Langmuir 1997, 13, 1501-1509
1501
Extractability and Solubilization Locus of Six β-Diketones and Their Iron(III) Complexes in Triton X-100 Micellar Solutions Kazuho Inaba The National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305, Japan Received August 15, 1996. In Final Form: November 12, 1996X The distribution equilibria of six β-diketones and their iron(III) complexes between Triton X-100 micellar and bulk aqueous phases at 298 K were measured spectrophotometrically. The partition constant of the extractants and the complexes, and the extraction constant of iron(III) in the micellar system were obtained simultaneously by a successive approximation calculation on the basis of a simple two-phase partition model. The values of partition constants for the extractants and for the complexes were compared to those obtained in conventional hexane-water and diethyl ether-water systems. The solute-solvent interaction due to the solvation of the acidic proton with oxygen atoms in the polyoxyethylene moiety was estimated in the experiments for the extractants. The effect of steric hindrance in the polyoxyethylene layer due to the formation of a hydrogen-bonded framework in the region was also observed. Hence, the extractants having phenyl substituent groups were mainly distributed in the nonpolar “core” region of the micelle while those having no bulky group or having a polar group in the substituent would penetrate into the polar “mantle” region. The extractability of the complexes with the extractants having trifluoromethyl groups was lower than that without the substituent group. Water molecules which penetrated into the micelle and formed a hydrogen-bonded framework would disturb the solubilization of the complexes with ligands having trifluoromethyl substituents in the structural region due to the low wettability of the substituent.
Introduction Surfactant molecules in aqueous solution form micelles, and the solution can dissolve many water-insoluble solutes in the internal sphere of the aggregates.1-4 The solution effects a kind of solvent extraction, and moreover, the micellar solution system can be easily and safely handled, as no mechanical agitation nor centrifugation process is necessary and no toxic and/or flammable organic solvent is used. Hence, the use of micellar solutions instead of liquid-liquid extraction systems has been developed in many fields of chemistry, and such systems appear to be suitable for the extraction of metals with water-insoluble chelating reagents. Many studies, focusing not only on applications5-7 such as spectrophotometric determination or cloud-point separation of metals but also on fundamental considerations8-12 on the equilibria and kinetics of the extraction, have been reported. There are a number of factors which influence the extractability of solutes, with the locus of solutes in the micellar phase being one of the most important factors which affect both the equilibria and kinetics in such systems. However, sysX Abstract published in Advance ACS Abstracts, February 15, 1997.
(1) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975; Chapter 2. (2) Watanabe, H. In Solution Chemistry of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol II, p 1305. (3) Georges, J. Spectrochim. Acta Rev. 1990, 13, 27. (4) Szymanowski, J.; Tondre, C. Solvent Extr. Ion Exch. 1994, 12, 873. (5) Hayashi, K.; Sasaki, Y.; Tagashira, S.; Kosaka, E. Anal. Chem. 1986, 58, 1444. (6) Watanabe, H.; Saitoh, T.; Kamidate, T.; Haraguchi, K. Mikrochim. Acta 1992, 106, 83. (7) Paradkar, R. P.; Williams, R. R. Anal. Chem. 1994, 66, 2752. (8) Tagashira, S. Anal. Chem. 1983, 55, 1918. (9) Saitoh, T.; Kimura, Y.; Kamidate, T.; Watanabe, H.; Haraguchi, K. Anal. Sci. 1989, 5, 577. (10) Tondre, C.; Claude-Montigny, B.; Ismael, M.; Scrimin, P.; Tecilla, P. Polyhedron 1991, 10, 1791. (11) Muralidharan, S.; Yu, W.; Tagashira, S.; Freiser, H. Langmuir 1990, 6, 1190. (12) Cai, R.; Freiser, H.; Muralidharan, S. Langmuir 1995, 11, 2926.
S0743-7463(96)00813-X CCC: $14.00
tematic information on the characteristics of micellar extraction such as extraction capacity or locus of both extractant and complex species in micelles, which is indispensable in developing suitable analytical conditions, is presently rather limited. Nonionic surfactants having polyoxyethylene chains form pseudo-double-layer micelles: a nonpolar hydrocarbon “core” region which is formed by the alkyl component of the surfactant and a polar but uncharged “mantle” region by the polyoxyethylene chain.13,14 The two parts have quite different natures as organic diluent, and thus, the partition behaviors of both the extractants and the complexes may be changed by their locus in the micelle. The relationship between the extractability of metals and their locus in the micelle has recently been reported. The author and co-workers have studied the extraction equilibria and kinetics of several lanthanides (Ln(III)) with bis(2,4,4-trimethylhexyl)phosphinic acid (HR) into a Triton X-100 micellar phase.15 The extraction in a conventional liquid-liquid system has been examined by many researchers, and it has been found to occur with three molecules of monovalent dimer anion, HR2-, into chloroform and heptane,16-18 whereas we found that two different types of uncharged complex are extracted into the micelle, Ln(HR2)3, which was found in the ordinary extraction, and the species with two monovalent dimers and one monovalent monomer anion, LnR(HR2)2. Moreover, the extractability of these species is very high, being 104 to 105 higher for Ln(HR2)3 than that in the chloroformwater system. We concluded that the high extractability of the metals into a Triton X-100 micelle is due to an interaction of the complexes with the polyoxyethylene chains in the “mantle” region. The extraction of a series (13) Robson, R. J.; Dennis, E. A. J. Phys. Chem. 1977, 81, 1075. (14) Streletzky, K.; Phillies, G. D. J. Langmuir 1995, 11, 42. (15) Inaba, K.; Muralidharan, S.; Freiser, H. Anal. Chem. 1993, 65, 1510. (16) Komatsu, Y.; Freiser, H. Anal. Chim. Acta 1989, 227, 397. (17) Inaba, K.; Freiser, H.; Muralidharan, S. Solvent Extr. Res. Dev. Jpn. 1994, 1, 13. (18) Li, K.; Freiser, H. Solvent Extr. Ion Exch. 1986, 4, 739.
