Faraday Discuss. Chem. SOC., 1984, 77, 1 15-126 Use of Microemulsions as Liquid Membranes Improved Kinetics of Solute Transfer at Interfaces BY CHRISTIAN TONDRE" AND ARISTOTELIS XENAKIS Laboratoire de Chimie Physique Organique, ERA CNRS 222, Universite de Nancy I, B.P. 239, 54506 Vandoeuvre-les-Nancy Cedex, France Received 28 th November, 1983 When the volume fraction occupied by the dispersed phase of a microemulsion is small, the microglobules making up this dispersed phase can be viewed as mobile carriers permitting the transport of substances which are either insoluble or very poorly soluble in the continuous phase. In this paper water-in-oil microemulsions composed of decane, water, tetraethylenegly- col dodecylether (TEGDE) and hexan-1-01 are used as liquid membranes and the micro- globules are shown to transport alkali-metal picrates between two aqueous phases.The effect of changing the initial picrate concentration in the source compartment has been investigated and the resulting flux can be adequately described by a classical model of facilitated transport (fast transfer and 'chemical reaction' coupled with slow diffusion), as has been observed previously for the transport of lipophilic substances by oil-in-water microemulsions. The presence of dicyclohexano- 18-crown-6 (DC 18C6) in the liquid membrane brings about an increase in the flux of alkali-metal picrates. At optimum conditions, the transport of K' picrate by the microemulsion alone is 5.1 times faster than with DC18C6 in pure decane, but it is 12.6 times faster when DC18C6 is added to the microemulsion.Although drastically reduced, selectivity for ion transport still exists in the latter situation. Liquid membranes have long been used as models of biological membranes for studying the transport of different solutes (salts, metabolites, drugs etc.) either facilitated by a carrier or not.'-9 Numerous applications have also been found in separation techniques which take advantage of their ability to perform selective permeations. * The transport of substances in such experiments is usually facilitated by the incorporation of a carrier molecule in the liquid membrane. Macrocyclic compounds which are either naturally occurring (antibiotics)2 or synthetic (crown ethers or cryptands for have often been used for this purpose.On the other hand, the use of liposomes or vesicles as carriers for intracelluIar delivery of drugs constitutes an active area of re~earch.'"'~ We have developed experiments to demonstrate that the very small microemul- sion droplets may behave like the mobile carrier molecules commonly used in studies of facilitated transport. In these experiments a microemulsion is used as a liquid membrane separating two liquid phases in thermodynamic equilibrium with it. We have previously examined the transport of lipophilic substances by oil-in-water microemulsion droplet^,^'-^^ but only very preliminary results have been given for the transport of hydrophilic solutes by water-in-oil microemulsions. " This paper will give a full account of the results obtained when water-in-oil microemulsion systems involving a non-ionic surfactant are used to transport alkali-metal picrates.The role of carrier of the microemulsion droplets has implications regarding the liquid-liquid extraction of metal ions as it can be responsible for an improvement 115116 MICROEMULSIONS TO IMPROVE KINETICS OF SOLUTE TRANSFER DECANE 34 H E x ANOL VL Fig. 1. Composition (filled circles) of the initial mixtures in the diphasic region of the pseudo-ternary diagram n-decane/ water/ tetraethyleneglycol dodecylether (3/4), hexan- 1-01 (1/4). Phase separation occurs according to the tie-lines joining the water apex to the open circles indicating the composition of the water-in-oil microemulsions. The dashed line delimiting the monophasic domain was drawn approximately according to ref. (22).in the kinetics of solute transfer at interfaces, as observed when surfactants are added to the extracting medium.18-20 For this reason we have