Langmuir 2005, 21, 2675-2681
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Dediazoniation in SDS/BuOH/H2O Reverse Micelles: Structural Parameters, Kinetics, and Mechanism of the Reaction Ma Jose Pastoriza-Gallego,† Carlos Bravo-Diaz,*,‡ and Elisa Gonzalez-Romero† Departamento Quimica Fisica and Departamento Quimica Analitica y Alimentaria, Facultad de Ciencias, Universidad de Vigo, 36200 Vigo, Pontevedra, Spain Received July 23, 2004. In Final Form: October 6, 2004 Dediazoniation of o-methylbenzenediazonium tetrafluoroborate was investigated in SDS/BuOH/H2O (SDS ) sodium dodecyl sulfate) reverse micelles, RMs, and, for comparison, in binary BuOH/H2O mixtures by employing a combination of spectrophotometric and chromatographic techniques. RMs were characterized by steady-state fluorescence; the data indicate that the aggregation number of the RMs increase upon increasing [SDS], while the radius of the water pool is mainly controlled by the amount of water in the system, and that the thickness of the interfacial region increases upon increasing the amount of BuOH in the system, in agreement with literature reports. Experimental evidence suggests that dediazoniation mainly takes place in the interfacial region of the RMs. Kinetic data show that a turnover from the heterolytic to the homolytic mechanism takes place about pH ) 5; the variation of the observed rate constants, kobs, with pH following an S-shaped curve. At pH ∼ 2, kobs values are insensitive to solvent composition both in RMs and in the binary mixture; however, kobs values in RMs are slightly lower than those in BuOH/H2O, probably due to the presence of SDS. High-performance liquid chromatography analyses of the reaction mixture indicate, in both RMs and in binary mixtures, the main dediazoniation products are the heterolytic ArOH and ArOBu, their yields depending on the composition of the system, and only small ( 12, water in excess of bound and interfacial molecules forms the central water pool, which has a comparatively higher mobility.8-10
wo )
[H2O] [SurfT]
(1)
As noted before, RMs have the property of solubilizing small amounts of water in their interior, providing a stable aqueous microenvironment in nonaqueous media. Because of this characteristic solubilization property and the high potential for experimental and industrial application,2,11-14 the fundamental characteristics of reverse micellar aggregates have long been of interest and the use of a variety (8) Riter, R. E.; Willard, D. M.; Levinger, N. E. J. Phys. Chem. B 1998, 102, 2705. (9) Pileni, M. P. Adv. Colloid Interface Sci. 1993, 46, 139. (10) Corbell, E. M.; Levinger, N. E. Langmuir 2003, 19, 7264. (11) Sanchez-Ferrer, A.; Garcia-Carmona, F. Enzyme Microb. Technol. 1994, 16(5), 409. (12) Silber, J. J.; Abuin, E.; Lissi, E. Adv. Colloid Interface Sci. 1999, 82, 189. (13) Fadnavis, N. W.; Deshpande, A. Curr. Org. Chem. 2002, 6(4), 393. (14) Texter, J. Microstructure Effects on Transport in Reverse Microemulsions. Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications; Surfactant Science Series; Dekker: New York, 2001; Vol. 95, p 241.
10.1021/la048143c CCC: $30.25 © 2005 American Chemical Society Published on Web 02/23/2005
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Table 1. Composition and Structural Characteristics of the Employed RMsa RM
% BuOHb
% BuOHc
[SDS]/[BuOH]b
N
rwp/(Å × 1022)
nwp/L-1
rc/Å
rc - rwp/Å
1 2 3 4 5 6 7 8
45 56 61.9 67.8 72.5 80.1 84.9 89.8
58.14 68.37 73.40 78.15 81.72 87.46 90.48 93.65
0.086 0.056 0.043 0.033 0.027 0.017 0.013 0.008
33 26 24 16 13 11 9 8
23.3 21.8 20.0 18.6 17.3 15.8 15.4 14.7
1.89 2.30 2.98 3.71 4.61 6.05 6.54 7.51
33.4 34.4 33.4 33.4 33.0 33.8 36.2 39.6
10.1 12.6 13.4 14.8 15.7 18.0 20.8 24.9
a N is the aggregation number; r , the radius of the water pool; n wp wp, the number of water pools per liter of aqueous phase; rc, the radius of the cell. rc - rwp indicates the thickness of the interfacial region where the surfactant and a fraction of the alcohol are located. b By weight. c By volume of total reverse micellar solution (see Experimental Section).
