Mixed Surfactant Systems - American Chemical Society

Chapter 13

Binding of Bromide Ion to Mixed Cationic—Nonionic Micelles Effects on Chemical Reactivity 1

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Sally Wright , Clifford A. Bunton , and Paul M . Holland

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Downloaded by COLUMBIA UNIV on February 16, 2013 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch013

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Department of Chemistry, University of California— Santa Barbara, CA 93106 General Research Corporation, Santa Barbara, CA 93111

Micellar enhancement of chemical reactivity has been used to probe counterion binding to mixed cationic/nonionic micelles composed of CTABr and C E by examining micellar rate effects on the reaction of bromide ion with methyl napthalene-2-sulfonate. The CTABr/C E mixed micellar system was also characterized by using conductivity measurements. Results show the addition of the nonionic surfactant C E leads to a marked decrease in the overall rate of demethylation of methyl napthalene-2-sulfonate by bromide ion, and that a simple pseudophase model can account for this effect. 10

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Micelles and other surfactant aggregates can affect chemical reaction rates via interactions with ionic reactants and hydrophobic substrates. The reaction rates can be enhanced (so called micellar catalysis), or inhibited depending on the charge type of the reaction. Because of the importance of interactions of ions at colloidal surfaces, micellar rate effects can be useful in probing ion binding (1-3). In the present work we consider the S 2 reaction of Br and methyl naphthalene-2sulfonate (MeONs) in water (4). N

MeONs

ONs

The rate of this reaction is increased by cationic micelles which concentrate the two reactants at their surface. Ions and polar molecules do not enter the hydrocarbon-like micellar core but reside at the micelle-water interface in a region that is often identified with the Stern layer. This concentration of reactants at colloidal surfaces 0097-6156/92/0501-O227$06.00A) © 1992 American Chemical Society

In Mixed Surfactant Systems; Holland, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

228

MIXED SURFACTANT SYSTEMS

is the major source of enhancements of rates of bimoleeular reactions by micelles, microemulsion droplets, vesicles and similar colloidal self assemblies. The data can be described quantitatively by assuming that water and the micelles behave as discrete reaction media, and that reactants partition between them as shown in Scheme 1, below.

s

+ D„ _^==r

w


k

s

M

/ K

w \

Products Scheme 1. The overall rate of reaction is the sum of the rates in each pseudophase (1-3). In this scheme, S is the substrate and D is micellized surfactant (detergent), and subscripts W and M denote aqueous and micellar pseudophases respectively. The rate depends on the distribution of the nonionic substrate, expressed by its binding constant n

^

-

(i)

The first order rate constants (with respect to substrate) are k ' and k' , and K is the binding constant of substrate with micellized surfactant D . The corrected overall first order rate constant, k/, is given by w

M

s

n

c

k *

w

M

* ** 1 Z.IDJ

(2)

+

where the superscript c on the overall rate constant indicates that a small correction has been made for reaction of MeONs with water molecules. The first order rate constants k ' and k' depend upon the concentrations of Br in the two pseudophases. For reaction in water: w

M

t

w

= k [Br ] w

w

(3)

where quantities in square brackets are molarities written in terms of total solution volume. It is convenient to write the concentration of Br in the micellar pseudophase as a mole ratio, so that the second order rate constant, 1^, has the dimensions of sec' , 1

jfc.

[ g r

= ** " [D„]

]

and substitution into equation 1 then gives


(4)

13. WRIGHT ET AL

Binding of Bromide Ion to Cationic-Nonionic Micelles 229 k [Br^] + w

k K [Brû] M

(5)

s

1 •


This treatment predicts that reaction rates in aqueous quaternary ammonium bromide surfactants should increase with increasing surfactant concentration as substrate becomes bound to the micellar surface where the concentration of Br is much higher than in water. The problem is then that of estimating the concentration of B r at the micellar surface, and it can be calculated in terms of various theoretical models (4,5). An alternative approach is to determine the fractional ionic coverage of the surface, β β =

(6) [CTABr ] M

and β = 1 - α, where a is the fractional micellar ionization which can be determined experimentally, for example from the Variation of conductance with surfactant concentration (4-8). The reaction in aqueous solutions of CTABr fits the pseudophase model (equations 1-5) and values of k / become independent of CTABr when all the substrate is micellar bound. Addition of a moderately hydrophobic alcohol, 1-butanol, slows the reaction by decreasing β and by increasing the volume of the micellar pseudophase. Both these effects decrease the concentration of Br at die micellar surface but the value of k,* is almost unaffected by addition of 1-butanol (7). This result was unexpected because the properties of a micellar surface should be affected by incorporation of a hydrophobic alcohol and reaction rates in solution are sensitive to changes in solvent properties. We therefore examined the reaction of Br with MeONs in a solution of CTABr plus the nonionic surfactant, C E (CioH 0(CH CH20) CH2CH20H) and measured a conductimetrically so that we could test the pseudophase model (equations 1-5) and we compare values of 1^ in solutions of CTABr, CTABr + 1-butanol, and CTABr + C E . This allows us to assess the effects of mixed CTABr/C E micelles on the reaction rate. 10