© 1997 American Chemical Society
1502 Langmuir, Vol. 13, No. 6, 1997
of organic compounds and metal chelates in a similar surfactant system has been examined by Saitoh et al.; it was concluded that the solutes are located in the “mantle” region of the micelle.19 An NMR approach for determining the locus of the solutes has been demonstrated using an 8-hydroxyquinoline derivative in n-dodecyloctaoxyethylene glycol monoether, and the locus of the ligand has been concluded to be in the “mantle” region while its zinc(II) complex resides in the “core” region.20 In these studies, however, most of the solutes have long alkyl chains or bulky substituent groups and the mobility or stability of the solutes in the micelle may be different from that of a small compound. The author has preliminarily reported that the extraction behaviors of two phenyl-substituted β-diketones, benzoylacetone and benzoyltrifluoroacetone, and their iron(III) complexes are very similar to those in the conventional hexane-water system.21 In the present study, the extraction equilibria of the other four β-diketones and their iron(III) complexes in Triton X-100 micellar solutions were measured. The extraction behaviors with the above two extractants were reexamined and their constants were recalculated. The results are compared to those in hexane and diethyl ether, which are analogs of the two parts of the micelle, to learn more fundamental information on the loci of the extractants and complexes in the micelle and on the factors that determine their position.
Inaba UV-160 spectrophotometer using a quartz cell with a 1- or 5-cm light path at the appropriate wavelength. The liquid-liquid extraction of iron(III) with β-diketones into hexane and diethyl ether was measured as follows. A certain volume of an acidic aqueous solution containing 1 × 10-4 M iron(III) and the same volume of an organic diluent containing 0.010.10 M of one of the β-diketones were taken into a stoppered glass tube and agitated vigorously by a mechanical shaker. After the extraction equilibrium was established, the solutions were centrifuged (2000 rpm × 5 min); then, the concentration of iron(III) in the organic phase was determined by spectrophotometry and that in the aqueous phase was determined by GFAAS. The hydrogen ion concentration of the resulting micellar or aqueous solution was measured potentiometrically by a Toa IM40S potentiometer using a glass electrode.
Theory In the present paper, the concentrations of the chemical species in the micellar phase are denoted by the subscript m, those in the bulk aqueous phase by no subscript, and those on the basis of total volume of the micellar solution by the subscript t. The concentrations in the organic phase in the case of conventional liquid-liquid extraction experiments are denoted by the subscript o. If whole distribution reactions in the micellar system can be assumed to be similar to those in the conventional liquid-liquid system, the following data treatment can be performed. The acid dissociation and partition constants for the β-diketones are expressed as
Experimental Section Reagents. All the reagents used were of analytical grade and were used without any further purification. The β-diketones (HA) used were acetylacetone (2,4-pentanedione; Haa), benzoylacetone (1-phenyl-1,3-butanedione; Hbza), dibenzoylmethane (1,3-diphenyl-1,3-propanedione; Hdbm), trifluoroacetylacetone (1,1,1-trifluoro-2,4-pentanedione; Htfa), benzoyltrifluoroacetone (1-phenyl-4,4,4-trifluoro-1,3-butanedione; Hbfa), and 2-thenoyltrifluoroacetone (1-(2-thienyl)-4,4,4-trifluoro-1,3-butanedione; Htta). Haa, Htfa, Hbfa, and Htta were supplied from Dojin Chemicals, Hbza was from Wako Pure Chemicals, and Hdbm was from Tokyo Chemicals. A nonionic surfactant, polyethylene glycol tert-octylphenyl ether (Triton X-100; Fluka, more than 99% pure), was used as the micelle-forming reagent. Two organic solvents, hexane and diethyl ether, were utilized as diluents in the liquid-liquid extraction experiments. Procedures. All experiments were carried out in a thermostated room at 298 ( 0.5 K. In order to avoid ion-pair extraction of charged metal species into the micellar phase, no additional salt was added to the solutions and the hydrogen ion concentration was controlled by sulfuric acid. The micellar extraction experiments were carried out as follows. The concentrations of Triton X-100 used in the present study were usually within the range 3.2 × 10-3 to 0.16 M (0.210.0%) except in the estimation of the partition constant of Fe(aa)3, where concentrations up to 0.318 M (19.9%) were used. The total metal concentration was kept lower than the number of micelles in the solution to avoid multiple occupation by an iron(III) complex in one micelle. An acidic micellar solution which contained 2 × 10-5 to 2 × 10-4 M iron(III) and 2 × 10-4 to 1 × 10-2 M of one of the extractants, whose concentrations were defined as for the total volume of the micellar solution, was kept standing until the complex formation equilibria were established. The resulting solution showed appropriate absorbance in the 400-500 nm range due to the formation of iron(III) complexes with β-diketones, and the absorbance was used for determining the concentration and distribution of the iron(III) complexes. The absorbance of the solution was measured by a Shimadzu (19) Saitoh, T.; Hoshino, H.; Yotsuyanagi, T. J. Chem. Soc., Faraday Trans. 1994, 90, 479. (20) McCulloch, J. K.; Fornasiero, D.; Perera, J. M.; Murray, B. S.; Stevens, G. W.; Grieser, F. J. Colloid Interface Sci. 1993, 157, 180. (21) Inaba, K. In Value Adding Through Solvent Extraction; Shallcross, D. C., Paimin, R., Prvcic, L. M., Eds.; University of Melbourne: Melbourne, 1996; Vol I, p 57.
