Langmuir 2003, 19, 5197-5203
Sodium Dodecyl Sulfate Micellar Effects on the Reaction between Arenediazonium Ions and Ascorbic Acid Derivatives U. Costas-Costas,† Carlos Bravo-Diaz,*,† and Elisa Gonzalez-Romero‡ Universidad de Vigo, Facultad de Ciencias, Departamento de Quimica Fisica and Departamento de Quimica Analitica y Alimentaria, 36200 Vigo, Pontevedra, Spain Received November 28, 2002. In Final Form: March 28, 2003 The effects of sodium dodecyl sulfate, SDS, micelles on the reaction between a number of arenediazonium, ArN2+, ions with hydrophilic L-ascorbic acid or vitamin C, VC, and the hydrophobic analogue 6-O-palmitoylL-ascorbic acid, VC16, have been investigated at different pH values. Previous results indicate that in aqueous acid solution, in the absence of surfactant, the reaction between ArN2+ and VC takes place through the rate-limiting decomposition of a diazo ether “complex” formed from the interaction of ArN2+ with the monoanion form of ascorbic acid, VC-, in a rapid pre-equilibrium step. The kinetic profiles found for the reaction of ArN2+ with VC or with VC16 together with high-performance liquid chromatography data suggest that the presence of SDS micelles does not alter the reaction pathway. Addition of SDS decreases the observed rate constant, kobs, for the reaction with VC up to a minimum after which further addition of SDS leads to a slight increase in kobs. The kobs values at the minimum are significantly above zero and much higher than those for the spontaneous thermal decomposition of ArN2+, suggesting that at high [SDS], a fraction of VC- ions are present in the micellar Stern layer. At high [SDS] ) 0.8 M, saturation kinetics is observed for the reaction with VC16 upon increasing [VC16], but at moderate [SDS] ) 0.16 M only the initial linear regions of the saturation profile are observed. The reaction is, however, inhibited upon increasing [SDS] at any fixed [VC16], with kobs approaching the value for the thermal decomposition of ArN2+. Results with VC and VC16 are consistent with the pseudophase model and are rationalized in terms of the electrostatic micellar-induced separation of reactants and co-ion (VC- ) incorporation into the micellar Stern layer and by assuming that the local VC16 concentration in the micellar pseudophase decreases because of the dilution effect caused by increasing [SDS] (VC16).
Introduction L-Ascorbic acid or vitamin C, VC, is a powerful antioxidant that efficiently protects important organic and biological molecules against oxidative degradation responsible for a number of chronic health problems, such as aging, cancer, atherosclerosis, and so forth, and for lipid peroxidation generating rancid odors and flavors in food systems, producing a significant decrease in food quality and safety.1-6 In addition to the protection against lipid peroxidation, VC also reacts with other components in the system such as trace metal ions to yield active oxygen species that are dangerous prooxidants.1-3,6 The biological activity and the efficiency of a particular antioxidant in multiphasic food or biological systems depends on a variety of factors, including the location of the antioxidant in the different phases.3,4,6,7 Due to the high solubility of VC in aqueous solution, it cannot be used to protect water-insoluble lipids such as fats, vitamins, and biomembrane lipids, but a number of lipophilic ascorbic acid derivatives, which have antioxidant properties similar to those of VC, have been successfully employed.1,8,9 Sodium dodecyl sulfate, SDS, micelles are often used as model systems for testing antioxidant activity.1,4,10,11 Indeed, the multifunctional environment
* To whom correspondence should be addressed. Fax: +34+ 986+812556. E-mail: [email protected]
† Departamento de Quimica Fisica. ‡ Departamento de Quimica Analitica y Alimentaria. (1) Liu, Z. L. Antioxidant Activity of Vitamin E and Vitamin C derivatives in membrane mimetic systems. In Bioradicals Detected by ESR Spectroscopy; Birkha¨user: Basel, Switzerland, 1995. (2) Frankel, E. N. Food Chem. 1996, 1996, 1. (3) Frankel, E. N. J. Oleo Sci. 2000, 50, 387. (4) Frankel, E. N.; Meyer, A. S. J. Sci. Food Agric. 2000, 80, 1925. (5) Jacobsen, C. Fett/Lipid 1999, 101, 484. (6) McClemments, D. J.; Decker, E. A. J. Food Sci. 2000, 65, 1270. (7) Frankel, E. N. J. Oleo Sci. 2001, 50, 387.