of techniques15,16 has clarified the important characteristics of the reverse micelle such as micellar size, structure, and aggregation number, etc. RMs have been widely applied to control chemical reactivity involving solubilized substances and catalysis, and the carrier function of reverse micelles has also become a major field in recent years.2,7,12 Ion distribution between the interface and the water pool of RMs has been of particular interest because it may affect the course of chemical and biochemical reactions.3,4,17,18 Romsted and co-workers first employed arenediazonium, ArN2+, ions to trap weak nucleophiles to probe the interfacial composition of association colloids and to map ion and polar organic molecules distribution in their interfaces.19,20 Das et al.3,4 determined, by chemical trapping, the local molar concentrations of water and Brions in the water pool of reversed cetyltrimethylammonium bromide (CTAB), isooctane, n-hexanol, and water reversed micelles, and Cuccovia et al.18 exploited the strikingly low selectivity of arenediazonium ions with a number of nucleophiles to analyze the local concentrations of Br- and Cl- ions in the water pool and micellar interface of CTAB/n-dodecane/CHCl3 and CTAB/isooctane/n-hexanol reverse micelles. Our laboratory has investigated dediazoniation reactions of a number of ArN2+ ions in a variety of aqueous and nonaqueous systems, including normal anionic and cationic micellar systems.21-26 In this work, we prepared and characterized by steady-state fluorescence a number of SDS/BuOH/water (SDS ) sodium dodecyl sulfate) reverse micelles and studied the dediazoniation reaction (15) Zana, R. Surfactant Solutions: New Methods for Investigation; Dekker: New York, 1985. (16) Alargova, R. G.; Kochijashky, I. I.; Sierra, M. L.; Zana, R. Langmuir 1998, 14, 5412. (17) Pal, T.; D., S.; Jana, N. R.; Pradhan, N.; Mandal, R.; Pal, A.; Beezer, A. E.; Mitchel, J. C. Langmuir 1998, 14, 4724. (18) Cuccovia, I. M.; Dias, L.; Maximiano, F. A.; Chaimovich, H. Langmuir 2001, 17, 1060. (19) Romsted, L. S. Interfacial Compositions of Surfactant Assemblies by Chemical Trapping with Arenediazonium Ions: Method and Applications. In Reactions and Synthethis in Surfactant Systems; Texter, J., Ed.; Dekker: New York, 2001. (20) Gunaseelan, K.; Romsted, L. S.; Gonza´lez-Romero, E.; BravoDı´az, C. Langmuir 2004, 20, 3047. (21) Bravo-Dı´az, C.; Gonza´lez-Romero, E. Reactivity of Arenediazonium Ions in Micellar and Macromolecular Systems; Current Topics in Colloid & Interface Science, ISSN 0972-4494; Research Trends: Trivandrum, India, 2001; Vol. 4, p 57. (22) Bravo-Dı´az, C.; Gonza´lez-Romero, E. Electrochemical Behavior of Arenediazonium ions. New Trends and Applications; Current Topics in Electrochemistry; Research Trends: Trivandrum, India, 2003; Vol. 9. (23) Costas-Costas, U.; Bravo-Dı´az, C.; Gonza´lez-Romero, E. Langmuir 2003, 19, 5197. (24) Costas-Costas, U.; Bravo-Dı´az, C.; Gonza´lez-Romero, E. Langmuir 2004, 20, 1631. (25) Pazo-Llorente, R.; Bravo-Dı´az, C.; Gonza´lez-Romero, E. Langmuir 2003, 19, 9142. (26) Pazo-Llorente, R.; Bravo-Dı´az, C.; Gonza´lez-Romero, E. Eur. J. Org. Chem. 2004, 3221.
