Langmuir 1996, 12, 3567-3573
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Trapping of Counterions and Water on the Surface of Cationic Micelles Giovanna Mancini* and Cesare Schiavo† Centro CNR di Studio sui Meccanismi di Reazione, Dipartimento di Chimica, Universita` degli Studi di Roma “La Sapienza”, P.le A. Moro 2, 00185 Roma, Italy
Giorgio Cerichelli* Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita` degli Studi dell’Aquila, Via Vetoio, Coppito Due, 67010 L’Aquila, Italy Received November 16, 1995. In Final Form: April 19, 1996X The concentration ratios of three nucleophiles, bromide and chloride ions and water, at the micellar surface were determined from the yields of trans-1,2-dibromocyclohexane, trans-1-bromo-2-chlorocyclohexane, and trans-2-bromocyclohexanol, respectively, products of the bromination of cyclohexene in aqueous (i) cetyltrimethylammonium bromide (CTAB), (ii) cetylpyridinium bromide (CPyB), and (iii) cetylpyridinium chloride (CPyC), with added NaX (X ) Br-, Cl-). The generally small values of [X-]/[H2O] obtained are attributed to a highly ordered interfacial structure in which the halide ions occupy well defined positions. A sphere to rod transition caused by the addition of NaX for all the surfactants was followed by the variation of the ratio [X-]/[H2O] and by 1H NMR.
Introduction The interface of the aggregates formed by ionic surfactants in water is located between the outer water bulk and the inner hydrophobic core. This anisotropic surface1 contains various species such as head-groups, counterions, water, and solubilizates. Investigations of the surface composition of aggregates should provide useful structural and chemical information because this is the region in which ionic reactions take place.2 Various methods3 are commonly employed for estimating the concentrations of counterions, water, and solutes at the aggregate-water interface. Traditionally each species requires a separate experimental method and theoretical assumptions. To our knowledge, the only method which simultaneously allows the estimation of the local interfacial concentrations of various nucleophiles at the surface of micellar aggregates was developed by Romsted.4 It is a chemical method, in that product yields from dediazoniation of a hydrophobic arenediazonium salt bound to a cetyltrimethylammonium halide (CTAX) are assumed to be proportional to the concentration of nucleophiles at the micellar interface. In a previous work5 we reported on the chemioselectivity of the bromination reaction of cyclohexene in aqueous cetyltrimethylammonium bromide (CTAB) micelles. This reaction yielded almost quantitatively the trans-2-bromocyclohexanol. Romsted’s results appeared to be dif† Present address: School of Chemistry, University of Birmingham Edgbaston, Birmingham B15 2TT, U.K. X Abstract published in Advance ACS Abstracts, June 15, 1996.
(1) A micelle-watre interface is constituted by a volume which can approximately be defined as a spherical surface. (2) Bunton, C. A. In Cationic Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Dekker: New York, 1991; p 323. (3) (a) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York; 1975. (b) Fendler, H. J.; Patterson, L. K. J. Phys. Chem. 1970, 74, 4608. Ericksson, J. C.; Gillberg, G. Acta Chem. Scand. 1966, 20, 2019. (c) Kandori, K.; McGreevy, R. J.; Schechter, R. S. J. Phys. Chem. 1989, 93, 1506. (d) Rehfeld, S. J. J. Phys. Chem. 1971, 75, 3905. (e) Cerichelli, G.; Coreno, M.; Mancini, G. J. Colloid Interface Sci. 1993, 158, 33. (4) (a) Loughlin, J. A.; Romsted, L. S. Colloids Surf. 1990, 48, 123. (b) Chaudhuri, A.; Romsted, L. S. J. Am. Chem. Soc. 1991, 113, 5052. (5) Bianchi, M. T.; Cerichelli, G.; Mancini, G.; Marinelli, F. Tetrahedron Lett. 1984, 25, 5205.
