Colloids and Surfaces, 48 (1990) 123-137 Elsevier Science Publishers B.V.. Amsterdam

123

A New Method for Estimating Counter-Ion Selectivity of a Cationic Association Colloid: Trapping of Interfacial Chloride and Bromide Counter-Ions by Reaction with Micellar Bound Aryldiazonium Salts

JOHN A. LGUGHLIN and LAURENCE S. ROMSTED*

Department of Chemistry, Wright-Rieman Laboratories, Rutgers, the State University of New Jersey, New Brunswick, NJ 08903 (U.S.A.)

(Received 26 July 1989; accepted 4 December 1989)

ABSTRACT

Yields of aryl halides and phenolic products from dediazoniation of a hydrophobic aryldiazon- ium salt bound to cetyltrimethylammonium halide, CTAX, micelles in 0.1 to 0.2 M HX with added NaX (X = Cl- and Br- ) are used to estimate, simultaneously, the quantity of halide counter-ions and water at the micelle surface. The interfacial counter-ion concentration increases, both in solutions of CTABr with added NaBr and CTACl with added NaCl with a proportionate decrease in interfacial water. The selectivity of CTAX micelles toward Br- and Cl- in solutions containing mixtures of the two ions is obtained directly, unlike all previous methods which require an esti- mate of the fraction of bound counter-ions. Counter-ion exchange constants, Kg;;, estimated from aryl bromide/aryl chloride yield ratios at high NaX, are consistent with published values. Our results show, for the first time, that K!$ is insensitive to ionic strength up to 3.0 M NaX when [Br- ]/ [Cl-] = 1.0, but has modest dependence on the mole fraction of Br- at constant total NaX. Kzi is also independent of surfactant concentration, after correcting for the fraction of bound counter-ions. In principle, product ratios from reaction of hydrophobic aryldiaxonium salts can be used to estimate the relative concentrations of all nucleophiles at the surfaces of association colloids, provided stable aryl substituted products are formed via rate determining loss of Nz.

INTRODUCTION

The interaction of counter-ions with charged interfaces of aqueous micelles and other association colloids strongly influences their aggregate structure and catalytic activity [ 1,2]. In aqueous solution, anionic or cationic surfactants spontaneously form micelles which are approximately spherical, dynamic ag- gregates with a highly anisotropic interface composed of head groups, counter-

*To whom correspondence should be addressed.

0166-6622/90/$03.50 0 1990 - Elsevier Science Pub1ishersB.V.

124

Br’

ur Cl‘

N2+ “20 H20

Cl’

50

Cl’

H20 Na*

H20

Br’

Fig. 1. Schematic of a small section of a cationic micelle in an aqueous salt solution containing mixtures of Br- and Cl-. The drawing illustrates the hydrocarbon, interfacial and aqueous regions of the micelle and probable locations of surfactant monomer, substrate, counter-ions, co-ions and water.

ions, solubilizates, and water between their hydrocarbon cores and the sur- rounding bulk aqueous phase (Fig. 1) . The interfaces of other types of asso- ciation colloids, inverse micelles, oil-in-water and water-in-oil microemul- sions, vesicles, and monolayers have very similar, but sometimes more complex, structures [ 11.

Two models for the distribution of counter-ions around ionic micelles and association colloids are commonly employed. Classical electrostatic theory treats the interface as a charged surface neutralized by counter-ions in the diffuse electrical double layer extending radially from the aggregate’s surface [ 3,4]. In the alternative pseudophase ion exchange model, the total volume of the aggregates in solution is treated as a separate “pseudophase” and counter- ions are assumed to be either “bound” to the aggregate or “free” in the sur- rounding water [ 1,2,5,6].

In applications of the pseudophase ion exchange model to micellar effects on reactivity [ 2,5], the interfacial counter-ion concentration is assumed to be constant because measured degrees of counter-ion binding, j$ are insensitive to surfactant and counter-ion concentration and vary only slightly with counter- ion type [ 7,8]. However, /3 values do depend upon the method of measurement [ 31, and the assumption of constant /I fails for very hydrophilic counter-ions such as OH- and F- [ 2,9]. In solutions containing two or more counter-ions, the selective association of ions with the micelle surface is described in terms of ion exchange [5,8] (Scheme 1, where subscripts w and m stand for the

Scheme 1

Br, + Cl, % Br, + Cl,,,

(1)

125

aqueous and micellar pseudophases respectively). Although this model under- states the complex microenvironment of the aggregates, it often gives both good qualitative and quantitative fits of micellar effects on reactivity [ 251. Because the model ignores specific structural features of the aggregate, it is readily generalized to other association colloids and its assumptions are sus- ceptible to experimental test.