also investigated the transport of alkali-metal picrates by a crown ether, which can be considered as a model sjstem, both when the membrane is a pure organic solvent and when the membrane is a water-in-oil microemulsion. Two alkali-metal cations with different stability constants2' for complexation with the crown ether have been tested in order to compare the selectivity of ion transport in both media. EXPERIMENTAL CHOICE OF CHEMICAL COMPOUNDS For these experiments, we required a water-in-oil microemulsion in thermodynamic equilibrium with an aqueous phase of composition as close as possible to pure water, i.e.a diphasic system of the so-called Winsor I1 type. In addition, the hydrophilic substance to be transported had to be able to be detected spectrophotometrically; this led us to choose the alkali-metal picrates. Because of the nature of the chosen hydrophilic solute we preferred to use a microemulsion system containing no salt, although most known diphasic systems of the Winsor I1 type require a salt in their formulation. This was intended to avoid an exchange of ions during the transport process. For this reason we sought a system involving a non-ionic surfactant and finally found a quaternary system meeting all the above requirements: decane/water/ tetraethyleneglycol dodecylether (TEGDE)/hexan- 1-01. The monophasic region of the pseudo-ternary phase diagram obtained when keeping the ratio of TEGDE to hexanol constant (TEDGE/hexanol = 3) has previously been described by Friberg et aLZ2 We found that the systems, the compositions of which are indicated in fig.1, separate into two perfectly clear phases according to the tie-lines indicated. Unfortunately we could not find a simple ternary system having such a property: if there was no hexanol the system did not produce clear phases even after several weeks. It thus seems that it is easier to obtain Winsor I1 systems by adding an alcohol, as previously observed by Winsor himself in the case of ionic surfactants in the absence of salt.23C. TONDRE AND A. XENAKIS 117 Table 1. Compositions (in wt%) of initial mixtures and microemulsion phases initial mixture & (water-in-oil microemulsion) d 7 7 system H 2 0 C10H22 C6HI30H TEGDE H 2 0 C10H22 C6HI30H TEGDE /gcmP3 /cp I 50 47 0.75 2.25 1.83 92.28 1.47 4.42 0.742 1.06 I1 53 42 1.25 3.75 3.40 86.32 2.57 7.71 0.753 1.30 I11 53 40 I .75 5.25 4.80 81.03 3.54 10.63 0.762 1.55 IV 54 38 2 6 5.76 77.85 4.10 12.29 0.770 1.77 V 50 40 2.5 7.5 6.47 74.82 4.68 14.03 0.771 1.81 VI 52 37 2.75 8.25 8.05 70.96 5.27 15.82 0.781 2.37 VII 50 37 3.25 9.75 10.20 66.45 5.84 17.51 0.789 2.8? Our choice of dicyclohexano- 18-crown-6 (DC 18C6) for the experiments in the presence of a classical extractant was for the following reasons: (i) it has good solubility in decane and poor solubility in water, and will thus stay in the liquid membrane, and (ii) it complexes both K+ and Na+ ions with different stability constants,21 which makes it convenient for testing the influence of the microemulsion on the selectivity of transport of alkali-metal ions.ORIGIN OF CHEMICALS Tetraethyleneglycol dodecylether (Nikko Chemicals, 'Japan), n-decane and dicyc- lohexano-l8-crown-6 (Fluka purum) and hexam- 1-01 (Fluka puriss.) were used as supplied. Potassium and sodium picrates were prepared from picric acid (Merck) according to the procedure described in ref. (24). The salts were recrystallized three times and the extinction coefficients measured in tetrahydrofuran were in agreement with previously published value^.