Langmuir 1994,10, 371-376
371
Articles Mechanism of Mass Transfer between Aqueous Phase and Water-in-Oil Microemulsion Pawel Plucinski’ and Walter Nitsch Technische Universitat Munchen, Institut fur Technische Chemie, 85747 Garching, Germany Received August 7,1992. In Final Form: November I , 199P The specific ion effect of the ion exchange in the Winsor I1 microemulsion system has been studied in the equilibrium state as well as in kinetic experiments using a two-phase stirred cell. The equilibrium results of an ion exchange revealed the importance of the specificion adsorption at the negative AOT layer. Generally, it was found that larger bare cationswere much better adsorbed at the interface (i.e. solubilized in reverse micelles) than smaller ones, independent of their valency. The differences in the ion exchange of particular cations are explained using the Grahame model of specific adsorption. The kinetic results of the metal ions solubilization allow an explanation for the influence of the number of reverse micelles on the solubilization rate. The relevant interpretation states that reverse micelles present in the organic phase colliding with the liquidlliquid interface form channels between the aqueous pool of the micelles and the bulk of aqueous phase (‘sticky collision”)in which mass transfer takes place. Then, aftersuccessful fusion, the micelles release themselves from the interface finishing the process of solubilization. The proposed mechanism could be seen as an alternative to the model of spontaneous aggregation at the interface. Additionally, it was observed that the measured ratio of equilibrium concentrations of the CaIZn mixture in the micellar phase, the ratio of Gibbs excesses at the liquidlliquid interface, and the ratio of the solubilization rates were equal. This result can be seen as an indication for the equilibrium state of newly formed reverse micelles at the interface at least for these defined species. Introduction
The Winsor I1 system has attracted great interest in the last decade because of its potential application in the field of liquid/liquid (1/1) extraction.lP2 In spite of numerous publications about the extractional equilibria and the separation possibility, the detailed mechanism of the solubilization in the 1/1 system was not clear until now.112 Thermodynamic studies are not able to describe the mechanism of the solute extraction (i.e. solubilization in reverse micelles (RMs)), but the knowledge of the mechanism promises the prediction of dynamics, which can be important for the separation technology. Therefore kinetic studies are necessary. The kinetics of the extraction (solubilization) in 1/1 systems with RMs have been studied in our laboratory for the last 5 years. Our results showed that the process of solubilization in the 1/1 system took place at the macroscopic 1/1 interfaces as a slow and therefore rate determining interfacial process. To explain the experimental results, a mechanism of rate controlling spontaneous formation of RMs at the 1/1 interface was proposed as a first approximation.5 For the ion exchange between the micellar and the aqueous phase the proposed mechanism states that the ion transfer out of the aqueous pool of the micelles into the aqueous bulk (reextraction)is controlled by the process Abstract published in Advance ACS Abstracts, January 1,1994. (1) Leodidie,E. B.; Hatton,T. A. InStructureandReactiuity inReuerse Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989; pp 270-302. (2) Hatton, T. A. In Surfactant-Based Separation Processes; Scamehorn, J. F., Harwell, J. H., Eds.;Marcek Dekker, Inc.: New York, 1989; pp 57-90. (3) Nitach, W.; Plucinski, P. J. Colloid Interface Sci. 1990,136,338. (4) Plucinski, P.; Nitsch, W. In Solvent Eztraction 1990, part A; Elsevier: Amsterdam, 1992; pp 847-852. (5) Plucineki, P.; Nitach, W. J. Colloid Interface Sei. 1992,154, 104. @
of coalescence of micelles at the macroscopic 1/1interface:>’ whereas the ion transfer into the water pool of micelles is limited by the encapsulation of the aqueous phase by the interfacial layer during the assumed spontaneous process of interfacial micellation.3-5 Such a mechanism means that the measurements of the ion exchange allow the calculation of the attachment (reextraction) and the formation (extraction) rates of micellesat the 1/1interface. The starting point to the conception of this work was the pronounced discrimination of the kinetics and of the interfacial behavior generated by transition and main group cati0ns.~*5Reverse micelles with counterions from transition group (Cd, Co, Mn, Ni, Zn) were formed about 1 order of magnitude faster than those with the main group counterions (Ba,Ca, Sr). The transition group counterions generated also larger sizes of the interfacially formed micelles and lower interfacial tensions.41~The explanation for this unexpected result should be pushed forward with ion exchange measurements of ion mixtures, which should allow an estimation of the degree of equilibration at the interface. Furthermore, we expected from such measurements an approach to the mechanism, enabling us the explanation of the increase of interfacial solubilization rate with the number of micelles. The concept of our work was not strictly restricted to the kinetic studies, measurements connected with the sizes of micelles,interfacial tension, and equilibria of ion exchange were measured too, to support the kinetic results. Experimental Section A stirred cell was used to measure the dynamics of the interfacialformation of RMs. Its construction and the evaluation of the experimental data are presented else~here.~ To measure (6) Bauech,T. E.; Plucinski, P. K.; Nitsch, W. J. Colloid Interface Sci. 1992,150,226.