S0743-7463(95)01039-0 CCC: $12.00
ferent from our result, so we decided to investigate the bromination reaction of cyclohexene in more detail. In this paper we report the use of the bromination reaction of cyclohexene (Scheme 1) for evaluating the concentration ratios of bromide and chloride ions and water at the surface of CTAB, cetylpyridinium bromide (CPyB), and cetylpyridinium chloride (CPyC) aggregates. Experimental Section All NMR spectra were recorded on a Varian XL 300 spectrometer. Gas chromatography analyses were obtained on a Varian Vista 6000 gas chromatograph by using a 25 m × 0.32 mm silica capillary column coated with cyano-propyl-phenyl silicone gum. Reagents. CTAB, CPyB, and CPyC (Fluka) were purified by the procedure of Duynstee and Grunwald.6 Cyclohexene of the highest purity (Fluka) was used without further purification. Standards. trans-1,2-Dibromocyclohexane (2a). Bromine was added dropwise to a vigorously stirred solution of cyclohexene (15 mmol) in CHCl3 (150 mL), up to persistence of color. The reaction mixture was washed with a saturated solution of aqueous NaHCO3 and dried over Na2SO4. Removal of solvent yielded the pure product as a pale-yellow oil (yield 97%). 1H NMR: δ ) 1.49 (m, 2H); δ ) 1.75-1.90 (m, 4H); δ ) 2.43 (m, 2H); δ ) 4.44 (m, 2H). 13C NMR: δ ) 22.32, C4, C5; δ ) 31.89, C3, C6; δ ) 55.09, C1, C2. trans-2-Bromocyclohexanol (2b). The bromohydrin was prepared according to Dalton and Jones.7 Cyclohexene (20 mmol) was dissolved in a mixture of DMSO (100 mL) and H2O (50 mmol). N-Bromosuccinimide (40 mmol) was added under N2. The reaction mixture turned yellow and evolved heat. After stirring for 15 min, the reaction was quenched by adding H2O (40 mL). The aqueous DMSO mixture was extracted with Et2O, and the organic layer was collected and dried over MgSO4. Removal of solvent yielded the pure product as a colorless oil (yield 90%). 1H NMR: δ ) 1.19-1.39 (m, 2H); δ ) 1.65-1.90 (m, 4H); δ ) 2.13 (m, 1H); δ ) 2.34 (m, 1H); δ ) 2.59 (m, 1H); δ ) 3.60 (m, 1H); δ ) 3.89 (m, 1H). 13C NMR: δ 24.12, C4; δ ) 26.68, C5; δ ) 33.53, C3; δ ) 36.22, C6; δ ) 61.86, C2; δ ) 75.32, C1. trans-1-Bromo-2-chlorocyclohexane (2c). Cyclohexene (10 mmol) was dissolved in 200 mL of an aqueous solution of 0.050 M cetylpyridinium cloride saturated with NaCl. Bromine (10 (6) Duynstee, E. F. S.; Grunwald, E. J. Am. Chem. Soc. 1959, 81, 4540. (7) Dalton, D. R.; Hendrickson, J. B.; Jones, D. Chem. Commun. 1966, 591.
© 1996 American Chemical Society
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Scheme 1. Bromination of Cyclohexene in the Presence of Various Nucleophiles
mmol) was added with stirring. After decolorizing, the reaction mixture was extracted with Et2O, and the organic phase was collected, washed with brine, and then dried over Na2SO4. Removal of solvent yielded a colorless oil as a mixture of two compounds in the ratio 2:1 (GLC analysis); the minor product was identified as 2-bromocyclohexanol on the basis of retention time. The major product (2c) was isolated by chromatography on silica gel using hexane as eluent (yield 52%). 1H NMR: δ 1.28 (m, 2H); δ ) 1.75-1.91 (m, 4H); δ ) 2.41 (m, 2H); δ ) 4.19 (m, 1H); δ ) 4.45 (m, 1H). 13C NMR: δ ) 22.55, C4; δ ) 23.33, C5; δ ) 32.76, C6; δ ) 33.29, C3; δ ) 55.45, C1; δ ) 62.85, C2. trans-2-Bromo-1-ethoxycyclohexane (2d). Bromine (30 mmol) was added dropwise to a vigorously stirred solution of cyclohexene (15 mmol) in absolute EtOH (150 mL). After removal of solvent the residue was dissolved in Et2O (100 mL), washed with saturated aqueous NaHCO3 solution, and then dried over Na2SO4. Removal of solvent yielded a colorless oil as a mixture of two compounds in a 3.8:1.0 ratio (GLC analysis and 13C NMR integration). The two compounds (2a and 2d) were identified in the mixture by 13C NMR; the major product was 2d. 13C NMR (2d): δ ) 15.51, CH3; δ ) 65.05, CH2O; δ ) 23.25, C4; δ ) 23.35, C5; δ ) 30.95, C3; δ ) 35.53, C6; δ ) 55.74, C2; δ ) 81.46, C1. Calibration of the Gas Chromatography Detector. The responses of the gas chromatograph detector (FID) to the products of the bromination reaction of cyclohexene were measured relative to trans-1,2-dibromocyclohexane. By injecting 1,2-dibromocyclohexane and 2-bromocyclohexanol or 1-bromo-2-chlorocyclohexane in different ratios, we determined a response factor of 1.00 for 1-bromo-2-chlorocyclohexane and 1.15 for 2-bromocyclohexanol relative to 1,2-dibromocyclohexane. For trans-2-bromo-1-ethoxycyclohexane, the response factor was estimated by comparing the integration of the 13C NMR spectrum and the GLC analysis of the mixture obtained in the preparation of trans-2-bromo-1-ethoxycyclohexane. This comparison yielded a response factor of 1.20 compared to 1,2dibromocyclohexane. The 13C NMR experiment was carried out under the following conditions: 30° pulse, 12 s repetition time, gated decoupled sequence to eliminate differences due to the heteronuclear 13C-1H NOE and to relaxation. All the measured responses were in good agreement with those calculated according to Musumarra et al.8 Bromination Reaction of Cyclohexene in Reference Solvents. Bromination Reaction of Cyclohexene in EtOH/H2O 80/20 and 60/40. Cyclohexene (0.29 mmol) and bromine (0.094 mmol) were added successively to a 20 mL solution of 0.40 M tetramethylammonium halide (TMAX; X ) Br-, Cl-) in EtOH/ H2O 80/20 (w/w) or EtOH/H2O 60/40 at 25.0 °C. Bromide and/or chloride ion concentrations were varied between 0.00 and 0.40 M, but TMA concentration was kept constant by mixing stock solutions of 0.40 M tetramethylammonium bromide (TMAB) and 0.40 M tetramethylammonium chloride (TMAC). Stock solutions were prepared in aqueous H2SO4 (5.0 × 10-3 M). After decolorizing, the reaction mixture was neutralized with a saturated aqueous solution of NaHCO3 and saturated with NaCl. Et2O (4 mL) was added, followed by dropwise addition of brine until phase separation. The aqueous phase was extracted with Et2O (2 × 3 mL). The organic layers were combined, dried over Na2SO4, and analyzed by GLC. Bromination Reaction of Cyclohexene in Dioxane/H2O 60/40. The procedure was analogous to that described for the EtOH/ (8) Musumarra, G.; Pisano, D.; Siskin, M.; Brons, G. Tetrahedron Comput. Methodol. 1989, 2, 17.
