5056
Langmuir 1999, 15, 5056-5064
Kinetic Studies of Metal Ion Complexation in Glycerol-in-Oil Microemulsions N. Zeynep Atay*,† and Brian H. Robinson‡ Department of Chemistry, Bogazic¸ i University, 80815 Bebek, Istanbul, Turkey, and School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. Received January 7, 1999. In Final Form: April 7, 1999 Nanodroplet dispersions containing polar solvents other than water have potential as a novel reaction medium for controlled synthesis. The kinetics of two model reactions, between divalent metal cations (M2+) and either the hydrophilic tridentate ligand murexide (Mu-) or the bidentate ligand pyridine-2-azo-pdimethylaniline (PADA) have been studied in Aerosol-OT (AOT)- or cetyltrimethylammonium bromide (CTAB)-stabilized, nanometer-sized glycerol droplets dispersed in an organic medium. The reactions are readily studied, because the viscosity of the reaction medium is determined by the low-viscosity organic solvent rather than by glycerol. The model systems are extremely well behaved, and useful insights into the significant kinetic factors that control reactivity in these systems are obtained. It is found that the rate of the reaction is very dependent on the metal ion, with Zn2+ > Co2+ > Ni2+. However, reactions are slower in dispersed glycerol as compared with dispersed water. The charge on the surfactant can significantly affect the rate of the forward (complex formation) reaction; reactions in glycerol nanodroplets stabilized by CTAB are considerably faster than those in glycerol nanodroplets stabilized by AOT. This is due to partitioning effects of the reactants after they have located into the same droplet. The determination of activation parameters confirms that diffusion is not determining the rates of complexation.
Introduction The aggregation behavior of surfactants in nonaqueous polar solvents has been studied in some detail over recent years.1 Favored solvents are ethylene glycol, glycerol, and formamide. In general, the critical micelle concentration (cmc) is increased as compared with water, and the aggregates which form are smaller. In contrast, glycerol and water behave in a very similar way when dispersed in alkane solvents, when Aerosol-OT (AOT) or cetyltrimethylammonium bromide (CTAB) is used as the surfactant.2,3 The advantage of an alkane continuous phase microemulsion is that glycerol can then be used as a solvent for chemical reactions, but the solvent will nevertheless have the low viscosity of the alkane continuous phase. To date, very few reactions have been studied with these solvent combinations, but there have been recent structural studies performed on sodium dodecyl sulfate (SDS)/formamide/hexanol/alkane4 and pentakis(ethylene glycol)dodecyl ether/glycerol/alkane5,6 systems. The AOT/ formaldehyde (with added NaBr)/styrene system has been used for studies of polymerization processes.7 Previous structural studies of AOT-stabilized nanodispersions of glycerol in n-heptane have shown that these systems are thermodynamically stable and consist of discrete droplets that are essentially spherical in shape.2 * To whom correspondence should be addressed. Tel: ++ 90 212 2631540 ext. 1440. Fax: ++ 90 212 2872467. E-mail: zatay@ boun.edu.tr. † Bogazic ¸ i University. ‡ University of East Anglia. (1) Warnheim, T. Curr. Opin. Colloid Interface Sci. 1997, 2, 472 and references therein. (2) Fletcher, P. D. I.; Galal, M. F.; Robinson B. H. J. Chem. Soc., Faraday Trans. 1 1984, 80, 3307. (3) Fletcher, P. D. I.; Galal, M. F.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2053. (4) Friberg, S. E.; Rong, G. Langmuir 1988, 4, 796. (5) Martino, A.; Kaler, E. W. J. Phys. Chem. 1990, 94, 1627. (6) Martino, A.; Kaler, E. W. Langmuir 1995, 11, 779. (7) Schubert, K. V.; Lusvardi, K. M.; Kaler, E. W. Colloid Polym. Sci. 1996, 274, 875.
Using photon correlation spectroscopy and small-angle neutron scattering, and for volume fractions of dispersed phase of less than 0.05, the hydrodynamic radii, rH, of the glycerol droplets were found to range typically from 1 to 10 nm. Precise values of rH for AOT-stabilized dispersions can be calculated2 using the formula
rH/nm ) 1.7 ((0.2) + 0.88 ((0.15)Rg
(1)
where Rg is the molar (mole) ratio of glycerol to surfactant (Rg ) [Gly]/[AOT]) and 1.7 nm is essentially the length of the surfactant molecule including the headgroup when the surfactant forms a reverse micellar structure (Rg ) 0). This equation applies over a range of temperatures of around 25 °C, provided that the composition is not located close to a phase boundary. A similar equation applies to aqueous nanodispersions stabilized by AOT.8 Glycerol can also be solubilized as thermodynamically stable nanodroplets in a solvent mixture of 1:1 (v/v) n-heptane/chloroform using the cationic surfactant CTAB.3 The resulting dispersions again contain discrete droplets of glycerol. The hydrodynamic radius, rH, of the spherical droplets can be calculated using the formula
rH/nm ) 1.8 ((0.4) + 1.2 ((0.2)Rg
(2)
where Rg is the molar (mole) ratio of glycerol to CTAB (Rg ) [Gly]/[CTAB]) and 1.8 nm is again essentially the thickness of the surfactant monolayer surrounding the droplet. The high viscosity of “bulk” glycerol makes it a very difficult solvent to work with for the study of reaction kinetics. However, this solvent is interesting as it is close to water in many of its properties and, furthermore, it (8) Nicholson, J. D.; Clarke, J. H. R. Proceedings of the International Symposium on Surfactants in Solution; Mittal, K., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 3, p 1663.
10.1021/la9900122 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/05/1999
Kinetic Studies in Glycerol-in-Oil Microemulsions
Langmuir, Vol. 15, No. 15, 1999 5057 Kos
M2+ + Mu- y\ z 1 kex
(3) (M2+/Mu-) y\ z (MMu)+ 2 inner-sphere complex outer-sphere complex When [M2+] . [Mu-], the pseudo-first-order rate constant for the reaction, kobs/s-1, is given by
a
kobs ) kf[M2+] + kb
(4)
and because step 1 is a rapid preequilibrium step, step 2 is rate limiting, with the result that
kf ) Koskex
b Figure 1. (a) PADA. (b) murexide.