(7) Albery, W. J.; Choudhery, R. A.; Atay,N. Z.; Robinson, B. H. J.
Chem. SOC.,Faraday Trans. 1 1987,83,2407.
0743-7463/94/2410-0371$04.50/00 1994 American Chemical Society
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Plucinski and Nitsch
Langmuir, Vol. 10,No. 2, 1994
Table 1. Eauilibria and Kinetic Results of the Solubilization of Different Mixtures of Metal Cations. components
no.
1
1 2 3 4 5 6 7 8 9 10 11
Ca Ca
cs
Sr Ni Ca
K K cs
K K
2 Zn Ni
( [Mezll[Mezl)es
([Medl[Mezlhb
jo,M.,ljo.m%
RiIRz
2.15 2.25 0.92 1.93 1.13 1.45 0.46 0.80 1.78 1.72 3.37
2.37 2.61 0.96 1.36 1.25 1.58 1.12 1.81 3.30-2.60 3.60-2.86 6.11-4.32
2.82 2.25 1.15 1.36 1.42 1.75 1.17 1.82 3.85 3.26 5.69
0.96 1.02 0.99
K Ca Zn Mg Ca Sr Ni Zn Li
1.00 0.94 0.96 0.80
0.80 0.81 0.77 0.87
r h 1.47 1.54 1.23 1.25 0.96 1.47 1.37 1.13 2.58 2.01 1.99
a [Me110 = [Me210 = 0.25kmol/m*;pH = 5.0; [AOT] = 0.050 kmol/ms; n = 150 min-l. j o , initial rate of the solubilization (kmol/(m28)); R, radius of hydrated cation (Conway, B. E. Ionic Hydration in Chemistry and Biophysics; Elsevier: Amsterdam, 1981);r, radius of bare cation in the aqueous solution (Marcus, Y.Chem. Rev. 1988,88,1475). Between 1st and 10th hour of the kinetic experiment.
*
the equilibrium conditions, equal volumes (usually 10mL) of the micellar phase and the aqueous solution were shaken with 175 min-’ strokes for 12 h. The probe was left for separation for a few days (2-4). If the phase separation was not complete (presenceof a small layer of emulsion separating the two phases, which appeared very often in the caseof transition-metalcations), the probe was centrifuged at 4000 rpm for 15 min to obtain clear phases. The organic solutions were sampled. The concentration of metal ions was measured by atomic absorption spectrometry (SP9 AAspectrometer,PyeUnicam, England),withtheaccuracy of h2 % . The water content of the micellar phase was determined by Karl Fischer titration with the precision f3% (633 KarlFischer Automat, Metrohm, Switzerland). Interfacial tension, always in the equilibrium conditions,was measured using a SITE 04 spinning drop tensiometer (Krtisa,FRG) with the error f4%. The densities of both phases, necessary for calculating the interfacial tension, were measured with a pycnometer. The kinetic experiments as well as the analyses were perfomed at 20.0 f 0.1 O C .