Mancini et al. H2O 60/40 mixture, except that dioxane replaced EtOH. In the workup of the reaction, the addition of NaCl alone caused phase separation. The aqueous phase was extracted with Et2O (2 × 3 mL), and the combined organic layers were dried over Na2SO4 and analyzed by GLC. Bromination Reaction of Cyclohexene in Dichloromethane. Cyclohexene (0.29 mmol) and bromine (0.094 mmol) were added successively to a solution of 0.40 M tetrabutylammonium halide (TBAX; X ) Br-, Cl-) (20 mL) in CH2Cl2 at 25.0 °C. Bromide and/or chloride concentration was varied between 0.00 and 0.40 M, but TBA concentration was kept constant, by mixing stock solutions of 0.40 M tetrabutylammonium bromide (TBAB) and 0.40 M tetrabutylammonium chloride (TBAC) in CH2Cl2. After decolorizing, the reaction mixture was washed with a saturated aqueous solution of NaHCO3 and then filtered through silica gel. The filtrate was analyzed by GC. Bromination Reaction of Cyclohexene in Self-Aggregating Surfactants. Bromination Reaction of Cyclohexene in CTAB in the Presence of Added Bromide or Chloride. Cyclohexene (0.076 mmol) and 330 µL of a stock solution of 0.15 M bromine (0.050 mmol) in water were added successively, at 25.0 °C, to a 20 mL aqueous solution of 0.030 M CTAB in the presence of variable concentrations of bromide ion up to 0.30 M, or chloride ion up to 2.7 M, obtained by mixing an aqueous stock solution of 0.030 M CTAB and an aqueous stock solution of 0.030 M CTAB and 0.30 M NaBr (or 2.7 M NaCl). After decolorizing, the reaction mixture was extracted with Et2O (3 × 8 mL), avoiding hard shaking to prevent emulsion formation. The organic fractions were washed with brine, dried over Na2SO4, and then analyzed (GC). The stock solution of bromine 0.15 M was prepared in doubly distilled water 5.0 × 10-3 M in H2SO4. Bromination Reaction of Cyclohexene in CPyB in the Presence of Added Bromide or Chloride. The procedure was analogous to that described for CTAB, with the exception of the concentration of the NaBr (0.050 M) and NaCl (2.5 M) stock solutions. Bromination Reaction of Cyclohexene in CPyC in the Presence of Added Bromide or Chloride. The procedure was analogous to that described for CTAB, with the exception of the concentration of the NaBr (0.10 M) and NaCl (1.6 M) stock solutions. Product Yields. All reported yields were normalized to the total quantity of products of the bromination reaction of cyclohexene. The concentration ratio of nucleophiles at the surface of aggregates can be calculated from yield data in a reference solvent, assuming that the reactivity ratios of the nucleophiles, Cl-, Br-, and H2O, do not change substantially. The reaction in EtOH/H2O 80/20 was carried out at various concentrations of halide ions as their tetramethylammonium salts (eqs 1-3),
%2aref ) k*2a[Br-ref]
(1)
%2bref ) k*2b[H2Oref]
(2)
%2cref ) k*2c[Cl-ref]
(3)
where ref ) reference solvent and k*x ) kx/k2a[Br-] + k2b[H2O] + k2c[Cl-] (x ) 2a, 2b, 2c). The selectivity constants were evaluated according to eqs 4-6 from the ratio of the percentage yields and that of the analytical concentration of nucleophiles in the reference solvent.
%2aref/%2bref ) SBr
%2aref/%2cref ) SBr %2cref/%2bref ) SCl
H2O[Br ref]/[H2Oref]
(4)
Cl-[Br ref]/[Cl ref]
(5)
H2O[Cl ref]/[H2Oref]
(6)
-
-
-
By assuming that the selectivity of the bromonium ion is the same at the micellar surface as it is in the reference solvent, we were able to calculate the ratios of the concentrations of the nucleophiles at the micellar surface (eqs 4a-6a)
%2am/%2bm ) SBr
-
%2bm/%2cm ) SBr
H2O[Br m]/[H2Om]
(4a)
Cl-[Br m]/[Cl m]
(5a)
-
Trapping on the Surface of Cationic Surfactants %2cm/%2bm ) SCl
-
H2O[Cl m]/[H2Om]
Langmuir, Vol. 12, No. 15, 1996 3569 (6a)
where m ) micellar phase. 1H NMR Experiments. 1H NMR spectra of D O solutions 2 of 0.030 M CTAB and 0.030 M CPyX ( X ) Br-, Cl-) in the presence of dioxane (10% molar ratio with respect to the surfactants, as internal standard) and of various concentrations of added NaX (X ) Br-, Cl-) were recorded at 25.0 °C. In CTAB solutions the relaxation time T2 of the signal of the trimethylammonium group was measured by using the Carr-Purcell-Meinboom-Gill sequence.