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Experimental Section We carried out all our experiments in solutions of cetyltrimethylammonium bromide (CTABr, n-C H NMe3Br) at 25.0 °C. Preparation and purification of 2-MeONs, CTABr, and CTA(S0 ), have been previously described. The C E used in this study was the single-species surfactant with a purity of 99.69% as detemined by gas chromatography. The reaction of Br with MeONs in micellar solution was followed spectrophotometrically at 326 nm. These experiments were designed such that surfactant concentrations were always much higher than the CMC. The fractional micellar ionization, a, was determined conductimetrically (6-8). 16

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230


Results Kinetics. Reactions were followed under conditions such that MeONs was essentially fully micellar bound (K^DJ > > 1) and reactions in the aqueous pseudophase could be neglected (4). We made a (small) correction for reaction of micellar-bound substrate with water by following the reaction in CTA(S0 ), . Reaction rates in mixed CTABr/C E micelles at constant CTABr concentrations of 0.05 M and 0.025 M are shown in Figure 1. These show a decrease in the reaction rate with the addition of CioE . In Figure 2 the reaction rate at a constant total surfactant concentration of 0.05 M shows a similar decrease with increasing C E in the surfactant ratio. The solid lines in Figures 1 and 2 show the theoretical fits for the variation of with concentrations of CTABr and C E (see discussion). 4

10

A

4

4


10

4

10

4

Estimation of a. Addition of nonionic solutes increases the fractional ionization, a, of ionic micelles by decreasing the charge density of the micellar surface. The conductivity increases as counterions are released from the micelles and a can be calculated from the slopes of plots of conductance against [CTABr] above and below the critical micelle concentration (CMC). This method involves assumptions regarding the contribution of micelles to conductivity and an alternative method of calculation includes a correction based on micellar size (9). We do not know the aggregation number or hydrodynamic radius of the mixed CTABr-C E micelles and therefore we used the simple "ratio of slopes" calculation which is a good indicator of changes in a (6). The conductivity method becomes unsatisfactory as a -* 1 and we limited our measurements to [C E ]/[CTABr] < 2.0. Results from the conductiv ity data using the ratio of slopes method are given in Table 1. The observed increase in fractional charge a is consistent with recent results on the change of overall micellar charge when β-dodecylmaltoside is added to sodium dodecyl sulfate (10). 10

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Table 1. Fractional Micellar Ionization, a, from Conductivity Data [C E ]/[CTABr] 10

4

a

0*

0.25

0.5

0.47

1.0

0.54

2.0

0.59

" Taken from ref 7. Discussion The pseudophase model treats the aqueous and micellar pseudophases as distinct reaction regions (Scheme 1), but provided that substrate is fully micellar bound and In Mixed Surfactant Systems; Holland, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

13.

Binding of Bromide Ion to Cationic-Nonionic Micelles 231

WRIGHT ET AL.


10

ο

1

1

0.00

1

0.02

1

0.04 [C

1 0

1

0.06 E ] M

1

0.08

0.10

4

Figure 1. Dependence of corrected first-order rate constants of reaction of Br with 2-MeONs upon [C E ] in constant [CTABr]. The solid lines are theoretical fits to Equation 7. ( • ) 0.025 M CTABr; ( • ) 0.05 M CTABr. 10

4

8

0

i

0

1

1

1

2 3 [C E ]/[CTABr]

1

1 0

.

»

4

5

4

Figure 2. Dependence of correctedfirst-orderrate constants of reaction of Br with 2-MeONs upon ratio of [C E ]/[CTABr] in constant total surfactant concentration of 0.05 M . The solid line is a theoretical fit to Equation 7 (the dashed portion is basal on an extrapolated value for β). 10

4


232


surfactant concentrations are much larger than the CMC, as in our experiments, equation 4 simplifies to (2-4) *;

= k βR

CO

M

where „ _

[CTABr]

(8)

[CTABr] + [C E ] Downloaded by COLUMBIA UNIV on February 16, 2013 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch013

