New Method for Estimating the Degree of Ionization and Counterion

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New Method for Estimating the Degree of Ionization and Counterion Selectivity of Cetyltrimethylammonium Halide Micelles: Chemical Trapping of Free Counterions by a Water Soluble Arenediazonium Ion Iolanda M. Cuccovia,* Idelcio N. da Silva, and Hernan Chaimovich Departamento de Bioquı´mica, Instituto de Quı´mica, Universidade de Sa˜ o Paulo, Caixa Postal 26077, CEP 05599-970, Sa˜ o Paulo, SP, Brazil

Laurence S. Romsted Department of Chemistry, Wright and Rieman Laboratories, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903 Received May 21, 1996. In Final Form: August 27, 1996X Products from spontaneous reaction of a short-chain, water soluble arenediazonium salt, 2,4,6trimethylbenzenediazonium tetrafluoroborate (1-ArN2BF4), in aqueous micellar solutions of cetyltrimethylammonium halides ((CTA)X (X ) Cl, Br)) are used to estimate the degree of ionization, R, and the ion exchange constant, KBr/Cl. The arenediazonium ion (1-ArN2+) reacts by rate-determining loss of N2 to give an aryl cation that traps available nucleophiles, i.e. H2O, Cl-, and Br-, to give stable phenol (1-ArOH) and halobenzene products (1-ArCl and 1-ArBr), respectively. Product yields are determined by HPLC from calibration curves obtained from independently prepared standards. Reproducible yields are obtained at very low halide ion concentrations, on the order of millimolar, well within the range needed to detect the “free counterions” in the aqueous intermicellar pseudophase. The basic assumption of the method is that 1-ArN2+ remains in the aqueous pseudophase at all (CTA)X and NaX concentrations. Trends in the Stern-Volmer constant for fluorescence quenching of Ru(bpy)32+ by 1-ArN2+ in (CTA)Cl/ NaCl solutions strongly support this assumption. The results obtained by this method are in good agreement with literature values: R ) 0.25 and 0.29 for (CTA)Br and (CTA)Cl, respectively, and KBr/Cl ) 2.65 ( 0.4. Potential applications of the method are briefly discussed.

Introduction Packing of head groups and counterions at aggregate interfaces, such as micelles, vesicles, and monolayers, produces a high local counterion concentration; typical estimates are >1 M, much greater than the counterion concentration in the surrounding aqueous pseudophase, which is usually in the range 0.001-0.01 M.1-7 The complex relationships between solution composition, asymmetric charge distribution in the vicinity of the micellar surface, and micellar properties such as the binding of solutes and counterions, aggregate size and shape, and phase stability are an active area of research.1 Experimental methods used for estimating ion distributions include solubility, conductivity, potentiometry, NMR, UV/visible, and fluorescence spectroscopies, and reaction kinetics.8-10 Counterion distributions are often described * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991. (2) Evans, D. F.; Wennerstrom, H. The Colloidal Domain: Where Physics, Chemistry, Biology and Technology Meet; VCH Publishers: New York, 1994; p 515. (3) Romsted, L. S. Ph.D. Thesis, Indiana University, 1975. (4) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2; p 1015. (5) Mukerjee, P. J. Phys. Chem. 1962, 66, 943. (6) Bocker, J.; Brickmann, J.; Bopp, P. J. Phys. Chem. 1994, 98, 712. (7) Chaudhuri, A.; Loughlin, J. A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8351. (8) Surfactant Solutions: New Methods of Investigation; Zana, R., Ed.; Marcel Dekker: New York, 1985. (9) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC, 1971. (10) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357.

S0743-7463(96)00498-2 CCC: $14.00

in terms of a pseudophase ion exchange, PPIE, model.10 In the pseudophase models, the totality of the aggregates in solution is treated as a separate phase (i.e. a micellar pseudophase), and a two-site model is used to describe counterion binding; i.e., counterions are either “bound” to the micellar pseudophase or “free” in the surrounding aqueous pseudophase.10 The degree of counterion dissociation, R, in an aqueous solution containing only an ionic surfactant is defined in the PPIE model as4,10

[Xw] - cmc R)

[Dt] - cmc

(1)

where square brackets, here and throughout the text, indicate concentration in moles per liter of solution volume, [Xw] is the concentration of free ions, [Dt] is the stoichiometric concentration of surfactant, cmc is the critical micelle concentration, and ([Dt] - cmc) is the concentration of micellized surfactant.9 Determination of cmc values is straightforward, and a wide variety of methods yield similar, although seldom identical, values for the cmc of a particular ionic surfactant. However, estimates of [Xw] and R are more dependent on the nature of the method, and estimates of R may vary by 50% even for widely used surfactants such as the cetyltrimethylammonium halides, (CTA)X.3,4,11,12 Ionic micelles also bind counterions specifically, as do other supramolecular aggregates such as polyelectrolytes (11) Gunnarsson, G.; Jonsson, B.; Wennerstrom, H. J. Phys. Chem. 1980, 84, 3114. (12) Kresheck, G. C. In Water: A Comprehensive Treatise: Aqueous Solutions of Amphiphiles and Macromolecules; Franks, F., Ed.; Plenum Press: New York, 1975; Vol. 4; p 95.