Ka )
[H+][A-] [HA]
Kd )
(1)
[HA]m
(2)
[HA]
The volume fractions of the micellar and bulk aqueous phases, Vm and V, can be written as
Vm ) φ(CTriton X-100 - Ccmc)
(3)
V ) 1 - Vm
(4)
Here, φ is the molar volume of the micelle (1.29 M-1),5 and CTriton X-100 and Ccmc are the total concentration of Triton X-100 and the critical micelle concentration of the surfactant (2.4 × 10-4 M),22 respectively. Using these equilibrium constants and the volume fractions, the mass balance of the β-diketone in the whole micellar system can be expressed as
[HA]t ) [HA]mVm + [HA]V + [A-]V )
{
}
(KdVm + V)[H+] + V [A-] (5) Ka
Since no additional salt is dissolved in the solution under the present experimental conditions, the charged metal species in the bulk aqueous phase (i.e., Fe3+, FeA2+, and FeA2+) are not extracted into the micellar phase. Thus, the distribution ratio of iron(III) between the micellar and the bulk aqueous phases can be expressed as
D)
[Fe(III)]m [Fe(III)]
[FeA3]m )
3+
2+
[Fe ] + [FeA ] + [FeA2+] + [FeA3]
(6)
Using equilibrium constants of the ligand and the complex species, the equation can be rewritten as (22) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
Six β-Diketones and Their Iron(III) Complexes D)
KexKd3[A-]3 Ka3(1 +
∑β [A ] ) - n
Langmuir, Vol. 13, No. 6, 1997 1503
β3Kdm[A-]3 ) 1+
n
(7)
∑β [A ]
- n
n
where βn is the stability constant of the nth complex in the aqueous phase, Kex is the extraction constant, and Kdm is the partition constant of the tris complex defined by
βn )
Kex )
[FeAn3-n]
(8)
[Fe3+][A-]n [FeA3]m[H+]3
(9)
[Fe3+][HA]m3
Kdm )
[FeA3]m
(10)
[FeA3]
As seen in eq 7 the distribution ratio, D, is dependent on the ligand anion concentration in the bulk aqueous phase, and [A-] is dependent on the volume fractions, Vm and V (see eq 5). The values of the constants, Kd, Kdm, and Kex, can be calculated by a successive approximation technique on the basis of eqs 5 and 7 using data obtained with several concentrations of Triton X-100. The concentrations of iron(III) in both phases, which are necessary to calculate the distribution ratio, can be obtained as follows. The absorbance of the whole micellar solution at a certain wavelength can be expressed by the sum of the absorbance of the metal species in the solution as
At ) 0V[Fe3+] + 1V[FeA2+] + 2V[FeA2+] + 3V[FeA3] + 3+
2+
3mVm[FeA3]m ) 0[Fe ]t + 1[FeA ]t + 2[FeA2 ]t + 3[FeA3]t + 3m[FeA3(m)]t (11) +
where n is the molar absorptivity for the nth complex based on the total volume of the solution. Using the stability constant of the complex species and the partition constant of the tris complex, Kdm, the equation can be rewritten as
At [Fe(III)]t
)
0 + 1β1[A-] + 2β2[A-]2 + 3β3[A-]3 + 3mβ3Kdm[A-]3 1 + β1[A-] + β2[A-]2 + β3[A-]3 + β3Kdm[A-]3
(12)
As seen in eq 5, the ligand anion concentration, [A-], depends on the volume fractions. The constants in the equations can be calculated using successive approximations, and thus, the concentration of the species in both phases can be determined.
Results and Discussion Determination of the Acid Dissociation Constants of β-Diketones and Their Liquid-Liquid Partition Constants Using a Two-Phase Distribution Method. The acid dissociation and the liquid-liquid partition constants of the β-diketones were measured by a liquidliquid distribution method using hexane-water and diethyl ether-water systems. The distribution ratio of an extractant, DHA, can be defined as the ratio of the concentration of extracted HA to the sum of the concentrations of the extractant in the aqueous phase presented in both undissociated and dissociated forms. Thus, the distribution ratio of the extractant is dependent on the hydrogen ion concentration of the aqueous phase as23
DHA )
Kd[H+]
[HA]o ) [HA] + [A-]
[H+] + Ka
(13)
Figure 1. Dependence of the distribution ratio of the six β-diketones in hexane-water (solid lines) and diethyl etherwater (broken lines) systems as a function of pH. The lines in the figure define the asymptotes for eq 13. Table 1. Summary of Equilibrium Constants for Six β-Diketones Used log Ka Haa Hbza Hdbm Htfa Hbfa Htta
log Kd(hexane)a
-8.97 ( 0.05 -0.05 ( 0.03 -8.73 ( 0.05 2.01 ( 0.03 -8.81 ( 0.07 3.92 ( 0.05 -6.44 ( 0.03 -0.54 ( 0.03 -6.07 ( 0.06 1.87 ( 0.05 -6.45 ( 0.05 0.69 ( 0.03
log log Kd(diethyl ether)b Kd(Triton X-100)c 0.50 ( 0.03 2.70 ( 0.03 4.70 ( 0.06 1.19 ( 0.05 3.71 ( 0.05 2.91 ( 0.05
0.65 ( 0.07 2.23 ( 0.10 4.0 ( 0.3 0.90 ( 0.09 1.78 ( 0.17 1.58 ( 0.16
a Hexane-water system. b Diethyl ether-water system. c Triton X-100-water system.
