Copper(II) Complexation by Hydrophobic Single- and Double-Alkyl

Langmuir 1997, 13, 5539-5543

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Copper(II) Complexation by Hydrophobic Single- and Double-Alkyl Chain Ligands Solubilized in Ammonium Surfactant Vesicles Marc Hebrant,† Paolo Tecilla,‡ Paolo Scrimin,§ and Christian Tondre*,† Laboratoire d’Etude des Syste` mes Organiques et Colloı¨daux (LESOC), Unite´ Associe´ e au C.N.R.S. no. 406, Institut Nance´ ien de Chimie Mole´ culaire, Universite´ Henri Poincare´ -Nancy I, BP no. 239, 54506 Vandoeuvre-les-Nancy Cedex, France, Dipartimento di Chimica Organica e Centro CNR Meccanismi di Reazioni Organiche, Universita` di Padova, Via Marzolo 1, 35131 Padova, Italy, and Dipartimento di Scienze Chimiche, Universita` di Trieste, Via L. Giorgeri 1, 34127 Trieste, Italy Received April 1, 1997. In Final Form: July 14, 1997X Copper complexation by hydrophobic ligands solubilized in vesicular systems has been studied as a means to carry out selective metal extraction and decontamination of aqueous effluents. The vesicular systems were constituted of dimethyldi-n-alkylammonium bromides with the alkyl chains being octadecyl (DODAB), hexadecyl (DHAB), or dodecyl (DDAB). The rate of complexation with a single-chain ligand, 6-[(hexadecylamino)methyl]-2-(hydroxymethyl)pyridine (C16NHMePyr), and a double-chain one, 6-[(din-dodecylamino)methyl]-2-(hydroxymethyl)pyridine (diC12NMePyr), was investigated by the stopped-flow technique. The results have shown that rigidifying the amphiphilic layer, compared to those of classical micelles, does not reduce the rate of complexation and that the melting of the alkyl chains (gel to fluid transition) has the result of decreasing the rate, but only as far as double-chain ligands are concerned. The observation of two kinetic processes in some cases is discussed in relation with the problems of lateral diffusion and flip-flop kinetics. The effective removal of copper ions was controlled by ultrafiltration of the vesicles containing the metal/ligand complexes. The yield of copper extracted was found to be lower than that with classical micelles under similar conditions, and only low metal content could be used due to stability problems (osmotic stress).

Introduction Surfactant-based separation processes are currently attracting much interest for analytical and environmental applications (see for instance the recent reviews of Pramauro and Bianco Prevot1 or of Hinze2 ). For what concerns the removal of contaminants from polluted aqueous solutions, different techniques have been extensively studied, among which one can mention cloud point extraction,1 hollow-fiber membrane extraction,3 micellarenhanced ultrafiltration,4 etc. More particularly micellar extraction processes coupled with ultrafiltration techniques (the so-called MEUF techniques)4 have demonstrated interesting potentialities for the removal of both metal ion contaminants and organic pollutants from waste waters. These processes have nevertheless one major drawback, which is due to the fact that the monomeric surfactant, usually present at a concentration close to the critical micellar concentration (cmc), is not retained by the ultrafiltration membrane.5 For this reason, some research works have considered the use of colloidal particles such as amphiphilic polymers. Block copolymer micelles, which do not have the preceding inconvenience of conventional surfactant micelles, have for instance been used to remove polycylic aromatic * To whom correspondence should be addressed. † Universite ´ Henri Poincare´-Nancy I. ‡ Universita ` di Padova. § Universita ` di Trieste. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Pramauro, E.; Bianco Prevot, A. Pure Appl. Chem. 1995, 67, 551. (2) Hinze, W. L. In Organized Assemblies in Chemical Analysis; JAI Press Inc.: Greenwich, CT, 1994; Vol. 1, pp 37-105. (3) Hurter, P. N.; Hatton, T. A. Langmuir 1992, 8, 1291. (4) Surfactant-Based Separation Processes; Surfactant Science Series Vol. 33; Scamehorn, J. F., Harwell, J. H., Eds.; Marcel Dekker: New York, 1989. (5) Tounissou, P.; Hebrant, M.; Rodehuser, L.; Tondre, C. J. Colloid Interface Sci. 1996, 183, 484.