To normalize the influence of changes of the micellar size on the kinetics of RMs formation (due to water solubilization),the organicphase was saturated with a so-called equilibrium amount of water, found previously in equilibrium experiments for the same conditions. All chemicals used were of analytical grade or better.
necessary to introduce the “discrete charge effect” of the specific adsorption.”l0 The specifically adsorbed ions in the inner Helmholtz plane are not hydrated,&1° so they may approach very close to the micellar interface.1° In general, the specific adsorption of an ion is enhanced by larger size and lower (i.e. hydration number and energy). In our series of experiments the larger bare ions were always better solubilized than smaller ones, independent of their valency (see Table 1, column 7). According to the literaturegJOthis indicates the specific adsorption at the oil/water micellar interface (Le. ion binding to AOT molecules), perhaps combined with a structural effect.12 The application of partially bare ions to describe the results of micellar kinetics in AOT reverse micelles was stated by Bardez et al.13 Wong et al.14stated also that the degree of binding of sodium to the micellar AOT interface was at least ca. 72 76. A detailed study of the equilibria of ion exchange was performed for a mixture of Ca and Zn. The selectivity of ion exchange defined as15 .1[ca2+l\
.
Results and Discussion Equilibrium Aspects. Metal Ion Concentration in the Micellar Phase. During the contact of the micellar phase with the aqueous solution of a mixture of different metal cations, ion exchange takes place. Sodium (AOT counterions) is substituted by different cations present in the aqueous phase. The results of this ion exchange are presented in Table 1. Pronounced differences in the capacity of RMs to exchange (Le. to solubilize) particular cations, even for cations of the same valency (e.g. Ca/Zn, Ca/Ni, Ca/Mg, Sr/Ca), can be observed. This indicates that the “classical”Poisson-Boltzmann treatment,”lo or a modified one,ll of cation interactions with the negative AOT layer is not sufficient in the case of reverse micelles. The model presented by Leodidis and Hattodl showed that ionic charge, hydrated size, and electronic properties were important to determine the selectivity of the water pool and that larger cations with greater energy of hydration could be exchanged in favor of the smaller cations. To explain the results presented in Table 1it is (8)Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, 1989;Chapters 6, 12,and 13. (9)Usui, S. In Electrical Phenomena at Interfaces; Kitahara, A., Watanabe, A., Eds.; Marcel Dekker, Inc.: New York, 1984; pp 15-46. (10)Devanathan, M. A. V.; Tilak, B. V. K. S. R.A. Chem. Rev. 1966. 65,635. (11)Leodidis, E. B.; Hatton, T. A. Langmuir 1989,5,741.
was calculated. Within a large range of concentrations of both cations in the aqueous phase this factor remained nearly constant (Figure 1). We can rewrite eq 1 in the form
The constant value of the selectivity coefficient (Sz 2.5) indicates that the interactions (or binding) of calcium with AOT are about 2.5 times stronger than those between zinc and AOT. According to the Grahame model of metal ion adsorption8 (extension of Stern models) the adsorption of metal ions in the inner Helmholtz plane requires an isotherm of the form (12)Eastoe, J.; Fragneto, G.; Robinson, B. H.; Towey, T. F.; Heenan, R. K.; Leng, F. J. J. Chem. Soo., Faraday Trans. 1992,88,461. (13)Bardez, E.;Larrey, B.;Zhu, X.X.;Valeur, B. Chem. Phys. Lett. 1990,171, 362. (14)Wong, M.; Thomas, J. K.; Nowak, T. J. Am. Chem. SOC.1977,99, 4730. (15)Treybal, R.E.LiquidExtraction; McGraw-Hill. New York, 1963; Chapter 3,pp 56-121.