If the [H+] in the aqueous phase at equilibrium is much higher than the value of Ka, DHA should equal Kd. If, on the other hand [H+] , Ka, the value of DHA becomes equal to KdKa-1[H+]. Hence, the value of Ka and Kd in the system can be calculated from the distribution data of the extractant at several pH values. The distribution ratio of the extractants in hexane-water and diethyl etherwater systems as a function of pH is plotted in Figure 1. The diethyl ether-water system formed an emulsion at high pH, and thus, the values of Ka could only be obtained in the hexane-water system. The values of Ka and Kd thus obtained for the six β-diketones are listed in Table 1. The values of Ka and those of Kd for the hexane-water system obtained are almost similar to those in the hexaneaqueous 0.1 M sodium perchlorate solution system.24 Analysis of the Distribution Equilibria of the Six β-Diketones and Their Iron(III) Complex Species in a Triton X-100 Micellar System. Previously, the author and co-workers have reported that the charged iron(III) complex species with Haa and Htfa cannot be neglected in the aqueous phase of a liquid-liquid system due to the low partition constants of their uncharged tris complexes.25,26 Similar phenomena occur in the present micellar system, and the equilibria among the complex species in the aqueous phase were examined and the stability constant and the molar absorptivity of the complex species existing in the bulk aqueous phase were calculated from experimental data obtained using single aqueous solutions. (23) Sekine, T.; Hasegawa, Y. Solvent Extraction Chemistry; Marcel Dekker: New York, 1977; Part I. (24) Sekine, T.; Hasegawa, Y.; Ihara, N. J. Inorg. Nucl. Chem. 1973, 35, 3968. (25) Sekine, T.; Inaba, K. Bunseki Kagaku 1982, 31, E291. (26) Inaba, K.; Itoh, N.; Matsuno, Y.; Sekine, T. Bull. Chem. Soc. Jpn. 1985, 58, 2176.
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Inaba
Figure 2. Changes in absorbance (normalized to total metal concentration) of iron(III) by formation of complexes with Haa at 440 nm and Htfa at 435 nm as functions of the ligand anion concentrations. The solid lines represent regressions of eq 14 onto the data (see Table 2). Table 2. Summary of Molar Absorptivities and Stability Constants of Iron(III) Complexes with Haa and Htfa 1 (M-1 cm-1) 2 (M-1 cm-1) 3 (M-1 cm-1) 3m (M-1 cm-1) log β1 (M-1) log β2 (M-1) log β3 (M-1)
Fe-Haa (440 nm)
Fe-Htfa (435 nm)
150 ( 70 1700 ( 300 3470 ( 300 3800 ( 80 10.94 ( 0.18 20.31 ( 0.14 27.40 ( 0.48
70 ( 30 1110 ( 100 3100 ( 300 3350 ( 130 6.70 ( 0.10 12.58 ( 0.11 17.17 ( 0.35
Figure 2 shows the changes in absorbance of aqueous iron(III) solutions with these ligands at 440 nm (Fe-Haa system) and 435 nm (Fe-Htfa system), which are normalized to the total iron(III) concentration, as a function of the ligand anion concentration. The system includes four types of iron(III) species (i.e., Fe3+, FeA2+, FeA2+, and FeA3), but the absorbance of the ferric ion is negligible in the visible region. The equation relating the absorbance to the ligand anion concentrations can be written as
1β1[A-] + 2β2[A-]2 + 3β3[A-]3
At [Fe(III)]t
)
1 + β1[A-] + β2[A-]2 + β3[A-]3
(14)
Values of β1, β2, β3, 1, 2, and 3 were obtained by regressions of eq 14 onto the data. The estimated values are listed in Table 2. To check the accuracy of the calculated values, values of 3 for both ligands were measured by dissolving a certain amount of the salts, Fe(aa)3 and Fe(tfa)3, synthesized by a conventional method.27 The values obtained were 3430 M-1 cm-1 for Fe(aa)3 at 440 nm and 3030 M-1 cm-1 for Fe(tfa)3 at 435 nm. With low solubility into water the value of 3 for Fe(tfa)3 could not be determined directly; hence, the value was estimated by an extrapolation of the values obtained in a series of methanol-water solutions (10-50% methanol). The values measured agree well with those calculated using eq 14 listed in Table 2. It is also possible to determine the values of 1 by separate experiments with the conditions that the metal concentration is largely in excess of the ligand concentration and the pH is high to form a monocomplex quantitatively. The values of 1 thus obtained were 250 M-1 cm-1 for Fe(27) Sekine, T.; Inaba, K. Bull. Chem. Soc. Jpn. 1984, 57, 3083.
Figure 3. Changes in absorbance (normalized to total metal concentration) of iron(III) by formation of complexes with Haa at 440 nm in (a) 10.0% and (b) 5.0% Triton X-100 solutions as a function of pH. The total concentrations of Haa are (1) 0.01 M, (2) 0.005 M, (3) 0.002 M, and (4) 0.001 M. The solid lines represent regressions of eqs 5 and 12 onto the data.