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and ion exchange resins.10,13 Many properties of surfactant aggregates such as the cmc, aggregation number, Krafft point, and chemical reactivity depend on counterion type.10 Quantitative comparisons of counterion selectivity are often expressed in terms of ion exchange, an approach originally developed as part of the pseudophase ion exchange, PPIE, model for interpreting the effects of surfactant aggregates on chemical reactivity.10 The selectivity of (CTA)X micelles toward two different counterions, here Cl- and Br-, is defined by an ion exchange constant:

KBr/Cl )

[Brm][Clw] [Brw][Clm]

Scheme 1

(2)

where the subscripts w and m indicate free and micellar associated ions, respectively. The average value of KBr/Cl determined by equilibrium methods based on bulk solution property methods is 5.2 ( 1.6.14 The value of KBr/Cl obtained by analyzing properties of thermal reactions that are slow compared with ion or monomer redistribution is 2.7 ( 1.5.14 To date, all methods used to estimate KBr/Cl measure the micellar or aqueous pseudophase concentration of only one of the two competing ions. Estimating the concentration of the second ion at the micellar surface requires a value for R.15 The factor of approximately two difference in the values of KBr/Cl obtained from the two approaches is usually attributed to differences in the bound/free population sensed by these methods, but part of the problem may be caused by the uncertainty in how to define R in the presence of two different counterions. In aqueous acid in the dark, arenediazonium ions decompose by rate-determining formation of a very reactive aryl cation that traps weakly basic nucleophiles competitively to form stable products that can be analyzed quantitatively by HPLC.7,16,17 This chemical trapping approach has already been used to determine, simultaneously, counterion, alcohol, and water concentrations at the surfaces of cationic micelles and microemulsions and also the distributions of alcohols between aggregates and bulk aqueous and oil phases in microemulsions from product yields from dediazoniation of aggregate bound 4-hexadecyl-2,6-dimethylbenzenediazonium ion (16-ArN2+).7,18-21 Here we demonstrate that yields of 2,4,6-trimethylchlorobenzene (1-ArCl) and 2,4,6-trimethylbromobenzene (1-ArBr) products from chemical trapping of Cl- and Brby 2,4,6-trimethylbenzenediazonium ion (1-ArN2+), in aqueous solutions of cetyltrimethylammonium halides ((CTA)X (X ) Cl, Br; Scheme 1), provide reasonable estimates of counterion concentrations in the intermicellar aqueous pseudophase. In these experiments, 1-ArX yields from chemical trapping by the aryl cation intermediate, (1-Ar+) are proportional to the halide ion concentration ([Xw]) in the intermicellar aqueous pseudophase because 1-ArN2+ is excluded from the aggregate surface as demonstrated by fluorescence quenching experiments. (13) Marcus, Y.; Kertes, A. S. Ion Exchange and Solvent Extraction of Metal Complexes; Wiley-Interscience: London, 1969. (14) Morgan, J. Ph.D. Thesis, University of Sidney, 1994. (15) Bunton, C. A. In Kinetics and Catalysis in Microheterogeneous Systems; Gratzel, M., Kalyanasundaram, K., Ed.; Marcel Dekker: New York, 1991. (16) Zollinger, H. Diazo Chemistry I: Aromatic and Heteroaromatic Compounds; VCH Publishers, Inc.: New York, 1994; p 1. (17) Swain, C. G.; Sheats, J. E.; Harbison, K. G. J. Am. Chem. Soc. 1975, 97, 783. (18) Romsted, L. S.; Yao, J. Langmuir 1996, 12, 2425. (19) Chaudhuri, A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8362. (20) Yao, J.; Romsted, L. S. J. Am. Chem. Soc. 1994, 116, 11779. (21) Loughlin, J. A.; Romsted, L. S. Colloids Surf. 1990, 48, 123.