created by micellar solutions has been used in many areas of chemistry for analysis, for the control of reactivity, and as models for biological membranes or systems.12-16 Reactions of natural reducing agents such as VC or catechol with arenediazonium ions, ArN2+, have attracted substantial attention in recent years from a biochemical point of view because antioxidants reduce ArN2+ ions via one-electron transfer processes17-20 to aryl radicals, which are believed to cause tumors or to react with important cellular constituents to generate mutagenic and carcinogenic products. Other studies point, however, to the genotoxicity of ArN2+ as a whole or a combination of ArN2+ and Ar•.17-19,21-25 ArN2+ ions are also useful to estimate interfacial compositions of micellar and other colloidal systems,26,27 to evaluate the concentration of VC in natural orange juices,28 to estimate antioxidant distributions in (8) Liu, Z. Q.; Ma, L. P.; Liu, Z. L. Chem. Phys. Lipids 1998, 95, 49. (9) Lo Nostro, P.; Capuzzi, G.; Pinelli, P.; Mulinacci, N.; Romani, A.; Vincieri, F. F. Colloids Surf., A 2000, 167, 83. (10) Pryor, W. A.; Strickland, T.; Church, D. F. J. Am. Chem. Soc. 1988, 110, 2224. (11) Pryor, W. A.; Cornicelli, J. A.; DEvall, L. J.; Tait, B.; Trivedi, B. K.; Witiak, D. T.; Wu, M. J. Am. Chem. Soc. 1993, 58, 3521. (12) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (13) Zana, R. Surfactant Solutions: New Methods for Investigation; Marcel Dekker: New York, 1985. (14) McIntire, G. L. Crit. Rev. Anal. Chem. 1990, 21, 257. (15) Russling, J. F. Electrochemistry in Micelles, Microemulsions and Related Microheterogeneous Fluids. In Electroanalytical Chemistry, Vol. 18; Marcel Dekker: New York, 1994. (16) Savelli, G.; Germani, R.; Brinchi, L. Reactivity control by Aqueous self-Assembling Systems. In Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Marcel Dekker: New York, 2001. (17) Brown, K. C.; Doyle, M. P. J. Org. Chem. 1988, 53, 3255. (18) Reszka, K. J.; Chignell, C. F. J. Am. Chem. Soc. 1993, 115, 7752. (19) Reszka, K. J.; Chignell, C. F. Chem.-Biol. Interact. 1995, 96, 223. (20) Costas-Costas, U.; Gonzalez-Romero, E.; Bravo Dı´az, C. Helv. Chim. Acta 2001, 84, 632.
10.1021/la026922s CCC: $25.00 © 2003 American Chemical Society Published on Web 05/22/2003
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Scheme 1. Proposed Mechanism for the Interaction between ArN2+ and VCa
Where DE stands for the 3-O-arenediazo ascorbic acid ether formed in a rapid pre-equilibrium step (ref 20).
food emulsions,29 and to estimate the association constants of electrochemically generated aryl radicals to micelles30 and cyclodextrins.31 Recently we explored the reaction of a number of ArN2+ ions with VC in aqueous solution at different pHs and proposed a mechanism for the reaction.20 The mechanism, Scheme 1, involves two competitive pathways: the spontaneous thermal decomposition of ArN2+ (A) and a clearly predominant inner sphere pathway whose slow step is the decomposition of a transient diazo ether intermediate formed from the reaction of ArN2+ with the monobasic form of ascorbic acid, VC-, in a rapid pre-equilibrium step (B). Here we compare the effects of SDS micelles on the reaction of the hydrophilic VC and its lipophilic derivative 6-O-palmitoyl-L-ascorbic acid, VC16. Their antioxidant properties are very similar, but their distributions in micellar solutions are very different; for example, lipophilic ascorbic acid ester derivatives self-micellize.9,32 The chemical structures of the ascorbic acid derivatives (VC and VC16) and those of the ArN2+ ions used here are shown in Scheme 2 and are the same as those in previous work.20 The results are pertinent to the food industry because of the extensive use of VCs as natural antioxidants. The experimental results are fully consistent with current micellar pseudophase models,12,16,33 which provide a unified theory to interpret micellar effects on chemical reactivity over a wide range of experimental conditions. Experimental Section 1. Instrumentation. UV-vis spectra and some kinetic experiments were followed on a Beckman DU-640 UV-vis spectrophotometer equipped with a thermostated cell carrier (21) Galli, C. Chem. Rev. 1988, 88, 765. (22) Toth, B.; Taylor, J.; Mattson, B.; Gannett, P. In Vivo 1989, 3, 17. (23) Zollinger, H. Diazo Chemistry I, Aromatic and Heteroaromatic Compounds; VCH: Weinheim, 1994. (24) Quintero, B.; Morales, J. J.; Quiros, M.; Martinez-Puentedura, M.; Cabeza, M. C. Free Radical Biol. Med. 2000, 29, 464. (25) Powell, J. H.; Gannet, P. M. J. Environ. Pathol., Toxicol., Oncol. 2002, 21, 1. (26) Romsted, L. S. Interfacial Compositions of Surfactant Assemblies by Chemical Trapping with Arenediazonium Ions: Method and Applications. In Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Marcel Dekker: New York, 2001. (27) Bravo-Dı´az, C.; Gonza´lez-Romero, E. Reactivity of Arenediazonium Ions in Micellar and Macromolecular Systems. In Current Topics in Colloid & Interface Science; 2001; Vol. 4. (28) Bravo-Diaz, C.; Gonzalez-Romero, E. Anal. Chim. Acta 1999, 385, 373. (29) Romsted, L. S.; Zhang, J. J. Agric. Food Chem. 2002, 50, 3328. (30) Gonza´lez-Romero, E.; Ferna´ndez-Calvar, M. B.; Bravo-Dı´az, C. Langmuir 2002, 18, 10311. (31) Gonza´lez-Romero, E.; Malvido-Hermelo, B.; Bravo-Dı´az, C. Langmuir 2002, 18, 46. (32) Lo Nostro, P.; Capuzzi, G.; Romani, A.; Mulinacci, N. Langmuir 2000, 16, 1744.