(aa)2+ at 440 nm and 120 M-1 cm-1 for Fe(tfa)2+ at 435 nm. The values of 1 experimentally obtained are slightly higher than those calculated. The pH of the solutions in the supplemental experiments was controlled higher than that during the whole species analysis in Figure 2; the difference in 1 may be due to the change in the pH of the solutions in the two different series of experiments (i.e., formation of hydrolyzed complex species). By introducing these 1 and 3 values into eq 14, the sets of the constants were calculated (2 ) 1500 M-1 cm-1, β1 ) 1010.66, β2 ) 1019.79, and β3 ) 1026.92 for the Fe-Haa system at 440 nm and 2 ) 850 M-1 cm-1, β1 ) 106.42, β2 ) 1012.49, and β3 ) 1017.24 for the Fe-Htfa system at 435 nm); however, the values of the standard deviation for the calculation for both systems were larger than those during the whole species analysis in Table 2, so the values in Table 2 were used in the following calculations. Previously, we measured the values of the stability constants and the molar absorptivities for the same series of complex species but in aqueous 4 M sodium perchlorate media.25,26 The values of β1, β2, and β3 reported were 1011.4, 1020.8, and 1026.7 with Haa and 107.7, 1014.4, and 1019.8 with Htfa, respectively. The values obtained here in the aqueous sulfuric acid solution without any additional salt are somewhat different from those reported in the aqueous 4 M sodium perchlorate solution. Similar effects due to the change of the aqueous media have been reported by many researchers.28 Since for the molar absorptivity, the monitored wavelength in the two different aqueous media is different, no quantitative comparison can be made. Using the above equilibrium constants, the changes of absorbance in Triton X-100 micellar solutions as functions of Triton X-100, the extractant, and hydrogen ion concentrations were analyzed (Figures 3 and 4). The values of 3m, Kd, and Kdm were analyzed simultaneously by fitting (28) Sille´n, L. G.; Martell, A. E. Stability Constants; The Chemical Society: London, 1964.
Six β-Diketones and Their Iron(III) Complexes
Langmuir, Vol. 13, No. 6, 1997 1505
Figure 5. Changes in absorbance (normalized to the total metal concentration) of Fe(aa)3 at 440 nm by partitioning in Triton X-100 solutions as a function of Triton X-100 concentration. The solid line gives the calculated curve obtained using eq 15.
Figure 4. Changes in absorbance (normalized to the total metal concentration) of iron(III) by formation of complexes with Htfa at 435 nm in (a) 10.0%, (b) 5.0%, and (c) 1.0% Triton X-100 solutions as a function of pH. The total concentrations of Htfa are (1) 0.025 M, (2) 0.02 M, (3) 0.01 M, (4) 0.005 M, (5) 0.0025 M, (6) 0.0013 M, and (7) 0.001 M. The solid lines represent regressions of eqs 5 and 12 onto the data.
eqs 5 and 12 to the data in Figures 3 and 4 using a successive approximation technique. The values thus obtained are listed in Tables 1-3. Since the water solubility of Fe(aa)3 is high enough so that its absorbance is measurable in aqueous solution, the values of Kdm and 3m can also be estimated in different concentrations of Triton X-100 under conditions where Fe(aa)3 is the dominant species in the aqueous phase. Figure 5 shows the absorbance at 440 nm of the micellar solutions containing Fe(aa)3 and an excess of Haa in several triton X-100 concentrations in the pH range 7.5-8.5. Here, [A-] is higher than 10-3 M, indicating that more than 99.9% of the whole iron(III) in the solution is Fe(aa)3 before the formation of the hydrolyzed iron(III) species occurs. The absorbance increases with increasing volume fraction of the micellar phase, and the change of absorbance can be explained as
At [Fe(aa)3]t
3V[Fe(aa)3] + 3mVm[Fe(aa)3]m )
) V[Fe(aa)3] + Vm[Fe(aa)3]m 3V + 3mKdmVm (15) V + KdmVm
Using the values of 3m and Kdm for Fe(aa)3 obtained from the data in Figure 3, the extraction curve can be drawn
Figure 6. Extraction curves of iron(III) with Haa (circle), Hbza (triangle), and Hdbm (square) as a function of the ligand anion concentration. The solid lines are the calculated extraction curves obtained using eq 7.
as seen in Figure 5. The curve calculated with the 3m and Kdm values obtained from the different series of the experiments gives good correspondence with the data. This indicates that the change of the surfactant concentration does not greatly affect the micelle characteristics such as size or shape under the present experimental conditions although the changes by a change of the surfactant concentration have been previously reported.13,14 The concentrations of Fe(aa)3 and Fe(tfa)3 in the micellar phase can be calculated from the equilibrium constants. Using these concentrations, the distribution ratios can be calculated. Figures 6 and 7 show the extraction curves of the iron(III) with these ligands. The values of Kex calculated on the basis of eqs 5 and 7 are listed in Table 3.