S0743-7463(97)00340-5 CCC: $14.00

hydrocarbons from aqueous solutions.3 To our knowledge very few studies have concerned the use of vesicles for such purposes.6 This was one of the objectives of the present work, in which we wanted to investigate the removal of metal ions by vesicular particles in which hydrophobic complexing agents have first been solubilized. The separation method considered here has some similarity with the metal ion separation using chelating mixed micelles,7-10 but the much larger size of the vesicles should allow the use of membranes with a larger pore size. This, in turn, should allow a higher flux of permeate, the latter being an important parameter in industrial applications.5 A second reason for our interest in these studies was related to the problem of performing kinetic separations of metal ions. It was shown in a previous work11 that a selective extraction of a certain metal ion in a mixture can sometimes be achieved by micellar ultrafiltration, relying on a kinetic approach rather than on the equilibrium thermodynamics (stability constants). This was made possible only because the rate of complex formation in microheterogeneous systems was considerably reduced compared to what it is in true homogeneous solution.12 The best example is that of Ni2+/Co2+ separation, which was achieved with cationic (and to a lesser extent with nonionic) micelles in which a lipophilic extractant, derived from the industrial extractant Kelex 100, was solubilized.7,11 (6) Annesini, M. C.; Cioci, F.; Lavecchia, R.; Marrelli, L. Ann. Chim. 1995, 85, 683. (7) Ismael, M.; Tondre, C. Langmuir 1992, 8, 1039. (8) Pramauro, E.; Bianco, A.; Barni, E.; Viscardi, G.; Hinze, W. L. Colloids Surf. 1992, 63, 291. (9) Tondre, C.; Son, S.-G.; Hebrant, M.; Scrimin, P.; Tecilla, P. Langmuir 1993, 9, 950. (10) Hebrant, M.; Bouraine, A.; Tondre, C.; Brembilla, A.; Lochon, P. Langmuir 1994, 10, 3994. (11) Ismael, M.; Tondre, C. J. Colloid Interface Sci. 1993, 160, 252. (12) Boumezioud, M.; Kim, H.-S.; Tondre, C. Colloids Surf. 1989, 41, 255.