Costas-Costas et al. Scheme 2. Chemical Structures of VC and VC16 and Those of the ArN2+ Ions Employed in This Work
attached to a computer for data storage. The fastest reactions were monitored on an Applied-Photophysics SX18MV-R stoppedflow spectrometer attached to a computer for data analysis and storage and outfitted with a thermostated cell. Product analysis was carried out on a Waters HPLC system including a 560 pump, a 717 automatic injector, a 486 VIS-UV 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 65/35 v/v MeOH/H2O containing 10-4 M HCl. The injection volume was 25 µL in all runs, and the UV detector was set at 210 nm (PMBD) and 220 nm (OMBD and MMBD). pH was measured by using a previously calibrated Metrohm 713 pHmeter equipped with temperature sensors. 1H NMR spectra were obtained on a Brucker ARX 400 spectrometer. 2. Materials. Reagents were of maximum purity available and were used as received. The surfactant SDS (99.9%), VC, the hydrophilic 6-O-ascorbyl palmitate (VC16), and the reagents used in the preparation of diazonium salts (as tetrafluoroborates) were purchased from Aldrich or Fluka. Other materials were from Riedel de Ha¨en. All solutions were prepared by using Milli-Q grade water. Diazonium salts were prepared under nonaqueous conditions as previously reported34 and were stored in the dark at low temperature to minimize their decomposition. Ascorbic acid and its derivatives are highly sensitive to various modes of degradation.35 Factors that may influence the degradative process include the temperature, pH, oxygen, and metal catalysts. To minimize degradation, stock solutions were prepared each day by dissolving solid VC in aqueous solutions containing HCl or the universal Britton-Robinson (BR) buffer and citric acid (CA). Additional details can be found elsewhere.20 Stock solutions of VC16 were prepared fresh daily by dissolving the appropriate amount of solid VC16 in an aqueous SDS solution of known concentration. Auxiliary spectrophotometric and polarographic experiments demonstrated that VC and VC16 are stable under such conditions for at least 24 h at T ) 35 °C. 3. Methods. Kinetic data were obtained spectrophotometrically by monitoring the disappearance of ArN2+ as previously described.20,36 Observed rate constants were obtained by fitting the absorbance-time data to the integrated first-order eq 1 using a nonlinear least-squares method provided by a commercial computer program,
(A - A∞) ) -kobst ln (A0 - A∞)
where A is the absorbance at any time t, A0 and A∞ represent the absorbance at zero and infinite time, respectively, and kobs is the measured rate constant. All runs were done at T ) 35 ( 0.1 °C, except where otherwise indicated, with the diazonium salts as limiting reagents. The good agreement between the optimized and experimental A∞ value confirmed that reactions are first (33) Fendler, J. H.; Fendler, E. F. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (34) Garcia-Meijide, M. C.; Bravo-Diaz, C.; Romsted, L. S. Int. J. Chem. Kinet. 1998, 30, 31. (35) Fennema, O. R. Food Chemistry; Marcel Dekker: New York, 1985. (36) Bravo-Diaz, C.; Soengas-Fernandez, M.; Rodriguez-Sarabia, M. J.; Gonzalez-Romero, E. Langmuir 1998, 14, 5098.
Effect of SDS Micelles on Reaction
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Scheme 3. Proposed Mechanism for the Reaction between ArN2+ and VC in SDS Micellar Systems
order with respect to ascorbic acid. Identification and quantification of reaction products was done by high-performance liquid chromatography (HPLC) analyses of the reaction mixtures once reaction was complete, that is, at infinite time following the procedure described elsewhere.20
Figure 1. Estimation of the association constant, KVC, of VC to SDS micelles according to eq 2 by monitoring the changes in the UV-vis spectrum. λ ) 255, [VC] ) 1 × 10-2 M, T ) 25 °C. (b) pH ) 2, (O) pH ) 4.
