulsion was measured in a stirred cell. The construction of the apparatus and the methodology of evaluationofthe experimentaldata are presented elsewhere.15J6 The lower part of the stirred cell was first filled with the aqueous phase (bufferedsolutionofTrp). Next the organic phase (micellar solution with injected pure water) was transferred slowly, to avoid emulsification at the liquiuiquid interface. The "switch on"ofbothrotorswas taken as the startingpointofan experiment. The agitation speed of the stirrer in the micellar solution was kept in the range 50-200 min-l, while the stirring speed in the aqueous solution was 20% higher to obtain the same volumetric flow in both parts of the stirred cell as measured using laserDoppler-anemometry.17 The equilibriumvalues were measured with standard methods.16J6The concentration of Trp in the micellar phase during the kinetic experiments was monitored . continuously at the wavelength 280 nm using a SP 800 Pye Unicam W/vis spectrophotometer. In the equilibrium measurementsthe Trp mass balancewas always proved with accuracy equal to 3~5%. The water content of the micellar phase was determined by a 633 Karl-Fischer Automat (Metrohm, Swiss). Interfacial tension (always for equilibrium conditions) and was measured using a SITE 04 spinning drop tensiometer (Kruss, FRG). Generallythe kinetic experiments as well as the analysis were performed at 20.0 i 0.1 "C. To normalize the influence of the change of micellar size on the kinetics of reverse micelle (RM) formation (due to water solubilization)the organic phase was previously saturated with a so-calledequilibrium amount of pure water. The aqueousphase was buffered with citrate buffer with a concentration of 0.20 (15)Plucinski, P.;Nitsch, W. Ber. Bunsen-Ges. Phys. Chem. 1989, 93,994. (16)Nitsch,W.; Plucinski, P. J . Colloid Interface Sci. 1990,136,338. (17)Waubke, M.;Nitsch, W. Chem. Ing. Tech. 1986,58,216.
0 1995 American Chemical Society
4692 Langmuir, Vol. 11, No. 12, 1995
Plucinski and Nitsch
10
12
8
4
0
/.
-I
I
20
16
r r ~ ]* lo3 ~ , [kmol/m*. ~
Figure 1. Solubilizationisotherm of tryptophan in AOT reverse micelles, pH = 2.0. 4 5 ,
-e
.-0
'y
0
t 4
Ot3
8
12
20
16
Trp concentration 103 [kmol/mq
Figure 2. The saturation-like isotherms of water ratio and interfacial tension, [AOT] = 0.050 km01/m3, pH = 2.0.
kmol/m3. Generally, in most of the experiments the pH of the aqueous phase was equal to 2.0. All chemicals used were analytical grade or better.
Results and Discussion Equilibrium Aspects. For the interpretation of kinetic data, the knowledge of various equilibrium properties is necessary. Figure 1 shows the equilibrium solubilization isotherm of Trp in AOT reverse micelles, which is similar to a Langmuir-type saturation isotherm and corresponds to eq 1:
(m) (1.752 + 2.979 NOTI =
I
I
150
200
250
Figure 3. The influence of the agitation speed on the initial mass transfer rate of tryptophan, [AOT] = 0.050 km01/m3.(a) 0, [Trp+]o= 1.76 x lo-' km01/m3,pH = 2.0;,. [Trp+lo= 7.06 x km01/m3,pH = 2.0; (b)0, [Trp+]o= 7.06 x 10-6kmol/m3, [Trp+]o= 6.00 x km01/m3, pH = 6.0. pH = 2.0; .,
0,l
15
mic
I
100
agitation speed [mini]
30
0
I
50
x
[Trplaq
(1)
This behavior is similar to the analogues equilibrium of Phe solubilization12 which, however, is shifted to higher concentrations of amino acid in the aqueous phase and is characterized by a similar saturation value (the saturation regions are reached for [Trpl,, 5 x kmol/m3,([Trpl/ [AOTI),,, = 0.57 and for [ P h e l , , ~2 x 10-2kmol/m3,([Phel/ [AOTl),,t = 0.56). The corresponding values of pKa.coo~ for both amino acids are p K ~ = p 2.38 and pKphe = 1.83.16 These results suggest that the final relation of amino acid and AOT reflects a structural interaction between the AOT anions and the amino acid cations in the (18)Cohn, E. J.;Edsall, J.T. Proteins, Amino Acids and Peptids as Ions and Dipolar Ions; Reinhold Pub. Co.: New York, 1943; Chapter 4.