© 1997 American Chemical Society

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The rate of complex formation in microheterogeneous systems is affected in different ways: (i) It can be very different depending on the nature of the metal ion, following the order of the water exchange rates in the first coordination shell, established by Eigen et al.13,14 (for instance the complexation of Ni2+ is almost two orders of magnitude slower than that for Co2+). (ii) It also depends on the lipophilicity of the extractant and consequently on its partitioning between the micellar and aqueous pseudophases (for instance the apparent rates for complex formation between Cu2+ and a series of 6-((alkylamino)methyl)-2-(hydroxymethyl)pyridines in CTAB micelles have been found to be 21 times slower with the hexadecylsubstituted ligand compared to those for the methylsubstituted one).9 While the separation of Co2+ and Ni2+ from their mixtures was practically quantitative, the separation of Co2+ and Cu2+ mixtures was only partial because both ions have quite rapid rates of complexation.11 The same difficulty would arise with all the metal ions whose rate of complexation is larger than that for Co2+. One way to decrease further this rate (see point ii) above) would be to increase the anchoring of the extracting agent inside the colloı¨dal particle, by rigidifying the amphiphilic membrane in which this extractant is solubilized. An attempt using polymerized micelles was not successful for reasons which have been previously discussed.15 In the present paper, we wanted to examine how the rate of complex formation occurring in vesicles (i.e. in bilayer amphiphilic membranes) can be compared with the rates measured in conventional micelles and how the gel to fluid transition influences this rate. Experimental Part Materials. Dimethyldi-n-octadecylammonium bromide (DODAB) and dimethyldi-n-dodecylammonium bromide (DDAB) were obtained from Sigma; dimethyldi-n-hexadecylammonium bromide (DHAB) was prepared according to Ueoka et al.16 Cetyltrimethylammonium bromide (CTAB) was purchased from Fluka (purum quality). Cu(NO3)2‚3H2O was analytical grade (Carlo Erba). The Cu2+ stock solution was titrated against EDTA. The synthesis of 6-[(n-hexadecylamino)methyl]-2-(hydroxymethyl)pyridine (C16NHMePyr) has been reported before.17 The synthesis of 6-[(di-n-dodecylamino)methyl]-2-(hydroxymethyl)pyridine (diC12NMePyr) is described below. To a solution of 6-((n-dodecylamino)methyl)-2-(hydroxymethyl)pyridine17 (0.5 g, 1.6 mmol) in 5 mL of EtOH were added 0.5 mL of 1-bromododecane (2.1 mmol) and 0.7 mL of N,Ndiisopropylethylamine (4.0 mmol). The reaction mixture was heated in a pressure vial at 80 °C for 20 h. The solution was then diluted with 25 mL of CHCl3 and washed first with a 5% solution of Na2CO3 and then with water. The evaporation of the dried (Na2SO4) organic phase gave a crude material that was purified by column chromatography (SiO2, CHCl3/MeOH 20/1), yielding 0.6 g of pure diC12NMePyr as a pale-yellow oil. 1H-NMR δ (CDCl3): 0.88 (t, J ) 6.2 Hz, 6 H, 2 × CH3), 1.25 (m, 36 H, 2 × (CH2)9CH3), 1.46 (m, 4 H, 2 × CH2(CH2)9CH3), 2.45 (t, J ) 6.9 Hz, 4 H, 2 × CH2(CH2)10CH3), 3.72 (s, 2 H, NCH2Py), 4.72 (s, 2 H, OCH2Py), 7.05 (d, J ) 7.7 Hz, 1 H, PyH5), 7.38 (d, J ) 7.7 Hz, PyH3), 7.63 (t, J ) 7.7 Hz, PyH4). The pH of the solutions was adjusted using HCl (Carlo Erba). Milli-Q water was used throughout this work. Techniques. The stopped-flow technique (Applied Photophysics) with optical detection was used for the kinetic measurements. Complex formation at various temperatures was detected (13) Eigen, M. Pure Appl. Chem. 1963, 6, 97. (14) Eigen, M.; Wilkins, R. G. In Mechanism of Inorganic Reactions; Advances in Chemistry Series Vol. 49; American Chemical Society: Washington, DC, 1965; p 55. (15) Hebrant, M.; Toumi, C.; Tondre, C.; Roque, J. P.; Leydet, A.; Boyer, B. Colloid Polym. Sci. 1996, 274, 453. (16) Ueoka, R.; Matsumoto, Y. J. Org. Chem. 1984, 49, 3774. (17) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem. 1991, 56, 161.