^ CHARACTERIZATION OF BIPHASIC SYSTEMS Biphasic systems with the compositions shown in fig. 1 and table 1 were prepared by weighing the components directly in a separating funnel which was then placed in a thermostat- ted bath regulated at 20 "C (*0.2 "C) until separation into two clear phases was achieved (this took 3 days to 3 weeks, depending on the system). The compositions of the initial mixtures were chosen so as to give approximately equal volumes of the two separated phases.Analysis of these phases was performed by the Karl Fisher method for the water content and by gas-phase chromatography for decane, hexan-1-01 and TEGDE, using columns 1 m long and in. in diameter containing 3% SE30 on chromosorb WAW 80/ 100 mesh with a linear variation of temperature from 50 to 280 "C. The composition of the superior phase & is given in table 1. The inferior phase is essentially water (no other component could be detected by gas-phase chromatography).For this reason the representative points of both phases can be shown in the same pseudo-ternary diagram (fig. 1). Densities and viscosities of & (table 1 ) were measured with a digital Anton Paar DMA 10 densimeter and with an Ubbelohde-type viscometer (Schott-Gerate with automatic timing), respectively. TRANSPORT EXPERIMENTS The setup used for the transport experiments is similar to that previously described when investigating oil-in-water microemulsions,'6~1 except that the shape of the cell used in the reverse of the previous one. As shown in fig. 2, it resembles an inverted U tube with the two arms filled with the aqueous phase and the microemulsion phase at the top. The volumes were 14.8, 18.6 and 35 cm3 for the source (S), receiving (R) and membrane (M) compartments,118 MICROEMULSIONS TO IMPROVE KINETICS OF SOLUTE TRANSFER ----- PECTAOMETER Fig.2. Schematic diagram of the apparatus used for the transport experiments. respectively. The light absorption of the receiving phase was measured continuously at 357 nm for K+ picrate and at 351 nm for Na+ picrate, thus allowing us to calculate from a standard curve the number of transported molecules at any time (Beer's law was found to be valid in the concentration range used). Peristaltic pumps (Masterflux) were used to ensure fast homogenization in both branches of the transport cell.9 The microemulsion compartment was stirred using a magnetic stirrer whose rotation speed was regulated at 120 r.p.m. The whole cell was thermostatted at 20 "C.RESULTS AND DISCUSSION TRANSPORT OF PICRATES BY PURE MICROEMULSIONS The transport of K' or Na+ picrates by the microemulsion systems referred to as I to VII in table 1 was first studied without adding a classical extractant to the membrane. Fig. 3 shows a plot of the number of moles of K' picrate transported against time for different initial concentrations of picrate in the source compartment, using system V. The curves look very much like those obtained for the transport of pyrene by oil-in-water microemulsions, with a time lag attributed to the time required for the picrate to reach an equilibrium concentration inside the membrane. The flux of picrate, calculated from the slopes of the straight lines observed when a steady state is established, is shown in fig.4 as a function of the initial picrate concentration. Blank experiments carried out with pure decane instead of the microemulsion in compartment M did not reveal any transport of picrates after 24 h. We chose a picrate concentration of 2 x mol dmP3 to study the influence of the composition of the microemulsion on the transfer rate of K' and Na' picrates. As can be seen in table 1 , the composition of the microemulsion was varied by changing the amount of amphiphile compounds in the initial mixture: the larger the amount of amphiphiles, the larger the amount of water incorporated in the microemulsion phase. Fig. 5 shows the variation of the flux of K' and Na+ picrates when increasing the volume fraction of water. The flux increases linearly up to a volume fraction of 0.05 and then decreases.Only a very small difference is observed between K+ and Na+ picrates, with the latter always giving a slightly lower value for the flux (table 2). The maximum observed in the variation of the flux was unexpected because when the corresponding experiments had been conducted with oil-in-water micro- emulsions (increasing percentage of oil), instead of a decrease, a dramatic increase of the flux of pyrene occurred for a certain volume fraction of Such aC . TONDRE AND A. XENAKIS 119 20 I I I I I