(19)Nitsch, W.Faraday Discuss. Chem. SOC.1984,77, 85.
architecture of the micellar shell. This conclusion is also supported by the characteristic change of the water ratio (w,= [HZO]/[AOTI)and the interfacial tension with Trp concentration (Figure 21, which correlates with the saturation behavior of the solubilization isotherms (Figure 1). Additionally, the results of the interfacial tension suggest a strong connection between the composition of the micellar shell and the macroscopic liquiaiquid interface. Similar changes of the interfacial tension and water uptake with the change in concentration of the phenylalanine12and cosurfactants (series of n-alkanols)21 were stated in our previous works. Kinetic Aspects. Znfluence of Convection. The methodological advantage of the applied stirred cell is the possibility to distinguish between the regimes ofinterfacial step (fluxjoindependent of agitation speedlg)and transport processes (fluxjo increases linearly with agitation speedlg) as rate controlling. The fluxes, defined according to eq 2,
for the solubilization of Phe were always independent of convection,12J3which means limitation by an interfacial process. Therefore it was astonishing that the change of molecular structure from Phe to Trp generates a different behavior: In the upper concentration range of the Trp+ cation (7.0 x 5 [Trp+]o5 1.75 x lo-' kmol/m3),the linear dependency between flux and agitation speed (Figure 3a) proves that transport processes are rate controlling, and only in the case of rather low Trp+ concentrations ([Trp+]o= 7.0 x km01/m3, pH = 2.0 or [Trp+lo= 6.0 x kmol/m3, pH = 6.0)is the expected interfacial solubilization rate determining (Figure 3b). However, in the presence of cationic species in the aqueous phase also for the high concentration level of Trp+([Trp'lo = 1.75 x kmol/m3),the region of flow independency of the Trp flux was obtained (Figure 4). The explanation
Langmuir, Val. 11, No. 12, 1995 4693
Kinetics of Mass Transfer of Tryptophan 7 ,
1
I
,/[Trpl= 1,76
+
Tm
I
10' kmol/m3
J;
Ca r r p q = 7,06
+
oy
i o 3 kmol/m3
0
Sr
I
I
I
I
50
100
150
200
250
agitation speed [min']
50
0
100
150
200
250
agitation speed [minl]
Figure 4. Dependence of the tryptophan solubilization rate on the agitation speed in the presence of metal cations in the aqueous phase, [AOTI = 0.050 kmol/m3,[Sr2+l= [Ca2+l= 0.10 km0Ym3, pH = 2.0.
r r p q = 1,76 10' kmol/m3 +
I /
rrpq = 7,06
l o 3 kmol/m3
.-m
.E
.-
F
c
0 0,oo
0,05
0,lO
0,15
0,20
0,25
AOT concentration [kmol/m3]
Figure 5. The influence ofthe AOT concentration on the initial solubilization rate, pH = 2.0,n = 150 min-I.
for this classical behavior of two-phase mass transferlg demands a detailed consideration of the transport and the interfacial process. Rate-Controlling Transport Processes. This kind of kinetics means Trp equilibrated buds a t th'e macroscopic 1iquiMiquid interface. For the evaluation of the results, the influence of the AOT concentration on the rate of solubilization is of a great interest (Figure 5). The high AOT concentration range is characterized by the independence of the mass transfer rate from the micellar concentration, which means that the mass transfer resistance has to be situated only in the aqueous phase. For this specific situation, in the relevant transport equation, JT,
- PTrp,aq( [TIT],,- [TIT].,*)
(3)
the interfacial concentration [Trp]*is negligiblelg and the values of the transport coefficient / ? T ~ become , ~ ~ accessible.15J6 In Figure 6 the calculated values to the mass transfer coefficients are compared with those found previously for the aqueous transport process of methylene blue (MB).16 The agreement of both values measured in the same apparatus confirms the postulate of the onesided resistance located in the aqueous phase for the high
Figure 6. Dependence of the aqueous phase mass transfer coefficienton the agitation speed, Trp, [AOT] = 0.20 km01/m3, km0Ym3,pH = 2.0; MB, [AOTI = 0.050 [Trp+lo= 1.76 x kmol/m3,pH = 5.0, [NaCl]= 0.10 km01/m3, [MBIo= 1.0 x kmol/m3.