In the pseudophase ion exchange model, counter-ion selectivity is expressed as an ion exchange constant [ Eqn ( 1) 1. Numerous estimates of ion exchange constants between different counter-ions have been published; primarily anion exchange constants for cationic micelles [ 10-121. However, none of the cur- rent methods for estimating counter-ion selectivity can measure the relative quantities of two different counter-ions at the micelle surface simultaneously (Fig. 1). For example, the amount of Cl- in solution cannot be determined in the presence of Br- using specific ion electrodes [ 131, light scattering [ 141 or conductivity [ 151. Micellar bound organic substrates typically react with counter-ions such as OH- or H30+, but are inert to other similarly charged counter-ions present in the same solution (e.g., Br- or Na+, respectively) [5]. Heavy atom fluorescence quenching occurs with Br-, but not Cl- [ 111. Con- sequently, ion exchange constants are usually obtained by experimentally de- termining surfactant or salt concentration effects on the distribution of one counter-ion. The distribution of the second counter-ion, which is usually in large excess, is obtained by assuming a single value for p and 1: 1 exchange between the two counter-ions [5,6,8,11,13].

Aryl [ 16-181 and alkyl [ 19,201 diazonium salts are known to react with nucleophiles present at the surface of cationic micelles. In general, aryldiazon- ium salts are believed to undergo thermal decomposition in aqueous acid in the dark via rate-determining formation of aryl cation intermediates followed by diffusion controlled reactions with all available nucleophiles [ 211 (Scheme 2). The diazonium ion exhibits only a slight selectivity toward different anionic nucleophiles over water, and the observed rate constant is almost completely insensitive to the polarity of the medium [21a]. Thus, product distributions from reaction of micellar bound aryldiazonium salts should be proportional to

Scheme 2

Nz

L kW SIOW

R

Cl-

Br-

a-l-- I) R

R-ArCI R-ArBr

P”

R-ArN,’ R-Ar+ ‘,-o\ + H’

2 R = 16 z -COOC,,H,,

R

R= 1 I -COOCH, R-ArOH

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the quantity of counter-ions and water at the micelle surface. Our results dem- onstrate that yield ratios from trapping of anionic nucleophiles by phenyl cat- ion intermediates in cationic micelles give reproducible estimates of the selec- tivity of the micelle surface toward Cl- and Br- over a wide range of ionic strengths and surfactant concentrations. Because the reaction is insensitive to medium effects, the approach should be applicable to all types of association colloids.

EXPERIMENTAL

Materials

3-Hexadecyloxycarbonylbenzenediazoniumte,16-ArN,+,was prepared by esterification of 3-nitrobenzoylchloride with 1-hexadecanol in pyridine [ 221 to give hexadecyl3-nitrobenzoate followed by quantitative cat- alytic hydrogenation with Pd/C in EtOAc to give hexadecyl3-aminobenzoate [ 231. The amine was converted to 16-ArN$ using t-butylnitrite and borontri- fluoride etherate in THF by the procedure of Doyle and Bryker [ 24].3-Meth- yloxycarbonylbenzenediazonium tetrafluoroborate, l-ArN,+ , was prepared by nitration of methylbenzoate to give methyl 3-nitrobenzoate, which was re- duced to the amine and converted to the diazonium fluoroborate salt by the same procedure used for 16-ArN$. Repetitive recystallization of l-ArN,+ and 16-ArN$ from dry acetonitrile solutions of diazonium salt at room tempera- ture by addition of anhydrous ether co-solvent yielded a white crystalline solid which undergoes no significant decomposition when stored at - 10” C in the dark. Degradation of both diazonium salts in aqueous acid containing halide salts gave the expected products in high yield, generally > 90%, based on HPLC analysis (see product yields) and the weight of diazonium salt.