f/min Fig. 3. Plots of the number of moles of K+ picrate crossing the second interface against time in microemulsion system V. The numbers indicate the initial picrate concentratbn in the source compartment S (mol dm-3).Cross-section of interface = 3.14 cm2. Fig.4. Plot of the flux of K+ picrate against the initial picrate concentration in the source compartment S. The curve is calculated from eqn (9) with the values of parameters given in fig. 6 . phenomenon was attributed to the percolation threshold of oil droplets. In the present situation a structural change of the microemulsion is probably responsible for the maximum observed. A possible interpretation is as follows: when the amount of water in the microemulsion is < 5 % , there is just enough water to hydrate the ethylene oxide groups constituting the hydrophilic part of the surfactant (we have between 2 and 2.4 water molecules per ethylene oxide group, in agreement with120 MICROEMULSIONS TO IMPROVE KINETICS OF SOLUTE TRANSFER 3 - - I .C E - 2 - z I 2 \ 4 0 2 4 6 a H,O(% V/V) Fig.5. Plots of the flux of K+ picrate ( X ) and Na+ picrate (0) against the percentage of water (v~E.‘) in the microemulsion phase. Initial picrate concentration = 2 x mol dmP3. previous e~timations’~,~~); at >5% water there is enough water to start forming water pools (the sudden increase in the ratio-of H20 to TEGDE can be seen in fig. 1 : it occurs when the representative points of the microemulsion phases depart from a straight line originating from the oil apex). In the first situation, only agregates of TEGDE and hexanol would exist, as previously postulated in comparable systems in the absence of a l c ~ h o l . ~ ’ Real water droplets (with unknown shape) with a palissade layer of amphiphile molecules would occur in the second situation.I3C n.m.r. relaxation time measurements currently in progress should enable us to say whether the mobility of the hydrophilic chains is in agreement with such a In this hypothesis it is clear that the equilibrium constant characterizing the interac- tion (solubilization or complexation) of the alkali-metal picrate with the ‘carrier’ will be very different if this carrier is an aggregate or a water droplet (in the latter case the ethylene oxide groups lining the inside of the droplet could act as a kind of crown ether towards the alkali-metal cations and make the releasing process more difficult). We will not attempt to speculate further on the maximum observed in fig. 5, but will merely try to interpret the results obtained in the linear part of the figure, i.e.where we can assume that there is only one sort of carrier (the exact structure of which does not matter). According to fig. 1 and table 1, the ratio between water and the amphiphile molecules is constant for all the systems containing <5% (v/v) water. Increasing the amount of amphiphile molecules (or water) can be considered as equivalent to increasing the concentration of carrier, which results in a linear increase of the flux. In the previous case of oil-in-water microemulsions we tested different possible mechanisms in order to determine which step was rate controlling: transfer of solute across the interface, solubilization inside the droplet or diffusion through the non-stirred layers. The results were consistent with a model in which the diffusion of the droplet having solubilized the probe is much slower than transfer andC.TONDRE AND A. XENAKIS 121 Table 2. Fluxes of transported picrates as a function of the nature of the liquid membrane and the presence or absence of a classical extractant flux"/ mol h- ' nature of water solute with DC 18C6 liquid membrane ("/o v/v) transported without DCl8C6 ( mol dm-') pure decane z microemulsion I microemulsion I1 microemulsion 111 microemulsion IV microemulsion V microemulsion VI microemulsion VTI ,O 1.36 2.55 3.63 4.41 4.98 6.27 8.1 1 KPi NaPi KPi NaPi KPi NaPi KPi NaPi KPi NaPi KPi NaPi KPi NaPi KPi NaPi 0.74 x 0.61 X 1.37 x lop6 0.87 x 1.48 X 2.15 X 1.33 x I 0-6 1.06 x 0.94 x 2.1 1 x lop6 - 4.21 x 1 0 - ~ 8.6 X 2.42 X lop6 0.85 X loy6 4.74 x 2.28 x 5.30 X 2.80 x lop6 2.61 x lop6 1.56 X lop6 " Cross-section of interface = 3.14 cm2. solubilization reactions.Similar treatment permits a satisfying description of the present results. The different steps involved in the transport process can be written as follows: interface I : Mesf + Pi, s Me', Picontinuous phase o f M Me+, Picontinuous phase of M +(c) (c, Me+, pi-) (2) diffusion of (C, Me+, Pi-) across the non-stirred layers (3) (c, Me+, pi-) * Me', Picontinuous phase of M +(c) (4) ( 5 ) where the subscripts M, S and R refer to membrane, source and receiving compart- ments, respectively, Me' is the alkali-metal ion, Pi- is the picrate ion and Me', Pi- is the ion pair, and (C) and (C, Me+, Pi-) are the 'carrier' and the carrier-solute complex, respectively.Steps (1) and ( 5 ) correspond to the formation of the ion pair through the interfaces characterized by an equilibrium constant k. Steps (2) and (4) are governed by the equilibrium constant K for the solubilization (or complexation) of the metal picrate in (with) the carrier. Step (3) is characterized by a diffusion coefficient 0, the thickness of the diffusion layer being L=2Z, where Z is the non-stirred layer on the organic side of both interfaces (as previously observed with oil-in-water microemulsions, changing the rotation speed of the magnetic stirrer influences the transfer rate, but changing the rotation speed of the peristaltic pumps does not affect the result). membrane: interface 2: Me', Pi~,ntinuous phase of == Me& + Pi; I122 MICROEMULSIONS TO IMPROVE KINETICS OF SOLUTE TRANSFER Solving the continuity equations for all the species present in the membrane with the assumptions that (i) the steady-state approximation is valid, (ii) the equili- brium characterizing steps (2) and (4) is always established and (iii) the unfacilitated transport of picrate is negligible, and with the following boundary conditions: [Me', Pi-]M = k[Me'],[Pi-1, = k[Pi-]: on interface 1 (6) [Mef, Pi-]M = k[Me+I2[Pi-l2 = k[Pi-]: on interface 2 (7) where [Pi-], and [Pi-]* are the concentrations outside the membrane but right beside it in compartments S and R, respectively, at the beginning of the steady state, leads to the following equation for the flux: As the concentration [Pi-I2 is practically zero at the beginning the equation reduces to the first term: DKk[(C)] [Pi-]: F = L 1 + ~ k [ ~ i - ] : ' (8) of the steady state, (9) The essential difference from the previously developed treatment comes from the ion-pair formation equilibrium, which introduces a squared term in the con- centration dependence of the If the model is suitable for describing the experimental results, a plot of 1/F against l/[Pi-]: should give a straight line with intercept L/D[(C)] and slope L/DKk[(C)].The concentration [Pi-], is not known and must be calculated first. It can be shown to be given by where [Pi-l0 is the initial picrate concentration in the source compartment, k' is the partition coefficient of the picrate ion between the aqueous phase and the microemul- sion phase as a whole (k' is a number without dimensions, not to be confused with k ) and the V are the volumes occupied by the different phases.k' was experimentally measured in system V and found to be equal to 0.84, so the complete correcting factor is 0.44. Fig. 6 shows that very good agreement exists between the theoretical prediction according to the proposed model and the experimental data. Combining the values of the intercept and slope enables us to determine the value of the product Kk= (0.98 f 0.2) X lo6 dm6 mol-'. It is difficult to discuss this value on a quantitative basis. Unfortunately we have no estimate of the equilibrium constant k, which could in principle be obtained from the solubility of picrate in water and in decane if one assumes that the continuous phase of the microemulsion is pure decane (it may also contain hexanol and TEGDE): we failed to detect any solubility in decane by measuring the absorption of