Kinetics Involving Divalent Metal Ions and Ligands in Surfactant Self

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Langmuir 2000, 16, 8685-8691

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Kinetics Involving Divalent Metal Ions and Ligands in Surfactant Self-Assembly Systems: Applications to Metal-Ion Extraction† Hanaa A. Gazzaz and Brian H. Robinson* School of Chemical Sciences, University of East Anglia, Norwich, Norfolk NR4 7TJ, U.K. Received March 1, 2000. In Final Form: June 14, 2000 This paper describes kinetic studies of metal complexation in the presence of micelles and vesicles of different charge type. The results are interpreted in terms of the effects of the surfactant self-assembly systems on the extraction of metal ions from an aqueous medium. It is found, in the case of anionic micelles, that the extracting ligand is preferentially located close to the surface of the surfactant aggregate, where it is held by hydrophobic interactions. In this location, it is accessible to the metal ion and so is readily complexed; there is no apparent tendency for the ligand to hide inside the micelle. The same situation is found for vesicles that are negatively charged to a similar surface potential. In contrast, when positively charged surfactants are used to form micelles, the metal ion is strongly repelled from the like-charged surface into the aqueous medium. Motion across a vesicle bilayer is found to be slow; furthermore, in our systems it was difficult to maintain a pH gradient for the times that are needed for the operation of an effective extraction procedure. The kinetic and thermodynamic behavior of ligands inside vesicles was further investigated for the dye pyridine-2-azo-p-dimethylaniline (used as the ligand in our model extraction studies), and some surprising results were obtained. Below the melting temperature of vesicles composed of the long-chain cationic surfactant dioctadecyldimethylammonium bromide, the dye is released from the vesicle into the aqueous solution. However, this is not always the case. The fluorescent dye probe 8-anilinonaphthalene sulfonate behaves very differently and shows complex kinetic behavior for insertion into a range of vesicles both above and below the melting temperature. The results demonstrate the importance in extraction of surface-charge effects and a possible control role for the bilayer melting transition, in the specific case of vesicular systems.

Introduction Metal-ion complexation in vesicular systems is studied to develop a metal-ion extraction system for the selective separation and concentration of metal ions.1-4 To further our understanding of fundamental aspects of the process, complex formation in micellar systems has also been studied, with the aim of comparing reactivity in the two types of surfactant assembly. The micelle-forming surfactants were sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB). For studies in vesicular systems, sodium 6-tridecylbenzenesulfonate (6-SLABS) and dioctadecyldimethylammonium bromide (DODAB) were mainly used. These vesicle-forming surfactants were chosen because vesicles can be formed spontaneously without injection of energy (e.g., by sonication), and these systems, especially 6-SLABS, have been well-studied from a kinetic viewpoint over recent years.5-7 * Author for correspondence. † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millenium”. (1) Stevens, G. W.; Perera, J. M.; Grieser, F. Curr. Opin. Colloid Interface Sci. 1997, 2, 629. (2) Van Zanten, J. H.; Chang, D. S. W.; Stanish, I.; Monbouquette, H. G. J. Membr. Sci. 1995, 99, 49. (3) Annesini, M. C.; Cioci, F.; Lavecchia, R.; Marrelli, L. Ann. Chim. 1995, 85, 683. (4) Hebrant, M.; Tecilla, P.; Scrimin, P.; Tondre, C. Langmuir 1997, 13, 5539. (5) Farquhar, K. D.; Misran, M.; Robinson, B. H.; Steytler, D. C.; Morini, P.; Garrett, P. R.; Holzwarth, J. F. J. Phys.; Condens. Matter 1996, 8, 9397. (6) Robinson, B. H.; Misran, M.; Bucak, S. Comprehensive Chemical Kinetics, Compton, R. G., Hancock, G., Eds.; 1999, Chapter 19, 683. (7) Brinkmann, U.; Neumann, E.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1998, 94, 1281.