Authentic samples of dediazoniation products were either purchased, l-Arc1 (Aldrich) and l-ArOH (Pfaltz and Bauer ), or prepared by reaction of the ap- propriate acid chloride with 1-hexadecanol, 16-ArCl and 16-ArBr, acid cata- lyzed esterification of 3-bromobenzoic acid in refluxing methanol, 1-ArBr, or by acid catalyzed esterification in refluxing toluene of 3-hydroxybenzoic acid and 1-hexadecanol, 16-ArOH. All intermediates and products were analyzed by NMR, IR and TLC. After purification, spectra were free of extraneous ab- sorbances and are completely consistent with the molecular structures. All compounds gave single spots by TLC.

CTABr (Aldrich) and CTACl, which was prepared from hexadecylchloride and trimethylamine [ 251, were recrystallized repeatedly from MeOH/Et,O. Critical micelle concentrations, c.m.c.‘s, for both surfactants were determined using a Fisher du Nouy Tensiometer and surface tension-log surfactant con- centration plots were without minima. C.m.c.‘s agreed with literature values. Salts and acids (standardized reagents) were A.C.S. grade and used without

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RESULTS

All reactions were carried out at 40’ C because CTABr has a Krafft point near room temperature [ 271, and precipitates easily, especially at high added NaBr. The first order rate constant, lz, for dediazonation of l-ArN$ in 0.01 M HX at 40°C is 2.8.10-5 s-l. The rate constant for dediazoniation of 3-alkyl- oxycarbonylbenzenediazonium salts has apparently never be measured, but our values compare favorably with an extensive study of substituent effects on dediazoniation [ 281. The observed rate decreases linearly with added NAX, but by only about 20% at 5 M NaX and is the same in NaCl and NaBr within experimental error. The values of k, for dediazoniation of 16-ArNg in 0.01 M CTAX and 0.1 A4 HX at 40” C are virtually independent of added NaX up to 4.0 M and show only a small dependence on counter-ion type: 2.5*10-5 s-l 24.1% (6 data points) in CTACl and 3.25*10m5 s-‘-I- 1.5% (6 data points) in CTABr. The small differences in these rate constants in the presence and absence of CTAX and NaX is consistent with the well established insensitivity of dediazoniation reactions to medium effects [21a] and supports our assump- tion that the reaction proceeds via unimolecular loss of N, in micelles as well as water.

Each dediazoniation reaction was initiated by the procedure described in the experimental section, and product distributions were determined after 10 half- lives by HPLC. Chromatograms were generally free of additional peaks, indi- cating that conversion to halo and phenolic products is quantitative and, as expected, acid hydrolysis of the ester linkage is insignificant [ 291. Percent yields based on the initial weight of diazonium salt averaged 94.7% (45 runs). Because no other products were observed in significant quantity in the chro- matograms, we assumed that the primary source of error in the yields is in the weighed amount of diazonium salt (typically 2.5-9.2 mg). Therefore, all re- ported yields are normalized to the total quantity of halo and phenolic products.

Figure 2 shows the percent yields of halo and phenolic products from reac- tion’of l-ArN2+ and 16-ArNz+ with increasing NaX in 0.01 M HX and 0.01 M CTAX, 0.1 M HX, respectively. The yield of halo products increases linearly with added NaX and the yield of phenol decreases proportionally. The yield of 16-ArBr from dediazoniation in CTABr is always higher than that of 16-ArCl from reaction in CTACl, and the opposite is true for the 16-ArOH produced in each reaction. Even in the absence of added NaX, the yields of halo products for 16-ArNz are much higher in CTAX than for l-ArN2+ in water.

We estimated the local counter-ion concentration at the surface of CTABr and CTACl micelles from yield data in solutions containing a single counter- ion. Assuming that product yields for 16-ArN,+ at the micelle surface are di- rectly proportional to product yields for l-ArN$ in aqueous acid, a least squares fit of the yield data for 1-ArBr and l-Arc1 can be used as a calibration curve

129

PJaXMW Fig. 2. Normalized product yields for dediazoniation of I-ArNz in 0.01 M HX+NaX and of 16&N: in 0.1 A4 HX, 0.01 M CTAX+NaX (X=Br-, Cl-). Phenolic products, section A: are l-ArOH ( 0 ) in NaCl, ( 0 ) in NaBr; and 16-ArOH ( 0 ) in CTACl, ( n ) in CTABr. Halo products, section B: are l-Arc1 (0), I-ArBr (0 ) obtained in NaCl and NaBr, respectively; and 16-ArCl ( 0 ), 16-ArBr ( n ) obtained in CTACl+ NaCl and CTABr + NaBr, respectively.

