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J . Phys. Chem. 1990, 94, 1624-1626
only occurs in clusters above a critical size, but it may suggest a steric barrier to reaction 4-ii at small cluster sizes that is absent above a certain critical size. That is, there may exist an orientational effect, which is only overcome by enhancing the probability of a properly positioned collision encounter (e.g., by surrounding the reactant ion by a full shell of solvent molecules). We hope to continue to study other fluorinated hydrocarbons in order to better understand the systematics of this cluster chemistry. Conclusion For van der Waals clusters of 1,l-difluoroethane, we have observed a reaction that forms a protonated cluster ion only above a certain cluster size. The generation of this product appears to also exhibit larger reaction efficiency with increasing cluster size. We have attempted to rationalize this result in terms of two possible reactive mechanisms: a solvated metastable parent ion
directly reacting with a neutral monomer (reaction 44) or a fragment ion, CH3CFH+, reacting with a neutral monomer to generate fluoroethylene (reaction 4 4 ) . Whatever the true nature of the reactive mechanism, it is clearly a process that has not been observed in bimolecular experiments, suggesting unique ionmolecule chemistry that can only occur in the "solvated" environs of a cluster. Such intramolecular processes are providing an important bridge between bimolecular gas-phase reaction dynamics and condensed-phase chemistry. Acknowledgment. This research was supported by the Office of Naval Research, which is hereby gratefully acknowledged. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. Registry No. C2H4F2,75-37-6.
Mechanbm of Solute Transfer across Water/Oil Interfaces in Biphasic Microemulsion Systems C. Tondre* and A. Derouiche Laboratoire d'Etude des Solutions Organiques et Colloidales (LESOC), UA CNRS No. 406, UniuersitC de Nancy I, B.P. No. 239, 54506 Vandoeuvre-les-Nancy Cedex, France (Received: July 7 , 1989)
The microdroplets constituting the dispersed phase of a microemulsion have numerous potential applications as transfer agents. The mechanism of loading/unloading of these droplets with substances to be transferred through liquid/liquid interfaces is not easy to ascertain. We compare in this work the transport of Ni2+ ions by two microemulsion systems: CI2EO4/1hexanol/ndecane/water (system I) and AOT/n-decanelwater (0.25 M salt) (system 11). From the effect of the anion associated with Ni2+on the measured flux, we can conclude that a 'direct" interfacial transfer prevails in the case of system I1 and an "indirect" transfer in the case of system I.
Introduction Microemulsion droplets have been shown to behave as possible carriers for the transport of substances through a medium where they are not (or poorly) This property has potential applications in many biological as well as technological fields, among which the most promising are the use of microemulsions as drug delivery systems: blood substitute^,^ and transfer agents in separation and hydrometallurgical processes? Our contribution to the development of such applications has been concerned with the characterization of the ability of the microdroplets to play the part of mobile carriers. We have applied for this purpose liquid membrane techniques to biphasic microemulsion systems consisting of a microemulsion phase in thermodynamic equilibrium with an external phase. This has allowed us to measure in different situations the flux of a solute transported from a source phase to a receiving phase across a microemulsion phase used as liquid membrane."-3 The data were found to be consistent with a classical facilitated diffusion model which implies the transfer of the solute across the first liquid-liquid interface, solubilization in a micro( I ) (a) Tondre, C.; Xenakis, A. Colloid Polymn. Sci. 1982, 260, 232. (b) Tondre, C.; Xenakis, A. In SurJucranrs in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 3, pp 1881-1896. (c) Tondre, C.; Xenakis, A. Faraday Discuss. Chem. SOC.1984, 77, 115-1 26. (2) (a) Xenakis, A.; Tondre, C. J . Phys. Chem. 1983, 87, 4737. (b) Xenakis, A.; Tondre, C. J. Colloid Inreface Sei. 1987, 117, 442. (3) Xenakis, A.; Selve, C.: Tondre, C. Tulunra 1987, 34, 509. (4) Fubini, 8.;G a m , M. R.;Gallarate, M. In?. J. Phorm. 1988,42, 19-26. Halbert, G. W.;Stuart, J. F. B.; Florence, A. T. Ibid. 1984, 21, 219-232. (5) Mathis, G.; Leempoel, P.; Ravey, J. C.; Selve, C.; Delpuech, J. J. J . Am. Chem. SOC.1984, 106, 6162-6171. Cecutti, M. C. Thesis, Toulouse, France, 1987. (6) Fourre, P.; Bauer, D. C.R. Acad. Sci., Ser. 2 1981, 292, 1077. Bauer, D.; Komornicki, J. Ini. Soluen? Extr. Conf. [Proc.] 1983, 315. Wu, C.-K.; et al. Scienria Sinica (Engl. Trunsl.) 1980, 23, 1533; Inr. Solvent Exrr. Conf., [Proc.] 1980,80-23. Osseo-Asare, K.; Keeney, M. E. Sep. Sei. Techno/. 1980, IS, 999. Kim, H. S.: Tondre, C. Sep. Sei. Technol. 1989, 24, 485.