Hebrant et al. at 310-320 nm. The kinetic curves obtained in CTAB micelles were perfectly first order for the C16NHMePyr and diC12NMePyr ligands. This was no longer true in vesicular solutions, in which a very slow increase of absorbance with time was always observed. This effect is likely to be related to the poor stability of the vesicular solutions in the presence of Cu2+ (the solutions precipitated after several hours when 5 mM of Cu2+ was added). The phenomenon, which can be attributed to the osmotic stress, was more pronounced at low temperature. The small turbidity increase was omitted in the treatment of the kinetic data, and under these conditions all the kinetic curves were correctly fitted with monoexponential functions, except those relative to the diC12NMePyr ligand in DHAB vesicles at temperatures below 20 °C. In the latter case the data were best fitted with biexponential functions. The vesicle solutions were prepared following a procedure derived from the one described by Cleij et al.18 The surfactant and ligand (15:1 molar ratio) were dissolved in methylene chloride; the solvent was evaporated under a gentle nitrogen stream, and the resulting solid was dried for 2 h under vacuum at room temperature. Acidified water (20-25 mL) (pH 3.5 ( 0.1) was then added, and the dispersion obtained was sonicated at 40 °C for 20 min (Branson sonifier 250, tip 13 mm, 45-50% power output). The vesicle solutions were filtered at room temperature through 0.45 µm Millipore membranes in order to remove the titanium particles coming from the immersion probe. The size of the vesicles was measured at 25 °C using a dynamic light scattering apparatus comprising a Nicomp 370 autocorrelator equipped with a Spectraphysics 2016 argon laser. The mean hydrodynamic diameters of the DHAB and DODAB vesicles were typically in the range 27 ( 5 nm, whatever the covesicallized molecule (DPH or ligand). The gel to fluid phase transition temperatures Tc were determined in several cases from fluorescence polarization measurements (Fluorimeter PE MPF 2A) using 1,6-diphenyl1,3,5-hexatriene (DPH) as a probe, under the following experimental conditions:19 λex ) 360 nm, λem ) 430 nm, [DPH] ) 3 × 10-5 M, [surf] ) 3 × 10-3 M, [ligand] ) 2 × 10-4 M. The fluorescence polarization P changes more or less abruptly at the phase transition when the temperature is varied. Tc was taken equal to the temperature at midtransition. The ultrafiltration experiments were performed with a 8010 Amicon stirred cell, using a YM 10 cellulosic membrane, under a nitrogen pressure of 2 bar. The ultrafiltration was stopped after 50% of the initial volume was filtered. No precipitation occured, although sometimes a very thin film formed at the surface of the retentate. No UV-visible absorption, due to a possible presence of free ligands, could be detected in the permeates. This is in line with the assumption that the dissolution of the ligands in the aqueous phase was negligible.

Results and Discussion The determinations of the transition temperatures Tc were carried out prior to the other experimental measurements. The results of the fluorescence polarization of DHP versus temperature are represented in Figure 1 for the different systems investigated, including the solubilized extractants. The critical temperatures are lower than those for the pure surfactants, and the transitions are broader. Indeed, the literature values for DHAB and DODAB are 28 and 45 °C respectively,20 whereas in the presence of the solubilized extractants in a 1:15 molar ratio we have obtained 21.3 and 35.5 °C. The presence of copper was avoided because of its effect on the stability of the vesicles, since the measurement of Tc requires several hours. The first-order rate constants kobs for Cu(II) complexation obtained from stopped-flow kinetics at different temperatures are reported in Figures 2 and 3. Figure 2 refers to the single-chain ligand C16NHMePyr in DODAB, (18) Cleij, M. C.; Scrimin, P.; Tecilla, P.; Tonellato, U. Langmuir 1996, 12, 2956. (19) Andrich, M. P.; Vanderkoi, J. M. Biochemistry 1976, 15, 1257. (20) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 709.

Copper(II) Complexation by Ligands

Figure 1. DPH fluorescence polarization versus temperature in covesicles of DHAB and diC12NMePyr (0), covesicles of DODAB and diC12NMePyr ([), and covesicles of DODAB and C16NHMePyr (2). [DPH] ) 3 × 10-5 M, [surf] ) 3 mM, [ligand] ) 0.2 mM.

Figure 2. Observed rate constants kobs versus temperature for the complexation of Cu(II) by C16NHMePyr solublized in vesicles of DODAB (9), DHAB (2), and DDAB (b) . [Cu2+] ) 5mM, [surf] ) 1.5 mM, [ligand] ) 0.1 mM, pH ) 3.5 ( 0.1, λ ) 320 nm.

Figure 3. Observed rate constants versus temperature for the complexation of Cu(II) by diC12NMePyr solublized in vesicles of DODAB (4), DHAB (slow, 0; fast, O), and DDAB ([) . [Cu2+] ) 5 mM, [surf] ) 1.5 mM, [ligand] ) 0.1 mM, pH ) 3.5 ( 0.1, λ ) 320 nm.