TABLE 1

Estimates of interfacial counter-ion concentration from normalized percent yields of 16-ArX from dediazoniation of 16-ArN,+ in 0.01 M CTAX and 0.1 M HX with added NaX

[XT] UW Normalized yields ( % ) [X-l (M)”

16-ArBr 16-ArCl Br- Cl-

0.12 25.2 15.5 3.3 2.6 0.62 18.2 3.1 1.12 32.3 21.6 4.3 3.8 1.12 32.1 4.4 - 1.62 34.0 23.3 4.6 4.1 2.12 25.5 4.5 3.12 42.2 24.2 5.7 4.3 4.12 33.3 6.0

“Concentrations are in mol 1-l of reactive volume within the micellar pseudophase not in mol I-’ of total solution volume. Concentrations were estimated from least squares fits (cc. = 0.99) of the normalized % yields of I-ArX in water.

for the effective local anion concentration at the micelle surface. This calcu- lation is not exact because the reactivities of l-ArN$ and 16-ArN,+ may be slightly different because of the short versus long chain ester substituents and we included no correction for changes in activity coefficients. The results listed in Table 1 are consistent with the commonly held assumption that the counter- ion concentration at the micelle surface is on the order of 3.5 M [ 7,301.

The linear increase in halo products and concomitant decrease in 16-ArOH

130

with added NaX (Fig. 2) shows that the counter-ion concentration at the mi- celle surface increases gradually with added salt. The slightly smaller yield of 16-ArOH in the presence of Br- compared to Cl- is the first clear demonstra- tion that a large increase in salt concentration is required to reduce the amount of interfacial water significantly and that the extent of the reduction depends on counter-ion type.

To calculate KEi (Scheme 1) using Eqn ( 1) , the yield ratio of halo products must first be corrected for the slight selectivity of the dediazoniation reaction toward different anions. We assumed that the yield ratio of halo-substituted products for l-ArN 2’ determined in aqueous solution as a function of added salt reflects the small inherent selectivity difference of the diazonium ion to- ward reaction with Cl - and Br-. The ratio of the slopes of the lines for halo substituted l-ArN$ dediazoniation products shown in Fig. 2 was used to define the selectivity constant, Snr- ” -0.75 [Eqn (2) ] and the concentration ratio of bound counter-ions is given by Eqn (3) [ 311.

sc, %l-ArCl

Br= %l-ArBr (2)

[Br,l jCiJ=

Sc,%16-ArBr

B’% 16-ArCl (3)

We carried out several types of experiments to determine the effect of solu- tion composition on the selectivity of the micelle surface toward Br - and Cl-. Values of KE; calculated from product yield ratios obtained in 0.01 A4 CTAX solutions containing mixtures of counter-ions, with increasing total halide concentration, [X,] , but constant [ CIT] / [ Brr ] = 1, are listed in Table 2. The subscript T stands for the total concentration of counter-ion in solution, i.e.,

TABLE 2

Normalized percent yields of 16-ArX and calculated values of Kg: from dediazoniation of 16-ArN 2’ in 0.01 M CTAX and 0.2 A4 HX with increasing [X,] at [ Clr] / [BrT] = 1

[XT] 0.f) Normalized yields ( % )

16-ArBr 16-ArCl 16-ArOH

0.21 1.10 17.3 5.6 0.41 1.05 20.8 5.6 0.71 1.03 22.5 6.3 1.21 1.02 24.2 6.5 1.71 1.01 26.7 7.1 2.21 1.01 28.3 7.1 3.21 1.01 31.6 7.8

77.1 2.55 73.5 2.92 71.2 2.76 69.3 2.84 66.2 2.85 64.6 3.01 60.6 3.06

Average 2.86? 4.2%

“CalculatedusingEqns (1) and (3),&$=0.75

131

[X,] = [CTAX] + [NaX] + [HX]. I n all runs, [NaX]+ [HX]>[CTAX], therefore the ratio of added salt sets the ratio of counter-ions in solution; i.e., [ NaXr ] z [ NaX,] and [Cl,] / [Brr ] = [Cl,] / [ Br,] . The slight increase in KE; with added NaX is within experimental error and Kgf = 2.66 + 4.2%. Note that the yields of both halo products have increased by about 20% at 3.21 M NaX with a concomitant decrease in the quantity of phenol; again indicating gradual dehydration of the micelle surface.