an aqueous solution of picrate before and after shaking with decane. On the other hand, the only results we know concerning the dynamics of solubilization of a picric acid probe in reversed micelles are consistent with an equilibrium constant of 2.3 x lo6 dm3 mol-', i.e.of the same order of magnitude as123 0 5 10 15 20 25 ( I / [ P ~ - ] , ) ~ / I O ' ~ cm6 molP2 Fig. 6. Plot of the reciprocal flux of K+ picrate against the square of the reciprocal concentra- tion of picrate at steady state in the source compartment S. Intercept=L/D(C)= 0.225 x 10" mol-' cm2 s; slope = L/DKk(C) = 2.29 x lo-' mol cmP4 s.the above value. This result was obtained by Tamura and Schelly2g in the system Aerosol OT+ benzene + water. TRANSPORT OF PICRATES BY MICROEMULSIONS CONTAINING A CLASSICAL EXTRACTANT When dicyclohexano- 18-crown-6 is added to the microemulsion being used as a liquid membrane, an increase in the flux of transported picrates is observed for both Na+ and K' salts. The results obtained with the different microemulsion systems containing mol dm-3 DC 18C6 are shown in fig. 7 and table 2, in which are also given for comparison the fluxes obtained when the liquid membrane is either pure decane containing 1 OP2 mol dm-3 DC 18C6 or the microemulsion alone. All the experiments in which a microemulsion is involved show a maximum in the flux for the same volume fraction of water. At this maximum (system V) the transport of K+ picrate by the microemulsion alone is 5.1 times faster than with DC18C6 in pure decane, but it is 12.6 times faster when DC 18C6 is added to the microemulsion.For the Na' picrate, the corresponding values are, respectively, ca. 245 and 325 because of the very weak flux obtained with DC18C6 in pure decane (the flux was so small that it was difficult to determine it accurately, which explains why the preceding values are approximate). A comparison of the results obtained for K' and Na' cations shows that, although drastically reduced, some selectivity for ion transport still exists in the presence of the microemulsion. This is of importance when considering the use of microemul- sions for liquid-liquid extraction of metal ions.124 MICROEMULSIONS TO IMPROVE KINETICS OF SOLUTE TRANSFER H20( O/o V/V) Fig.7. Plots of the flux of K+ picrate (X, -) and Na+ picrate (0, - - -) against the percentage of water (v/v) in the liquid membrane containing (or not containing) dicyclohexano- 18-crown- 6. Initial picrate concentration = 2 x 1 0-3 mol dm-3 ; concentration of DC 18C6 = 1 0-2 mol d ~ n - ~ . Note also that when the system contains both the classical extractant and the microemulsion globules, the resulting flux of K' as well as Na+ picrates is not equal to the sum of the fluxes obtained with the individual carriers. The accelerating effects of microemulsions on the kinetics of the extraction of metal ions by a classic extractant have been reported by Fourre and Bauer,I8 who used a two-compartment cell of the type described by Lewis29 and Allen3' to study the extraction of gallium by Kelex 100.They suggested that the reason for the improvement in the extraction kinetics could be found in the fact that the micro- globules of the microemulsion act as a relay between the aqueous phase and the extractant contained in the organic phase. According to this explanation the follow- ing simplified mechanism can be proposed for the present results: A Pi(aqueous phase S ) * Pi(microemu1sion 'carrier') - Pi(aqueous phase R) @ 11 + ether carrier) * If there were no coupling between the two carrier complexes a simple additive effect would be expected on a first approximation. The coupling between the two species has the result of reducing the effective membrane thickness L and thus it improves the transfer rate according to equations similar to eqn (9).The detailed mechanism can be regarded as follows: the diffusion of DC18C6 in the non-stirred layer is certainly much faster than the diffusion of the microemulsion globule, so when the empty macrocycle diffuses back towards interface 