1506 Langmuir, Vol. 13, No. 6, 1997
Inaba
D)
AtV (3m[Fe(III)]t - At)Vm
(16)
The wavelength measured and the molar absorptivity of the extracted FeA3 with the above three β-diketones based on the total volume of the solution were 3820 M-1 cm-1 at 470 nm for Fe(dbm)3, 3980 M-1 cm-1 at 445 nm for Fe(bfa)3, and 3890 M-1 cm-1 at 485 nm for Fe(tta)3. In the case of Hbza extraction, on the other hand, the shape of the absorption spectra between 350 and 550 nm changed in accordance with the ligand anion concentration, as was reported previously.21 This may have been due to the presence of complex species in the aqueous phase. Here, the complex species in the aqueous phase can be assumed to be the monocomplex species, Fe(bza)2+, because of the high Kdm value estimated for the tris complex.26 The concentrations of extracted FeA3 and FeA2+ in the aqueous phase can be analyzed by a doublewavelength measurement.21 The molar absorptivity of these complexes was obtained experimentally under conditions such as the metal or ligand anion concentration being in a large excess and the pH being high enough to promote the formation reactions, the values being 3650 M-1 cm-1 at 450 nm and 2910 M-1 cm-1 at 490 nm for the extracted Fe(bza)3 (3m) and 750 M-1 cm-1 at 450 nm and 900 M-1 cm-1 at 490 nm for Fe(bza)2+ (1). The concentration of the extracted tris complex, [Fe(bza)3]m, can be calculated as Figure 7. Extraction curves of iron(III) with Htfa (circle), Hbfa (triangle), and Htta (square) as a function of the ligand anion concentration. The solid lines are the calculated extraction curves obtained using eq 7. Table 3. Summary of Partition and Extraction Constants for Iron(III) Complexes with Six β-Diketones in Three Different Solution Systems hexane-water
diethyl etherwater
Triton X-100water
Haa Hbza Hdbm Htfa Hbfa Htta
-0.26 ( 0.10 4.77 ( 0.08 10.16 ( 0.08 2.46 ( 0.20 8.12 ( 0.19 6.95 ( 0.06
(a) log Kdm 1.01 ( 0.10 5.91 ( 0.15 11.11 ( 0.19 5.55 ( 0.14 9.61 ( 0.14 9.18 ( 0.05
1.10 ( 0.07 5.51 ( 0.10 11.52 ( 0.11 2.61 ( 0.09 7.73 ( 0.17 7.48 ( 0.16
Haa Hbza Hdbm Htfa Hbfa Htta
0.37 ( 0.10 -0.08 ( 0.08 -0.63 ( 0.08 1.92 ( 0.20 1.48 ( 0.19 2.69 ( 0.06
(b) log Kex -0.01 ( 0.10 -1.00 ( 0.15 -2.03 ( 0.19 -0.17 ( 0.14 -2.56 ( 0.14 -1.73 ( 0.05
-0.37 ( 0.07 0.02 ( 0.10 0.47 ( 0.11 -2.24 ( 0.09 1.35 ( 0.17 0.56 ( 0.16
In the cases of extraction with the other four β-diketones a large partition constant for the uncharged tris complex species of iron(III) and a low concentration of the charged complex species in the bulk aqueous phase can be safely assumed on the basis of the liquid-liquid extraction data previously obtained.26 The extraction behavior in the micellar solutions was analyzed as follows. The shape of the visible spectra in the range 350-550 nm of the solutions of the Hdbm, Hbfa, and Htta systems was similar when the concentrations of the extractant and/or hydrogen ion were varied. As was previously reported, this may suggest that only one complex species exists in the whole solution; thus, the extraction data can be treated as if only the Fe3+ species exists in the bulk aqueous phase and only the FeA3 species exists in the micellar phase.21 The distribution ratio, D, can then be expressed as
[Fe(bza)3]m )
1(450)At(490) - 1(490)At(450) 1(450)3m(490) - 1(490)3m(450)
(17)
Using this equation, the distribution ratio of the iron(III) in the Hbza extraction system can be obtained. In the cases of extraction with these four β-diketones the values of the stability constants in the aqueous solution are unknown. However, it has been reported that the values of the stability constants of a metal with a series of homologous ligands seem to be similar if the values of the acid-dissociation constants of the ligands are similar;29-31 therefore, the values of the stability constants of iron(III) with Hbza and Hdbm can be estimated from those with Haa, and those with Hbfa and Htta, from those of Htfa. Using the estimated βn values, the distribution constant for the extractant, Kd, and that for the uncharged tris complex species, Kdm, as well as the extraction constant, Kex, can be obtained by successive approximation on the basis of eqs 5 and 7. The values of the constants thus obtained are listed in Tables 1 and 3. The calculated distribution ratio of the metal in the micellar system is shown in Figures 6 and 7 as a function of the ligand anion concentration obtained by the calculation. The lines in the figures are the extraction curves calculated using eq 7 with the values of the constants given in Tables 1 and 3. The slopes of the curves for the extraction with Hdbm, Hbfa, and Htta give +3, and the facts suggest that the formation of charged complex species in the bulk aqueous phase is negligibly small. On the other hand, the slope with Hbza gives +2, indicating that the monocomplex, Fe(bza)2+, is the dominant species in the bulk aqueous phase. Thus, the initial assumption of the existence of charged monocomplex species in the bulk aqueous phase for the Fe-Hbza system is valid. (29) Sekine, T.; Murai, R.; Niitsu, M.; Ihara, N. J. Inorg. Nucl. Chem. 1974, 36, 2569. (30) Sekine, T.; Iwahori, S.; Murai, R. J. Inorg. Nucl. Chem. 1977, 38, 363. (31) Sekine, T.; Iwahori, S.; Johnsson, S.; Murai, R. J. Inorg. Nucl. Chem. 1977, 39, 1092.
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Langmuir, Vol. 13, No. 6, 1997 1507
Figure 8. Changes of the extraction constant, Kex, of iron(III) with the six β-diketones as a function of the number of extractant molecules in one micelle. The broken lines give the average values of Kex determined in the present study.