Mas$ Tramfer in Microemulsion System 5
-
I
I
Langmuir, Vol. 10, No. 2, 1994 373 Zn concentration [kmol/m3]
I
1.4
4 '
0.0
0.1
0.2
0.3
0.4
0.5
0.6
I
I
I
I
I
I
1
0
I
I
- o - [Zn2+]=0.250kmoVm3
01 0.01
I
0.1
,
1
,
10
1
0
0 4
100
~
[Zn2'] = 0.25 kmol/m
0 2
00
Figure 1. Selectivity of the CdZn solubilization in the Winsor 11system: [AOTI = 0.060kmol/ma; [Caz+l= 0.25 kmol/ms, [Znz+I = 0.010-0.500 kmol/ma; [Zna+l= 0.25 kmol/m*,[Ca2+l= 0.0100.500 kmol/ms. ai
= zierin: exp
(-29 -
03
05
0 4
06
Ca concentration [kmol/m3]
(4)
where ci is the charge density of i-species, Z i is a valency, e is an elementary charge, ri is the radius of adsorbed ion, nio is the number of ion adsorbed per unit area, and AGah is the energy of adsorption. We can express now the ratio of Ca and Zn concentrations in the form
,[Ca2+1,,,
0 2
01
--
30
) 2
[Ca2+1aq = exp(AGah%- AGahCa) (5) [Zn2+ldi, \ [zn2+1,, J eg Thus, the value of the selectivity coefficient in this particular case of micellar ion exchange depends on the ratio of ion radii and the exponent of the adsorption energy difference. The distribution of both ions in the reverse micelles depends on their size and electrostatic and sometimes chemical interactions (e.g. chelate formation'6). Interfacial Tension and Water Uptake. In the previous section it was shown from equilibria results that calcium ions interact much stronger with AOT molecules than zinc ones. A similar conclusion can be drawn investigating the influence of the Ca/Zn ratio on the interfacial tension and water uptake (Figures 2 and 3). The addition of small amount13 of Ca strongly increases the interfacial tension and decreases the water ratio WO. The addition of zinc to a calcium solution shows only a very small effect. Very interesting is the dependence of the interfacial tension on the Zn2+molar fraction in the aqueous phase (Figure 4). For both total ion concentrations, straight lines were obtained with very similar slopes. This means that the gradient of the interfacial tension (-dy/dzz,J is constant and not sensitive to the total ion concentration. The gradient of the interfacial tension is defined by the well-known Gibbs adsorption equation17
I
0
'
10'
I
0.0
0.1
,
0.2
,
0.3
,
0.4
0.5
C a concentration [kmol/m3]
Figure 3. Influence of the addition of cation in the aqueous phase on the water uptake (symbols and conditions as in Figure 1). 1.6 1.4
-
1.2
-
1.0
-
0.8
-
0.6
-
0.4
-
0.2
I
I
I
l
I
I
I
,
I
I
,
0.0
0.2
0.4
0.6
0.8
1 .o
Zn2+molar fraction [ - 1
The chemical potential depends on the activity of the adsorbed species (16) Eicke, H.F.; Kvita, P. In Reverse Micelles; Luisi, P. L., Straub, B. E., Eds.; Plenum Press: New York, 1984. (17) Hiemenz, P. C. Principles of Colloid and Surface Chemistry; Marcel Dekker,Inc.: New York, 1986, Chapter 7, pp 363426.
Figure 4. Influence of the zinc molar fraction in the aqueous phase (in equilibrium state) on the value of interfacial tension. [AOTI = 0.050 kmol/ma.
-d7=C(ridlnxi+ridlnfi) I
where i = (AOT-,C1-; Na+; Zn2+; Ca2+, solvent).
(7)
Plucimki and Nitsch
374 Langmuir, Vol. 10, No. 2, 1994 During the contact of the phases 93% (for Zn only) to 97% (for Ca only) of the initial amount of sodium from the organic phase (AOT counterion, 5 X kmol/mg) was reextracted. Thus we have assumed the total Na+ concentration in the aqueous phase to be nearly constant. The adsorption of the organic solvent was neglected as discussed by Leodidis and Hatton.Is The amount of the surfactant as well as chloride (great excess) ions is also assumed to be constant in a first approximation. Then we can write: d In XAOT = 0; d In XNa = 0, and d In cci 0. If one assumes a constant activity coefficient for a constant ionic strength, then d In f i = 0. Equation 7 is simplified
calculated ratio of Gibbs excesses a - measured concentration ratio of Ca and Zn in the shell region of reverse micelle
-dy = (r, d In xZn+ rCa d In xCa)
(8) For the mixture of Ca and Zn X , + ZCa = 1 and dxca -dxzn. We can rewrite eq 8 in the form
0 0
02
0 4
0 6
08
10
Zn molar fraction [ - ]
Figure 5. Comparison of the measured concentration ratio in the bulk and the calculated ratio (eq 12) at the liquid/liquid interface. [AOT] = 0.050 kmol/mg. 7 , , I I
The gradient of the interfacial tension is
or
The gradient of the interfacial tension for the linear course of changes is equal -dr/dxzn = -Ay, where (Ay = TCa . ) , y With this value it is possible to calculate the ratio of Gibbs excesses for calcium and zinc ions according to the equation
-
**
r
5.