Results We carried out the bromination reaction of cyclohexene in a reference solvent to measure the selectivity constants SBr-H2O, SBr-Cl-, and SCl-H2O (eqs 4-6). We chose the EtOH/ H2O 80/20 mixture as a reference solvent because its polarity ( ) 32.8)9 is similar to that of the aggregate surface.10 We used TMAX as the nonaggregating salt because it is more soluble than NaX and because it is a better mimic of the aggregate/water interface. We kept TMAX at a constant concentration (0.40 M) to avoid solvent reorganization effects on the activation energy of the reaction11 and varied the concentration of Br- between 0.00 and 0.40 M and that of Cl- between 0.40 and 0.00 M.12 The results of the reaction carried out in the reference solvent are reported in Figure 1 and Table 1. The percentage of trans-1,2-dibromocyclohexane has been corrected by 6.4% (yield at [TMAB] ) 0), which probably corresponds to the yield of 1,2-dibromocyclohexane formed by the collapse of the ion pair “bromonium ion/bromide ion” in the first step of the bromination reaction.13 The fact that the 2b/2d ratio does not depend on the relative amounts of Cl- and Br- in solution and that the selectivity of bromonium ion toward water versus ethanol is independent of Cl-/Br- ratio demonstrates either that the formation of a free bromonium ion takes place or, if the “bromonium ion/halide ion” ion pair is the reactive species, that the reactivity of the bromonium ion is independent of halide ion (Br- or Cl-) in the ion pair. The selectivity constants (Table 2) have been calculated according to eqs 4-6, assuming that the concentration of water is constant (8.90 M).14 Analogous experiments were carried out in (i) EtOH/H2O 60/40 ( ) 43.4),9 (ii) dioxane/H2O 60/40 ( ) 28.0)15 in the presence of TMAX 0.40 M, and (iii) CH2Cl2 ( ) 9.02)16 in the presence of tetrabutylammonium halide (TBAX) 0.40 M. As before, the concentration of water was assumed to be constant in both the solvents: 18.7 M in EtOH/H2O 60/40 and 22.0 M in dioxane/H2O 60/40. (9) Hazlet, S. E.; Collison, E. R. B. J. Am. Chem. Soc. 1944, 66, 1248. (10) (a) Van der Langkruis, G. B.; Engberts, J. B. F. N. J. Org. Chem. 1984, 49, 4152. (b) Bunton, C. A.; Minch, M. S.; Hildago, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262. (c) Menger, F. M.; Yoshinaga, H.; Venkatasubban, K. S.; Das, A. R. J. Org. Chem. 1981, 46, 415. (d) Ramachandran, C.; Pyter, R. A.; Mukerjee, P. J. Phys. Chem. 1982, 86, 3198. (e) Al-Lohedan, H.; Bunton, C. A.; Mhala, M. M. J. Am. Chem. Soc. 1982, 104, 6654. (f) Bunton, C. A.; Ljungrens, S. J. Chem. Soc., Perkin Trans. 2 1984, 355. (11) Ritchie, C. D.; Skinner, G. A.; Badding, V. G. J. Am. Chem. Soc. 1967, 89, 2063. (12) The concentration 0.40 M corresponds to the solubility of TMAB in the reference solvent. (13) Since we are interested in the reactivity ratio of free nucleophiles in solution, we did not take into consideration the amount of product whose formation is not due to free ions. All other percentages were normalized with respect to the corrected percentage of 1,2-dibromocyclohexane. (14) This is an average of the molarities of water in the two stock solutions of TMAX (8.89-8.91 M, respectively, for X ) Cl- and X ) Br ). (15) D’Aprano, A.; Fuoss, R. M. J. Phys. Chem. 1968, 72, 4710. (16) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 2nd ed.; Harper & Row Publisher: New York, 1981; Chapter 2.
Figure 1. Plot of the yield data of the bromination reaction of cyclohexene in EtOH/H2O 80/20 in the presence of TMAX 0.040 M versus the concentration of TMAX at 25.0 °C. Table 1. Percentage Yields of the Bromination Reaction of Cyclohexene in EtOH/H2O 80/20 in the Presence of TMAX 0.40 M at 25.0 °C %2aa (Br-)
%2ba (H2O)
%2ca (Cl-)
%2da (EtOH)
6.4 (0.00) 14.8 (8.10) 23.5 (17.5) 32.3 (27.0) 40.2 (35.5) 48.2 (44.2) 55.7 (52.3) 63.3 (60.5) 70.7 (68.4)
19.0 (20.7) 18.0 (19.4) 17.0 (18.3) 16.0 (17.0) 14.7 (15.9) 14.0 (15.1) 13.0 (14.0) 12.0 (12.9) 10.9 (11.7)
42.0 (45.5) 37.0 (39.9) 31.2 (33.6) 25.3 (27.3) 20.1 (21.6) 14.2 (15.2) 9.3 (10.1) 4.3 (4.7)
32.0 (34.8) 30.3 (32.6) 28.4 (30.6) 26.7 (28.7) 25.0 (27.0) 23.6 (25.5) 22.0 (23.7) 20.4 (22.0) 18.0 (19.9)
[TMAB], [TMAC], M M 0 0.050 0.10 0.15 0.20 0.25 0.30 0.35 0.40
0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.050 0
a Percentages obtained after correction of dibromide yields due to the cage reaction12 are reported in parentheses.