Robson and Dennis have reported that one Triton X-100 micelle is formed with 143 molecules of the surfactant monomer,13 and thus the number of aggregates in the solution, {MTriton X-100}t, can be calculated as
{MTriton X-100}t )
CTriton X-100 - Ccmc 143
(18)
The change of the extraction constant, Kex, as a function of the number of extractant molecules in one micelle is shown in Figure 8. Although the values of Kex show deviations in all the extractant systems by an error in the data analysis, no obvious change in the extraction behavior of iron(III) with the six β-diketones into the Triton X-100 micellar phase was found when the number of the extractant molecules in one micelle was less than 10. This may indicate that the internal sphere of the micelle is rather roomy and the nature of the micelle and the interface, such as the framework and structure of the surfactants, may not be affected largely by the solubilization of at least 10 molecules of the extractant. Muralidharan et al. have reported the number of 8-hydroxyquinoline molecules in one Triton X-100 micelle at saturation being 60.11 The specific volume of the micelle which is calculated from the Vm and the volume of the weighed pure surfactant dissolved in the solution becomes 2 although the density of the surfactant itself is 1.06. This also suggests that the internal sphere of the micelle is not tightly packed. Extraction Equilibria of Iron(III) with Six β-Diketones in Hexane-Water and Diethyl Ether-Water Systems. In order to analyze the characteristics of the two regions in the micelle, the extractability of the complex species in conventional liquid-liquid extraction systems
Figure 9. Extraction curves of iron(III) with (a) Haa (circle), Hbza (triangle), and Hdbm (square) and (b) Htfa (circle), Hbfa (triangle), and Htta (square) into hexane (open symbols) and diethyl ether (closed symbols) as a function of the ligand anion concentration. The lines give the calculated extraction curves obtained using eq 7.
using hexane and diethyl ether was determined. Since the rate of solvent extraction of iron(III) with these β-diketones in a conventional liquid-liquid system is slow,26 the two-phase system was agitated for 8 h until the extraction equilibria were established. The distribution ratio of iron(III) as a function of the ligand anion concentration is shown in Figure 9a and b. The extraction data were analyzed in a similar manner as in the micellar system using eq 7; the values obtained are summarized in Table 3. The lines in the figure are the extraction curves calculated using the obtained constants. In the case of Haa extraction, the slope of the extraction curve decreases with increasing ligand anion concentration. This suggests that the unextracted complex species, Fe(aa)n3-n, becomes dominant in the aqueous phase at high ligand anion concentrations when the partition constant of the tris complex is low. On the other hand, the curves for the other extractants give straight lines with a slope of +3, indicating that the extractability of iron(III) with these five β-diketones into hexane and diethyl ether is efficiently high so that no complex species is found in the aqueous phase in the ligand anion concentration range of the present study. Solubilization Locus of β-Diketones and Their Iron(III) Complexes in a Triton X-100 Micellar Phase. As described in the former section the Triton X-100 micelle has two different types of sites in the internal sphere; the micellar phase seems to have the natures of a nonpolar diluent and of a solvating-type one. Solutes can partition into each of the two organic phases according to their chemical and physical properties such as acidity, basicity, and bulkiness. Hence, an estimation of the
1508 Langmuir, Vol. 13, No. 6, 1997
Figure 10. Relationship for the partition constant of β-diketones (a) between hexane-water and diethyl ether-water systems and (b) between hexane-water and Triton X-100water systems.
solubilization locus of a solute in the micelle may be possible by comparing the extraction behavior of the solute in the micellar system with those in the two different diluent-water systems. The relationships of the partition constants of the extractants, Kd, (a) between a hexane-water system and a diethyl ether-water system and (b) between a hexanewater system and a Triton X-100-water system are shown in Figure 10. It is well-known that the partition constant of a solute into a nonpolar solvent is mainly affected by (i) the solubility into water (hydrophilicity) and (ii) the insolubility into water (hydrophobicity).32 The former factor is due to the van der Waals forces between the solvent and solute molecules being stronger in aqueous solutions than in nonpolar organic solvents, while the latter is due to the hydrogen-bonded molecular network in water being stronger than the van der Waals forces in nonpolar organic solvents. Among a series of homologous ligands the partition constant of a stronger acid is lower than that of a weaker acid due to the high solubility in water but that of a bulky solute is higher than that of a smaller one due to low solubility. If the organic diluent is a solvating-type one, the partition constant is affected not only by the above two terms but also by the solubility into the organic solvent (organophilicity).33 A solute which can form a stronger interaction (solvation) with the organic solvent may have larger solubility into the solvent than that having a weaker interaction. Hence, an increase in (32) Diamond, R. M.; Tuck, D. G. Prog. Inorg. Chem. 1960, 2, 109. (33) Sekine, T.; Inaba, K.; Morimoto, T. Anal. Sci. 1986, 2, 535.