Results 1. Estimation of the Association Constant of VC to SDS Micelles. The association constant, KVC, of VC to SDS micelles, equilibrium A in Scheme 3,37 was estimated spectrophotometrically by monitoring the shift in the absorbance of its UV-vis spectrum at λ ) 255 nm at two different pH values and by fitting the data to eq 2 by using a commercial computer program,
1 1 1 ) + A - A0 (A∞ - A0)KVCDn A∞ - A0
where A is the measured absorbance at any surfactant concentration, A0 is the initial absorbance (no added surfactant), A∞ is the absorbance value at high [SDS] (i.e., when the substrate has been incorporated into the micelle), and Dn is the concentration of micellized surfactant, that is, Dn ) [SDS]T - cmc (cmc, critical micelle concentration). Figure 1 shows the linear plots obtained at constant ionic strength (I ) 0.25) from which values of KVC ) 6.1 ( 0.6 (pH ) 2) and KVC ) 3.1 ( 1.0 (pH ) 4) can be estimated. These low KVC values reflect the high hydrophilicity of VC due to the large number of -OH groups in their molecules. The estimated KVC value at pH ) 4 is probably lower than that at pH ) 2 because of the ionization of VC, which is a diprotic acid with pKa values of ∼4.2 and ∼11.8 for the C3 and the C2 hydroxyl groups, respectively. Hence VC is primarily located in the aqueous phase and only a small fraction, ∼ 15%, is incorporated into the micellar aggregates at the highest [SDS] employed (0.03 M) at pH ) 2. 2. Micellar Effects on the Spontaneous Dediazoniation of ArN2+. In aqueous acid solution, in the dark, and in the absence of catalysts, dediazoniation is believed to take place through a DN + AN mechanism, that is, ratedetermining decomposition of ArN2+ to yield a highly reactive aryl cation that is trapped by weakly basic nucleophiles in the vicinity, Scheme 1A.20,34,38 Upon (37) Scheme 3 illustrates some of the relevant equilibria involved when VC, VC-, and ArN2+ ions are present in the system. We intentionally did not include that for exchange between Na+ and H+ (either from VC or from the buffer) ions for the sake of clarity and because it was not studied directly in the present work, e.g., by addition of NaCl. Hence, the data in the paper provide no evidence in support of, or against, ion exchange. The primary effects of increasing [SDS] on kobs are caused by dilution of the reactants in the micellar pseudophase and not by ion exchange (vide infra). (38) Pazo-Llorente, R.; Rodriguez-Sarabia, M. J.; Gonzalez-Romero, E.; Bravo-Dı´az, C. Int. J. Chem. Kinet. 1999, 31, 73.
Figure 2. Effect of SDS on kobs for the spontaneous decomposition of 3-MBD. [3-MBD] ) 1 × 10-4 M, pH ) 2, T ) 35 °C. The solid line was obtained by fitting the experimental data to eq 3.
addition of SDS, the observed rate constants, kobs, for the spontaneous decomposition of either 2-, 3-, or 4-MBD (MBD, methylbenzenediazonium) are slightly depressed. Figure 2, chosen as representative, shows that kobs for 3-MBD decreases only about 30% on going from [SDS] ) 0 to [SDS] ) 0.1 M. Because ArN2+ ions are cationic and somewhat hydrophobic, their distribution between the aqueous and micellar pseudophases, equilibrium C in Scheme 3, depends on SDS concentration, for example, Figure 2. Assuming that all solutes and surfactant are at thermal equilibrium throughout the reaction, kobs is equal to the sum of rates of concurrent reactions in each pseudophase,9,32 eq 3,
kw + kMKsDn 1 + KsDn
where kw and kM refer to the rate constants in the aqueous (w) and micellar (M) pseudophases and Ks is the association constant for ArN2+, equilibrium C in Scheme 3. By fitting the data to eq 3, values of Ks ) 325 ( 12 M-1 and kM ) (5.7 ( 0.3) × 10-4 s-1 for 3-MBD are obtained. This Ks value and those for 2-MBD (Ks ) 240 ( 10 M-1) and 4-MBD (Ks ) 1200 ( 40 M-1) are very similar to those obtained previously by other methods.36 Product distributions, obtained from HPLC analysis of the samples once reaction is essentially complete, that is,
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Figure 3. (A) Effects of [VC] on kobs for decomposition of 3-MBD at selected [SDS]. pH ) 2, (b) [SDS] ) 0 M, (O) [SDS] ) 5 × 10-3 M, (9) [SDS] ) 0.01 M, (0) [SDS] ) 0.05 M. T ) 35 °C, [3-MBD] ∼ 10-4 M. (B) Total yields for the reaction between 3-MBD and VC at selected [SDS]. (O) [SDS] ) 0.01 M, (b) [SDS] ) 0.04 M, (0) [SDS] ) 0.1 M. Samples were analyzed after ∼10 h. Solid lines were fitted to aid the eye.