The yields of halo and phenolic products obtained in 0.01 M CTAX and 0.2 M HX when [ CIT] / [ BrT] is varied are shown in Fig. 3 as a function of the mole fraction of Br- at three different total ionic strengths. Again, the yields of halo products increase with ionic strength and phenol yields decrease. Val- ues of Kg; calculated from this data show a small but significant decrease with increasing Br - mole fraction (Fig. 4), indicating an increase in selectivity toward Cl- by the micelle surface despite the large increase in the fraction of surface covered by Br- at high Br- mole fraction. All KE; values fall on a single line, cc. = 0.91 (Fig. 4)) within approximately + 5% (error bars, Fig. 4), even at 0.21 M NaX, where the assumption that [NaXr ] x [NaX,] is least valid. This observation is without precedent and is inconsistent with the operational assumptions of the pseudophase ion exchange model [ 6,8] and a recent Pois- son-Boltzman treatment, that includes specific counter-ion binding [ 41.

Table 3 lists yields of 16-ArBr and 16-ArCl and estimates of KEf for increas- ing [ CTAX] at 0.2 M HX at constant total counter-ion [XT] = 1.0 M and [ CIT J / [Brr ] = 1.02. Values of K!$ were first calculated as above, assuming that the concentration ratio of added salt set the concentration ratio in the aqueous phase; Kgf = 3 at 0.01 M CTAX when [NaX]>>[CTAX], but de- creases to Kgf = 2 in 0.4 M CTAX, when almost half the stoichiometric amount

100 A

0 0.2 0.4 0.6 0.8 1.0

I& l/WI Tl + PrTl) . . Fig. 3. Normalized product yields for dediazoniation of 16-ArN,+ in 0.2 M HX and 0.01 M CTAX plottedagainst BrT mole fraction at [XT]=0.21 M (O), 1.11 M (m), 2.11 M (A). Section A, phenolic products; section B, halo products (open symbols 16-ArCl, closed symbols 16-ArBr).

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4.0 -

3.6 -

Liz 3.2- l A

a 2.6 -

A

phh 2.4 -

2.0

0 0.2 0.4 0.6 0.6 1.0

[Br, 1 1 WI J + IBr, I)

Fig. 4. Values of KE; calculated from the data in Fig. 3; at [XT] =0.21 M (O), 1.11 M (m) and 2.11 M (A). Least squares line has a slope=-1.31, intercepk3.50 and cc.=O.91. Error bar illustrates an assumed error of + 5% in the yield of 16-ArBr.

TABLE 3

Normalized percent yields of 16-ArX and calculated values of Kg; from dediaxoniation of 16-ArN $ in CTAX solutions containing 0.2 M HX at [Xr] = 1.0 M and [Cl,] / [Brr] = 1.02

[CTAXI (M) Normalized yields (% ) Kga K-p

16-ArBr 16-Arc1 16-ArOH

0.01 25.1 6.2 68.1 3.10 0.11 22.7 6.6 70.1 2.63 0.21 18.8 5.8 75.4 2.48 0.31 19.5 7.6 72.9 1.96 0.41 17.9 6.9 71.5 1.98

*Assuming [C~]/[Br,]=[Clr]/[Brr]. bIncludes correction for preferential binding of Br-, c.m.c. =O, /3= 1, see text.

3.12 2.94 3.11 2.61 3.16

of anions are micellar bound. At high [ CTAX] , the ratio of added salt no longer sets the equilibrium counter-ion ratio in the aqueous phase because of prefer- ential binding of Br- by the micelles. We corrected for this effect by estimating the equilibrium concentrations of counter-ions using the product yields and S’gp Corrected values of KEf are essentially constant and independent of sur- factant concentration (Table 3 ) .

* [Br,] and [Cl,] were calculated from product yields, Sgi and the mass balance equation for bound counter-ions, setting j?= 1 for simplicity. [CL] and [Br,] were obtained by the difference between their total and micellar bound concentration.