Figure 1. Typical scheme for liquid-membrane extraction.

There has, however, been only limited work on DODAB systems.8 Research to date on metal-ion extraction processes has focused on liquid-membrane extraction systems,9-11 shown schematically in Figure 1. When applied specifically to vesicles, the scheme is more appropriately represented in Figure 2. The basic reaction involves metal (M) ions being extracted from an aqueous feed on the outer surface of the membrane using a ligand L. This step is then followed by assisted transfer, as an ML species, through the membrane to the more acidic water core. The metal ion is stripped from the ML species (8) Robinson, B. H.; Bucak, S.; Fontana, A. Langmuir, submitted for publication, 1999. (9) Nakashio, F. J. Chem. Eng. Jpn. 1993, 26, 123. (10) Wodzki, R.; Wyszynska, A.; Narebska, A. Sep. Sci. Technol. 1990, 25, 1175. (11) Kakoi, T.; Horinouchi, N.; Goto M.; Nakashio, F. Sep. Sci. Technol. 1996, 31, 381.

10.1021/la0002942 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/04/2000

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Gazzaz and Robinson

Figure 3. Chemical structure of the ligand PADA.

Figure 2. Scheme for metal-ion extraction using vesicles (vesicles not drawn to scale).

at the lower pH inside the bilayer. The ligand L is continuously recycled within the vesicle bilayer. Important features of the scheme that need to be addressed experimentally if a successful scheme is to be developed are the following: (a) It is necessary to maintain a pH gradient across the membrane over long periods of time (hours). (b) The extracted metal ions should be retained within the aqueous core of the vesicle; that is, metal ions must not leak out over a period of time. (c) Both the ligand L and the metal-ligand complex ML should be mobile within the vesicle bilayer. (d) The metal-ligand complexation reaction should be sensitive to pH to provide the driving force for the overall reaction, M2+out f M2+in. The surface charge of the self-assembly system (provided by the ordered arrangement of surfactant headgroups) is expected to be important in the first step of complexation. The structure of the bilayer should be such as to allow facile transport of the metal ion (in the form of ML) through the bilayer. L and ML also need to be sufficiently hydrophobic such that they do not exit from the vesicle. To further investigate the factors that influence this process, ligand binding to vesicles has been studied both below and above the bilayer phase-transition (melting) temperature. Vesicle-forming surfactants with C12-C18 chain lengths display a phase transition from a rigid to a more fluid phase as the temperature is increased. This transition might also be expected to affect the properties of the system. Finally, we need to establish and then retain a pH gradient across the vesicles. The strategy adopted is to explore, in a series of experiments, different stages in the overall reaction. An azo dye ligand has been chosen to study the complexation reaction on the aggregate surface. This is done for experimental convenience in these model studies because complexation is readily detected. In a working system, it would be necessary to use other ligands, for example, oximes, as described in the series of papers by Tondre and coworkers.12-14 The structure of the ligand (L) used, pyridine-2-azo-p-dimethylaniline (PADA), is given in Figure 3. PADA is a relatively hydrophobic ligand, with a solubility in water of ∼2 × 10-4 mol dm-3. The ligand partitions very strongly to micellar hydrophobic domains; for example, in SDS micelles. Metal-ion complexation can (12) Hebrant, M.; Bouraine, A.; Tondre, C.; Brembilla, A.; Lochon, P. Langmuir 1994, 10, 3994. (13) Richmond, W.; Tondre, C.; Krzyzanowska, E.; Szymanowski, J. J. Chem. Soc., Faraday Trans. 1 1995, 91, 657. (14) Cierpiszewski, R.; Hebrant, M.; Szymanowski, J.; Tondre, C. J. Chem. Soc., Faraday Trans. 1 1996, 92, 249.