Table 2. Selectivity Constants in Various Solvents
SBr-H2O SBr-ClSCl-H2O
EtOH/H2O 80/20 ( ) 32.84)9
EtOH/H2O 60/40 ( ) 43.40)9
dioxane/H2O 60/40 ( ) 27.96)15
100 1.6 60
67 1.3 53
90 1.6 55
CH2Cl2 ( ) 9.02)16 1.2
The values of the selectivity constants obtained in all the reference solvents are reported in Table 2. We then carried out the bromination reaction of cyclohexene in the presence of (i) CTAB (0.030 M) and (ii) CPyX (X ) Br, Cl) (0.030 M) with or without added NaX in both cases. The results of these experiments, reported in Tables 3-8, as percentages of products obtained, were used to calculate the concentration ratios of nucleophiles at the micellar surfaces (Tables 3-8; Figures 2-7) by using the selectivity constants (eqs 4a-6a) measured in the reference solvent. The experiments in aqueous CPyC in the presence of added NaCl (Table 8) show that the percentage of trans1,2-dibromocyclohexane due to the collapse of the ion pair “bromonium ion/bromide ion” is less relevant relative to the bromination reaction carried out in the reference solvent and its contribution decreases with increasing concentration of NaCl. In these experiments it was possible to take into consideration only the products due to the free nucleophiles (2b and 2c). As shown in Table 8, this correction does not affect the [Cl-]/[H2O] ratio.
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Table 3. Percentage Yields of the Bromination Reaction of Cyclohexene in Aqueous CTAB 0.030 M in the Presence of Added NaBr at 25.0 °C [NaBr], M
%2a
%2b
104[Br-m]/[H2Om]
0 0.00010 0.00030 0.00050 0.00060 0.00075 0.0010 0.0030 0.0050 0.010 0.030 0.10 0.30
11.0 13.2 13.3 13.5 13.6 14.0 14.7 15.6 16.1 16.1 19.3 19.6 20.6
89.0 86.8 86.7 86.5 86.4 86.0 85.3 84.4 83.9 83.9 80.7 80.4 79.4
13.0 15.2 15.3 15.6 15.7 16.3 17.2 18.5 19.2 19.2 23.9 24.4 25.9
Table 4. Yield Data of the Bromination Reaction in Aqueous 0.030 M CTAB in the Presence of Added NaCl at 25.0 °C [NaCl], M
%2a
%2b
0 0.0010 0.0030 0.0050 0.0070 0.010 0.050 0.10 0.25 0.30 0.40 0.80 1.0 1.2 1.3 1.4 1.6 1.8 2.0 2.5 2.7
11.0 10.8 11.5 15.3 15.4 15.2 13.5 12.1 9.0 7.2 5.0 4.0 4.0 3.50 3.50 3.50 3.40 3.30 3.40 3.40 3.40
89.0 89.2 88.5 84.7 84.6 84.8 82.0 81.0 81.0 80.3 80.0 79.0 70.9 64.0 62.0 59.7 58.6 57.5 56.7 56.3 56.3
%2c
104[Br-m]/ [H2Om]
4.5 6.9 10.0 12.5 15.0 17.0 25.1 32.5 34.5 36.8 38.0 39.2 39.9 40.3 40.3
13.0 12.1 12.9 18.1 18.2 17.9 16.5 14.9 11.1 8.97 6.25 5.06 5.64 5.47 5.65 5.86 5.80 5.74 6.00 6.04 6.04
104[Cl-m]/ [H2Om]
Table 6. Yield Data of the Bromination Reaction of Cyclohexene in Aqueous 0.030 M CPyB in the Presence of Added NaCl at 25.0 °C [NaCl], M %2a %2b %2c 104[Br-m]/[H2Om] 103[Cl-m]/[H2Om] 0 0.0010 0.0050 0.010 0.030 0.10 0.30 0.40 0.60 0.80 1.0 1.3 1.6 1.8 1.9 2.0 2.5
15.1 14.7 13.6 13.6 12.5 11.2 6.4 5.6 5.5 5.4 5.4 4.8 4.8 4.6 4.4 4.4 4.4
84.9 85.3 86.4 86.4 84.2 82.1 80.7 79.9 77.0 71.0 66.7 61.5 54.4 53.1 53.2 53.1 53.1
3.3 6.7 12.9 14.5 17.5 23.6 27.9 33.7 40.8 42.3 42.4 42.5 42.6
17.8 17.2 15.7 15.7 14.8 13.6 7.93 7.01 7.14 7.61 8.10 7.80 8.82 8.66 8.27 8.25 8.19
0.653 1.36 2.66 3.02 3.79 5.54 6.97 9.13 12.5 13.3 13.3 13.3 13.4
Table 7. Yield Data of the Bromination Reaction of Cyclohexene in Aqueous 0.030 M CPyC in the Presence of Added NaBr at 25.0 °C [NaBr], M %2a %2b %2c 104[Br-m]/[H2Om] 104[Cl-m]/[H2Om]
9.15 14.0 20.6 25.9 31.3 35.9 59.0 84.6 92.7 103 108 114 117 119 119
Table 5. Yield Data of the Bromination Reaction of Cyclohexene in Aqueous 0.030 M CPyB in the Presence of Added NaBr at 25.0 °C [NaBr], M
%2a
%2b
104[Br-m]/[H2Om]
0 0.0010 0.0030 0.0050 0.0070 0.010 0.015 0.020 0.030 0.035 0.040 0.050
15.1 15.1 15.0 15.1 15.1 15.1 15.1 15.8 16.6 17.0 17.5 18.5
84.9 84.9 85.0 84.9 84.9 84.9 84.9 84.2 83.4 83.0 82.5 81.5
17.8 17.8 17.6 17.8 17.8 17.8 17.8 18.8 19.9 20.5 21.2 22.7
Consequently this correction was avoided in all other experiments in micelles. To detect the range of concentrations in which the sphere to rod transition takes place, we carried out a 1H NMR investigation in the D2O solutions of CTAB and CPyX with and without added NaX. We followed the broadening of the 1H NMR signals of the trimethylammonium headgroups and the alkyl chain of the surfactants17 relative to the signal of dioxane (as internal reference) as a function of NaX concentration. Whenever possible we used also the Carr-Purcell-Meinboom-Gill sequence in order to measure T2 of the trimethylammonium head-group of CTAB. The two methods were in pretty good agreement. Results are reported in Figures 2-7.