does not freeze easily so that a supercooled medium is easily achieved. We have chosen to study an elementary reaction, that of metal/ligand substitution, in a glycerol dispersion in n-heptane, or n-heptane/CHCl3. Some of the problems associated with the high viscosity of glycerol were alleviated to some extent in previous studies by Caldin and Grant, who studied ligand-substitution reaction kinetics in glycerol by a laser temperature-jump method9 so that initiation of the reaction by mixing was not required. Solutions were, however, prepared at 90 °C, at which temperature the solvent is relatively mobile. The reaction observed was the complexation of the dye ligand pyridine-2-azo-p-dimethylaniline (PADA; Figure 1a) with divalent metal ions such as Ni2+, Zn2+, Co2+, and Cu2+. The reaction rate was found to be essentially independent of the metal ion studied, which suggested that the kinetics could be controlled by the viscosity of the solvent. However, the derived rate constants were considerably below the diffusion-controlled limit, suggesting the ring-closure effects may also be important. Metal ion/ligand complexations taking place in the dispersed phase (glycerol) of glycerol-in-oil (g/o) microemulsions can be readily followed by rapid-mixing techniques, such as stopped flow, even at room temperature, because the viscosity of the medium corresponds closely to that of the continuous phase, which in our case is n-heptane. Because glycerol is dispersed as spherical and discrete nanodroplets, the high viscosity of “bulk” glycerol no longer presents a problem; nevertheless, reactions involving metal ions will take place in the polar glycerol “pseudophase”. Metal ion/ligand complexations between the divalent aquo-metal ions zinc, cobalt and nickel (Zn2+, Co2+, and Ni2+) with the indicator murexide (Mu-; Figure 1b) can be readily studied in g/o microemulsions. This system was chosen because the extent of Mu- partitioning from the glycerol pseudophase into the continuous alkane phase is insignificant. For aqueous systems, the reaction scheme, in both bulk and dispersed water, has been shown to be consistent with the following mechanism:10-14 (9) Caldin, E. F.; Grant, M. W. J. Chem. Soc., Faraday Trans. 1 1973, 69, 1648. (10) Eigen, M.; Tamm, K. Z. Elektrochem. 1962, 66, 107.
(5)
where kf/(dm3 mol-1 s-1) is the second-order rate constant for the overall forward reaction, kb/s-1 is the rate constant for the backreaction (dissociation of murexide from (MMu)+), Kos/(dm3 mol-1) is the association equilibrium constant for outer-sphere complex formation, and kex/s-1 is identified with the rate constant for exchange of a water molecule between the solvation shell of the metal ion and bulk water. Ring closure to form the inner-sphere complex in aqueous systems is normally considered to be rapid. It is generally assumed that three water molecules are released during the formation of the final inner-sphere complex with murexide in water, with the release of the first one associated with kex. Unfortunately, for the reasons already given, no comparable reactions (i.e., metal ion/ murexide complexation) have been followed in glycerol as the solvent. To make a direct comparison between reaction rates in bulk and dispersed glycerol, the complexation reaction between divalent metal ions and the ligand PADA has also been studied in g/o microemulsions. Because PADA has a considerable solubility in the hydrocarbon continuous phase, ligand partitioning between the oil continuous region and the nanodroplet has then to be taken into account when interpreting the kinetic data. Reactions between PADA and divalent aquo-metal ions, such as Zn2+, Co2+, and Ni2+, have already been extensively studied in aqueous solution,15-18 SDS micellar solutions,18-22 and AOT-stabilized w/o microemulsions.21,22 The mechanism of this reaction is also consistent with the Eigen-Tamm mechanism,10,11 where the hydrated metal ion and the ligand diffuse together in a fast preequilibrium step to form an outer-sphere complex. Reaction then proceeds with rate-determining water loss from the metal ion. The mechanism in aqueous solution (11) Eigen, M.; Wilkins, R. G. In Mechanism of Inorganic Reactions; Gould, R. F., Ed.; Advances in Chemistry Series 49; American Chemical Society: Washington, DC, 1965; p 55. (12) Robinson, B. H.; Steytler, D. C.; Tack, R. D. J. Chem. Soc., Faraday Trans. 1 1979, 75, 481. (13) Fischer, M.; Knoche, W.; Robinson, B. H.; Maclagan Wedderburn, J. H. J. Chem. Soc., Faraday Trans. Trans. 1 1979, 75, 119. (14) Fischer, M.; Knoche, W.; Fletcher, P. D. I.; Robinson, B. H.; White, N. C. Colloid Polym. Sci. 1980, 258, 733. (15) Cayley, G. R.; Hague, D. N. Trans. Faraday Soc. 1971, 67, 786. (16) Cobb, M. A.; Hague, D. N. Trans. Faraday Soc. 1971, 67, 3069. (17) James, A. D.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1978, 74, 10. (18) Robinson, B. H.; White, N. C. J. Chem. Soc., Faraday Trans. 1 1978, 74, 2625. (19) Holzwarth, J.; Knoche, W.; Robinson, B. H. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 1001. (20) Reinsborough, V. R.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1979, 75, 2395. (21) Fletcher, P. D. I.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1983, 79, 1959. (22) Fletcher, P. D. I.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2417.
5058 Langmuir, Vol. 15, No. 15, 1999
is represented by the following equation:18
Atay and Robinson
The hydrophilic indicator murexide and hexaquo-nickel, -zinc, and -cobalt nitrates (AR, 98%) were obtained from Fisons. AerosolOT was purchased from Sigma and used without further purification, following our usual tests for purity.23 n-Heptane was distilled over sodium metal, stored over a type 4A molecular sieve, and filtered before use. Chloroform (AR, containing 1.5% ethanol) was also obtained from Fisons. CTAB (stated purity 99%) was purchased from Sigma and was used without further purification.