/
1
e
0
50
1
I
100
1
I
I
150
200
250
stirring rate [rpm]
We can assume that one calcium molecule at the interface “occupies” double the area as an AOT molecule. The average AOT cross section can be assumed in the first apprOXimatiOn’g asf A o T = 70 A2,thus fCa = 140A2and r C a = 1.186 X 10-6 mol/m2. The calculated ratio of the interfacial concentrations of both cations was next compared to the equilibrium concentration ratio in the micellar phase (Figure 5). Both values agree very well. This might be an indication confirming the statement that the ratio of concentrations of solubilized ions at the shell of reverse micelles and on the planar 1/1 interface are equal. Although the assumption of a constant activity coefficient for such ionic strength is rather rough, it is commonly used.20 Kinetic Aspects. Rate ControllingProcess. From the results of the ion exchange between the aqueous and the micellar phase (Figure 6), it can be stated that in the convection independent range (plateau region) an interfacial process is rate controlling. This conclusion is experimentally justified for our stirred cell calibrated for different extractional systems.21 In this apparatus transport processes are characterized by a linear relation between mass flux and stirring speed. Additionally it was (18)Leodidis, E. B.; Hatton, T. A. J. Phys. Chem. 1991, 95, 5957. (19) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. In The Structure, Dynamics and Equilibrium Roperties of Colloid System; Bloor, D. M., Wyn-Jones, E.,Eds.;Kluwer: Netherlands, 1991; pp 557-581. (20) Atkins, P. W. Physical Chemistry; Oxford University Press: Oxford, 1986, Chapter 11. (21) Nitech, W.; Kruis, B. J. Znorg. Nucl. Chem. 1978,40, 857.
Figure 6. Proof of the interfacial step as a limiting one for the solubilization of a mixture of calcium and zinc. IAOTI = 0.050 kmol/m3; [Ca2+] = [Znw] = 0.26 kmul/ma. found that the rate of reextraction (i.e. flux of solubilized components out of the micelles into the aqueous phase) is always running independent of the convection at a higher stirring speed! which also indicates a limitation by an interfacial process. Therefore a mechanistic treatment of the exchange process in the flow independent region must be related to events taking place at the macroscopic 1/1 interface. Exchange Mechanism. In previous publications we treated the coalescence of micelless (reextraction) and the spontaneous self-organizationof micelles at the 1/1interfaces (extraction)as separate processes for the explanation of the “communication”between the aqueous core of the reverse micelles and the aqueous bulk phase. This intuitive and rough assumption of spontaneous aggregation at the macroscopic interface cannot sufficiently explain the increase of the ion exchange rate with the increase of micelle concentration (Figure7,and our previous works‘*s). The AOT coverage of the liquid/liquid interface, determined by a low and concentration-independent interfacial tension (Table 21, corresponds to a saturated palisadelike AOT layer. For such a state of the interface the influence of the micellar concentration on spontaneous aggregations at the interface is difficult to understand. Therefore a suitable interfacial mechanism of ion exchange has to involve the number of collisions of RMs with the macroscopic AOT-covered interface in order to explain the increase of the extraction rate with the micellar
1
1
Mass Transfer in Microemulsion Systems 4 ,
5. E >
E
Y
I
~
I
o -zinc 0
-calcium -Zn+Ca
I
I
I
I
Langmuir, Vol. 10, No. 2, 1994 375
I
I
3
water
high AOT concentration 9!