DHAB, and DDAB vesicles, and Figure 3 concerns the double-chain ligand. A first observation is that the rate of copper complexation with the single-chain ligand is faster in DODAB compared to DHAB and DDAB, for which similar values of kobs were obtained. In all cases the curves of kobs versus temperature relative to the single-chain ligand do not present any peculiarity at the critical temperature and their exponential increase is consistent with a classical Arrhenius behavior. A completely different situation exists with the doublechain ligand (Figure 3), for which a spectacular break in the kinetic curves, fairly well correlated with the critical

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Figure 4. Signal amplitudes A in OD units associated with the values of kobs given in Figure 3: DODAB (O), DHAB fast reaction process (9), DHAB slow reaction process (4).

transition temperature, is observed in DODAB and in DHAB. For the DDAB vesicles we have only measured the kinetics of copper complexation at temperatures higher than Tc (14 °C for the pure surfactant).20 The interesting feature is that the rate of the reaction seems to be mainly controlled by the organization of the alkyl chains in the bilayer membrane: whatever the type of vesicles, we have a single curve of kobs in the gel state (i.e. at temperatures below Tc) and another single curve in the fluid state (i.e. at temperatures larger than Tc). The second characteristic time, which was only observed for DHAB at low temperatures, gives kobs values which fall quite nicely on the second curve. This last observation suggests that we have in this case a temperature domain, close to the transition temperature, in which there is a coexistence of the two states of the bilayer membrane. Such an interpretation is consistent with the fact that the fluorescence polarization P is much lower for DHAB than for DODAB (see Figure 1) , which is an indication that there is much less order in the system. Another interesting observation, which was not at all expected, is the fact that the melting of the alkyl chains in the bilayer is associated with a decrease of the rate of complex formation. Our expectation was that the more rigid the bilayer the more efficient should be the insertion of the complexing agent inside the particle and consequently the slower should be the reaction. In fact the reality is completely different and a likely explanation is the following: the solubilization site of the double-chain ligand is located more deeply in the bilayer membrane when the chains are fluid than when they are in the gel state. In the latter situation the polar head of the ligand could protrude outside the particle, thus making the approach of the copper ions easier. In fact a quite similar observation had been reported before for the kinetics of enantiospecific cleavage of R-amino acid esters in vesicles.18 A dramatic effect of the fluidity of the membrane on the reactivity was noticed, and in most cases a rate decrease was shown to occur in the phase transition interval. The preceding interpretation is in agreement with the fact that for the same surfactant the rate of copper complexation is slightly slower with the single-chain ligand C16NHMePyr than with the double-chain diC12NMePyr (compare results in Figures 2 and 3). The penetration in the double-layer membrane is of course easier with the former one, in spite of the much larger number of methylene groups in the double-chain ligand. Nevertheless, when we compare secondary amines and tertiary amines, one should keep in mind that the change of basicity and of the steric hindrances may also affect in some way the rate of complex formation. If we now look at the amplitudes of the absorbance changes (Figure 4) obtained from the theoretical treatment