Figure 4. Diagram showing site of reaction on micelles. The thickness of the volume element contained within the dotted lines is ∼1 nm.

be complicated, but under a particular set of conditions (e.g., concentration ratio, pH) the process can be represented by a simple equilibrium relation shown in Scheme 1 below. Scheme 1

In the presence of micelles, it is necessary to establish the preferred reaction site, shown in Figure 4. For example, metal ions can be Coulombically attracted or repelled by the micelle surface, and a hydrophobic ligand could be preferentially located inside the hydrophobic core of the micelle, or close to the micelle surface. Previous workers15-18 established that, in the presence of SDS micelles, ligand and metal ion coexist in the general vicinity of the micelle surface. In a typical experiment, there would be one to a few metal ions associated with each micelle, and the average number of ligands on each micelle may well be less than one, so the nature of the micelle is not expected to be significantly changed. On the (15) James, A. D.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1978, 74, 10. (16) Holzwarth, J.; Knoche, W.; Robinson, B. H. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 1001. (17) Reinsborough, V. C.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1979, 75, 2395. (18) Fletcher, P. D. I.; Reinsborough, V. C. Can. J. Chem. 1981, 59, 1361.

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Langmuir, Vol. 16, No. 23, 2000 8687 Table 1. Wavelengths of Maximum Absorption of PADA When Totally Transferred to Micelles/Vesicles solution

λmax

H2O SDS (mic) TTAB (mic) 6-SLABS (ves) DODAB (ves)

465 460 450 465 460

Temperature ) 25 °C

Figure 5. Equilibrium constant Kass/dm3 mol-1 (and forward rate constant kf/dm3 mol-1 s1) for Ni2+/PADA complexation reaction at different SDS concentrations above and below the CMC.

time scale of complexation, metal ions will have time to “jump” between the individual micelles so that the discrete nature of the micelle surface is not important in determining the kinetics. It was also demonstrated that anionic micelles could dramatically enhance both the association equilibrium constant Kass and the forward rate constant kf, the effects being particularly pronounced just above the critical micelle concentration (CMC) of the surfactant. A quantitative treatment appropriate to a surface reaction was developed to interpret the experimental data.15 The general behavior of the system is indicated schematically in Figure 5. It can be seen that the rate and equilibrium constant for complex formation are increased by a maximum factor of ∼103 just above the CMC. In the presence of micelles formed by a cationic surfactant, the (electrically neutral) ligand interaction with the micelle, being essentially hydrophobic, is similar to that for the anionic micelle system. However, the metal ion will now be repelled from the surface, resulting in complexation being more difficult than in bulk water. Some measurements have previously been carried out on such systems.19 Thus it can be concluded that the vesicle surface charge is likely to be important in determining the efficiency of the initial complexation step in metal-ion extraction using vesicles, as indicated in Figure 2. Experimental Section TTAB was obtained from Sigma. 6-SLABS has two similar-sized alkyl chains and was specially prepared as a pure linear isomer, and was a gift from Dr. Peter Garrett of Unilever, Port Sunlight Laboratory. Vesicle sizes were typically in the 0.1-1-µm range. DODAB was obtained from Acros (99+% pure). 6-SLABS vesicles form spontaneously in aqueous solution5 on addition of NaCl in excess of 0.025 mol dm-3. The time required for spontaneous vesicle formation was ∼10 min. DODAB vesicles were prepared as follows: DODAB (10-2 mol dm-3) was dispersed in water at 60 °C, left overnight, and then extruded at the same temperature through polycarbonate (19) Tondre, C.; Hebrant, M. J. Mol. Liq. 1997, 72, 279, and references therein.