of o-methylbenzenediazonium, OMBD, tetrafluoroborate. For this purpose, a fixed wo ) 42.7 value was chosen, allowing variations in the percentage of BuOH in the system range of 45-90% (w:w). The aim of the manuscript is 2-fold. For one side, dediazoniation solvolysis studies in binary mixtures throughout the whole composition range are limited by the solubility of the alcohol in water, and the use of RMs provides a nice opportunity to explore these reactions under experimental conditions that cannot be attained in binary mixtures. On the other hand, relatively little dediazoniation work that has been carried out in reversed micelles, and even less in those RMs prepared by employing anionic surfactants,3,4 and thus comparisons with normal micelles can be done expanding the scope of our previous investigations. OMBD was chosen because most of the expected heterolytic and homolytic dediazoniation products are commercially available and because substantial knowledge of its behavior in binary ROH/H2O mixtures and in micellar systems is available. Experimental Section Materials. Reagents were of maximum purity available and were used without further purification. Sodium dodecyl sulfate, SDS; pyrene; N-cetylpiridinium chloride, (NCP)Cl; and the reagents used in the preparation of OMBD were purchased from Aldrich. BuOH (HPLC grade, F ) 0.81 g cm-3) was purchased from Merck or Riedel de Hae¨n. Other materials employed were from Riedel de Ha¨en. All solutions were prepared by using Milli-Q grade water. In binary mixtures, percentages of alcohol will be given hereafter by volume, and the small excess volume of mixed solvents was ignored as in previous studies.27 OMBD was prepared under nonaqueous conditions following a literature procedure28 and stored in the dark at low temperature to minimize its decomposition and recrystallized periodically from acetonitrile-cold ether. Stock solutions were prepared by dissolving the arenediazonium salt in the appropriate acidic (HCl) solution to minimize diazotate formation,29 to give final concentrations of about 10-2 M and [HCl] ) 3.6 × 10-3 M. Stock solutions were generally used immediately or within a short period of time with storage in an ice bath to minimize decomposition. SDS/BuOH/H2O reverse micelles were prepared by keeping wo ) 42.7 according to the phase diagram given by Jobe et al.30 by mixing the appropriate amounts of SDS, BuOH, and aqueous HCl ([HCl] ) 2.1 × 10-2 M), determined by weight, to get the desired compositions, Table 1. For comparison purposes, one of the samples did not contain BuOH; i.e., a normal SDS micellar system was prepared. Instrumentation. Kinetic experiments were performed on a Beckman DU-640 UV-vis spectrophotometer equipped with a (27) Pazo-Llorente, R.; Bravo-Dı´az, C.; Gonza´lez-Romero, E. Eur. J. Org. Chem. 2003, 17, 3421. (28) Garcia-Meijide, M. C.; Bravo-Diaz, C.; Romsted, L. S. Int. J. Chem. Kinet. 1998, 30(1), 31. (29) Zollinger, H. D. Chemistry I, Aromatic and Heteroaromatic Compounds; VCH: Weinheim, Germany, 1994. (30) Jobe, D. J.; Dunford, H. B.; Pickard, M.; Holwarth, J. F. In Reactions in Compartmental Liquids; Knoche, W., Ed.; SpringerVerlag: Heidelberg, Germany, 1989.
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thermostated cell carrier attached to a computer for data storage. Product analysis and some kinetic experiments were carried out on a WATERS HPLC system that included a 560 pump, a 717 automatic injector, a 2487 dual wavelength detector, and a computer for data storage. Products were separated on a Microsorb-MV C-18 (Rainin) reverse-phase column (25 cm length, 4.6 mm internal diameter, and 5 µm particle size) using a mobile phase of 70/30 (v:v) MeOH/H2O containing 10-4 M HCl. The injection volume was 25 µL in all run, and the UV detector was set at 220 nm. Emission experiments were carried out at T ) 25 °C by employing a Bio-Tek Kontron SFM-25 spectrofluorimeter equipped with a thermostated cell carrier attached to a computer for data storage. Fluorescence measurements were obtained by employing 1 × 1 cm sample cells thermostated by circulating water through a jacketed cell-holder device by employing pyrene as fluorescent probe and N-cetylpyridinium chloride as static quencher. Methods. The aggregation numbers and the size of the prepared RMs were estimated by employing steady-state fluorescence and by following the method proposed by Turro and Yekta.31 For this purpose, emission espectra (λexc ) 334 nm) were recorded and the logarithm of the intensity ratio in the absence, and in the presence, of quencher at a specific wavelength within the spectral emission range was calculated at different quencher concentrations (see results). Kinetic data were obtained both spectrophotometrically and chromatographically. Observed rate constants were obtained by fitting the absorbance-time or percent yield-time data to the integrated first-order eq 3 using a nonlinear least-squares method, where M is the measured magnitude of the UV-vis absorbance or product yields.