E
2 -
t
oil
IIII
' d
ib 5 I I 1I I I I &
\I4
water
low AOT concentration 0.00
C.05
0.10
C.15
0.20
0.25
0.30
0.35
AOT concentration [kmol/m3]
Figure 7. Influence of the AOT concentration on the solubilization rate of Zn2+and Ca2+[Ca2+]= [Zn2+]= 0.25 kmol/ms, pH = 5.0, n = 150 rpm. A
i4
\ . v
-.Na
0
I
L-
AlAl& 34All
I Li d 4
NaL
,' 44
-.Me
L
l l l f ~ ~ ~ I~l l l&l Ai11 l Me
Figure 8. Mechanistic model of the ion exchange during the sticky collisionof reverse micelleat the macroscopicliquid/liquid interface. Table 2. Influence of the AOT Concentration on the Value of Interfacial Tension. [AOT] X1@[kcal/m31 15 25 50 75 100 200 300 y X 109 [N/ml
1.06 1.10 1.11 1.06 1.12 1.20 1.27
* [Ca2+] = [Zn2+] = 0.25 kmol/ms, volume ratio 1:l. concentration. Such a mechanism of extraction, involving the collisions of micelles with the macroscopic interface, leads to an interface covered with buds, which are in contact with the aqueous phase (Figure 8). If one defines that a successful collision of a micelle with the interface (sticky collision) means a micelle with an aperture toward the aqueous phase (state A in Figure 8) and that during the residence time of such a micelle the ion exchange takes place (state B), then the fusion of the micellar shell determines the micellar desorption or fission (state C), finishing the mass transfer process. In such a model the assumption of a spontaneous micellation (cytosis) is replaced by the assumption that micelles conserve a certain degree of individuality at the interface. With this mechanism the increase of the micellar concentration in the bulk of the organic phase means an increase of the number of buds (shown schematically in Figure 9) and therefore an increase of extraction rate, corresponding to the result of the experiments (Figure 7). (An alternative to the proposed mechanism could be a consecutive process, in which after a sticky collision the original micellar shell merges with the interfacial layer at position x 1 enforcing the formation of a new bud at the position x2. However, such a mechanism should always demand the formation of equilibrium micelles,which does not agree with every experimental result presented in this work (see Table l).)
-
high buds density
.'
'e
AI I I I I I I I I3 4 I I I I I
-
low buds density
Figure 9. Schematicinfluence of the number of reverse micelles in the bulk of organic phase on the number of interfacial buds.
A formation of a channel between two microemulsion nanodroplets during their collision in the bulk of organic phase was proposed by Eicke et a1.22and then considered to be based on the change of spontaneous curvature by Fletcher et alS23and K o z l o ~ . ~ ~ Equations for the calculation of the mass flux for the mechanism proposed above have to consider the processes of collision,mass transfer, and fusion. The process of mass transfer cannot be described in detail until the fusion and the residence time of the buds at the interface are better understood. Also the influences caused by a possible contribution of local changes of interfacial tension are not accessible now. Therefore, in a final approach, the assumption of the equilibrium condition a t the interface could be done. The process of sticky collision should be treated as follows Nb = F( 1 - fA)CYbz (13) according to the classical collision theory.25 The rate of sticky collisions Nb can be calculated with the sizes of micelles from the ion exchange (reextraction). The fraction of effective collision CYb ( a b = exp(--hEArBextr/RT))is accessible from the temperature dependence of reextraction: F means the macroscopic area, 2 the collision number, and f A corresponds to the covered part of the interface cf, cross section of one bud, and A , number of buds per area unit). The calculation of A from suitable measurements of the reextraction process (eq 13) could be a key for checking the proposed mechanism. But until now because of uncertainty concerning the calculation of theoretical (2)and experimental (ab) parameters, the estimation of the very sensitive term (I -fA) is not possible. This is also the reason that the evaluation of S (see below) is postponed at the moment. The step of desorption directed by the fusion of the bud's shell formally corresponds to a unimolecular react i ~ nwhich , ~ ~ suggests the equation Nr = FAa$ (14) as a plausible first approach. F and A are defined above, afmeans the fraction of effective fusion, accessible from the activation energy of extraction5 (af= exp(-AEAexk/ RT))and from the analogywith the unimolecular reaction% the factor S in eq 14 should be a frequency of the bud fluctuation. The process of fusion should be determined (22)Eicke, H. F.; Shepherd, J. C. W.; Steinemann, A. J . Colloid Interface Sci. 1976, 56, 168. (23)Clark, S.;Fletcher, P. D. I.; Ye, X.Langmuir 1990,6, 1301. (24) Kozlov, M.J. Chem. SOC.,Faraday Trans. 1991,87, 3776. (25) Bamford, C. H.; Tipper, C. F. H. In Comprehensiue Chemical Kinetics, Vol. II, The Theory of Kinetics; Elsevier: Amsterdam, 1969; Chapters 3 and 4.