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The solid lines in Figures 1 and 2 are calculated by using Equation 7 and interpolated values of β and 1^ = 1.07 χ 10" s*. The results for reactions with a constant [Br] of 0.05 M and variable [surfactant], data not shown, are fitted reasonably with k = 1.30 χ 10" s*. We think that the difference between the two sets of values of 1^ is due to our using values of β, based on our conductivity experiments, that are low in the latter case because of the difficulty in estimating a at high [Br]. In water, addition of Br to CTABr slightly increases β so that our values of β that are determined conductimetrically in the absence of added Br are probably too low in experiments with added NaBr. The data point at [C E ]/[CTABr] = 4 (Figure 2) was not fitted because we have no value of a at this concentration ratio. The simple pseudophase model (Scheme 1 and equations 1-5) fits rate effects of mixed micelles of CTABr and C E over a range of surfactant concentration. Values of k are little affected by marked changes in micellar composition. For aqueous CTABr (4) k = 9.6 χ 10" s , in CTABr 4- 1-butanol (7) k = 9.1 χ 10" s , and in CTABr + C E k = 10.7 χ 10^ s and 13.0 χ 10 s in the absence and presence of added NaBr, respectively. These comparisons are based on our writing concentrations of Br at the micellar surface as mole ratios. It should be pointed out that SN2 reactions of nucleophilic anions are not very sensitive to small changes in the properties of the medium such as polarity or dielectric constants, so that changes in the polarity of the micellar surface by addition of C E are probably not very important kinetically. None the less, these similarities in values of k were unexpected because addition of C E should change the properties of the micellar surface, and we need more evidence before we can decide whether they apply only to S 2 reactions or are more general. 3

3

1

1

M

10

10

4

4

M

4

1

4

M

1

M

1

10

4

-4

1

M

10

4

M

10

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N

Conclusions The addition of the nonionic surfactant C E to micellar solutions of CTABr leads to a marked decrease in the overall rate of demethylation of 2-MeONs by bromide ion. At constant CTABr concentration there is a monotonie decrease in the rate on addition of CioE . The good fit of a simple psuedophase model to the results suggests that the observed effect of C E on the rate is due to a combination of Br being expelled from the micellar region as C E is added and dilution of the phase volume of the reaction region at the micellar surface. The observation that the second order rate constants at the micellar surface are nearly the same for CTABr, CTABr + 1-butanol, and 10

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13.

WRIGHT ET AL.

Binding of Bromide Ion to Cationic—Nonionic Micelles

CTABr + C E was unexpected, in view of differences in the structures of surfactants and the alcohol. 10

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Legend of Symbols D K k' k' k/ k k R S S α

n

8 w


M

w

M

w

M

β

micellized surfactant binding constant of substrate based on concentration of micellized surfactant first order rate constant with respect to substrate in aqueous phase first order rate constant with respect to substrate in micellar pseudophase corrected overall first order rate constant second order rate constant with respect to substrate in aqueous phase second order rate constant with respect to substrate in micellar pseudophase ratio of CTABr in total surfactant substrate in aqueous phase substrate in micellar pseudophase degree of fractional micellar ionization fractional counterion binding (neutralization) of micelle, β = 1 - a

Acknowledgements Support by the National Science Foundation (Organic Chemical Dynamics Program) is gratefully acknowledged. The authors also wish to thank Dr. Robert G. Laughlin of The Procter & Gamble Company for supplying us with the C E used in this study. 10

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Literature Cited 1. Romsted, L . S. In Surfactants in Solution; Mittal, K. L . ; Lindman, B.; Eds.; Plenum Press: New York, NY, 1984; Vol.2., p. 1015. 2. Bunton, C. Α.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. 3. Bunton, C. A. In Cationic Surfactants: Physical Chemistry; Rubingh, D. N.; Holland, P. M . ; Eds.; Surfactant Science Series 37; Marcel Dekker, Inc.: New York, NY, 1990; pp 323-405. 4. Bacaloglu, R.; Bunton, C. Α.; Ortega, F. J. Phys. Chem. 1990, 94, 5068. 5. Bunton, C. Α.; Moffatt, J. R. J. Phys. Chem. 1988, 92, 2896. 6. Zana, R. J. Colloid Interface Sci. 1980, 78, 330. 7. Bertoncini, C. R. Α.; Nome, F.; Cerichelli, G.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5875. 8. Neves, F. de F.S.; Zanette, D.; Quina, F.; Moretti, M . T.; Nome, F. J. Phys. Chem. 1989, 93, 4166. 9. Evans, H. C. J. Chem. Soc. 1956, 579. 10. Bucci, S.; Fagotti, C.; Degiorgio, V; Piazza, R. Langmuir 1991, 7, 824. RECEIVED January 6, 1992


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Mixed Surfactant Systems - American Chemical Society

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