filters according to the method of Jung et al.20,21 This procedure resulted in the formation of unilamellar vesicles of ∼200 nm in diameter. Solutions of Ni2+(aq) were prepared from hexahydrated nickel nitrate, obtained from Sigma. PADA was purchased from Lancaster Synthesis, and used without further purification, since CHN analysis was consistent with a pure compound. The extinction coefficient of the free dye in water has been determined as 2.40 × 104 dm3 mol-1 cm-1 at 460 nm,22 and this value was used to calculate the precise ligand concentration in water. On complexation of the unprotonated form of PADA with a range of divalent metal ions, there is a large shift of the visible absorption peak to longer wavelengths (from 460 nm to ∼540 nm), which is only slightly dependent on the specific nature of the metal ion. There are also small but significant changes in the spectra of both the free ligand and the complex when they are associated with charged micelle and vesicle surfaces (details in Table 1). It is important to establish the optimum pH conditions for complexation because for pH values in excess of 8, the metal ion will be present as the more reactive Ni(OH)+ species, before precipitation as the increasingly insoluble neutral hydroxide at even higher pH values. There is also a complication at low pH because the ligand becomes significantly protonated at pH values less than 6 in water. The protonated ligand complexes only weakly with metal ions, and on complexation a proton is lost, because the spectrum of the product is identical with that measured at higher pH values. In very acid solutions, (pH ∼2), the spectrum of the dye changes again suggesting the development of a diprotonated PADA species. It is therefore necessary to work in a relatively narrow pH window in water (pH ) 6-8) to ensure the species involved are limited to only the divalent metal ion and the neutral ligand. There is a further complication when the reaction is carried out in the presence of SDS anionic micelles. The “local” pH in the vicinity of the micelle surface is decreased by two pH units because of the high surface potential (-120 mV) of the micelle surface; the pKa of the ligand is therefore also apparently shifted by 2 units as a result of attracting hydrogen ions to the surface. Therefore reactions in anionic micellar and anionic vesicle solutions should preferably be carried out within the pH range (as measured with a glass electrode) of pH ) 8-10. In the case of cationic surfactant assemblies, to keep the equilibrium simple a bulk pH of ∼6 was found to be appropriate. To determine a Kass value in the surfactant solutions, the same equation is used to evaluate Kass as was used in aqueous solution (See Scheme 1). In the case of SDS micelles, we also make the reasonable assumption (see (20) Jung, M.; Hubert, D. H. W.; Bomans, P. H. H.; Frederik, P. M.; Meuldijk, J.; VanHerk, A. M.; Fischer, H.; German, A. L. Langmuir 1997, 26, 66877. (21) Jung, M.; Hubert, D. H. W.; van Veldhoven, E.; Frederik, P. M.; Blandamer, M. J.; Briggs, B.; Visser, A. J. W. G.; van Herk, A. M.; German, A. L. Langmuir, in press. (22) Klotz, I. M.; LohMing, W. C. J. Am. Chem. Soc. 1953, 75, 4159.

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below for justification) that there is negligible free ligand and metal ion present in aqueous solution, so that the reaction for surfactant concentrations greater than the CMC is simply:

It should be noted that this approximation is only valid if the partitioning of both the metal ion and PADA to the surface is strong. In a similar way, we can interpret the data in the presence of the cationic surfactants (positively charged surface). It can be assumed that, for concentrations greater than CMC, most of the ligand is bound to the micelle surface (governed by K3) and the metal ion distributes between aqueous and micellar environments with an equilibrium constant K2. Then we have:

Gazzaz and Robinson

through the following argument: just below the CMC the hydrophobic dye can associate with surfactant to give a negatively charged (DmSn)n- species in which a number m of dye molecules are located in a small volume element with some surfactant molecules. The negative charge on this aggregate means that it will attract Ni2+; however, the high L/Ni2+ stoichiometry is such that bis(Ni2+L2) and possibly higher-order species will be formed in addition to the 1:1 complex. This shoulder disappears and reverts to the normal Ni2+(PADA) spectrum on addition of excess Ni2+ and by further addition of SDS (but still less than the CMC) or polar solvents such as acetone or ethanol, which remove the hydrophobic driving force for the dye-surfactant complex. This information can most simply be interpreted according to the scheme below:

Ni2+ + PADA S Ni(PADA)2+ Ni(PADA)2+ + PADA S Ni(PADA)22+

Additionally, we have investigated the interaction of the dye probe 8-anilinonaphthalenesulfonate (ANS) with the vesicle forming surfactant DODAB by spectrofluorimetry. A Hi-Tech stopped-flow instrument was used to follow kinetic processes for time scales of 20 × 10-3 mol dm-3, essentially no complexation is possible. This is in marked contrast to the situation with the anionic surfactant system. The ligand is strongly bound to the micelle for concentrations greater than the CMC. In this particular case, it is possible to measure the equilibrium constant for binding of PADA to the cationic TTAB micelle from the change in the visible absorption spectrum when PADA binds to the micelle surface in the absence of Ni2+. The spectra are shown in Figure 7-a. There is a clear shift in the ligand spectrum to shorter wavelengths on ligand association with the micelle. The isosbestic behavior suggests simple binding behavior, which can be fitted to the equation given below. Figure 7-b shows a plot of absorbance versus TTAB concentration. As can be seen from the calculated line, the fit to the model is satisfactory, and a value of K3 of 1.3 ×103 dm3 mol-1 at 25 °C is obtained.

Figure 7. (a) UV-visible spectra for the binding of PADA (2 × 10-5 mol dm-3) to TTAB micelles (0-7 × 10-3 mol dm-3). (b) Plot of absorbance at 440 nm against TTAB concentration from which K3 is obtained.

K3 ) [PADA]surf/{[PADA]water ([TTAB] - CMC)} Reaction in Presence of 6-SLABS Vesicles. Figure 8 clearly shows a similar shift in the pKa of the ligand in the presence of the 6-SLABS vesicles, as was seen for SDS micelles. The apparent pKa is found to be 6.3 ( 0.2 at 25 °C. The binding constant for Ni2+(aq) to the ligand indicates a similar value for K1 as in the case of SDS micelles, in that K1 is greatly enhanced compared with the value in the absence of surfactant. The kinetic data indicate a new feature in the vesicle system not seen in the micellar reaction in that the complexation reaction is characterized by two separable relaxation times τf and τs. The data were analyzed using conventional Hi-Tech software for the analysis of a double exponential decay. τf is found to be linearly dependent on the metal ion concentration and inversely proportional to the surfactant concentration. A similar dependence was observed previously for the same reaction on SDS micelles, suggesting that this is association of the metal ion with the ligand on the external vesicle surface. In contrast, τs is independent of both the metal ion and surfactant concentrations, as shown in Figure 9. Analysis of the slope for the faster transient gives kf, which is found to be a factor of ∼4 slower than that measured in the SDS case. This means that the metal and ligand react in a rather similar way on the micelle

Figure 8. UV-visible spectra for PADA (2 × 10-5 mol dm-3) in 6-SLABS vesicles (1 × 10-3 mol dm-3 6-SLABS + 3 × 10-2 mol dm-3 NaCl) at different pH values (4-10).

and vesicle surfaces, suggesting a similar ligand location in the two self-assembly systems. The slow relaxation, associated with τs, generally has a similar amplitude to that associated with τf. It can be identified with either (a) nickel ion migration through the membrane followed by reaction with L on the inner surface of the membrane, or (b) Transport of L through the membrane from the inside to the outside, followed by subsequent reaction with

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Figure 9. Observed rate constant for reaction between Ni2+ and PADA (2 × 10-5 mol dm-3) in 6-SLABS vesicles (1 × 10-3 mol dm-3 6-SLABS + 3 × 10-2 mol dm-3 NaCl) at pH ) 8.5.

Gazzaz and Robinson

Figure 11. UV-visible spectra for the reaction of PADA (2.5 × 10-5 mol dm-3) with different Ni2+ concentrations in DODAB vesicles (2 × 10-3 mol dm-3) at 25 °C.