133

DISCUSSION

The long chain aryldiazonium salt is a “pseudomonomer” [ 321, of like charge and structurally similar to the surfactant molecule. We found that it is insol- uble in water at very low concentrations, < 10m4 M, and therefore does not self-micellize. 16-A$ is more hydrophobic than p-nitrophenyloctanoate, which has only a 7 carbon chain, but a very large binding constant, KS= 1.5010~ M-’ [33]. Thus 16-ArN$ is certainly > 99% bound and reaction is limited to the micellar pseudophase. The high [ CTAX ] / [ 16-ArN$ ] concentration ratio, 50/l or greater, prevents perturbation of micelle structure and the positive charge of the diazo head group should minimize specific interactions with sur- factant monomer within the micelle.

The results in CTACl and CTABr (Fig. 2, Table 1) provide the first direct and simultaneous experimental estimate of the concentration of counter-ion and water at the micelle surface. Most methods provide information on the fraction of bound counter-ions, but not their interfacial concentration [ 13- 151. In 0.1 M HX with no added NaX, the estimated halide concentrations at the micelle surface are consistent with other published values [ 7,321. However, the steady increase in counter-ion concentrations with added NaX to nearly double their initial values at 4-5 M added NaX contradicts the assumption of constant interfacial counter-ion concentration used in the pseudophase ion exchange model [5-81. However, this observation is consistent with the re- cently observed steady decrease in 7gBr NMR linewidth in CTABr up to 1.0 M added NaBr [ 341. It is not possible to determine if the increase in surface X- concentration is caused by increased packing of interfacial head groups and counter-ions or by invasion of the interface by NaX at high [ NaX] , as occurs with ion exchange resins in concentrated salt solution [ 351. The consistently higher yield of 16-ArBr over 16-A&1 demonstrates clearly that the interfacial concentration of Br- at the surface CTABr micelles is always higher than Cl- on CTACl micelles.

Our estimates of Kg; are important in several respects. To a first approxi- mation, Kgf = 3, is in good agreement with other estimates which range from 3-5 [lo]. More importantly, Kg; is virtually independent of ionic strength (Table 2, Fig. 4) and surfactant concentration (Table 3). This means that the specific interactions responsible for selective binding of Cl - and Br- either do not depend upon the ionic strength of the aqueous phase or that the inter- actions change to the same extent. However, the specific interactions must depend, to some extent, upon nearest neighbors because Kgf decreases with increasing Br - mole fraction (Fig. 4). Such changes in ion exchange constants have been observed with ion exchange resins and are usually related to changes in the swelling of the resin or water penetration [ 361, but is is not clear how these properties of cross-linked resins relate to dynamic aggregates like micelles.

Measured product ratios of aqueous dediazoniations show small differences

134

in selectivity toward different anionic nucleophiles at high salt concentrations [21a,37,38], as in our results for l-ArN,+ (Fig. 2). These differences probably reflect small differences in activity coefficients for Cl- and Br-*. Neverthe- less, KEi is insensitive to changes in activity coefficients of Cl - and Br- in the aqueous phase, because Kg; depends on the ratio of the activity coefficients of these ions, which changes slowly with added NaX in aqueous solution [ 391. The high and gradually changing concentrations of micellar bound Cl - and Br-, Table 1, indicate that the activity coefficient ratios for these ions should be nearly constant.

Fabre and coworkers demonstrated that radioactive tracer self-diffusion measurements can be used to estimate, in separate experiments, the fraction of bound Cl- ( [Cl,] / [Cl,] ) and the fraction of bound Br- ( [Brm] / [ Brr ] ) in micellar solutions of CTACl/CTABr mixtures containing only small amounts of added NaX [ 401. Their results show that the fraction of bound Br- is always larger than the fraction of bound Cl- at all CTAC/CTABr ratios in 5,10 and 15% CTAX with only a trace of NaX carrying the radioactive label. We esti- mated KE; from their data and found that KEf= 2.9 2 7.4% (29 data sets). This value is the same as ours at [Cl,] / [ Brr ] = 1 within experimental error (Table 3), but unlike our results in Fig. 4, their Kgf values shown no trend with in- creasing Br - mole fraction. This difference may be related to the very different conditions of the two sets of experiments; high [ CTAX] , low [ NaX] in theirs and the reverse in ours.