Glycerol was obtained from Fisons (>98%). In the g/o microemulsions, there are likely to be trace amounts of water present, which is primarily introduced with AOT, because this surfactant contains 0.2 mol of water/mol of AOT. However, the metal ion can be safely taken to be solvated by glycerol rather than water inside a glycerol droplet. PADA was also purchased from Sigma. It was found to be unnecessary to buffer the solutions. (Buffers are avoided because of possible side reactions with metal ions.) All kinetic data were obtained using an in-house-designed small-volume stopped-flow instrument with spectrophotometric (visible) detection. Transients were always found to be single exponentials, from which pseudo-first-order rate constants, kobs/s-1, could be readily calculated. Equilibrium measurements were made on a PYE UNICAM SP8-200 UV/vis spectrophotometer, with temperature control to (0.2 °C. Preparation of AOT-Stabilized Dispersions Containing the Reactants M2+and Mu-. The hexaquo-metal nitrate salts were dissolved, with occasional shaking, in neat glycerol at 70 °C. At this temperature the viscosity of the solvent is considerably lower than that at room temperature, so solubilization is greatly facilitated. Following addition of an appropriate aliquot of the reactant solution into an AOT solution in n-heptane, the resulting dispersions were sonicated for 5-10 min, with occasional shaking, until thermodynamically stable, transparent g/o microemulsions were formed. Slightly longer periods of sonication were needed for dispersing glycerol droplets containing higher concentrations of metal ions (e.g., [Ni2+]g ) 0.1 mol dm-3). (The subscript “g” indicates that the concentration is expressed in the glycerol phase.) The sonication procedure serves to accelerate the attainment of the equilibrium state, with the microemulsions so obtained being thermodynamically stable systems. Murexide-containing glycerol dispersions in oil were prepared in the same way as described above. Murexide was found to be more stable (i.e., color-fast) in glycerol than in water, and solutions of murexide in glycerol could be stored for over a week without any observable photochemical or acid-base-catalyzed decomposition.24,25 The λmax of murexide (520 nm in aqueous solution) is shifted to 495 nm in the AOT-stabilized g/o microemulsions. Reactions were also carried out in “mixed” microemulsion systems, in which the dispersed phase consisted of a 1:1 v/v mixture of water and glycerol. These (glycerol + water)-in-oil ((g + w)/o) microemulsions were prepared by mixing a w/o microemulsion with an equal volume of g/o microemulsion in such a way that after mixing the volume of the dispersed phase remained unchanged. To achieve this, the R values used for both glycerol and water dispersions are related. For AOT-stabilized w/o microemulsions, rcore ) 0.175Rw, and for AOT-stabilized g/o microemulsions, rcore ) 0.88Rg, resulting in comparable droplet sizes when Rw/Rg ) 5.03. The w/o microemulsion was therefore prepared with Rw ) 5.03Rg in order to maintain essentially the same volume fraction of dispersed phase after mixing the two solutions. The majority of experiments were performed with Rg ) 3, so that the volume fraction of glycerol was ∼3%. This would be the case for “mixed” systems as well. Preparation of CTAB-Stabilized Dispersions Containing the Reactants M2+ and Mu-. Glycerol stock solutions were prepared as described previously. Both glycerol and surfactant were weighed into a volumetric flask before making up to the mark with a 1:1 v/v n-heptane/chloroform mixture. Sonication and vigorous shaking of microemulsions containing murexide again produced clear solutions within minutes. However, complications arose in the preparation of dispersions containing metal ions within the CTAB-stabilized glycerol droplets. The lower phase transition temperature (LPTT) in CTAB/glycerol/n-heptane-chloroform (1:1, v/v) microemulsions at Rg ) 3 is close to room temperature (cf. 21 °C).3 Addition of nickel to the system changes the phase diagram in such a way that dispersions containing concentrated Ni2+ are no longer single phase at 25 °C. It was, therefore, only possible to make optically clear systems at room temperature using low concentrations of Ni2+, where
(23) Fletcher, P. D. I.; Perrins, N. M.; Robinson, B. H.; Toprakcioglu, C. In Reversed-Micelle-Technological and Biological Relevance; Luisi, P. L., Straub, B., Eds.; Plenium Press: New York, 1984; p 69.
(24) Mead, J. Ph.D. Thesis, University of Kent at Canterbury, U.K., 1985. (25) Knoche, W.; Rees, N. H. J. Chem. Educ. 1984, 61 (8), 726.
Kos
(H2O)6M2+ + NN y\z kex
(H2O)6M+/NN (H2O)5M2+NN y\ + z kdiss outer-sphere complex monodentate complex krc
z (H2O)4M2+NN + H2O (6) H2O y\ k-tc bidentate complex where NN represents PADA. Because PADA is a bidentate ligand, a ring-closure step is again necessary for the formation of the final product. The rate constant, krc, for this step is usually assumed to be greater than that of the dissociation of the monodentate complex, kdiss, so that the monodentate complex is not present in significant concentrations at final equilibrium after the reaction. For their experiments in bulk glycerol, Caldin and Grant9 suggested that “some property” of the solvent, rather than of the metal ion, was rate limiting. They concluded that the rate-determining step contained contributions from both “geometrical” factors governing the metal ion/PADA complex and “hindered” ring closure. In bulk glycerol, the following reaction scheme4 for metal ion complexation with a bidentate ligand LL (omitting charges) was proposed, with glycerol acting as a tridentate ligand in the absence of LL:
G′ and G′′ represent reduced coordination by glycerol on binding of LL. Because the solvent-exchange behavior is not expected to be different for bulk and dispersed media, it is a reasonable assumption that in the case of dispersed glycerol the metal ion solubilized within the glycerol droplet will retain some complexing with glycerol. In the case of a tridentate reactant like murexide, it is likely that a glycerol molecule will be totally substituted in the final step. We have previously studied the kinetics of metal-ligand substitution reactions in microemulsions21,22 and compared the results with those obtained in bulk water. An important general point which emerges is that if reaction step 2 in eq 3 is much slower than the mass transport step 1 bringing the reactants together, the dispersed phase of the microemulsion can then be regarded as a pseudo continuous phase for the purposes of a kinetic analysis. In the microemulsion, the initial state of the reactants is a situation where they are encapsulated in separate droplets, but this communication process to locate the reactants in the same droplet has already been shown to be relatively rapid and so is not to be rate-limiting in a water dispersion.22 Materials and Methods