at t g 4t1/2 (data not shown), indicate that the phenol derivative is the major dediazoniation product and no peaks associated with reduction products were detected. 3. Micellar Effects on the Reaction between ArN2+ and VC. As shown previously,20 the reaction between VC and ArN2+ depends on pH and [VC], so the effects of SDS micelles on the reaction were determined by using buffered control pH. Figure 3A shows the effects of increasing [VC] on kobs at selected [SDS]. In the absence of SDS, kobs appears to be approaching a plateau consistent with previous results.20 The saturation kinetic pattern is maintained even at high [SDS], suggesting that SDS micelles do not change the mechanism of the interaction between ArN2+ and VC. This point, however, was further confirmed by HPLC analyses of the reaction mixtures after ∼10 h; no extraneous peaks other than the front peak and those for the heterolytic (ArOH, major product) and homolytic (ArH, yield < 10%) derivatives showed up in the chromatograms. Figure 3B shows that total (ArOH + ArH) yields decrease upon increasing [SDS]. Nonquantitative conversion to products was previously observed in the absence of SDS20 and is attributed to the formation of the stable E-diazo ether derivative, which elutes with the front peak.39 At constant [VC], kobs for 3-MBD passes through a shallow minimum with increasing [SDS]. Figures 4 and 5 show the effects of SDS micelles on kobs at selected [VC] and pH values for 2-, 3-, and 4-MBD. In all cases, the lowest kobs values are significantly above zero and much higher, ∼10-fold, than kobs for the spontaneous thermal decomposition of ArN2+ in the presence of SDS, for example, for 3-MBD, Figure 2.36 4. Micellar Effects on the Reaction between ArN2+ and the Hydrophobic VC16. At very high [SDS] ) 0.8 M, Figure 6A, saturation kinetics patterns are obtained suggesting that the interaction between ArN2+ and VC16 is the same as that with VC; however, note that the kobs values for a given [VC16] are significantly lower than that for the reaction with VC at the same [SDS]. For example, at [VC16] ) [VC] ) 2.1 mM, kobs ) 3.45 × 10-3 s-1 (VC16) and kobs ) 5.86 × 10-3 s-1 (VC). At moderate (39) The reaction of ArN2+ ions with alkoxide or phenoxide ions takes place through the very rapid formation of the corresponding Z-diazo ether. In a second step, some of the Z-diazo ether decomposes to yield reduction products (ArH) and the rest is converted to the E-diazo ether, which is much more stable (ref 20).
Figure 4. Effects of [SDS] on kobs for the decomposition of 3-MBD at selected [VC]. T ) 35 °C, pH ) 2, [3-MBD] ) 2 × 10-4 M, (O) [VC] ) 100 × [3-MBD], (b) [VC] ) 50 × [3-MBD]. Solid lines were fitted to aid the eye.
[SDS] ) 0.16 M, Figure 6B, reaction with VC16 is strongly accelerated, by a factor of ∼10, compared to that with VC at the same pH, and the increase in kobs with increasing [VC16] is essentially linear. For comparison purposes, the effect of [VC] on kobs at the same [SDS] is included, inset in Figure 6B. The presumed saturation limit could not be reached because of limitations caused by the stoppedflow technique and solubility at higher [VC16]. Figure 7A shows the effect of [SDS] on kobs for the interaction of VC16 with 2-, 3-, and 4-methylbenzene-
Effect of SDS Micelles on Reaction
Figure 5. Effects of [SDS] on kobs for the decomposition of 2-, 3-, and 4-MBD ions with VC. T ) 35 °C. (A) [VC] ) 100 × [2-MBD] ) 2 × 10-2 M, (b) pH ) 3. Inset, (O) pH ) 2. (B) [VC] ) 10 × [3-MBD], pH ) 3. (C) pH ) 2, [4-MBD] ) 2 × 10-4 M, (O) [VC] ) 100 × [4-MBD], (b) [VC] ) 50 × [4-MBD], (0) [VC] ) 20 × [4-MBD]. Solid lines were fitted to aid the eye.