Enhanced Br- binding has been suggested to occur at the sphere/rod tran- sition for CTABr [41]. Our results, like those of Fabre et al. [40], do not show enhanced binding of Br- over Cl- at high Br- concentrations, at high Br- mole fractions or at high [ CTAX]. This may be a consequence of our low [ CTAX] in most of the experiments. However, even when [ CTAX] becomes high, selectivity toward Br- decreases (compare yields of 16-ArBr and 16-ArCl in Table 3). Nor do our results give any indication of the coexistence of rod- like micelles of CTABr enriched in Br- and spherical CTACl micelles enriched in Cl- [ 421. Indeed, values of Kg; are independent of ionic strength at any one [Cl,] / [BrT ] ratio, contrary to what one would expect if added Br- enhanced Br- binding and induced micelle growth.

The major current limitation of our diazonium salt probe is that the acid strength of all micellar solutions must be kept at 10.1 M HX to suppress formation of a yellow side product which appears at lower acidities. Our pre- liminary control studies indicated that when dediazoniation of 16-ArN$ was carried out in 0.01 A4 HBr, 0.01 M CTABr, a new strong absorption was ob- served at ;2,,= 370 nm, k, is 30% larger, and the total yield of halo and phe-

*We found that the small difference in yield between 1-ArBr and l-ArCl disappears when the yields are plotted against the activities of N&r and NaCl, using literature values for their activity coefficients [ 39 1.

135

nolic products decreased to ~20%. This additional UV absorption band is completely absent in the dediazoniation of 16-ArNz in 0.1 M HBr, 0.01 M CTABr and in the dediazoniation of l-ArN2+ in aqueous 0.01 M HX. As noted above, product yields under both sets of conditions are nearly quantitative. In an additional control experiment, [ CTAX] was increased to 0.1 M at 0.01 M HX, thus diluting 16-ArN,+ and its products within the micellar pseudophase ten fold, and the yield of 16-ArOH and 16-ArX was again high (approximately 85% ). This observation is consistent with our suspicion that the new product is formed in a competing micellar induced bimolecular reaction whose rate depends upon the volume of the micellar pseudophase. The reaction might be azo dye formation between unreacted 16-ArNz and the product, 16-ArOH, which is activated toward electrophilic substitution by the -OH group [ 37,381. We did not attempt to identify this side product because it does not interfere with product analysis under routine experimental conditions (i.e., 2 0.1 M HX and 2 0.01 M CTAX).

The requirement that the solution acidity be 2 0.1 M HX for micellar reac- tions of 16-ArNz prevents the study of counter-ion selectivity at low salt con- centrations and low acidity, typical conditions of most studies of micellar ef- fects on reactivity. However we recently synthesized a new hydrophobic diazonium salt which undergoes dediazoniation free of competing reactions down to 0.0001 M HX [ 43 1. We are now using this substrate to explore counter- ion selectivity in the absence of added salt.

CONCLUSIONS

Dediazoniation product distributions in solutions containing mixtures of counter-ions are “snapshots” of the reactive volume surrounding the aryldi- azonium probe molecule at the micellar surface. The use of diazonium salts as trapping reagents for interfacial nucleophiles has a number of potential appli- cations. Our approach can be used with a variety of cationic interfaces includ- ing o/w and w/o microemulsions, inverse micelles and mixed micelles of zwit- terionic and cationic surfactants. The selectivity of cationic micelles toward other counter-ions such as I-, SCN-, RCO,, or any other nucleophile which reacts via rate determining loss of N, can also be determined from product distributions. The interfacial concentration of added alcohols and their effect on interfacial counter-ion and water concentration can also be monitored di- rectly. Finally, the effect of counter-ions such as tosylate, which have pro- nounced effects on micelle structure [ 441, but may not react with aryldiazon- ium salts, can still be explored in terms of their effect on interfacial water and counter-ion concentration.

136

ACKNOWLEDGEMENTS

We wish to thank Judy Waidlich for the preparation of the short diazonium salt, l-ArN$. We also wish to thank the following organizations for financial .support: the Research Council and Biological Sciences Research Fund of Rut- gers University, the National Institutes of Health, GM32972, the National Science Foundation U.S.-Latin American Cooperative Program-Brasil, The Petroleum Research Fund and the Research Corporation.

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