Kinetic Studies in Glycerol-in-Oil Microemulsions
Langmuir, Vol. 15, No. 15, 1999 5059
[Ni2+]g e 10-3 mol dm-3. The LPTT increased for higher concentrations of nickel ion, in such a way that glycerol droplets containing [Ni2+]g ) 0.1 mol dm-3 could not be dispersed in CTABstabilized microemulsions at temperatures below 40 °C. Similar problems had to be overcome with solutions containing Co2+ in the dispersed glycerol phase. In contrast, microemulsions containing Zn2+ in the glycerol phase formed easily at 20 °C. Spectrophotometric measurements showed that λmax for murexide in the CTAB-stabilized g/o microemulsions shifts to 530 nm, in the opposite direction to that observed with AOT, suggesting that the charge of the surfactant counterion in the core of the microemulsion affects the spectrum. Microemulsions were found to be stable over long periods of time, but decomposition of murexide (again in contrast to AOT) caused some difficulties in the CTAB systems. Fresh solutions containing murexide were therefore prepared daily and used within 12 h. Preparation of AOT-Stabilized g/o Microemulsions Containing PADA. PADA21,22 is at least a factor of 12 more soluble in n-heptane than in water. Therefore, g/o microemulsions were prepared by dispersing “empty” glycerol droplets in an oil continuous solution, which already contained a known concentration of PADA in the n-heptane phase. The metal-containing solutions were prepared as described previously. Because the metal ion has no solubility in oil, PADA was expected to partition between the oil and glycerol phases and react with the metal ion preferentially in a region associated with the glycerol droplet/ surfactant interface, because this is the preferred location of this ligand. The cationic reactant will also locate to the interface, when AOT is the surfactant. Stock solutions of PADA in n-heptane were freshly prepared each day because of a slight decrease in absmax of PADA, which was observed on leaving solutions overnight. λmax of PADA shifts by about 27 nm to 413 nm in AOT-stabilized g/o microemulsions as compared with bulk aqueous solution (λmax ) 440 nm in water). The “Ni2+PADA” complex had a λmax of 540 nm, similar to that in water. The kinetics were followed at 540 nm. CTAB-Stabilized g/o Microemulsions Containing PADA. The oil continuous phase of the CTAB-stabilized systems consisted of a 1:1 (v/v) mixture of n-heptane and chloroform. The solubility of PADA increases with increasing chloroform content in the liquid mixture. It was therefore expected that PADA would partition much more strongly into the oil pseudophase when chloroform was present. Mechanism of Metal Ion/PADA Complexation in g/o Microemulsions. The M2+PADA complex, being charged, is only expected to be formed in the glycerol droplet. The likely mechanism is therefore as follows:
s-1), and the dissociation rate constant, kb/s-1, are given by the following expressions:
kf ) k2/{1 + (Kass[AOT])-1}
(12)
kb ) k-2
(13)
and
There are two extreme cases which can be considered: (a) All of the PADA is in the interfacial region of the glycerol phase: Then [PADA]oil ) 0; and hence Kass f ∞. Therefore, kf ) k2, and we have the same case as that for murexide. (b) All of the PADA is in the oil phase: [PADA]int ) 0; hence, Kass ) 0. Therefore, kf ) 0. In the n-heptane/chloroform (1:1, v/v) mixtures of the CTABstabilized g/o microemulsions, no metal ion/PADA complexation was observed. This suggests that PADA had, effectively, totally partitioned into the oil continuous phase (i.e., case b above). Unfortunately, it was not possible to reduce the chloroform concentration to decrease the solubility of PADA in the continuous phase, because at lower percentages of chloroform the stability map of the single-phase g/o microemulsions shifts to lower temperatures, making it impossible to prepare thermodynamically stable glycerol dispersions in n-heptane/chloroform (>1:1, v/v) mixtures. For AOT-stabilized g/o microemulsions, with [AOT]ov ) 0.1 mol dm-3, in which the oil phase always consisted of 100% n-heptane, it was possible to measure the association constant, Kass, using spectrophotometric data. The value of Kass at 25 °C was found to be 21 ( 3 dm3 mol-1. Because all of the kinetic measurements were carried out in systems prepared using [AOT]ov ) 0.1 mol dm-3, it was possible to calculate k2 simply by using eq 12, because kf ) k2/1.48. (Note that the subscript “ov” relates to an overall concentration of the surfactantsi.e., moles of surfactant per cubic decimeters of the dispersion medium.)
Results Equilibrium Measurements. Spectrophotometric Determination of the Equilibrium Constant, Kspec, for Metal Ion/Murexide Complexation in g/o Microemulsions. The spectrophotometric data for M2+/Mucomplexation were always consistent with the following simple stoichiometric reaction: Kspec
M2+ + Mu- y\z (MMu)+
(14)
so that, ignoring activity coefficients: where KP is a dimensionless equilibrium constant for the partitioning of PADA between the oil and the glycerol (g) phases.
KP ) [PADA]g/[PADA]oil
(9)
However, it is possible to define KP more explicitly and relate it to an association constant for binding, Kass, of PADA to the interfacial region of the glycerol droplet:
PADAoil + AOT T PADAint
(10)
Kass/(dm3 mol-1) ) [PADA]int/([PADA]oil[AOT])
(11)
Then
and [PADA]int is essentially equivalent to [PADA]g. Following the approach discussed previously by Fletcher and Robinson,21,22 it follows that the “overall” second-order rate constant for the formation of the M2+PADA complex, kf/(dm3 mol-1
Kspec ) [MMu+]/([M2+][Mu-])
(15)
The spectra are very similar for all of the metal ions and show a clear isosbestic point. Representative data for Zn2+/Mu- complexation are shown in Figure 2. (At pH > 9 in water, deprotonation of the (MMu)+ complex occurs and the isosbestic point is lost.) Values of Kspec at 25 °C for the AOT systems were obtained, in general, from gradients of plots of (AbsT Abs0)/[M2+]T versus AbsT, where AbsT is the total absorbance, Abs0 is the initial absorbance (without added metal ion) at a given wavelength, and [M2+]T is the total metal ion concentration in the dispersed phase (in all cases, [M2+]T . [Mu-]). The same values for Kspec were obtained from analysis at different wavelengths. These values are compared, in Table 1, with equilibrium constants obtained from the kinetic data for the same metal ion/murexide complexation in the g/o microemulsions (Kkin ) kf/kb). The agreement is seen to be satisfactory. This is an important
5060 Langmuir, Vol. 15, No. 15, 1999
Figure 2. Spectrophotometric data for zinc ion/murexide complexation in g/o microemulsions: [AOT]ov ) 0.1 mol dm-3, [Mu-]g ) 5 × 10-4 mol dm-3, Rg ) 3.0, T ) 25 °C. (a) No Zn2+. (b) [Zn2+]g ) 10-3 mol dm-3. (c) [Zn2+]g ) 2 × 10-3 mol dm-3. (d) [Zn2+]g ) 5 × 10-3 mol dm-3. (e) [Zn2+]g ) 10-2 mol dm-3.