(
ln
)
Mt - M∞ ) -kobst M0 - M∞
(2)
All runs were done at T ) 35 ( 0.1 °C with arenediazonium salts as the limiting reagents. Duplicate or triplicate experiments gave average deviations lower than 7%. Spectrophotometric kinetic data were obtained by following the disappearance of OMBD at an appropriate wavelength to minimize interference mainly from dediazoniation products or intentionally added electrolytes. Beer’s law plots (not shown) for ArN2+ aqueous and BuOH solutions up to 4.0 × 10-4 M are linear (correlation coefficient, cc g 0.999). Reaction solutions were prepared by dissolving OMBD in the appropriate acidic (HCl) mixture to minimize diazotate formation,29 to give final concentrations of about 1 × 10-4 M and [HCl] ) 3.6 × 10-3 M. Calibration curves (not shown) to convert HPLC peak areas into concentrations were obtained by employing authentic samples of the expected dediazoniation products, and in all cases linear relationships (cc > 0.99) between the measured peak area and concentration were obtained. All samples were analyzed in triplicate, obtaining average values with deviations lower than 3%. Chromatographic kinetic data for all dediazoniation products were obtained following a well-established procedure described elsewhere.28,32-34 Dediazoniations were quenched at convenient times with an aliquot of a stock quenching solution prepared by dissolving the sodium salt of 2-naphthol-6-sulfonic acid, 2N6S, in a solution containing TRIS buffer ([TRIS] ) 0.05 M) as described elsewhere. After mixing, the final 2N6S concentration was about 20-fold excess over that of arenediazonium salt and the final pH was about pH ) 8 because naphthoxide ions are much more reactive than their protonated forms, but as pH increases, the competing reaction of arenediazonium ions with OH- to form diazotates becomes significant.
Results (1) Fluorimetric Characterization of the SDS/ BuOH/H2O Reverse Micelles. In steady-state fluores(31) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951. (32) Bravo-Diaz, C.; Soengas-Fernandez, M.; Rodriguez-Sarabia, M. J.; Gonzalez-Romero, E. Langmuir 1998, 14, 5098. (33) Bravo-Dı´az, C.; Sarabia-Rodriguez, M. J.; Barreiro-Sio, P.; Gonzalez-Romero, E. Langmuir 1999, 15, 2823. (34) Bravo-Diaz, C.; Romsted, L. S.; Harbowy, M.; Romero-Nieto, M. E.; Gonzalez-Romero, E. J. Phys. Org. Chem. 1999, 12, 130.
Figure 1. Illustrative determination of RM parameters, according to eq 3, for RM2. λexc ) 334 nm; λem ) 389 nm; [pyrene] ∼2 × 10-5 M; T ) 25 °C.
cence measurements, the investigated system is continuously irradiated and the spectrum of the emitted light is recorded. The basis, advantages, and limitations of the method have been recently reviewed.16 We followed the mentioned method proposed by Turro and Yekta,31 which has been widely employed to determine structural parameters in aggregation colloidal systems,15,16 by determining the ratio between the emission intensity at a given wavelength in the absence, Io, and in the presence (IQ) of a static quencher according to eq 3,
()
ln
I0 [Q] NA[Qm] )N ) IQ Dn nwp
(3)
where N is the aggregation number, [Q] is the quencher concentration, [Dn] ) [SURFT] - CMC stands for the micellized surfactant, [Qm] is the concentration of the quencher per liter of aqueous phase, and nwp is the number of water pools per liter of aqueous phase. Parts A (top) and B (bottom) of Figure 1 illustrate the evaluation of N and nwp, respectively, for a representative RM. Structural parameters were estimated by assuming that spherical cells, containing spherical water pools, form the whole reverse micellar solution and by employing the equations that relate the different structural parameters with N and nwp given elsewhere.35,36 Estimated parameters for (35) Rodenas, E.; Pe´rez-Benito, E. J. Phys. Chem. 1991, 95, 9496. (36) Rodenas, E.; Pe´rez-Benito, E. J. Phys. Chem. 1991, 95, 4552.