diazonium ions, and Figure 7B shows the effect of [SDS] on kobs for the interaction of 3-MBD with VC16 at two different concentrations of VC16. For any one of the ArN2+ ions investigated, kobs values decrease sharply and continuously upon increasing [SDS], with kobs approaching that for the spontaneous decomposition of ArN2+ at high [SDS]. This behavior contrasts with that in the presence of VC, where a minimum was observed, Figures 4 and 5. Product yields, Figure 8, were obtained by HPLC analyses of the reaction mixtures after 14 h. No extraneous peaks other than the front peak and those for the ArOH and ArH derivatives were found. Figure 8A,B shows the effect of [VC16] on product yields at selected [SDS]. Large amounts of ArH are found, in contrast with the results obtained in the presence of VC, but yields decrease upon increasing [VC16]. Figure 8A,B also shows that total yields depend on [SDS] for any given [VC16]. This observation prompted us to investigate the effects of [SDS] on the product distribution, Figure 8C. As shown, increasing [SDS] favors the formation of the reduction ArH product, and saturation is found when [SDS] > 0.2 M. Formation of ArOH is consistent with the observed decrease in kobs, Figure 7, because at [SDS] > 0.2 M the spontaneous decomposition of ArN2+ becomes significant compared to that with VC16.40 Discussion The estimated Ks values for 2-, 3-, and 4-methylbenzenediazonium ions with SDS micelles show that a substantial fraction of ArN2+ ions are incorporated into the SDS micellar aggregates even at low [SDS]. HPLC an-
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Figure 6. (A) Effects of [VC16] (O) and [VC] (b) on kobs for the decomposition of 3-MBD at [SDS] ) 0.8 M. T ) 35 °C, pH ) 3, [3-MBD] ) 2 × 10-4 M. (B) Variation of kobs for the reaction of 3-MBD with VC16 at [SDS] ) 0.16 M and at different pH [(O) pH ) 3, (0) pH ) 4]. Inset: kobs for the reaction of 2 × 10-4 M 3-MBD with VC at the same [SDS] and pH ) 3. Solid lines were fitted to aid the eye.
alyses of the product distribution, in the absence of VC or VC16, show that no unexpected products other than the phenol derivative are formed and only a slight decrease in kobs has been detected, for example, Figure 2. Hence SDS micelles do not change the mechanism for the spontaneous decomposition of the employed ArN2+ ions, in agreement with previous findings for these and other arenediazonium ions in anionic, cationic, and nonionic micelles.26,27 The results obtained for the interaction between ArN2+ and VC in the presence of SDS micelles, Figures 3A, 4, and 5, are consistent with the proposed mechanism in aqueous acid solution, in the absence of surfactant, combined with the pseudophase ion exchange, PIE, model, Scheme 3.37 A complete quantitative treatment was not attempted because a number of approximations are needed. A comprehensive set of equations for solving Scheme 1 (or similar reaction schemes)41-44 and details and the basic assumptions of the PIE model can be found elsewhere.12 (40) HPLC results with VC16 are somehow unexpected and deserve further investigation because, apparently, it is not easy to rationalize why SDS micelles induce formation of the reduction product in the presence of VC16 but not when VC is added, although they are different compounds and the results may not be the same. The bond-rotating mechanism to transform the Z-isomer into the much more stable E-derivative has been described in aqueous solution (see, for example: Hanson, P., et al. J. Chem. Soc., Perkin Trans. 2 2002, 1135 and references therein), but nothing is known about the micellar effects on the reactivity and stability of such diazo ethers. The presence of SDS prevents electrochemical detection of the formation of the transient diazo ether, as in aqueous acid solution (ref 20), because the polarographic peak of SDS shows up in the same potential region as that for the expected polarographic peak of the transient diazo ether.
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Figure 9. Variation of kmin with [VC]; kmin values are the minimum k value determined from the kobs vs [SDS] plots for the reaction of 3-MBD with VC (see the text).
Figure 7. (A) Effects of SDS on kobs for the reaction of 2-MBD (b), 3-MBD (0), and 4-MBD (O) with VC16 at pH ) 3. [VC16] ) 6.7 × 10-4 M, [ArN2+] ∼ 1 × 10-4 M, T ) 35 °C. (B) Effects of [SDS] on the reaction of VC16 with 3-MBD. pH ) 3, T ) 35 °C, (O) [VC16] ) 4 × [3-MBD] ) 6.6 × 10-4 M, (b) [VC16] ) 10 × [3-MBD] ) 1.3 × 10-3 M. Solid lines were fitted to aid the eye.
Figure 8. HPLC product distribution for the reaction between 3-MBD and VC16. (O) ArOH, (b) ArH, (0) total (ArOH + ArH). Samples were analyzed after 14 h at T ) 35 °C, pH ) 3 (BR buffer), [3-MBD] ∼ 2 × 10-4 M. (A) [SDS] ) 0.16 M, (B) [SDS] ) 0.8 M, (C) [VC16] ) 2 × 10-3 M. Solid lines were fitted to aid the eye.