0 0.0050 0.010 0.030 0.060 0.080 0.10
3.8 4.9 7.1 10.4 14.6 15.5 21.8
86.8 88.3 87.4 87.0 84.2 83.4 77.1
9.4 6.8 5.4 2.6 1.1 1.1 1.1
4.40 5.55 8.12 12.0 17.3 18.6 28.3
18.0 12.8 10.3 4.98 2.18 2.20 2.38
Table 8. Yield Data of the Bromination Reaction of Cyclohexene in Aqueous 0.030 M CPyC in the Presence of Added NaCl at 25.0 °C [NaCl], M
%2a
%2ba
%2ca
104[Cl-m]/[H2Om]b
0 0.0050 0.0100 0.030 0.060 0.10 0.30 0.60 1.0 1.3 1.6
3.8 3.7 3.7 3.6 3.1 2.8 2.1 2.0 1.8 1.5 1.4
86.8 (90.2) 85.9 (89.2) 85.5 (88.8) 84.5 (87.7) 84.1 (86.8) 83.1 (85.4) 79.4 (81.1) 74.3 (75.8) 66.4 (67.6) 59.1 (60.0) 51.9 (52.6)
9.4 (9.8) 10.4 (10.8) 10.8 (11.2) 11.8 (12.3) 12.8 (13.2) 14.1 (14.6) 18.5 (18.9) 23.7 (24.2) 31.8 (32.4) 39.4 (40.0) 47.0 (47.4)
18.0 (17.6) 20.2 (20.2) 21.1 (21.0) 23.3 (23.5) 25.4 (25.3) 28.3 (28.3) 38.8 (38.8) 53.2 (53.2) 80.0 (79.8) 111 (111) 150 (150)
a Percentages obtained after correction of dibromide yields due to the cage reaction12 are reported in parentheses. b Ratio obtained after correction of dibromide yields is reported in parentheses.
Discussion The results reported in Table 2 support our assumption that the selectivities of the bromonium ion toward the nucleophiles on the surface of the aggregate are similar to those in the reference solvent because large changes of solvent polarity do not change the selectivity constants significantly. The values of the [Br-m]/[H2Om] and [Cl-m]/[H2Om] ratios are significantly different from those expected. If we assume a concentration of H2O ∼ 40 M (which is lower (17) The growth of a micellar aggregate (sphere to rod transition) is characterized by a tighter packing of the monomers and by a reduced mobility of the larger aggregate; these phenomena can be followed in an NMR investigation by shortening of T2 and by a consequent broadening of NMR signals. The mobility of the bulk solvent and of reference substances that are present only in the bulk is scarcely or not affected by the growth of the aggregates. If in a micellar solution with a bulk reference a phase transition is promoted, the areas of the signals of the surfactant and of the reference will be constant, while the relative heights will change due to the broadening of the surfactant signals. At the same time, having an internal standard, we take care, at least partially, of the fact that real broadening measures T2* and not T2.