Values of [Xw], calculated from 1-ArCl and 1-ArBr yields by using standard calibration curves, are used to estimate R values (eq 1) for (CTA)Cl and (CTA)Br micelles. Yields of 1-ArCl and 1-ArBr obtained in (CTA)X solutions containing mixtures of Cl- and Br- over a range of (CTA)X and NaX concentrations are used to estimate KBr/Cl. The R values obtained for each surfactant are within the range of published values, and the average value of KBr/Cl agrees with those obtained by kinetics. Experimental Section Materials. (CTA)Br (Merck, Darmstadt, Germany) and (CTA)Cl (Kodak, Rochester, NY) were recrystallized from methanol-acetone mixtures.22 The cmc’s determined by conductivity are 1.2 × 10-3 M for (CTA)Cl and 9 × 10-4 M for (CTA)Br, in good agreement with literature values.9 2,4,6-Trimethylbenzenediazonium tetrafluoroborate (1-ArN2BF4) was prepared as previously described.7 Dediazoniation products (2,4,6-trimethylchlorobenzene (1-ArCl), 2,4,6-trimethylbromobenzene (1ArBr), and 2,4,6-trimethylphenol (1-ArOH)) and NaCl and tris(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate (Ru(bpy)3Cl2‚ 6H2O; Aldrich Chemical Co. Inc., Milwaukee, WI) were reagent grade and used as received. Distilled deionized water was used in the preparation of all solutions. The Cl- and Br- concentrations of all surfactant and salt stock solutions were determined by the method of Schales and Schales.23 Methods. Fluorescence Measurements. The effect of added 1-ArN2+ on the fluorescence intensities of Ru(bpy)3 Cl2 solutions was determined in the ratio mode by using a Hitachi F-200 fluorescence spectrophotometer fitted with a Xe lamp and glass cuvettes thermostated at 25 °C, at the following settings: λexc ) 450 nm, slitexc ) 10, λem ) 595 nm, slitem ) 10. Aqueous solutions of various concentrations of (CTA)Cl and NaCl with a constant concentration of Ru(bpy)32+ were prepared from stock solutions of (CTA)Cl (0.2 M) and NaCl (0.2 M), which were obtained by dissolving their respective solids in aqueous solutions of 3 × 10-6 M Ru(bpy)32+. Successive aliquots of 10 µL (up to 0.1 mL) of an aqueous 0.032 M 1-ArN2BF4 stock solution containing 3 × 10-6 M Ru(bpy)32+ were added to 3.0 mL of the (CTA)Cl/NaCl/Ru(bpy)32+ solutions, and the emission intensity was recorded after each addition. The 1-ArN2BF4 solution was kept frozen (-20 °C) until just before use to minimize spontaneous decomposition of 1-ArN2+. Dediazoniation Reactions. 1 mL aqueous solutions of HX, (CTA)X, and NaX, (X ) Cl, Br) were prepared by pipetting stock solutions of each component and water into 2 mL Teflon-stoppered volumetric tubes, 50 µL of an aqueous 0.01 M 1-ArN2BF4 solution was added, and then 50 µL of cyclohexane was layered on top before stoppering the tubes. The final concentrations of 1-ArN2+ and H+ in all solutions are 5 × 10-4 M and 2 × 10-4 M, respectively. All samples were kept in a constant temperature bath at 30 °C in the dark for about 12 h, g10t1/2.18 After reaction, each sample was diluted to 2 mL with 1-propanol to give a homogeneous (22) Florenzano, F. H.; Santos, L. G.; Cuccovia, I. M.; Scarpa, M. V.; Chaimovich, H.; Politi, M. J. Langmuir 1996, 12, 1166. (23) Schales, O.; Schales, S. S. J. Biol. Chem. 1941, 140, 879.

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solution that was stored at 5 °C until product analysis. Aqueous stock solutions of 1-ArN2BF4 were prepared just before use and kept in ice baths to minimize dediazoniation. The layer of cyclohexane is crucial. In our preliminary experiments and in previously published results7 the total yields of halobenzenes and phenol (Scheme 1) were generally well below 100% and the halobenzene, but not phenol, yields were irreproducible in the absence of cyclohexane. This problem is attributed to the significant vapor pressures of 1-ArBr and 1-ArCl and their low solubilities in water.7 The cyclohexane layer prevents their loss by vaporization both into the head space of the volumetric tubes and during workup. Product Analysis. Product yields were determined chromatographically by using a Shimadzu HPLC equipped with a SPD10A UV/visible detector, C-R6A integrator, LC-6 AD pump, 20 µL sample loop, and a Microsorb C18 (Rainin) reversed phase column (4.6 mm i.d. × 25 cm; 5 mm particle size). The sample loop was overfilled with product solution, eluted with 82% MeOH/ 18% H2O (v/v) (1.0 mL/min, P ) 137 kgf/cm2), and detected at 225 nm. Typical retention times for 1-ArOH, 1-ArCl, and 1-ArBr are 4.4, 14.4, and 15.2 min, respectively, consistent with published results.7 Reproducible calibration curves for peak area, obtained by computer integration, versus concentration of the standard solution of each product are linear. The equations used to calculate concentrations from peak areas are area1-ArOH ) 1.6 × 1010[1-ArOH], area1-ArCl ) 1.92 × 1010[1-ArCl], and area1-ArBr ) 2.18 × 1010[1-ArBr]. Total product yields based on the weight of added 1-ArN2+ are quantitative within experimental error (ca. (5%). Normalized product yields were calculated as the percent of the total phenol and halobenzene products. Equation 3 illustrates the calculation of the percent yield of 1-ArCl in terms of total products:

%1-ArCl )

[1-ArCl] × 100 (3) [1-ArOH] + [1-ArBr] + [1-ArCl]

Similar expressions were used to calculated %1-ArBr and % 1-ArOH.

Results Parts A and B of Figure 1 show that reproducible yields of 1-ArCl and 1-ArBr are obtained at NaCl and NaBr concentrations as low as 1 mM and that their percent yields are linear functions of [NaX] up to 0.12 M added salt with slopes of 23% M-1 and 45% M-1, respectively. The error in %1-ArX at 10 mM NaX is no greater than 10% and decreases to about 1% above 10 mM NaX. All 1-ArX yields obtained in CTAX solutions (Figures 4 and 5) are within this range. The insets in Figure 1A and B show that 1-ArCl and 1-ArBr yields have the same dependence on added sodium salts of Cl- and Br- as previously observed with their tetramethylammonium salts at 40 °C.24 Estimates of the aqueous pseudophase or “free” counterion concentration in (CTA)X solutions by chemical trapping are based on the assumption that an insignificant fraction of hydrophilic and positively charged 1-ArN2+ and its ion pair, (1-ArN2+‚X-), is associated with cationic micelles. Association of a small fraction of the arenediazonium ion with the micelles would make a significant contribution to the observed 1-ArX yield because the concentration of X- at the surface of (CTA)X micelles is usually two to three orders of magnitude greater than the stoichiometric surfactant concentration. To test for possible association of 1-ArN2+ to cationic micelles, we measured the effect of added (CTA)Cl, in the presence and absence of added NaCl, on the quenching of Ru(bpy)32+ by 1-ArN2+. Solutions of (CTA)Cl and NaCl were used because the relative fluorescence intensity of Ru(bpy)32+ (24) The downward curvature of the percent yield versus salt concentration plots has been attributed to changes in the activity coefficients of ground state 1-ArN2+ and X- ions and 1-ArN2+‚X- ion pairs with increasing ionic strength.7

Figure 1. Yields of 1-ArX from dediazoniation of 5 × 10-4 M 1-ArN2+ in aqueous NaX solutions containing 2 × 10-4 M HX at 30 °C. Insets show 1-ArX yields up to 3 M NaX. (A) X ) Cl; (B) X ) Br.

is independent of their concentrations.25,26 Experimental data for the quenching of Ru(bpy)32+ fluorescence by 1-ArN2+ were analyzed by using the Stern-Volmer equation:25

Φ0 ) 1 + KSV[1-ArN2+] Φ

(4)

where Φ0 and Φ are the fluorescence intensities in the absence and presence of 1-ArN2+, respectively. Typical linear Stern-Volmer plots for the effect of added 1-ArN2+ on Φ0/Φ ratios are shown in the inset of Figure 2. Micellar exclusion of dicationic Ru(bpy)32+ is well documented,25,27 and we assume that, under our experimental conditions, it resides exclusively in the aqueous pseudophase. Association of 1-ArN2+ to (CTA)Cl micelles would reduce its aqueous concentration, reduce its quenching efficiency of Ru(bpy)32+, and decrease the value of KSV. However, Figure 2 shows that KSV increases sharply with added (CTA)Cl above the cmc (1.2 × 10-3 M CTACl) and appears to approach a plateau at higher (CTAC)Cl concentrations. Added NaCl increases KSV values for the quenching of Ru(bpy)32+ by 1-ArN2+ (Figure 3), which is (25) Quina, F. H. Tese de Livre Doceˆncia. Instituto de Quı´mica, Universidade de Sa˜o Paulo, Sa˜o Paulo, Brasil, 1977. (26) Demas, J. N.; Addington, J. W. J. Am. Chem. Soc. 1976, 98, 5800. (27) Chaimovich, H.; Bonilha, J. B.s.; Politi, M. J.; Quina, F. H. J. Phys. Chem. 1979, 83, 1851.

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A

B Figure 2. Effect of added (CTA)Cl on the Stern-Volmer constant. Inset shows effect of added 1-ArN2+ on the fluorescence intensity ratio (Φ0/Φ) at different [(CTA)Cl]: (O) 0 M (CTA)Cl; (0), 0.0234 M (CTA)Cl; (4) 0.0468 M (CTA)Cl at 25 °C.