consistency check (see the next section) and confirms that the reaction is uncomplicated, as proposed in eq 14. Kinetic Studies. (a) Metal Ion/Murexide Complexation in g/o and (g + w)/o Microemulsions Stabilized by AOT. The experimental rate constants, kobs/s-1, were determined under pseudo-first-order conditions (i.e., [M2+] > [Mu-]), and second-order rate constants were derived from the slopes of the plots of kobs versus [M2+]g plots. The concentrations used in the analysis, both reactant and droplet concentrations, are chosen such that the droplets do not contain (on average) more than one ion of the reactants, although this is not a necessary requirement. The droplet concentrations in all experiments ranged from 4.7 × 10-4 to 4.2 × 10-3 mol dm-3 of the dispersion medium, so the average occupancy by the metal ion was less than unity. Rate and equilibrium constants for metal ion/murexide complexation are given in Table 1. Complexation between metal ions and murexide in dispersed glycerol shows the same trend, in terms of variation of the metal ion, to that in bulk aqueous solution (in that the rate constants are in the sequence Zn2+ > Co2+ > Ni2+). The results (a typical study being shown in Figure 3) clearly show that the complex formation reaction is slowest in the case of Ni2+ (by a factor of 70 with respect to Co2+ and 160 with respect to Zn2+), indicating that the nature of the metal ion has a large effect on the reaction kinetics. The kinetic and equilibrium spectrophotometric data are reasonably consistent in the computed values of Kspec and Kkin. The following experimental variations have been made to the microemulsions: (a) changing droplet size, by varying Rg but keeping the overall concentration of AOT, [AOT]ov, constant; (b) changing the droplet concentration, by changing the AOT concentration at fixed Rg. The results of following these procedures are indicated in Table 2. The kinetics depend significantly on the size of the droplets. However, no change in rate parameters was
Atay and Robinson
observed on changing the droplet concentration as long as the droplet size (Rg) remained unchanged. Complexation with Ni2+ was studied in greater detail in glycerol + water mixtures dispersed in oil. The results for Ni2+/Mu- complexation in “mixed” microemulsions are given in Table 3, with those measured in the corresponding bulk solvent given for comparison. The results in Table 3 indicate an increase in reaction rates as the water content of the glycerol + water, (g + w), dispersed phase increases. The rate constants measured in the dispersed glycerol + water phase are closer to those in the dispersed aqueous phase than in dispersed glycerol. The rate constants in all mixed systems are, in fact, the same, provided that a correction factor is included in the calculation of kf for the situation where the two reactants are initially in different phases. When Ni2+ is initially in the dispersed aqueous phase prior to mixing, the kf value needs to be multiplied by Vg+w/Vw (where V is the volume of the dispersed phase); if the metal ion is initially in the glycerol phase, then the factor is Vg+w/Vg. When this correction is applied, the kf/(dm3 mol-1 s-1) values for the complexation reactions in all three systems turn out to be 291, 290, and 293, with the average kf being 290 ( 3 dm3 mol-1 s-1. (Strictly speaking, the concentrations should be adjusted rather than the rate constants. However, the conclusion is the same. We thank a reviewer for pointing this out.) The activation energy, EA, and the transition theory parameters ∆Gq, ∆Hq, and ∆Sq for nickel ion complexation in g/o and (g + w)/o microemulsions are given in Table 4, and those for the (NiMu)+ dissociation reaction are given in Table 5. Values of EA are obtained from a plot of ln k versus T (K)-1; ∆Sq was calculated from eq 16
∆Sq ) R{ln(k/T) + ∆Hq/RT - ln(kB/h)}
(16)
∆Gq from eq 17
∆Gq ) ∆Hq - T∆Sq
(17)
and ∆Hq from eq 18
∆Hq ) EA - RT
(18)
The value of EA for a diffusion-controlled rate process in neat glycerol can be calculated and is +53 kJ mol-1. (The calculation is based on the temperature dependence of the viscosity of the solvent.) Values of EA are large enough to indicate that complexation/dissociation reactions are not diffusion-controlled in these glycerol-based microemulsions. (b) Metal Ion/Murexide Complexation in g/o and (g + w)/o Microemulsions Stabilized by CTAB. Kinetic measurements were only possible using Ni2+ as the metal ion, because in these systems both Zn2+ and Co2+ react with murexide at rates too fast to be followed by the stopped-flow method. The kinetics of complexation in g/o microemulsions were followed at 35 °C because of the problem already mentioned of stabilizing nickel ioncontaining microemulsions at room temperature. The kinetics were again followed under pseudo-first-order conditions (i.e., [Ni2+]g . [Mu-]g). The reaction was also followed in the following media at the same temperature: (a) both reactants in a glycerol + water phase of (g + w)/o microemulsions (as for the AOT system discussed above), (b) nickel ion in the aqueous phase of a w/o microemulsion and murexide in the glycerol phase of a g/o microemulsion and (c) both reactants in the dispersed water phase of a w/o microemulsion.
Kinetic Studies in Glycerol-in-Oil Microemulsions
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Table 1. Rate and Equilibrium Constants for Metal Ion/Murexide Complexation in AOT-Stabilized g/o Microemulsions at Rg ) 3.0 ([AOT]ov ) 0.1 mol dm-3, [Mu-]g ) 5 × 10-4 mol dm-3, [M2+]g ) (5-20) ×10-3 mol dm-3, T ) 25 °C) M2+
kf/(dm3 mol-1 s-1)
kb/s-1
Kkin/(dm3 mol-1)
Kspec/(dm3 mol-1)
Zn2+
3200 ( 100 1370 ( 40 20.3 ( 0.5
5.0 ( 0.5 3.5 ( 0.5 0.02 ( 0.01
640 ( 80 390 ( 70 1015 ( 130
440 ( 40 380 ( 40 1180 ( 160
Co2+ Ni2+
emulsions for the metal ions Ni2+ and Co2+, at fixed [AOT] and Rg. Reactions with Zn2+ were too fast to study by the stopped-flow method. The reactions were again followed under pseudo-first-order conditions; i.e., [M2+] . [PADA]. The second-order complex formation rate constants, obtained from the gradients of kobs versus [M2+]g plots, are indicated in Table 8, where kf ) k2/1.48, as shown previously. Discussion
Figure 3. Effect of metal ion on observed rate constants, kobs, for metal ion/murexide complexation in g/o microemulsions: [AOT]ov ) 0.1 mol dm-3, [Mu-]g ) 5 × 10-4 mol dm-3, Rg ) 3.0, T ) 25 °C. Table 2. Effect of Droplet Size (Rg) and Droplet Concentration ([AOT]ov) on Reaction Kinetics for Metal Ion/Murexide Complexation in AOT-Stabilized g/o Microemulsions ([Mu-]g ) 5 × 10-4 mol dm-3, [M2+]g ) (5-50) × 10-3 mol dm-3, [AOT]ov ) 0.1 mol dm-3, T ) 25 °C) M2+
Rg
kf/(dm3 mol-1 s-1)
Zn2+
1.0 2.0 3.0 1.0 2.0 3.0 1.0 1.5 2.0 3.0 3.0b 3.0c
933 ( 40 2250 ( 70 3200 ( 100 395 ( 15 765 ( 50 1370 ( 40 8.5 ( 0.3 11.8 ( 0.4 15.0 ( 0.4 20.3 ( 0.5 20.3 ( 0.5 20.3 ( 0.5
Co2+ Ni2+
kb/s-1 a 5.0 ( 0.5 3.5 ( 0.5 0.02 ( 0.01
Kkin/(dm3 mol-1) 190 ( 30 450 ( 60 640 ( 80 113 ( 20 220 ( 40 390 ( 70 425 ( 60 590 ( 80 750 ( 95 1015 ( 130 1015 ( 130 1015 ( 130
a Result for all R . b [AOT] -3 c g ov ) 0.05 mol dm . [AOT]ov ) 0.2 mol dm-3.