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of the kinetic data with mono- or biexponential functions, the following comments can be made. The amplitudes are small (around 0.03 OD units) compared to what was previously observed at the same pH, in CTAB for C16NHMePyr (about 0.2-0.4 OD units depending on the CTAB concentration)21 and also compared to what was observed in this work for the ligand diC12NMePyr in CTAB (0.2 ( 0.05 OD units). This difference may have several origins, but a large part is due to the effect of the nature of the anion (excess of Br- in the studies in CTAB and of NO3- in the studies in vesicles). Indeed the absorbance enhancement effect of Br- ions on the UV-visible spectrum of the complexes of copper with CnNHMePyr was noticed before, whereas in nonionic surfactants (i.e. in the absence of Br-) signal amplitudes on the order of 0.1 ((0.03) OD units had been reported.21 A comparison of the data of Figure 4 with this last value suggests that another part of the effect may be due to the partitioning of the ligand between the two amphiphilic layers of the vesicles,22 only the fraction solubilized in the outside layer being involved in complex formation. This fraction was previously shown to be on the order of 62-69%.23 Finally the change of stability constant between the complexing sites including a tertiary amine instead of a secondary amine may affect the measured signal amplitude. The phase transition temperature only weakly affects the signal amplitude in the case of DODAB. The situation with DHAB is completely different because of the existence of two relaxation processes: the total amplitude decreases when approaching Tc, but this behavior is mainly related to the disappearance of the slow process just before the melting of the alkyl chains. What is the true origin of this slow process is a difficult question. If we assume, as was said above, that we have two different solubilization sites corresponding to fluid and gel domains, respectively, this also implies that the rate of exchange of the ligand molecules between these two kinds of domains is slow. This would certainly be the case if there is a coexistence of two populations of vesicles, in the gel and fluid states, respectively. The other alternative is to have different domains in the same vesicle. In that case the transfer of a ligand molecule from one type of domain to the other type will be controlled by the rate of lateral diffusion. For lipids in the fluid phase this diffusion rate is known to be rapid, with diffusion coefficients on the order of 10-7 cm2 s-1.24 The lateral diffusion is likely to be much slower in the gel-fluid coexistence region. This is demonstrated by the times required for the equilibration of mixtures of two phospholipids depending on whether they are in the fluid phase or in the gel-fluid transition region.25 The complete equilibration takes about ten times more time in the latter case. This suggests that the exchange rate of the ligand molecules will become slower than that in the pure fluid phase, but we do not know how the exchange time will compare with the time scale of the complexation kinetics. The effective removal of copper by ultrafiltration, after complexation with the vesicle-solubilized extractants, is reported in Figure 5. In this figure we have plotted the extraction yield, defined as Y ) 100([Cu2+]0 - [Cu2+]per)/ [Cu2+]0, as a function of temperature ([Cu2+]0 and [Cu2+]per (21) Son, S.-G.; Hebrant, M.; Tecilla, P.; Scrimin, P.; Tondre, C. J. Phys. Chem. 1992, 96, 11072. (22) Fendler, J. H. Membrane Mimetic Chemistry; John Wiley & Sons: New York, 1982; p 129. (23) Ghirlanda, G.; Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem 1993, 58, 3025. (24) Raudino, A.; Castelli, F. In Vesicles; Surfactant Science Series Vol. 62; Rosoff, M., Ed.; Marcel Dekker: New York, 1996; p 124. (25) Davidson, S. M. K.; Liu, Y.; Regen, S. L. J. Am. Chem. Soc. 1993, 115, 10104.

Hebrant et al.

Figure 5. Yield of copper(II) extraction versus temperature: C16NHMePyr solubilized in CTAB micelles ([CTAB] ) 25 mM) (b) ; covesicles of C16NHMePyr and DODAB (O), covesicles of diC12NMePyr and DODAB (9);covesicles of diC12NMePyr and DHAB (0). [Cu2+] ) 7.87 × 10-5M, [surf] ) 1.5 mM, [ligand] ) 0.1 mM, pH ) 5.2 ( 0.2.