Plucinaki and Nitech
376 Langmuir, Vol. 10, No. 2, 1994 100
I
-
0
I
I
- Ca const., Zn changes
- o - Zn const., Ca changes
0.01 0.01
01
10
100
Figure 10. Comparison of the ratios of the initial solubilization rates and bulk equilibrium concentrations in the micellar phase (the same conditions as in Figure 1).
by the balance of forces at the neck of the bulge (bud).6 The critical distance D which controls the fusion should correspond to the range of the hydrophobic (Le. attractive) interactions. The mechanistic concept presented above explains a possible way of the “communication”between the aqueous pool of the RM and the aqueous bulk phase yet only in qualitative principle, but a certain analogy to the mass transfer between reverse micelles in the bulk2224can be regarded as a support. Nevertheless the proposed mechanism should be discussed, because little is known about the detailed process in such a system, and the number of variables is so large, that a plausible mechanism could be a contribution to further discussions. It should be mentioned, that the proposed bud mechanism describes the process of interactions of reverse micelles with the macroscopic interface and is therefore relevant for extraction and reextraction. Mass-Transfer of Binary Ion Mixtures. In the mechanism of the interfacial solubilization described above, nothing was said about the ion exchange between the “buds”and the aqueous interfacial layer, only equilibrium
was assumed. It was the idea to prove this assumption with the simultaneous ion exchange from the binary mixture of cations in the aqueous phase. Unfortunately, the results listed in Table 1 are difficult to judge. Only in the case of Ca2+/Zn2+mixture does the plot in Figure 10 show the large scale agreement between equilibrium and kinetic data. For the other binary mixtures nos. 2 to 6 in Table 1 equilibrium and kinetic data seem to agree more or less, whereas for the cation groups nos. 7 to 11 (mixtures of mono- and divalent cations) the kinetic and equilibrium results are very different. It is evident, that the degree of equilibration (ion exchange) during the residence time of an individual bud at the interface depends on the type of involved ion. However, for a final conclusion it is necessary to take the possible transport of water into account, which can change the evaluation of kinetic data. Nevertheless the observed minimum for the exchange of water in the case of reextractions means nonequilibrium conditions for the micellar exchange at the interface and such a result is difficult to interpret with the alternative mechanism of spontaneous aggregation. Conclusions This work shows that kinetic studies are necessary to understand the phenomena taking place at the interface of Winsor I1 microemulsion systems. More than one solubilized species can be used to estimate the degree of equilibration. The pronounced effect of the AOT concentration &e. number of RMs) on the rate of an interfacial solubilization leads to a new proposal of the “dynamic communication”between the micellar and aqueous phase. The assumption of the presence of buds at the 1/1interface is a logic, physical, and plausible conclusion being in agreement with experimental results. The question remains open concerning factors determining the interfacial equilibration. More knowledge is necessary about the influenceof individual ions on the ion exchange rate inside the adsorbed buds and about the process of fusion, which determines the residence time of an individual bud. Acknowledgment. This work has been supported by a grant from Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 266). Pawel Plucinski is grateful to Dr. Robert Aveyard for helpful discussion.