Trapping on the Surface of Cationic Surfactants
Langmuir, Vol. 12, No. 15, 1996 3571
Chart 1
than that of pure water but certainly much higher than that expected on the surface of micelles4,18) on the surface of the micellar aggregate, we estimated a range of concentrations of both Br- (0.05-0.1 M) and Cl- (0.070.6 M) from [Br-m]/[H2Om] and [Cl-m]/[H2Om], respectively, which is below the stoichiometric concentration of each anion in their respective solutions. These values are about one order of magnitude lower than those estimated theoretically18 and experimentally by Romsted.4 On the other hand, if we estimate the ranges of H2O concentration on the basis of the Cl- and Br- concentrations reported by Romsted, we obtain values of 200-1500 M ([Cl-m]/ [H2Om]) and 1000-3000 M ([Br-m]/[H2Om]) at the micellar surface. This approach indicates either that the concentration of halide ion is lower than the analytical concentration in water or that the concentration of water is impossibly high. Alternatively we may think that halide ions are localized near head-groups by noncovalent bonding interactions such as ion-ion electrostatic and dispersive forces and that the bromonium ion may be formed in an area of the micellar interface which has a high concentration of water. As for the formation and localization of bromonium ion we have to consider both a situation in which, Br- being in excess over Cl-, the brominating agent is the tribromide ion and a situation in which, Cl- being in excess over Br-, the brominating agent is molecular bromine. The former case is illustrated in a very simplified way in Chart 1, in which a view of the micellar surface is seen from the bulk aqueous phase. In this case the formation of the bromonium ion takes place in close proximity to the bromide ion which, in the presence of molecular bromine, has formed the tribromide ion (i.e. the brominating agent19). The reactive intermediate has the possibility either of reacting, after rearranging, within the “bromonium ion/bromide ion” ion pair or of reacting with the most available nucleophile on the back side of the halonium ion, that is water; in fact, both bromide and chloride ions are bound to well defined positions far away from the bromonium ion. In the latter case (i.e. chloride ion predominates on the surface) molecular bromine (the brominating agent) is likely to be located closer to the halide ion than the tribromide ion, since it has no charge and is less polarizable (i.e. it is not strongly bound to well defined positions on the surfce of the aggregate as it is tribromide ion). This hypothesis might explain the fact that with equal quantities of total bromide or chloride ion (Tables 5 and 8) we estimate concentrations of chloride ion higher than those of bromide ion. This result is rather unexpected, as it is known3c,4 that the binding constant of chloride ion, to these aggregates, is lower than that of bromide ion. Alternatively, it might be the case that the nucleophilicity of water and chloride and bromide ions on the surface of the aggregate changes with regard to that in the reference solvent, revealing that the assumption made (18) Mukerjee, P. J. Phys. Chem. 1962, 66, 943. (19) Cerichelli, G.; Grande, C.; Luchetti, L.; Mancini, G. J. Org. Chem. 1991, 56, 3025.
Figure 2. Variation of the ratio [Br-m]/[H2Om] as a function of NaBr concentration in the bromination reaction of cyclohexene in aqueous 0.030 M CTAB at 25.0 °C. Variation of the ratio of the intensities of the 1H NMR signals (dioxane/alkyl chain and dioxane/trimethylammonium) and of the relaxation time (T2) of the 1H NMR signal of trimethylammonium as a function of NaBr concentration in a D2O solution of 0.030 M CTAB at 25.0 °C.
Figure 3. Variation of the ratio [Cl-m]/[H2Om] as a function of NaCl concentration in the bromination reaction of cyclohexene in aqueous 0.030 M CTAB at 25.0 °C. Variation of the ratio of the intensities of the 1H NMR signals (dioxane/alkyl chain and dioxane/trimethylammonium) and of the relaxation time (T2) of the 1H NMR signal of trimethylammonium as a function of NaCl concentration in a D2O solution of 0.030 M CTAB at 25.0 °C.
about selectivity was not correct. However, that would be in contrast with the result obtained which indicates that variations of the polarity of the medium do not change the selectivity constants significantly (Table 2). The graphs in which we report the ratios [Br-m]/[H2Om] versus the concentration of bromide ion and [Cl-m]/[H2Om] versus the concentration of chloride ion (Figures 2-7) are consistent with saturating the surface of the aggregate by counterions followed by a sphere to rod transition. Added halide ions lower the repulsive interactions between the cationic head-groups, decreasing the space between the head-groups and releasing water (higher values of [X-m]/[H2Om]), giving tighter packing and a rodlike structure. Note that the transition occurs at lower
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Figure 4. Variation of the ratio [Br-m]/[H2Om] as a function of NaBr concentration in the bromination reaction of cyclohexene in aqueous 0.030 M CPyB at 25.0 °C. Variation of the ratio of the intensities of the 1H NMR signals (dioxane/alkyl chain) as a function of NaBr concentration in a D2O solution of 0.030 M CPyB at 25.0 °C.
Figure 5. Variation of the ratio [Cl-m]/[H2Om] as a function of NaCl concentration in the bromination reaction of cyclohexene in aqueous 0.030 M CPyB at 25.0 °C. Variation of the ratio of the intensities of the 1H NMR signals (dioxane/alkyl chain) as a function of NaCl concentration in a D2O solution of 0.030 M CPyB at 25.0 °C.
concentrations of Br- than Cl- consistent with the higher binding specificity of bromide ion compared to chloride ion.3e In addition, further experimental evidence for such a phenomenon was obtained by 1H NMR experiments in which a sphere to rod transition in an aqueous solution of CTAB and CPyX was observed by variation of relaxation time and consequently of the line width17 of proton signals following addition of NaX. As shown in Figures 2-7, chemical trapping and NMR results are in pretty good agreement in evidencing the phase transition. In particular when we have a smooth transition promoted by high concentrations of added chloride (Figures 3, 5, and 7), the agreement is very good; when we have a sharp transition promoted by a low concentration of added bromide, the sensitivities of the chemical and spectrometric tools are slightly different. A comparison of the results concerning the two surfactants (CTAX and CPyX) evidences a different structure
Mancini et al.