The kinetic results are indicated in Table 6. The energy parameters for nickel ion/murexide complexation and dissociation reactions in CTAB-stabilized (g + w)/o microemulsions are indicated in Table 7. (c) Studies with PADA in g/o Microemulsions Stabilized by AOT. The kinetics of metal ion/PADA complexation were studied in AOT-stabilized g/o micro-
AOT System. In w/o microemulsions, fast reactant exchange (communication) between the water pools is followed by the actual reaction within a particular droplet. The mechanism and energetics of metal ion/murexide complexation within a dispersed water droplet were found to be similar to those observed in bulk aqueous solution, i.e., rapid communication between the droplets, followed by rapid outer-sphere complex formation within the droplet, and then the rate-determining transition from the outer-sphere complex to the inner-sphere complex, associated primarily with loss of water from the innercoordination sphere of the metal ion. (The two reactants will have located to the same droplet by a process involving fast fusion/fission of the droplets.) The observation that kf again primarily depends on the nature of the metal ion, i.e., the reaction rates follow the order Ni2+ < Co2+ < Zn2+, further supports the assertion that reactant transfer into and within the droplets is not rate-determining, nor is ring closure. The kf values for complexation of metal ions with murexide in bulk water,26-29 in dispersed water,24 and in dispersed glycerol are compared in Table 9. The kinetic results show that metal ion/murexide complexation in the glycerol phase of g/o microemulsions proceeds in a broadly similar way (with respect to a comparison of metal ion reactivity) to bulk and dispersed water. In bulk water, murexide complexes with zinc approximately 7000 times faster than with nickel.11,26-29 However, in g/o microemulsions this rate enhancement factor is only 160, so the differences in rates of complexation for different metal ions are not as pronounced in dispersed glycerol; however, the trend is still the same. The second-order rate constant for reactant transfer between glycerol droplets, in n-heptane stabilized with AOT, has been previously measured as 2 × 106 dm3 mol-1 s-1 30 at 25 °C (here the concentration term refers to the droplets) and is essentially independent of the (small) chemical species being transferred. This rate constant is somewhat less than that for reactant transfer between dispersed water droplets (cf.1.4 × 107 dm3 mol-1 s-1 at 25 °C).31 It is, however, sufficiently fast for the actual metal complexation step inside a droplet to be rate-limiting, rather than the communication step. It is known that (26) Geier, G. Ber. Bunsen-Ges. Phys. Chem. 1965, 69, 617. (27) Lin, C. T.; Bear, J. L. J. Phys. Chem. 1971, 75, 3705. (28) Jost, A. Ber. Bunsen-Ges. Phys. Chem. 1975, 79, 850. (29) Hewkin, D. J.; Prince, R. H. Coord. Chem. Rev. 1970, 5, 45. (30) McDonald, J. A., University of East Anglia, unpublished results. (31) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 985.
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Atay and Robinson
Table 3. Nickel Ion/Murexide Complexation in Various AOT-Stabilized Microemulsions and Bulk Reaction Media ([Mu-] ) 5 × 10-4 mol dm-3, [Ni2+] ) (5-50) × 10-3 mol dm-3, [AOT]ov ) 0.1 mol dm-3, Rg ) 3.0 and Rw ) 15.1, T ) 25 °C) reaction medium
kf/(dm3 mol-1 s-1)
kb/s-1
Kkin/(dm3 mol-1)
and in the glycerol phase of g/o microemulsions Ni2+ in the aqueous phase of w/o and Mu- in the glycerol phase of g/o microemulsions 2+ Ni and Mu in the glycerol + water phase of (g + w)/o microemulsions Ni2+ in the glycerol phase of g/o and Mu- in the aqueous phase of w/o microemulsions Ni2+ and Mu- in the aqueous phase of w/o microemulsions Ni2+ and Mu- in bulk glycerol + water, containing (a) 0.3 mol dm-3 NaCl and (b) 0.5 mol dm-3 NaBra Ni2+ and Mu- in bulk water, containing 0.5 mol dm-3 NaCla
20.3 ( 0.5 322 ( 30 293 ( 30 261 ( 30 392 ( 30 (a) 1180 ( 40 (b) 1146 ( 85 1290 ( 50
0.02 ( 0.01 1.0 ( 0.1 1.0 ( 0.1 1.0 ( 0.1 3.0 ( 0.1 (a) 3.0 ( 0.1 (b) 3.0 ( 0.1 3.0 ( 0.1
1015 ( 130 322 ( 60 293 ( 60 261 ( 60 131 ( 15 (a) 393 ( 30 (b) 382 ( 30 430 ( 30
Ni2+
a
Mu-
NaCl and/or NaBr were added to the bulk solvents to mimic the high ionic strength environments within the droplets. Table 4. Energy Parameters for the Nickel Ion/Murexide Complexation in AOT-Stabilized g/o and (g + w)/o Microemulsions (T ) 25-35 °C for Calculations of EA; T ) 25 °C for Calculations of ∆Hq, ∆Gq and ∆Sq) reaction medium
EA/(kJ mol-1)
∆Hq/(kJ mol-1)
∆Sq/(J K-1 mol-1)
∆Gq/(kJ mol-1)
AOT-stabilized g/o microemulsions AOT-stabilized (g + w)/o microemulsions
79 ( 5 44 ( 1
77 ( 5 41 ( 1
38 ( 5 -60 ( 6
66 ( 4 59 ( 6
Table 5. Energy Parameters for the Nickel Ion/Murexide Dissociation Reaction in AOT-Stabilized g/o and (g + w)/o Microemulsions (T ) 25-35 °C for Calculations of EA; T ) 25 °C for Calculations of ∆Hq, ∆Gq, and ∆Sq) reaction medium
EA/(kJ mol-1)
∆Hq/(kJ mol-1)
∆Sq/(J K-1 mol-1)
∆Gq/(kJ mol-1)
AOT-stabilized g/o microemulsions AOT-stabilized (g + w)/o microemulsions
114 ( 23 111 ( 35
112 ( 22 109 ( 32
98 ( 34 121 ( 47
83 ( 13 73 ( 7
Table 6. Nickel Ion/Murexide Complexation/Dissociation Rate Constants in Various CTAB- and AOT-Stabilized Microemulsions ([Mu-] ) 5 × 10-4 mol dm-3, [Ni2+] ) (0.5-50) × 10-3 mol dm-3, [CTAB]ov ) 0.1 mol dm-3, Rg ) 3.0 and Rw ) 15.1, T ) 35 °C) reaction medium
kf/(dm3 mol-1 s-1)
kb/s-1
Ni2+ and Mu- in the glycerol phase of CTAB-stabilized g/o microemulsions Ni2+ and Mu- in the glycerol phase of AOT-stabilized g/o microemulsions Ni2+ and Mu- in the glycerol + water phase of CTAB-stabilized (g + w)/o microemulsions Ni2+ and Mu- in the glycerol+water phase of AOT-stabilized (g + w)/o microemulsions Ni2+ and Mu- in the aqueous phase of CTAB-stabilized w/o microemulsions Ni2+ and Mu- in the aqueous phase of AOT-stabilized w/o microemulsions19
1620 ( 20 57.5 ( 1.0 2630 ( 158 530 ( 4 2630 ( 158 1287 ( 24
0.01a 0.12 ( 0.01 6.5 ( 0.1 3.5 ( 0.1 6.5 ( 0.1 7.9 ( 0.5
a The equilibrium constant (K spec) for the reaction taking place in dispersed glycerol (under the same experimental conditions), obtained from spectrophotometric data, was found to be 1.5 × 105 dm3 mol-1. Using this value and the forward rate constant, the rate constant for dissociation, kb/s-1, was calculated.