Figure 10. Schematic representation for complexation in the presence of 6-SLABS vesicles.

the metal ion on the external bilayer surface. Kinetic studies do not allow us to easily distinguish between the two possibilities, but the latter mechanism is clearly preferred; from intuition, faster transport of a neutral hydrophobic species through the membrane is more likely than that for a doubly charged hydrated small cation. The transport rate constant for the ligand then has a value of ∼4 s-1. Applying the Einstein equation for translational diffusion across a bilayer with dimensions of ∼4 nm gives a translational diffusion coefficient of the order of 10-17 m2 s-1. This suggests that the transport step is much less than diffusion-controlled within the membrane, where a value of ∼10-10 m2 s-1 might be predicted for diffusion in a medium similar to that of a liquid alkane. We conclude that there is an interface interaction energy for the ligand, which has to be overcome before migration can occur; we tentatively associate this with desolvation of the ligand at the surface prior to transport through the membrane. This transport step may therefore become rate-determining in the overall process of metal-ion extraction through thicker membranes. A proposed scheme for the reaction studied is shown in Figure 10. Reaction in Presence of DODAB Vesicles. DODAB vesicles have an additional feature not found in the SLABS system. At ∼ 44 °C, there is a transition within the bilayer from a more structured to a less structured phase;23 this is the so-called gel w liquid-crystal transition (Tm). This might be expected to influence the extraction process. Figure 11 shows visible spectra for the complexation reaction at a temperature less than Tm. It should be noted that K1 is very similar (at ∼2 × 104 dm3 mol-1) to that measured in water in the absence of micelles/vesicles [2.5 ( 0.2 × 104 dm3 mol-1 at 25 °C]. This suggests that the metal ion is repelled from the surface of the positively charged vesicle, and, surprisingly, that the ligand also

does not associate with the vesicle. In contrast, for temperatures T > Tm, the extent of complexation is reduced, suggesting that when the bilayer is in the liquidcrystalline phase, the ligand associates with the vesicle and is therefore separated from the metal ion, as was observed previously for the TTAB system. There is a small but significant change in the visible absorption spectrum of the dye in the presence of DODAB vesicles for T > Tm, the wavelength of maximum absorption moving from 460 to 440 nm in the presence of excess DODAB. This behavior is shown in Figure 12. From the spectra, it is possible to calculate the equilibrium constant K′ for binding of the ligand to the vesicle. A value of ∼103 dm3 mol-1 is obtained, assuming that a pair of surfactant molecules in the layer of the vesicle contributes one binding site for the ligand L, and that all sites are equivalent. The simplest scheme for the reaction is shown below (Scheme 2).

(23) Nascimento, D. B.; Rapuano, R.; Lessa, M. M.; Carmona-Ribeiro, A. M. Langmuir 1998, 14, 7387.

The kinetics of binding can also, in principle, be studied. An attempt was made to do this using the stopped-flow

Figure 12. UV-visible spectra of PADA (2.5 × 10-5 mol dm-3) at different DODAB concentrations at 50 °C.

Scheme 2 vesicle sites

– vesicle sites

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method, but the reaction was over within the dead-time of the instrument (∼1 ms). A lower estimate of kf and kb can, however, be obtained using the following argument:

k ) kf [vesicle sites] + kb > 103 s-1 K′ )

kf ∼ 103 dm3 mol-1 kb

So at T > Tm and for [vesicle sites] ∼ 10-3 mol dm-3:

kf > 5 × 105 dm3 mol-1 s-1 kb > 5 × 102 s-1 These results indicate fast entry and exit of the ligand from the DODAB vesicle, consistent with a weak external binding site for the ligand on the external surface of the bilayer. Binding of ANS to DODAB Vesicles. The negatively charged fluorescent probe ANS was also used to study the binding of a dye to the vesicle bilayer. It is known that the fluorescence intensity of ANS is dramatically enhanced when it is incorporated into phospholipid bilayers.24 Figure 13 shows the enhancement in fluorescence intensity of ANS in the presence of different micellar and vesicular systems at 20 °C. The positively charged systems apparently have a much more pronounced effect on the fluorescence intensity than the neutral or negatively charged ones with very little enhancement observed for negatively charged micelles and vesicles. This is likely to be due to repulsion between the latter and the negatively charged dye. The cationic vesicles have the highest intensities, over all the studied systems, both above and below the phase-transition temperature, with the maximum intensity around Tm. This indicates that the ANS is probably located within the bilayer, both above and below Tm. The behavior of ANS is therefore in contrast to that observed for PADA; ANS is bound both above and below Tm, whereas PADA only binds to the vesicles for temperatures greater than Tm. The fluorescence intensity reaches a plateau with increasing vesicle concentration. The kinetics of binding of ANS to the cationic vesicles DODAB and dihexadecyldimethylammonium bromide (DHDAB) have also been studied. For DHDAB the binding is temperature-dependent and the rate of incorporation increases dramatically as the temperature is increased from 25 to 29 °C. This enhancement is associated with the change in structure of the vesicles. Similar behavior is observed for DODAB, with fluorescent changes being observed over a period of up to 200 s. The Tm is ∼ 44 °C and the kinetics change considerably in this temperature region. However, for both systems the kinetics are surprisingly complex and there is evidence for at least two relaxation times in most cases. A detailed analysis of the data is therefore needed before firm conclusions can be made about the nature of the binding sites. Discussion Metal complexation in the presence of micelles or vesicles shows similarities, but also some important differences. For anionic surfaces, complexation is facilitated, and for cationic micelles, complexation is reduced. (24) Tsong, T. Y. Biochemistry 1975, 14, 5409.

Figure 13. Fluorescence spectra of ANS (5 × 10-6 mol dm-3) with different micellar and vesicular systems at 20 °C, excitation wavelength ) 360 nm.

This is simply interpreted in terms of Coulombic attraction or repulsion of the metal ion with the surface. For anionic vesicles formed by 6-SLABS, the ligand can apparently locate on both sides of the membrane, and it is then possible, from the kinetics, to determine the transport time of the dye between the internal and external surfaces of the bilayer. The value obtained indicates that there is a restriction to transbilayer movement. DODAB contains two C18 chains, and consequently the bilayer is expected to be thicker (and accordingly less penetrable). It is of interest to note that no incorporation of the ligand PADA in the bilayer is possible at temperatures less than Tm; indeed if the vesicle solution is cooled through Tm, we see the ligand expelled from the vesicle into the aqueous solution. We therefore conclude that to have an effective system for metal-ion extraction with a ligand migrating inside the bilayer, the temperature should exceed Tm; this is achieved in general using shorter alkyl chains than C18. However, this can introduce a further difficulty. To operate effectively, it is necessary to maintain a pH gradient between the outside and the inside of the vesicle over a long period of time. It might seem that this would be most easily achieved using a thick bilayer at low temperature, but there is then the problem that L and ML transport within the bilayer would be severely restricted. In the case of 6-SLABS, it appears that the bilayer is not thick enough to sustain the pH gradient. In addition, there appears to be a particular problem with DODAB in that the vesicle surface does not show a smooth continuous curvature, but there are edge discontinuities (facets)20 through which hydrogen (and other) ions could flow from the inner compartment to the outside. It should be pointed out that some success in maintaining a pH gradient has been achieved by other workers using phospholipid liposomes,25 but these systems are not practicable for application in a metal-ion extraction system. Further work is now in progress to devise a system in which a pH gradient can be sustained. Acknowledgment. We thank the Saudi Arabian Government for the award of a scholarship to H.A.G., and Professor Josef Holzwarth (FHI, Berlin) for useful discussions on the DODAB system. LA0002942 (25) Haran, G.; Cohen, R.; Bar, L. K.; Barenholz, Y. Biochim. Biophys. Acta 1993, 1151, 201.