Figure 6. Variation of the ratio [Br-m]/[H2Om] as a function of NaBr concentration in the bromination reaction of cyclohexene in aqueous 0.030 M CPyC at 25.0 °C. Variation of the ratio of the intensities of the 1H NMR signals (dioxane/alkyl chain) as a function of NaBr concentration in a D2O solution of 0.030 M CPyC at 25.0 °C.
Figure 7. Variation of the ratio [Cl-m]/[H2Om] as a function of NaCl concentration in the bromination reaction of cyclohexene in aqueous 0.030 M CPyC at 25.0 °C. Variation of the ratio of the intensities of the 1H NMR signals (dioxane/alkyl chain) as a function of NaCl concentration in a D2O solution of 0.030 M CPyC at 25.0 °C.
of the interface. In particular when the bromination was carried out in an aqueous solution of CPyB and CTAB in the absence of added bromide ion, a higher value of [Br-m]/ [H2Om] was observed with the former surfactant. If we assume that both CTAB and CPyB have the same degree of neutralization charge β (eq 8)3e
β ) [Br-m]/[Dn]
(7)
(where Dn ) micellized surfactant), a higher value of [Br-m]/[H2Om] indicates a lower content of water at the interface. This hypothesis seems to be supported by comparison of the results obtained from the bromination reaction carried out in aqueous CTAB and CPyB in the presence of added NaCl (Tables 4 and 6). In the case of CTAB (Table 4) the ratio [Br-m]/[H2Om] increases at low concentrations of added NaCl and decreases at higher concentrations of added salt. This result may indicate
Trapping on the Surface of Cationic Surfactants
that low concentrations of added NaCl release water from the surface of the aggregate, bromide ion being exchanged at higher concentrations of added salt. Conversely the same phenomenon is not observed in the case of bromination carried out in aqueous CPyB (Table 6). With this surfactant, the ratio [Br-m]/[H2Om] decreases even at low concentrations of added bromide ion, suggesting that water is not released from the surface of the aggregate. This observation is consistent with a lower content of water, at the surface, to give CPyB a tighter structure than that of CTAB. That could be due to stronger interactions between bromide ion and the cationic head-groups of CPyB which are more polarizable. In addition further evidence for these stronger interactions in CPyB as compared to those in CTAB was revealed by the X-ray resolution of the two crystalline structures CTAB20 and CPyB21 which put in evidence a shorter bromide-bromide distance in the case of CPyB; we believe that it is reasonable to think that also in aqueous solution the aggregates formed by CPyB have a tighter structure than those formed by CTAB. Conclusions The results we have obtained give interesting information on the structure of the micelle/water interface. We observed that the concentration ratios of nucleophiles on the surface of the micellar aggregates yield anomalous concentration values. Our results can be explained by the hypothesis that the specific interactions between bromide or chloride ions and an ammonium head-group keep the ions in a mobile but well defined position on the surface of the aggregate. This evaluation is generally applicable on the basis of our experience to both charged and neutral species,19,22 as each species has its own defined dynamic location on the surface of a micellar aggregate; this location depends on the structure of the solute as well as on that of the surfactant. (20) Campanelli, A.; Scaramuzza, L. Acta Crystallogr. 1989, C42, 1320. (21) Coiro, V. Personal communication. (22) Cerichelli, G.; Luchetti, L.; Mancini, G. Tetrahedron Lett. 1989, 30, 6209. Cerichelli, G.; Luchetti, L.; Mancini, G. Tetrahedron 1994, 50, 3797.
Langmuir, Vol. 12, No. 15, 1996 3573
The bromination reaction of cyclohexene takes place in an area of the micellar surface which is rich in water. As a consequence we observe an increase of the nucleophilicity of water and a decrease of that of chloride and bromide ions with respect to the reference solvent. The different specificities of chloride and bromide (i.e. chloride less specific than bromide) ion could also account for the observed higher nucleophilicity of the former ion. The results reported by Romsted4 are in agreement with theoretical previsions yet are still not in contrast with our results; in fact the reactive species he used to trap the nucleophiles is a cationic surfactant. Consequently, bromide and chloride ion should be located (as counterions) in close proximity to the cationic species. It is evident from both our and Romsted results that chemical trapping detects the local concentration as a snapshot of the microscopic situation in the close surroundings of the chemical probe. Moreover our results have shown (i) a sphere to rod transition promoted by addition of salt (in this transition the compression of volume between the cationic headgroups yields a decrease of the amount of water on the surface of the aggregate) and (ii) the difference between the structure of CPyB and CTAB, the former being tighter and less hydrated. Acknowledgment. Support of this work by CNR, MURST, and NATO is gratefully acknowledged. Supporting Information Available: Tables S1-S3 providing product yields for the bromination reaction of cyclohexene in reference solvents (i) EtOH/H2O 60/40 in the presence of 0.40 M TMAX (S1); (ii) dioxane/H2O 60/40 in the presence of 0.40 M TMAX (S2); and (iii) CH2Cl2 in the presence of 0.40 M TBAX (S3) and Tables S4-S6 providing the results of 1H NMR experiments carried out on D2O solutions of 0.030 M CTAB (S4), 0.030 M CPyB (S5), and 0.030 M CPyC (S6) in the presence of added NaX (6 pages). Ordering information is available on any current masthead page. LA951039Y