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Figure 2. Effects of pH (defined as - log [H+]) on kobs for dediazoniation of OMBD in a 97.3% BuOH/H2O (v:v) binary mixture. [OMBD] ∼3 × 10-4 M; T ) 35 °C. Scheme 1. (A) Competitive Reaction Mechanism for Solvolysis of OMBD in BuOH/H2O Mixtures; (B) Basic Representation of the Spontaneous DN + AN Dediazoniation Mechanism Predominant at Low pHa
a k and k stand for the thermal rate constant and for the w c first-order rate constant for the decomposition of the ArN2OBu complex, respectively, and K stands for the equilibrium constant for complex formation.
the different RMs employed are given in Table 1, and the obtained values are very similar to those previously reported.36 (2) Effects of pH and Temperature on kobs at a Given BuOH/H2O Ratio. Previous solvolytic studies in EtOH/H2O mixtures indicate that a turnover from the heterolytic to the homolytic mechanism may take place under acidic conditions.26 To investigate this behavior in the present system, a number of kinetic experiments were carried out to determine the effect of pH on kobs, and as shown in Figure 2, a variation in the pH of the solution leads to an S-shaped variation in kobs. This S-shaped profile is typical of acid-base processes, where both forms are reactive, and has been interpreted in terms of two competitive mechanisms, the thermal decomposition of ArN2+, that predominates at low pH, and the rate-limiting homolytic scission of an unstable diazo ether intermediate, namely, Ar-NdN-OR, formed from reaction between ArN2+ ions and ROH, a mechanism that is predominant at higher pH, Scheme 1A.26 From Scheme 1A, eq 4 can be derived
kobs )
kw[H+] + kcK1 [H+] + K1
(4)
where kw and kc are the rate constants for the spontaneous
thermal decomposition of OMBD and the rate constant for decomposition of the complex, respectively, and K1 ) K[BuOH] with K standing for the equilibrium constant for complex formation. From eq 4, and by considering limits, when [H+] . K1, kobs ≈ kw; i.e., the reaction proceeds through the DN + AN mechanism; meanwhile when [H+] , K1, kobs ≈ kc; i.e., the reaction proceeds through the ArN2OBu complex. The solid line in Figure 2 was obtained by fitting the corresponding data to a HendersonHasselbach type equation, from where a value of pK1 ) 5.0 ( 0.1 can be obtained. This value is substantially higher than those reported for EtOH/H2O mixtures (pK1 ∼ 3.6).26 To further confirm that under our experimental conditions only the heterolytic mechanism is operating, the activation parameters for the reaction were determined by measuring kobs at different temperatures at a number of solvent compositions and by means of the theory of absolute rates. Table 2 shows the activation parameters in 98% BuOH and the corresponding rate constants at T ) 40 °C. Table 2 also shows, for OMBD, literature values in pure water and in different ROH/H2O mixtures for the sake of comparisons. In all cases, values of ∆Hq are relatively high compared with those for bimolecular reactions and the entropic term is clearly positive, values totally consistent with the DN + AN mechanism.27 (3) Effects of the Percentage of BuOH on kobs for ArN2+ Loss and for Product Formation in Binary BuOH/H2O and Reverse Micelles. Table 3 shows the effect of increasing BuOH on kobs for dediazoniation in the binary mixture and in RMs. For the binary mixture, only a slight increase in kobs, not higher than 10% with respect to the value in absence of BuOH, is observed so that kobs can be considered essentially constant with an average value of kobs ) 9.7 × 10-4 s-1. When the reaction is carried out in the presence of RMs, Table 3, kobs values also increase slightly (