Figure 4. Increase in [Xf] with added (CTA)X in 2 × 10-4 M HX at 30 °C and no added salt. (A) X ) Br, (B) X ) Cl.

In a solution of a ionic surfactant, without added salt, the relationship between the concentration of halide ion in the aqueous pseudophase, ([Xw]) and the degree of ionization (R; eq 1) is given by:4,10,29

[Xw] ) R([(CTA)X] - cmc) + cmc + [HX]

Figure 3. Change in KSV with added NaCl: (O) (CTA)Cl ) 0 M; (3) 0.0234 M; (4) 0.0468 M; (0) 0.0936 M.

characteristic behavior for salt effects on quenching of positively charged fluorescent molecules by like-charge ionic quenchers.25,26,28 However, the effect of added (CTA)Cl on KSV, (Figure 2) cannot be attributed solely to a saltinduced increase in aqueous counterion concentration caused by micellar dissociation. Assuming R ) 0.29 (see below) for (CTA)Cl micelles, the effective [Cl-w] at 0.1 M (CTA)Cl is about 0.029 M. At this NaCl concentration, KSV ≈ 450 M-1 (Figure 3), but in 0.1 M (CTA)Cl, KSV ≈ 600 M-1. The large increase in KSV with added (CTA)Cl strongly supports our assumption that 1-ArN2+ is excluded from cationic micelles (see Discussion). Note that KSV increases with added NaCl even at ca. 0.1 M (CTA)Cl (Figure 3), indicating that 1-ArN2+ does not associate with (CTA)X micelles at the highest surfactant and salt concentrations used in these experiments. (28) Balzani, V.; Moggi, L.; Manfrin, M. F.; Bolletta, F.; Laurence, G. S.; Coord. Chem. Rev. 1975, 15, 321.

(5)

where [HX] is the stoichiometric concentration of added acid (see Methods). Equation 5 predicts that plots of [Xw] against ([(CTA)X] - cmc) should be linear with the slope ) R and the intercept ) cmc + [HX]. Parts A and B of Figure 4 show results obtained for (CTA)Br and (CTA)Cl, respectively. Note that [Xw] ≈ 0 at ([(CTA)X] - cmc) ≈ 0, because in the absence of micelles the concentration of free halide ions in solution is very low i.e. [HX] ) 2 × 10-4 M and the cmc values are 0.9 × 10-3 M for (CTA)Br and 1.2 x10-3 M for (CTA)Cl. The values of R obtained from the slopes in Figure 4 are 0.25 and 0.29 for (CTA)Br and (CTA)Cl, respectively. A unique feature of the chemical trapping method is that the concentrations of two anions, here Cl- and Brin the aqueous pseudophase, can be estimated simultaneously.7,21,30 Thus, unlike the case for other methods, KBr/Cl is calculated from product yield ratios using eq 2 without needing estimates of, or assumptions about, the value of R. In a solution containing (CTA)Cl with added NaBr, [Brf] and [Clf] were determined experimentally, as described above. Concentrations of micellar bound counterions are obtained from the difference of the (29) Quina, F. H.; Chaimovich, H. J. Phys. Chem. 1979, 83, 1844. (30) Chaudhuri, A.; Loughlin, J. A.; Romsted, L. S. Atualidades de Fisico-Quimica Organica-1991; Florianopolis, Brazil, 1991; p 176.

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Table 1. Data Used to Estimate KBr/Cl from 1-ArBr and 1-ArCl Product Yields Obtained from Dediazoniation of 5.0 × 10-4 M 1-ArN2+ in (CTA)X Solutions Containing NaX and 2.0 × 10-4 M HX at 30 °C [(CTA)Cl], M