Table 7. Energy Parameters for the Nickel Ion/Murexide Complexation and Dissociation Reactions in CTAB-Stabilized (g + w)/o Microemulsions (T ) 25-35 °C for calculations of EA; T ) 25 °C for calculations of ∆Hq, ∆Gq, and ∆Sq) reaction
EA/(kJ mol-1)
∆Hq/(kJ mol-1)
∆Sq/(J K-1 mol-1)
∆Gq/(kJ mol-1)
nickel ion/murexide complexation nickel ion/murexide dissociation
33 ( 1 72 ( 3
31 ( 1 70 ( 3
-89 ( 8 0.42 ( 0.03
57 ( 4 70 ( 3
Table 8. Rate Constants for Metal Ion/PADA Complexation in AOT-Stabilized g/o Microemulsions at Rg ) 3.0 ([AOT]ov ) 0.1 mol dm-3, [PADA]ov ) 10-5 mol dm-3, [Ni2+]g ) (1-5) × 10-2 mol dm-3, [Co2+]g ) (1-10) × 10-3 mol dm-3, T ) 25 °C) metal ion
kf/(dm3 mol-1 s-1)
k2/(dm3 mol-1 s-1)
kb/s-1
Co2+ Ni2+
2820 ( 186 20.7 ( 0.8
4175 ( 275 30.6 ( 1.2
6.2 ( 0.2 0.07 ( 0.01
lateral diffusion within the AOT surfactant monolayer is only decreased 2-fold when water is replaced by glycerol, so that the surfactant does not experience the full effect of the change in viscosity, and the exchange rate constant is similarly not much affected.32 Similar observations have been made for micellar solutions in glycerol.33 As in bulk aqueous and w/o microemulsion solutions, the hypothesis that “desolvation” of the metal ion is ratedetermining in the glycerol system is further supported by the observation that the value of kf measured for each metal ion is substantially different. (32) Fletcher, P. D. I.; Robinson, B. H.; Tabony, J. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2311. (33) Fletcher, P. D. I.; Gilbert, P. J. J. Chem. Soc., Faraday Trans. 1 1989, 85, 147.
Table 9. Values of kf for Metal Ion/Murexide Complexation in Bulk and Dispersed Systems ([Mu-] ) 5 × 10-4 mol dm-3, [M2+] ) (5-50) × 10-3 mol dm-3, [AOT]ov ) 0.1 mol dm-3, Rg ) 3.0 and Rw ) 15.1, T ) 25 °C) metal ion
kf/(dm3 mol-1 s-1) in bulk water26-29
Zn2+ Co2+ Ni2+
2 × 107 1.6 × 105 3 × 103
kf/(dm3 mol-1 s-1) in AOT-stabilized w/o microemulsion (kobs > 103 s-1)12 392 ( 30
kf/(dm3 mol-1 s-1) in AOT-stabilized g/o microemulsion 3200 ( 100 1370 ( 40 20.3 ( 0.5
Metal Ion/Murexide Complexation in Mixed (Glycerol + Water) Systems. In the case of (g + w)/o microemulsions, there are three possible reaction environments where metal ion/murexide complexation can take place: (a) In a water droplet following murexide transfer:
Kinetic Studies in Glycerol-in-Oil Microemulsions
(b) In a glycerol droplet following metal ion transfer
(c) In a mixed glycerol + water environment
In all of the cases shown above, the reactants are initially in different “neat” dispersed phase media. If the reaction takes place in an aqueous environment following Mu- transfer into a water pool without solvent mixing, then the forward rate constant, kf, might be expected to be similar to that measured in a w/o microemulsion. If, however, the reaction takes place in an allglycerol environment, the measured kf should be the same as that observed in g/o microemulsions. The other possibility, and a likely one based on our knowledge of communication dynamics in microemulsions, is shown in environment c. In this scenario, in which the reactants are initially in different solvent environments, when the two solutions are mixed, a dispersed phase consisting of a mixture of 1:1 (v/v) glycerol/water is rapidly formed as a result of the fast fusion/fission of the droplets. Complexation will then take place in a mixed environment, and the rate constant is then expected to be close to that observed in (g + w)/o microemulsions. This is, in fact, what happens. A rapid mixing of the aqueous and glycerol droplets takes place, and the result is the formation of transparent, stable (g + w) droplets dispersed in n-heptane prior to the ligand being bound. The complexation reaction then takes place in this mixed-solvent environment. Values of kf measured in mixed glycerol + water droplets are higher (by a factor of 14.3) than those in g/o microemulsions but still lower than those in w/o microemulsions. The metal ion/murexide complexation in (g + w)/o microemulsions will proceed through a mechanism similar to that observed in aqueous solution: rapid outersphere complex formation, followed by rate-determining desolvation of the metal ion. In the case of (g + w)/o microemulsions, the solvation of the reactants is likely to be complex, with preferential solvation of the divalent metal ion likely to play a role. The metal ion might be expected to be preferentially solvated by water, although not exclusively so. This could explain why the measured kf is closer to that in water than that in glycerol. Droplet Size Effect on Metal Ion/Murexide Complexation Reactions. For all metal ion/murexide complexation reactions studied in AOT-stabilized g/o microemulsions, kf increases with increasing Rg, i.e., increasing droplet size. If the effect of the “limited” volume of glycerol was of no importance in the kinetics, then kf should be independent of the droplet size, because the rate constant is defined in terms of concentrations in the pseudo glycerol phase. This, however, is not observed. Because kf is
Langmuir, Vol. 15, No. 15, 1999 5063