AOT concentration ([AOT]= 0.20 kmol/"), which agrees with the observed linear dependence of the solubilization rate on the Trp concentration. The other borderline case of a one-sided resistance in the microemulsion phase is realized for low AOT concentrations, as the solubilization rate increases proportionally with the AOT concentration (Figure 5). Rate-Controlling Interfacial Processes. Concerning the kinetics of solubilization of hydrophilic species in microemulsions of AOT, the behavior of Trp is so far unique,12-16,20 because of the fact that depending on the concentration level of Trp in the aqueous phase the transport processes or the interfacial process can be rate determining. The interpretation of this observed change of mechanism requires analogous to the treatment of the usual chemical extraction in the case of limiting transport, that equilibrium conditions be established at the interface. If the interfacial process is the rate-controlling step, the kinetic situation will be characterized by the gradient of chemical potential across the interface (nonequilibrium conditions). According to the bud mechanism, in the case of transport-limited mass transfer, one has to reckon with equilibrium a t the interfacial buds, whereas in the case of a rate-controlling interfacial step, the equilibration of the buds is not established. For a first approach to an interpretation of our experimental data the three steps of the bud mechanism should be considered.12J4 Concerning the limitation of the mass transfer process the first step "sticky collision'' is considered to be meaningless, because the equilibration of an individual bud is independent of the collision number. However,the degree of equilibration depends on the residence time of the individual bud, which is controlled by the rate of fusion: the greater the residence time, the higher the chance for equilibration. Assuming this plausible mechanistic interpretation, one has to conclude that the cationic Trp species in the shell of the buds govern the residence time of a bud. A small Trp fraction in the shell shorterns the residence time, and equilibrium cannot be established; that means that the interfacial process is rate determining. According to this interpretation a n increase of Trp (20) Plucinski, P.; Nitsch, W. J.Colloid Interface Sci. 1992,154,104. (21)Plucinski, P.; Reitmeir, J. Colloids Surf. A 1996,97, 157.
4694 Langmuir, Vol. 11, No. 12, 1995 concentration in the shell leads to slow fusion, equilibration, and finally transport-controlled solubilization. The results presented in Figure 4 support the interpretation presented above: in the presence of additional cationic species (strontium or calcium) the interfacial process becomes rate controlling in spite of high Trp concentrations in the aqueous phase (compare Figures 3 and 4). The reason for such behavior is a competitive adsorption of Trp and strontium or calcium ions, which decreases the fraction of Trp in the interfacial AOT layer and therefore again the residence time.
Conclusions Compared with a series ofprevious experimental studies about the kinetics of the solubilization in Winsor I1 system, Trp so far shows remarkable behavior as different kinetic borderline cases, transport or reaction limitation are obtained, depending on the concentration level. Therefore, the aspect of the degree of equilibration a t the interface becomes important. The proposed plausible interpretation concerns the bud mechanism, giving a special emphasis to the rate of fusion. Of course our knowledge about the mechanism of fusion, the importance of the neck position of a bud, and its composition is rather underdeveloped, and therefore further experiments will be necessary for final statements.
Plucinski and Nitsch
Acknowledgment. This work has been supported by the Deutsche Forschungsgemeinschaft (Grant SFB 266). Symbols Used A .i t [Trpl V
wQ
P Y
Interfacial area (4.42x m2)(cross sectional area of stirred cell), m2 solubilization rate, kmoY(m2s) time, s Trp concentration, kmoYm3 volume of organic (micellar) phase (4.22x m3), m3 water ratio, dimensionless mass transfer coefficient, m/s interfacial tension, N/m
Subscripts and Superscripts aq mic sat Trp
*
0
aqueous micellar in a saturation region of solubilization isotherm tryptophan interfacial initial
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