[NaBr], M

1-ArCl, peak area, 10-4 µV

1-ArBr, peak area, 10-4 µV

[1-ArCl],a 106 M

[1-ArBr],a 106 M

1-ArCl,b %

1-ArCl,b %

[Clw],c 102 M

[Brw],c 102 M

[Clm]/ [Clw]d

[Brm]/ [Brw]d

0.022 0.032 0.043 0.054 0.064 0.086 0.108 0.043 0.064 0.108

0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.079 0.079 0.079

2.53 3.52 4.61 4.71 5.33 6.85 7.14 4.98 6.84 8.95

9.95 11.05 6.72 7.34 6.00 5.08 4.91 12.03 10.51 7.82

1.31 1.73 2.40 2.45 2.77 3.56 3.72 2.59 3.56 4.66

4.56 5.07 3.06 3.37 2.75 2.33 2.25 5.52 4.82 3.59

0.37 0.49 0.69 0.70 0.79 1.02 1.06 0.74 1.02 1.33

1.30 1.45 0.88 0.96 0.79 0.66 0.64 1.58 1.38 1.02

1.63 2.13 2.91 3.30 3.43 4.43 4.61 3.22 4.43 5.78

2.86 3.25 2.22 2.17 1.75 1.47 1.42 3.51 3.07 2.27

0.32 0.51 0.48 0.63 0.88 0.94 1.33 0.34 0.46 0.85

1.12 0.86 1.73 1.79 2.46 3.12 3.27 1.24 1.56 2.47

[(CTA)Cl], M

[NaBr], M

1-ArCl, peak area, 10-4 µV

1-ArBr, peak area, 10-4 µV

[1-ArCl],a 106 M

[1-ArBr],a 106 M

1-ArCl,b %

1-ArCl,b %

[Clw],c 102 M

[Brw],c 102 M

[Clm]/ [Clw]d

[Brm] [Brw]d

0.025 0.038 0.050 0.062 0.075 0.100 0.119 0.025 0.050 0.075 0.100

0.056 0.056 0.056 0.056 0.056 0.056 0.056 0.089 0.089 0.089 0.089

6.14 6.28 8.26 9.19 9.20 11.09 11.73 4.53 7.61 9.70 13.85

6.88 5.27 4.79 5.22 4.41 4.69 4.02 11.54 7.76 7.13 7.36

2.82 2.88 3.79 4.21 4.22 5.09 5.38 2.08 3.50 4.44 6.35

3.58 2.74 2.50 2.72 2.30 2.44 2.09 6.01 4.40 3.71 3.83

0.81 0.82 1.08 1.20 1.21 1.45 1.54 0.59 1.00 1.27 1.81

1.02 0.78 0.71 0.78 0.65 0.69 0.59 1.72 1.15 1.06 1.08

4.40 3.90 3.57 3.90 3.30 3.50 3.00 8.60 5.77 5.30 5.40

1.79 2.05 2.71 3.01 3.01 3.63 3.84 1.48 2.50 3.17 4.53

0.28 0.44 0.57 0.44 0.70 0.61 0.69 0.14 0.53 0.64 0.64

0.40 0.83 0.84 1.08 1.50 1.76 2.10 0.64 1.00 1.37 1.21

a See Experimental Section for calibration curves for converting HPLC peak areas into molarities. b See eq 3. c From the slopes of the lines in Figure 1A and B. d [Clm] ) [Clt] - [Clw]; [Brm] ) [Brt] - [Brw].

Figure 5. Plots of molar ratios of micellar bound to free Bragainst Cl- at four to seven different [(CTA)X] and [NaX] values at 30 °C and 2 × 10-4 M HX (see Table 1): (b) 0.0606 M (CTA)Cl + NaBr; (9) 0.079 M (CTA)Cl + NaBr; (2) 0.056 M (CTA)Br + NaCl; (1) 0.080 M (CTA)Br + NaCl.

stoichiometric concentration of each anion and its measured concentration in the aqueous pseudophase; i.e., [Brm] ) [NaBrT] - [Brf] and [Clm] ) [(CTA)ClT] - [Clf], where [NaBrT] and [(CTA)ClT] are the stoichiometric concentrations of NaBr and (CTA)Cl, respectively. A plot of [Brm]/[Brw] against [Clm]/[Clw] should be linear with the slope ) KBr/Cl (eq 2) if the selectivity is independent of counterion concentration and the ratio of the concentrations of the two counterions. Table 1 shows how [Brm]/ [Brw] and [Clm]/[Clw] values are obtained from HPLC peak areas for 1-ArBr and 1-ArCl, respectively, in solutions containing variable amounts of (CTA)Cl, (CTA)Br, NaCl, and NaBr. Each point in Table 1 is an average of at least two independent determinations. The results are plotted in Figure 5. Within experimental error the data describe a straight line and the average value of KBr/Cl obtained from the slope is 2.65 ( 0.4. Discussion Chemical trapping by aggregate bound 16-ArN2+ is a novel method for estimating interfacial compositions that