dependent upon Rg, and kf ) Koskex, it would appear that either Kos or kex (or both) is changed by changing the droplet size. It is unlikely that kex will be affected by the change in Rg, so the best approach to interpreting the data is in terms of fast “internal partitioning” of the reactants inside the glycerol droplets. This concept of “partitioning” has previously been used successfully to explain the variation of rate constants with composition for nickel ion/murexide complexation in micellar13 and w/o microemulsion24 systems. However, there is no quantitative theory presently available to calculate ion distributions inside the nanoglycerol droplets. A glycerol droplet may be considered to consist of an interface and a core region; it is also reasonable to assume that the surfactant headgroups, together with their specifically adsorbed counterions, form the “Stern” layer at the interface. In the case of AOT, the sulfonate headgroups at the glycerol/surfactant interface are negatively charged. This negative charge will affect the distribution of the ions within the droplet; charged surfaces in a lower dielectric constant medium such as glycerol are expected to show a higher tendency to adsorb counterions as compared with water, especially those with a high charge number. Therefore, in an AOT-stabilized g/o microemulsion the doubly charged metal ions such as Zn2+, Co2+, and Ni2+ would be expected to be mainly situated at the interface rather than in the core. On the other hand, ions with the same charge as the surface would be repelled from the interface into the core. Murexide, being both hydrophilic and negatively charged, would be expected to preferentially locate to a “bulklike” glycerol environment, which is more likely to be found in the core of the droplet, away from the charged interface. Measured rate constants in the dispersed phases are somewhat lower than those in bulk solutions, and this is likely to be associated with this unfavorable internal partitioning of the reactants, which in classical kinetics/thermodynamics would manifest itself as an activity coefficient correction. The droplet size will therefore affect the partitioning tendency of the reactants within the droplet; the smaller the droplet, the greater the tendency for the metal ion to partition to the interface. The kinetic results are in agreement with this in that the percentage of interfacebound nickel decreases as the droplet size increases, and hence an increase in the rate of complexation is observed. (The same trend was observed previously for reactions in water droplets.) In the case of PADA, the rate constants for complexation of the divalent metal ions nickel or cobalt with PADA are slightly faster than those with the hydrophilic ligand murexide. In the case of Ni2+, complexation with PADA is 1.5 times faster than with murexide (comparing kf(Mu-) with k2(PADA); Tables 3 and 8), so that overall the ligand dependence is not very significant, and it would appear that PADA is totally available for reaction at the interface. Also Co2+ complexes with PADA 140 times faster than Ni2+ but only 70 times faster when the ligand is murexide. A noticeable difference between the reactions observed with AOT-stabilized w/o and g/o microemulsions is the much lower value of kb in the case of dispersed glycerol systems. The difference is as high as a factor of 50 when the metal ion is Ni2+ (Table 6) and shows that the high viscosity of glycerol is an effective inhibitor of ring opening. CTAB System. The rate of the Ni2+/Mu- reaction in the CTAB-stabilized system is much faster than that in the AOT-stabilized system. In CTAB-stabilized g/o systems (Rg ) 3.0) Ni2+ complexes with Mu- ∼30 times faster than it does in AOT-stabilized g/o microemulsions. For a glycerol droplet stabilized by CTAB, the headgroups at the droplet
5064 Langmuir, Vol. 15, No. 15, 1999
surface are positively charged. Therefore, the positively charged nickel ions will be repelled by the surface and are expected to be preferably located in the “core” of the glycerol droplet. Although murexide is negatively charged, because its hydrophilicity and bulky structure, it is also expected to be situated in the core. Because of these increased “effective” concentrations of the reactants in this local reaction domain, there is a considerable rate enhancement in CTAB-stabilized g/o microemulsions when compared with the AOT-stabilized g/o microemulsions. This observation, therefore, provides further evidence, following ref 21, that partitioning effects can significantly change rates of complexation in microemulsions. The results in Table 6 also show that rates of metal ion/ligand complexation in the CTAB-stabilized systems decrease, as expected, when the reaction takes place in dispersed glycerol rather than in dispersed water. However, the rate enhancement observed in w/o microemulsions with respect to g/o microemulsions is relatively low, particularly when compared with systems stabilized by the anionic surfactant AOT. In CTAB-stabilized systems the complexation rate is only 1.6 times faster in the dispersed water than in the dispersed glycerol phase, with the rate enhancement being a factor of 20 when observed in AOT-stabilized systems. Differences between reaction rates in CTAB and AOT reverse micelles for other reactions have been observed previously.34
Atay and Robinson
The dissociation rate constant is slow compared to water and is similar in CTAB- and AOT-stabilized droplets, as might again be expected. The Ni2+/Mu- complexation reaction was also observed in bulk glycerol + water mixtures of high ionic strength. The purpose of this experiment was to observe what effect Br- might have on the system. In fact, the presence of Brin the system did not alter the reaction kinetics to a significant extent (Table 3). The effect of ethanol, which is present at 1.5% in CHCl3, was also assessed. It was suspected that ethanol would be located in the droplet core. When the volume percentage of ethanol in the chloroform, used in CTAB-stabilized microemulsions, was increased from 1.5% to 3%, no change in the reaction kinetics was noted. We therefore consider that ethanol plays no significant role in complexation. We have collected an extensive table of activation parameters from the temperature dependence of the kinetics. The activation energy is in all cases appreciable, with evidence of enthalpy-entropy compensation in the forward direction, and is inconsistent with a mechanism based on rate-limiting transport of the reactants, where the activation energy would be consistent with that for diffusion (19 kJ mol-1 in water and 53 kJ mol-1 in glycerol). LA9900122 (34) Goto, A.; Kishimoto, H. Bull. Chem. Soc. Jpn. 1989, 62, 2854.