Inaba
the value of Kd from that obtained in a nonpolar diluent system may be larger for a stronger acid than for a weaker one. As shown in Figure 10a, the plots of Kd for Haa, Hbza, and Hdbm, which have no trifluoromethyl substituent group and pKa values of about 9, show good correlation between hexane-water and diethyl ether-water. It is also found that the plots for the other three extractants having one trifluoromethyl substituent group show good correlation but their values in the diethyl ether-water system are much higher than those with Haa, Hbza, and Hdbm. This indicates that the extractants having one trifluoromethyl substituent group are stronger acids than those without the substituent group and can form stronger interactions in the diethyl ether phase. The linear relationship for each series of the three extractants suggests that there is no obvious effect with the change in bulkiness of the solutes in the two systems. The plots of Kd between the hexane-water system and the Triton X-100-water system (Figure 10b), on the other hand, have a different relationship from that in Figure 10a. If the extractants mainly partition into the hydrocarbon “core” region and no other factor affects the partitioning, the plots may have a linear relationship for these two systems with all the extractants. On the other hand, if the extractants mainly distribute into the polyoxyethylene “mantle” region and the characteristics of the sphere are similar to those of diethyl ether, the plots in Figure 10b may show two different lines for the extractants with and without the trifluoromethyl substituent group, as seen in Figure 10a. The plots for Hbza, Hdbm, and Hbfa have a good linear relationship although Hbfa is a stronger acid than Hbza and Hdbm. This suggests that these three extractants mainly distribute into the “core” of the micelle. Since these extractants have one or two phenyl substituent groups and their molecular size is larger than the others, steric hindrance may be an important factor in the “mantle” region (organophobicity). The plots for Haa and Htfa, on the other hand, show higher extractability into Triton X-100 micelles than the above three extractants, indicating that they can distribute into the “mantle” region. These two extractants have no bulky substituent group, and their molecular shape is rather linear. They can penetrate into the polyoxyethylene region, and their acidic proton can interact with oxygen atoms in the surfactant. The larger deviation of Kd in Htfa between hexane-water and Triton X-100-water systems than in Haa suggests interaction in the “mantle” region. In the case of Htta, the plot shows high extractability into Triton X-100 micelles and indicates the occurrence of the interaction although the thienyl substituent group is as bulky as the phenyl substituent group. The reason for this deviation is unclear; however, it may be due to interaction between the sulfur atom in the thienyl substituent group and the polyoxyethylene chain. Both the sulfur atom and the oxygen atom in the polyether are electron donors; it is speculated that this results in a small amount of water dissolving into the “mantle” region. The penetration of water molecules into Triton X-100 micelles has been reported by other researchers.13,14 The relationship of the partition constant of the tris complex species, Kdm, (a) between hexane-water and diethyl ether-water and (b) between hexane-water and Triton X-100 micelle-water is shown in Figure 11. The extractability into diethyl ether for the three complexes formed with the ligands having a trifluoromethyl substituent group is higher than that of the other complexes. Iron(III) has six coordination sites, and those in the extracted complex species are filled by three molecules of bidentate β-diketonate anion, and moreover, the values
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Langmuir, Vol. 13, No. 6, 1997 1509
methyl-substituted extractants may not easily penetrate into the polyoxyethylene part because of their low wettability. A similar change of behavior in the extraction kinetics of iron(III) with Hbza and Hbfa has been found;21 the whole extraction rate was controlled by the formation of the monocomplex in the bulk aqueous phase for the Hbza system while that was dependent on the triton X-100 concentrations for the Hbfa system. It was estimated that the decrease in the extraction rate with decreasing Vm may be due to a difficulty in the interfacial transport for the Fe(bfa)3 complex. The kinetic study for the other extractants is now ongoing, and the results will be reported elsewhere. Saitoh et al. have reported that the partition constants of solutes between the Triton X-100 micellar phase and the bulk aqueous phase do not become higher than 103.2 because of a steric effect in the “mantle” region.19 However, the values of the partition constant obtained in the present study are higher than this value. They measured the partition constants of several phenols and benzoic acids, chelating reagents having a large substituent group, and their iron(II), nickel(II), and cobalt(III) complexes. All of the materials they measured are bulky, and moreover, most have a polar part such as a hydroxyl, nitro, carboxyl, thiazolylazo, and/or pyridylazo group. Since the polar solutes may interact with the oxyethylene part, most of the materials examined should distribute into the “mantle” region although the bulky solutes have a steric disadvantage in the structural region. In contrast, the extractants used in the present study are rather small and their iron(III) complexes have no additional polar substituent group on the surface of the molecules except in the case of Htta. This may be one of the reasons for the difference in the solubilization locus of the solutes in these two studies.
Figure 11. Relationship for the partition constant of tris(βdiketonato) complexes (a) between hexane-water and diethyl ether-water systems and (b) between hexane-water and Triton X-100-water systems.
of the stability constants for these tris complexes are high. For the above reasons, the formation of an adduct with diethyl ether seems to be negligible. The reason for the higher extractability of Fe(tfa)3, Fe(bfa)3, and Fe(tta)3 into diethyl ether than that of Fe(aa)3, Fe(bza)3, and Fe(dbm)3 is unclear, but it may be due only to a change of solubility of these complexes by the change of dielectric constant of the organic diluents. The extractability of Fe(aa)3, Fe(bza)3, and Fe(dbm)3 into the Triton X-100 micellar phase is quite similar to those into diethyl ether while that for Fe(tfa)3, Fe(bfa)3, and Fe(tta)3 becomes lower than those into the nonpolar and the solvating-type diluents. This change of behavior in the extraction with the three trifluoromethyl-substituted extractants suggests that the nature of the polyoxyethylene “mantle” region is not similar to that of diethyl ether. As was already discussed above, the “mantle” region forms a hydrogen-bond framework and may dissolve some water molecules; the complexes formed with trifluoro-
Conclusions The distribution equilibria of six β-diketones and their iron(III) complexes between Triton X-100 micellar and bulk aqueous phases can be well analyzed by successive approximations on the basis of the two-phase partition model used in the conventional liquid-liquid system. The extractability of the extractants in the micellar system is largely different from that in conventional hexane-water and diethyl ether-water systems. The change in extractability is mainly due to the solubilization efficiency of the extractants into the polyoxyethylene “mantle” region. Two types of factors determine this efficiency in the “mantle” region: the solute-solvent interactions and solvent-solvent interactions. The former acts as a positive factor while the latter acts as a negative factor for the extraction. The extractability of the tris complexes into Triton X-100 micelles is affected by the substituent group of the ligands in the complexes. The change of wettability of the complexes is an important factor in determining the solubilization into the micelle. Acknowledgment. The author is grateful to Professor Henry Freiser and Drs. Subramaniam Muralidharan and Roger Sperline of the University of Arizona, who first stimulated the author’s interest in the field of micellar extraction. LA960813U