provides estimates of ion and molecule concentrations in the interfacial regions of aggregates that are difficult to obtain by other methods.7,18-21,30 1-ArN2+ does not associate with cationic micelles (see Results). Thus estimates of R and KBr/Cl obtained from product yields from dediazoniation of 1-ArN2+ in the aqueous solutions of cationic micelles should agree with published results because the selectivity of 1-Ar+ toward different nucleophiles in the intermicellar region will be the same as its selectivity toward these nucleophiles in the absence of micelles, i.e. in bulk aqueous solutions. The results in Figures 1, 4, and 5 and Table 1 show that HPLC is a convenient analytical method for obtaining reproducible estimates of halide product yields from dediazoniation of 1-ArN2+ in solutions containing millimolar concentrations of halide ions. The Cl- and Brconcentrations used here are about an order of magnitude below previously published results.7 At such low 1-ArCl and 1-ArBr yields, the protocol must include layering of cyclohexane over the aqueous phase to prevent product loss by vaporization followed by addition of sufficient 1-propanol after reaction to ensure formation of a homogeneous solution of all components prior to HPLC analysis. A fraction of the cyclohexane will dissolve in the (CTA)X solutions, but NMR chemical shift and line width results show that cyclohexane is located primarily in the micellar core of (CTA)Br micelles.31 Effects of added cyclohexane on R values and the counterion binding properties do not appear to have been measured, but added CCl4, which is more polarizable and more likely to be present at the micellar interface than cyclohexane, does not have a significant effect on R.12 A variety of theoretical and experimental evidence supports our conclusion that 1-ArN2+ is effectively excluded from the micellar pseudophase under the conditions used in these experiments. Theoretical models such as PPIE10 and Poisson-Boltzmann equation (PBE)15,32 provide estimates for the extent of co-ion exclusion. The PPIE model has been used extensively to describe counterion (31) Eriksson, J. C.; Gllberg, G. Acta Chem. Scand. 1966, 20, 2019. (32) Feitosa, E.; Agostinho Neto, A.; Chaimovich, H.; Cuccovia, I. M. Langmuir 1993, 9, 702.

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distributions in terms of two-site ion exchange; i.e., counterions are either micellar bound or free in the aqueous pseudophase. In a few cases, the PPIE model has been successfully adapted for treating co-ion distributions by assuming the counterion effects on co-ion distributions are described by a Donnan equilibrium.33-36 The PBE model gives excellent fits of observed rate constants for reactions in which one reactant is a co-ion and the other is a micellar bound organic substrate.36 It also provides direct estimates of co-ion distribution in micellar solutions and, by defining a thickness of a layer for “bound” ions at the micellar surface, the interfacial concentration of counter and co-ions.8,36 Co-ion concentrations estimated from the PBE model show that interfacial co-ion concentrations are a small fraction of the bulk concentration that increases with added salt.36 For example, the interfacial concentration of a co-ion that does not bind specifically to SDS micelles, with 0.01 added salt, is about 0.01% of its concentration in the surrounding aqueous phase.36 At 1 M added salt, the co-ion concentration at the surface of the SDS micelles is about 1% of the aqueous phase concentration.36 In the experiments described here, concentrations of (CTA)X and NaX never exceeded 0.1 M and, in the absence of specific interactions between (CTA)X micelles and 1-ArN2+, the fraction of micellar bound 1-ArN2+ should always be much less than 1%. In support of this conclusion, the observed rate constant for OH attack on N-methyl-4-cyanopyridinium ion, which has the same charge and similar hydrophilicity to that of 1-ArN2+, is completely unaffected by added (CTA)X.37 The quenching of Ru(bpy)32+ fluorescence by 1-ArN2+ in (CTA)Cl/NaCl solutions also supports the absence of significant binding of 1-ArN2+ to (CTA)Cl micelles. The marked increase in KSV with added (CTA)Cl (Figure 2, is consistent with an excluded volume effect; i.e., the volume available to Ru(bpy)32+ and 1-ArN2+ in the aqueous pseudophase decreases with added (CTA)Cl. The molar volume of (CTA)Cl micelles is ca. 0.32 L‚M-1, assuming their densities are ca. 1 g/mL.38 The maximum increase in the aqueous phase concentrations of Ru(bpy)32+ and 1-ArN2+ would be about 3% at 0.1 M (CTA)Cl. However, at 0.1 M (CTA)Cl, KSV is about twice that obtained at 0 M (CTA)Cl. This suggests that the excluded volume is considerably greater than the physical volume of the micelles. KSV also increases with added NaCl (Figure 4), as observed for salt effects on the quenching of other likecharged ions.25,26,28 The effective intermicellar ionic strength of a 0.1 M CTACl solution is ca. 0.03 (see Results). The expected value of KSV from a pure ionic strength effect (Figure 3) is ca. 450 M-1. The value of KSV in 0.1 M CTACl is ca. 600. Experimental KSV values are higher than expected on the basis of NaCl addition, the excluded volume, or the sum of both. The effect of (CTA)Cl on KSV is suggestive of a combination of effects that include ionic strength and quencher concentration in the intermicellar aqueous phase produced both by physical exclusion and electrostatic repulsion from the micelles. The effects of (CTA)Cl and salt on quenching of Ru(bpy)32+ fluorescence (33) He, Z.-M.; Loughlin, J. A.; Romsted, L. S. Bol. Soc. Chil. Quim. 1990, 35, 43. (34) Miola, L.; Abakerli, R. B.; Ginani, M. F.; Berci-Filho, P.; Toscano, V. G.; Quina, F. H. J. Phys. Chem. 1983, 87, 4417. (35) Quina, F. H.; Politi, M. J.; Cuccovia, I. M.; Martins-Franchetti, S. M.; Chaimovich, H. In Solution Behavior of Surfactants: Theoretical and Applied Aspects; Mittal, K. L., Fendler, E. J., Ed.; Plenum Press: New York, 1982; Vol. 2, p 1125. (36) Blasko, A.; Bunton, C. A.; Armstrong, C.; Gotham, W.; He, Z. M.; Nikles, J.; Romsted, L. S. J. Phys. Chem. 1991, 95, 6747. (37) Quina, F. H.; Politi, M. J.; Cuccovia, I. M.; Baumgarten, E.; Martins-Franchetti, M.; Chaimovich, H. J. Phys. Chem. 1980, 84, 361. (38) Mukerjee, P. J. Phys. Chem. 1962, 66, 1733.