Previous 1H NMR studies36 indicate that the reactive -N2+ group of ArN2+ ions is located in, or very close to, the micellar surface, and as noted above a significant fraction is micellar bound even at low surfactant concentrations. By contrast, VC binds very weakly, KVC ∼ 3-6, and its reactive anion form, VC-, will be almost exclusively in the aqueous pseudophase because it is a co-ion to the SDS micelles. Therefore, the observed inhibition can be
interpreted in terms of the decrease in the local concentration of VC- in the vicinity of ArN2+ due to micellarinduced compartmentalization of reactants. A similar inhibition is observed for the coupling reaction of a number of ArN2+ ions with naphthoxide ions in SDS micelles.45 However, the gradual increase in kobs above the minimum upon increasing [SDS], Figures 4 and 5, shows that VC- ions are not totally excluded from the Stern layer. The assumption of ascorbate ions having access to the Stern layer of the SDS micelles is consistent with a number of reports that conclude that ionic micelles inhibit, but do not completely suppress, reactions between bound substrates and co-ions.12,41,46,47 In addition, direct experimental evidence of the incorporation of co-ions in micellar systems has been obtained by HPLC analysis of the product distribution of the reaction between ArN2+ ions and a number of inorganic co-ions such as Cl- and Br-.26,48 The minima in Figures 4 and 5 can be rationalized by taking into account the different equilibria indicated in Scheme 3. At very high [SDS], ArN2+ ions are totally bound to the micellar aggregate and an increase in [SDS] leads to an increase in binding of VC given that KVC is low but not negligible, equilibrium A in Scheme 3. Concomitantly, at constant pH, that is, at constant [H3O]+, increasing surfactant concentration reduces the concentration of H3O+ ions at the micellar surface. A decrease of the acidity in the Stern layer should increase [VC-]M, equilibrium C in Scheme 3, increasing kobs. Unpublished experiments indicate that micelles shift the pKa of VC by a factor of only 0.1 pKa units; hence it is reasonable to assume that (pKA)w ≈ (pKA)M, Scheme 3, and therefore the increase in VC-M probably is not caused by a micellar-induced pKA shift. Additional evidence comes from the linear increase in kobs values at the minimum of the plots kobs versus [SDS], Figures 4 and 5, for dediazoniation of 3-MBD with increasing [VC], Figure 9. The intercept value i ) (5.9 ( (41) Romsted, L. S. Micellar Effects on Reaction Rates and Equilibria. In Surfactants in Solution; Mittal, K. L., Lindman, J., Eds.; Plenum Press: New York, 1984. (42) Romsted, L. S.; Zanette, D. J. Phys. Chem. 1988, 92, 4690. (43) Romsted, L. S. J. Phys. Chem. 1985, 89, 5113. (44) Romsted, L. S. J. Phys. Chem. 1985, 89, 5107. (45) Pazo-Llorente, R.; Rodrı´guez-Menacho, M. C.; Gonza´lez-Romero, E.; Bravo-Dı´az, C. J. Colloid Interface Sci. 2002, 248, 169. (46) Menger, F. M.; Doll, D. W. J. Am. Chem. Soc. 1984, 106, 6. (47) Ranganathan, R.; Okano, L. T.; Yihwa, C.; Alonso, E.; Quina, F. H. J. Phys. Chem. B 1999, 103, 1977. (48) Cuccovia, I. M.; Agostinho-Neto, A.; Wendel, C. M. A.; Chaimovich, H.; Romsted, L. S. Langmuir 1997, 13, 5032.
Effect of SDS Micelles on Reaction
Langmuir, Vol. 19, No. 13, 2003 5203
Scheme 4. Proposed Mechanism for the Reaction between ArN2+ and VC16 in the Presence of SDS Micellesa
a The subscript M stands for the micellar pseudophase and DE for the diazo ether formed (see Scheme 1).