1 it will meet microemulsion globules carrying a picrate ion, and if exchange occurs the driving force will pull it towards interface 2 before it has reached interface 1. The effective layer is thus decreased for both carrier species.C. TONDRE AND A. XENAKIS 125 CONCLUSIONS We have attempted to give a clear demonstration of the carrier properties of water-in-oil microemulsion globules by studying the transport of hydrophilic solutes (alkali-metal picrates) through an oil continuous phase.The behaviour observed is very similar to that previously reported for the transport of lipophilic substances by oil-in-water microemulsions. A quantitative interpretation is nevertheless more difficult because, contrary to the case of oil-in-water microemulsions, very little is known concerning the structure of the microemulsion systems used here, which involve a non-ionic surfactant. This work also tries to clarify the mechanisms by which microemulsions can improve the kinetics of the liquid-liquid extraction of metal ions. It has been shown that transfer through a liquid membrane can help to test for the optimum conditions, such as the optimum water content of the organic phase.Nevertheless, when there is enough water to form water pools, the non-ionic detergent molecules present in the microemulsion system probably act as a strong chelating agent, and other systems will have to be found in order to investigate further the role of microemulsions in the mechanism of ion-extracting processes. We thank J. L. Fringant and J. L. Vasseur for their technical assistance in the building of the experimental apparatus. ' H. L. Rosano, P. Duby and J. H. Schulman, J. Phys. Chem., 1961, 65, 1704. R. Ashton and L. K. Steinrauf, J. Mol. Biol., 1970, 49, 547. W. I. Higuchi, A. H. Ghanem and A. B. Bikhazi, Fed. Proc., Fed. Am. SOC. Exp. Biol., 1970,29,1327. K. H. Wong, K. Yagi and J. Smid, J. Membr. Biol., 1974, 18, 379. J. D. Lamb, J.J. Christensen, S. R. Izatt, K. Bedke, M. S. Astin and R. M. Izatt, J. Am. Chem. SOC., 1980, 102, 3399. Y. Kobuke, K. Hanji, K. Horiguchi, M. Asada, Y. Nakayama and J. Furukawa, J. Am. Chem. Soc., 1976, 98, 7414. M. Kirch and J. M. Lehn, Angew. Chem., Int. Ed. Engl., 1975, 14, 555. E. Pefferkorn and R. Varoqui, J. Colloid Interface Sci., 1975, 52, 89. l o N. N. Li and A. L. Shrier, in Recent Developments in Separation Science, ed. N. N. Li (CRC Press, Cleveland, 1972), vol. I, p. 163. I ' N. N. Li, Ind. Eng. Chem., Process Des. Deu., 1971, 10, 215. l 2 N. N. Li, AIChEJ., 1971, 17, 459. l 3 J. N. Weinstein, Pure Appl. Chem., 1981, 53, 2241. l4 J. H. Fendler and A. Romero, Li$e Sci., 1977, 20, 1109. I s C. Tondre and A. Xenakis, Colloid Polym. Sci., 1982, 260, 232. '' A. Xenakis and C. Tondre, J. Phys. Chem., 1983, 87, 4737. l7 C. Tondre and A. Xenakis, in Surfactants in Solution, ed. K. L. Mittal (Plenum, New York, 1983), '' P. Fourre and D. Bauer, C.R. Acad. Sci., Ser. B, 1981, 292, 1077. l 9 P. Fourre, D. Bauer and J. Lemerle, Anal. Chem., 1983, 55, 662. 2o J. Komornicki, Thesis (UniversitC de Paris, 1981). '' J. Lamb, R. M. Izatt, J. J. Christensen and D. Eatough, in Coordination Chemistry ofMacrocyclic ' C. F. Reusch and E. L. Cussler, AIChE J., 1973, 19, 736. vol. 3, p. 1881. Compounds, ed. G. A. Melson (Plenum Press, New York, 1979). S. Friberg, I. Lapczynska and G. Gilbert, J. Colloid Interface Sci., 1976, 56, 19. 22 23 P. A. Winsor, Trans. Faraday Soc., 1948, 44, 376. 24 M. Coplan and R. Fuoss, J. Phys. Chem., 1964, 68, 1177. G. Mathis, J. C. Boubel, J-J. Delpuech, J. C. Ravey and M. Buzier, in Magnetic Resonance in Colloid and Interface Science, ed. J. P. Freissard and H. A. Resing (D. Reidel, Dordrecht, 1980), p. 597. 25126 MICROEMULSIONS TO IMPROVE KINETICS OF SOLUTE TRANSFER 26 M. Buzier, Thesis (Universitk de Nancy, 1979). 27 C. Tondre, A. Xenakis, A. Robert and G . Serratrice, to be published. 2R K. Tamura and Z. A. Schelly, J. Am. Chem. Soc., 1981, 103, 1018. 29 J. B. Lewis, Chem. Eng. Sci., 1954, 3, 260. 30 K. A. Allen, J. Phys. Chem., 1960, 64, 667.