0022-3654/90/2094- 1624$02.50/0
droplet, diffusion of the droplet through the stagnant layers, and the reverse operations at the second liquid/liquid interface.lcv2a It is very important to know the mechanism of interfacial transfer precisely. This is particularly true if we are interested in the optimization of the transport process. It may also considerably influence the effectiveness of liquid membrane separation based on the use of microemulsions. Previous experiments had suggested to us that there may be no general mechanism valid for any microemulsion system.' Instead, different transfer modes could be operative depending mainly on the nature of the surfactant. We were missing so far a clear and convincing demonstration. We think that the results reported below provide a definitive evidence. Experimental Section The surfactants had the following origins: Aerosol OT (sodium bis(2-ethylhexyl) sulfosuccinate, AOT) was purchased from Sigma and tetraethylene glycol dodecyl ether (C12E04)from Nikko Chemicals (Tokyo, Japan). They were both used without further purification. The liquid membrane transport experiments are relative to two different quaternary systems, both leading after phase separation to a water-in-oil microemulsion phase in equilibrium with an external phase mainly constituted of water. The first system (I) consisted of CI2EO4/1-hexanol/n-deanelwater with weight ratios 7.5/2.5/40/50. The composition of the second system (11) was AOT/n-decane/H,O, 0.25 M KBr (7/40/53). The microemulsion phases obtained after phase separation included 6.5% water in the first case and 8.5% in the second one. Following the previously described procedure,lCq3we have used the reversed micelles to carry Ni2+ ions. The nickel salt was introduced in the source aqueous (7) Derouiche, A.; Tondre, C. J . Chem. Soc., Faraday Trans. I 1989,85, 3301.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1625
Solute Transfer across Water/Oil Interfaces
250
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time (min) Figure 1. Concentration (in ppm) of Ni2+ions transported versus time for system I (C,2E0,/1-hexanol/n-decane/water): (A) SO>-,(*) CI-, (0) Ac-, (0) NO NOT > C1- > SO4*-. This order is well-known to express the effect of these anions on the water structure, with implications in the way they affect the cloud point temperature of nonionic s ~ r f a c t a n t s . ~ Large J~ ions like C104- (or SCN-), at one end of the series, can qualify as water structure breakers whereas at the other end are found structure maker ions. The lyotropic number characterizing the acetate anion is close to that of chlorine.II The reason why we have considered the acetate anion, which is the only organic ion in the series, is because it was found to be the best ligand, among several others tested, for accelerating metal extraction.l* The extractability of salts from an aqueous phase into a pure organic phase is expected to be in close relation to the lyotropic series: the weaker the structuring effect of the anion on water (8) Cayias, J. L.; Schechter, R. S.;Wade, W. H. ACS Symp. Ser. 1975, No. 8. 234. (9) Firman, P.; Haase, D.; Jen, J.; Kahlweit, M.; Strey, R. Langmuir 1985, I , 718-724. Luck, W. A. P. Top Curr. Chem. 1976, 64, 113. (IO) Tokiwa, F.; Matsumoto, T. Bull. Chem. SOC.Jpn. 1975, 48, 1645. Schott, H.; Han. S. K. J . Pharm. Sci. 1975, 64,658. Schott, H.; Royce, A. E.; Han, S. K. J. Colloid Interface Sci. 1984, 98, 196. ( I I ) Underwood, A . L.; Anacker, E. W. J . Colloid Interface Sci. 1987, I 17, 242. (12) Gu, 2. M.; Wasan, D. T.; Li, N. N. Sep. Sci. Technol. 1985, 20, 599-61 2.
100
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500
600
Figure 2. Concentration (in ppm) of Ni2+ions transported versus time O:-, (0)CI-, for system I1 (AOT/n-deanelwater (0.25 M KBr)): (A) S (*) Ac-, (0)NO