0.5) × 10-4 s-1 represents the kobs value in the absence of VC, and it is equal, within experimental error, to kobs for dediazoniation of 3-MBD in SDS micelles at high [SDS], kM ) (5.7 ( 0.3) × 10-4 s-1, Figure 2. If no VC- ions were present in the micellar pseudophase, then the kM values would be independent of [VC] and the kobs value should be that for the thermal decomposition of ArN2+ in SDS micelles at all [VC]. Hydrophobic 6-O-ascorbic acid ester derivatives such as VC16 are almost insoluble in water at room temperature, although their solubility increases at higher temperatures due to self-micellization, with the reactive ascorbic ring exposed to the aqueous medium.9,32 On the basis of the effect of the hydrocarbon chain length on the association constant of the substrates and micelles,49 it can be expected that the value of KVC for VC16 with SDS should be higher than 105 M-1 and VC16 should be totally bound to the SDS micelles under our reaction conditions; that is, mixed SDS-VC16 micelles are present. The association constant, Ks, values for all ArN2+ ions obtained indicate that they are fully associated to the SDS micelles. Hence reaction between the anion of VC16 and ArN2+ ions takes place almost exclusively in the interface region of the SDS micelles, where the reactants are brought together. Bearing in mind that both VC16 and ArN2+ are micellar bound, Scheme 4 is appropriate, and the observed rate is given by eq 4:
d[ArN2+] ) kM[ArN2+]M + kVC16[DE] dt
where the subscript M stands for the micellar pseudophase and DE stands for the diazo ether complex. Taking into account the corresponding mass balances, the ionization (KA) of VC16, and that we have worked under pseudofirst-order conditions, eq 5 can be derived:
kM + kVC16B[VC16]T 1 + B[VC16]T
fulfilled even at pH ) 4, Figure 6B, because the pKa of VC16 in the presence of SDS micelles is ∼5.7;50 thus B ≈ KKA/[H3O]+ ≈ 10 and hence B[VC16]T , 1. Unfortunately, the data do not allow estimations of kVC16 and B independently but only for the product term because the saturation limit could not be attained. Further investigations with less hydrophobic VC derivatives are in progress and will be part of a future communication. In conclusion, the evidence is consistent with ArN2+ ions and VC16 reacting at the micellar interface and with the observed increase in kobs at fixed [SDS] due to the concentration effect exerted by the micelles, as found for a number of bimolecular reactions.12,51 The observed inhibition found at fixed [VC16] upon increasing [SDS], Figure 7, represents the dilution of the reactants within the micellar pseudophase upon increasing micelle concentration in solution. Note that the results shown in Figure 6A indicate that at high [SDS] ) 0.8 M, kobs for the reaction of 3-MBD with VC16 is lower than that with VC at the same [SDS], in keeping with the above assumptions because VC- ions are incorporated into the micellar psudophase, as discussed before, leading to an increase in kobs, Figures 4 and 5; meanwhile, the dilution effect exerted by micelles is perceptible even at high [SDS], Figure 7, that is, kobs values approaching that for the spontaneous decomposition of ArN2+ at high [SDS]. The results obtained with VC16 contrast with those reported for other reactions of VC16, where a rate-limiting diffusion of reactants has been proposed to explain experimental results for the reaction of VC16 with the hydrophobic R-tocopherol (vitamin E) and lipophilic nitroxides such as 4-palmitoyl-2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO-16) in SDS systems.1,52 Surfactant monomers, organic molecules, and ions associate with micelles at essentially diffusion-controlled rates, typically in the k1 ∼ 108 -109 M-1 s-1 order of magnitude, and they do not change significantly with variation in the chain length, but they exit at a rate which is basically governed by the strength of binding of the particular substrate.12 Aniansson et al.53 reported that the exit rates for alkyl sulfates at T ) 25 °C range from k-1 ∼ 109 L mol-1 s-1 for the hexyl derivative to k-1 ∼ 104 L mol-1 s-1 for the hexadecyl derivative. The actual value of k-1 for VC16 should not be very different from 104 L mol-1 s-1 since VC16 has the same hydrophobic tail as the hexadecyl sulfate. Either k1 or k-1 for both VC16 and ArN2+ is certainly much higher than the values found in this work for kobs, and thus the assumption that micellized surfactant is at thermal equilibrium with solutes throughout the reaction is completely fulfilled.12,26,51
where B is given by eq 6,
KKA [H3O+] + KA
where K is the equilibrium constant for the formation of the transient diazo ether complex DE (Scheme 4) and KA is the acidity constant of VC16. Equation 5 predicts that for those cases where the spontaneous decomposition of ArN2+ is negligible compared to product formation via the complex, that is, kM < kVC16B[VC16]T, kobs is independent of [VC16] at very high [VC16]. Alternatively, one expects a linear increase in kobs upon increasing [VC16]T when B[VC16]T , 1. This condition is achieved when [H3O+] is higher, by a power of ∼2, than the acidity constant of VC16 because K values for the ArN2+ ions employed are in the 102-103 M range.20 The assumption appears to be
Acknowledgment. Financial support from the following institutions is acknowledged: MCYT of Spain (BQU2000-0239-C02), Xunta de Galicia (XUGA 38301A92 and XUGA 38305A94), and Universidad de Vigo. U.C.C. thanks Xunta de Galicia for a postgraduate research training grant. LA026922S (49) Quina, F. H.; Alonso, E.; Farah, J. P. S. J. Phys. Chem. 1995, 99, 11708. (50) Bravo-Dı´az, C. Unpublished results. (51) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (52) Liu, Z. L.; Han, Z. X.; Yu, K. C.; Zhang, Y. L.; Liu, Y. C. J. Phys. Org. Chem. 1992, 5, 33. (53) Aniansson, E. A. G.; Wall, S. N.; Almgrem, M.; Hoffman, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem. 1976, 80, 905.