Cuccovia et al.

are consistent with our assumption that 1-ArN2+ does not bind to (CTA)X micelles under the conditions used in chemical trapping experiments.39 The best demonstrations of micellar exclusion of 1-ArN2+ are the R values estimated for (CTA)Cl and (CTA)Br micelles. The values obtained here, R ) 0.29 for (CTA)Cl and R ) 0.25 for (CTA)Br are well within the range of R values reported by other authors using a variety of methods.3,12 At 0.05 M (CTA)Cl, we estimate [Clw] ) 0.014 M, assuming R ) 0.29 and cmc ) 0.0012 M. The local concentration of Cl- at (CTA)Cl surfaces estimated by chemical trapping is about 1.5 M.7 If 1% of 1-ArN2+ were micellar bound, the estimated value of [Clw] from the yield of 1-ArCl would increase by 0.015 M or almost 100%, but we did not obtain unusually high values. Our estimate of R is insensitive to less than 0.1% of probe binding, since the error in the determination of 1-ArX concentration is of the order of 5%. In sum, the reasonableness of the R values, the (CTA)Cl and NaCl induced increases in KSV, and the minimal binding of co-ions under the experimental conditions predicted by the PBE model and indicated by literature results are consistent with exclusion of 1-ArN2+ from (CTA)X surfaces. Thus, product yields obtained by chemical trapping reflect the concentrations of halide ions in the intermicellar aqueous pseudophase. Values of KBr/Cl in (CTA)X micelles have been estimated by a variety of methods.14,21,40 Our value of 2.65 ( 0.4 obtained by chemical trapping of Cl- and Br- ions in the aqueous pseudophase is well within published values and within the range found by chemical trapping using the micellar bound analog.21 The chemical trapping method will work with a wide variety of weakly basic, anionic nucleophiles. Preliminary unpublished results indicate that the dediazoniation rate of 1-ArN2+ is not significantly affected by high concentrations of I-, HSO3-, SO42-, RCO2-, and ROPO3H-, indicating that the reaction mechanism is the same in the presence of these ions. However, more basic nucleophiles such as N3-, SO32-, CN-, and OH- react within the mixing time, consistent with attack at the terminal nitrogen instead of rate-determining loss of N2.16,41 Chemical trapping in the aqueous pseudophase with 1-ArN2+ or in aggregates with its long-chain analog (16ArN2+) should work with virtually any type of association colloid, and one or both probes can be used in vesicles, polyelectrolytes, and monolayers either as a co-surfactant (16-ArN2+) or as a co- or counterion, 1-ArN2+. Detection limits may be enhanced by one to two orders of magnitude by using a fluorescence detector, provided the substituent does not quench the fluorescence. This approach should make the chemical trapping method particularly useful in more complex multicomponent systems because products are formed competitively and the yield of each product will be proportional to its concentration. Acknowledgment. L.S.R is grateful for financial support from the National Science Foundation (Grant CHE 95-26206) and the NSF U. S.-Latin American Cooperative ProgramsBrasil. I.M.C. and H.C. are grateful for support from the following Brazilian agencies: FAPESP, CNPq, and FINEP (PADCT). The authors are grateful to Dr. V. B. Junqueira for the use of HPLC facilities in early stages of this work. LA960498B (39) A full treatment of the results in Figures 2 and 3 is beyond the scope of this paper. (40) Morgan, J. D.; Napper, D. H.; Warr, G. G.; Nicol, S. K. Langmuir 1994, 10, 797. (41) Hegarty, A. F. In The Chemistry of the Diazonium and Diazo Groups, Part 2; Patai, S., Ed.; John Wiley & Sons: New York, 1978; p 511.