are the total copper concentration and the concentration in the permeate, respectively). The experiments were carried out at pH 5.2 ((0.2), and the other experimental conditions are given in the caption of Figure 5. We have also shown in the figure the result previously obtained 9 at pH 5.0 and 25 °C with the ligand C16NHMePyr in CTAB micelles when the ligand to metal ratio [L]0/[Cu2+]0 was equal to 1. This ratio was slightly above 1 in the present case, but the surfactant concentration was considerably lower (1.5 × 10-3 M instead of 2.5 × 10-2 M). The results reported in Figure 5 call for the following comments: (i) There is no significant change of the yield of extraction associated with the gel-fluid transition in the vesicular systems. This suggests that the partitioning of the complex is not affected by the chain melting. (ii) The comparison of the yields obtained for the monoalkyl chain ligand in CTAB micelles (75%)9 and in DODAB vesicles (35 ( 5%) suggests that only the part of the ligand solubilized in the outside layer of the bilayer is reacting with copper during the time of ultrafiltration (about 15 min). Indeed, the yield of 35 ( 5% (compared to 75% in a single layer) is close to what you would expect if you consider a 60/40 partitioning between the outside and inside layers of the bilayer,23 the contribution of the flipflop mechanism being expected to remain small during the time scale of the ultrafiltration experiment.26 This also makes unlikely the formation of complexes of 1:2 stoichiometry, although this particular point will deserve further investigations. (iii) The difference in the yields measured in DODAB for C16NHMePyr (∼35%) and diC12NMePyr (maximum of 13%), respectively, is likely to be attributed to the decrease of the stability constant of the ligand/metal complexes, since in this case the partitioning of the ligand should play a similar part in both cases. (iv) The decrease of the extraction yield observed from DODAB to DHAB for the dialkyl chain ligand may be due to the fact that the solubilization of the complex is slightly less favorable in the latter case. Another important feature, which was completely omitted up to this point of the discussion, has to be considered. It concerns the real nature of the DODAB suspensions, since it has been claimed that DODAC molecules (the same molecules as here, except that Brreplaced Cl-) in water solutions are organized in small open structures equivalent to membrane fragments or flat disklike bilayers.27 The present work is too prelimi(26) Kornberg, R.; McConnell, H. M. Biochemistry 1971, 10, 1111. (27) Pansu, R. B.; Arrio, B.; Roncin, J.; Faure, J. J. Phys. Chem. 1990, 94, 796.

Copper(II) Complexation by Ligands

nary to pretend it brings new information regarding this problem for the case of DODAB. In any case the significance of our kinetic results should not be too much affected by the presence of membrane fragments in which the solubilization of the ligand/metal complexes should be comparable to the solubilization in true vesicles. Nevertheless the reasoning concerning the accessibility of ligand molecules depending on whether they are solubilized in the inside or outside layer of the bilayer would be questionable in case we would be dealing with open vesicles instead of closed structures. It is also clear that the retention of the DOBAB particles by the ultrafiltration membrane could be less efficient in the situation where the membrane fragments coexist in solution with vesicles. Conclusion The use of vesicular systems solubilizing complexing agents capable of removing metal ions has been studied from two different viewpoints: complexation kinetics and effective extraction using ultrafiltration. Concerning the first aspect, it turned out that: contrary to what was expected, rigidifying the amphiphilic layer brings about a slight increase in the rate of complexation. The melting of the alkyl chains has the result of decreasing this rate only when dialkyl chain ligands are concerned. This observation is to be put in the context of the effect of the membrane fluidity, which has been previously reported for the rate of enantioselective cleavage of R-amino acid esters catalyzed by copper(II): it demonstrates that the rate of copper binding itself is similarly affected by the

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melting of the alkyl chains. On the other hand, the present results show that we cannot expect with vesicular systems an improvement of the kinetic selectivity of metal complexation, compared to what was reported before in classical micellar or microemulsion systems. The analysis of the differences in the kinetic behavior of mono- and dialkyl ligands was complicated by the fact that we were dealing with a secondary amine in the first case and a tertiary amine in the latter one. Further studies will have to involve molecules in which the complexing sites are kept identical, which means that the second alkyl chain should be grafted in such a position that it does not alter the value of the stability constant. With respect to the second purpose of this work, the effective removal of metal ions by ultrafiltration, it should be stressed that, if the advantages of vesicular systems are the absence of a cmc (which should reduce the leakage of monomeric surfactant) as well as their larger size compared with that of micelles, their major drawback is the stability problem caused by the presence of electrolytes (osmotic stress). For this reason only low metal content could be tested and the yields of copper extraction obtained were less than those obtained using classical micelles. Acknowledgment. Funding for this research was provided by the Ministry of University, Scientific and Technological Research (MURST, Italy), the CNR (Italy), and CNRS (France). The authors acknowledge reciprocal travel grants in the framework of the binational “Galileo” Project. LA9703407