Effects of Amine Oxide Surfactants on Reactions of Bromide and Hydroxide Ions with Methylnaphthalene-2-Sulfonate

Journal of Colloid and Interface Science 211, 179 –184 (1999) Article ID jcis.1998.6006, available online at http://www.idealibrary.com on

Effects of Amine Oxide Surfactants on Reactions of Bromide and Hydroxide Ions with Methylnaphthalene-2-Sulfonate Lucia Brinchi,* Chiara Dionigi,* Pietro Di Profio,* Raimondo Germani,* Gianfranco Savelli,*,1 and Clifford A. Bunton† *Dipartimento di Chimica, Universita` di Perugia, 06100 Perugia, Italy; and †Department of Chemistry, University of California, Santa Barbara, California 93106 Received October 15, 1997; accepted November 30, 1998

The SN2 reaction of Br2 with methylnaphthalene-2-sulfonate (MeONs) in water is accelerated by micelles of tetradecyldialkyl amine oxide (alkyl 5 methyl, n-propyl) and rates increase sharply in HBr due to increased binding of Br2 to the protonated amine oxide. Second-order rate constants at the micellar surface are similar to those at surfaces of trialkylammonium and sulfobetaine micelles. The reaction of OH2 with MeONs is weakly inhibited by amine oxide micelles, showing that dispersive, as well as coulombic and charge-dipole, forces play a major role in the association of ions with surfaces of micellar aggregates. © 1999 Academic Press Key Words: amine oxide surfactants; micelles; dispersive forces; SN2 reactions.

INTRODUCTION

Rate enhancement of bimolecular reactions of counterionic reagents by micelles is readily understandable because concentration of a counterion at the micellar surface speeds its reaction with a micellar-bound substrate. The micellar rate effects are treated quantitatively by models that take into account reactant concentrations at the micelle–water interface (1). Nonionic micelles have little effect on reactions of ions with moderately hydrophilic substrates but they inhibit reactions of very hydrophobic substrates, probably because they bind in an apolar region from which ions are excluded (2). Zwitterionic micelles of betaine surfactants behave similarly to cationic micelles in their effects on rates of spontaneous reactions (3). They may inhibit or accelerate these reactions depending on their mechanism. Micelles of sulfobetaines inhibit, but do not suppress, bimolecular reactions of very hydrophilic anions, e.g., OH2 and F2, and accelerate reactions of less hydrophilic ions, e.g., Br2 and iodosobenzoate ion (4). Amine oxide surfactants are protonated in dilute acid and then form cationic micelles, (5) but in neutral or alkaline solutions they generate micelles that may be regarded as nonionic or zwitterionic and can react nucleophilically (6). As a result reactions of added nucleophiles may depend on the extent of

protonation of an amphiphilic amine oxide. Some kinds of long-tail amine oxides have been reported to form monolayers, bilayers, and vesicles under particular conditions (7). Properties of the head groups can be controlled by protonating them, with predictable effects on reactions of water and nucleophilic anions.

In the present work we examine the effects of nonionic and protonated amine oxides on the SN2 reaction of Br2 with methylnaphthalene-2-sulfonate (MeONs) and of the nonionic amine oxides on the corresponding reaction of OH2. There may also be effects on its reaction with water. This substrate is relatively hydrophobic and binds readily to micelles, even in dilute surfactant.

These reactions are well studied in cationic and sulfobetaine micelles (1, 4, 8). The amine oxides are C14H29N1R2–O2; R 5 Me, n-Pr, DMMAO, and DPMAO respectively, where MAO denotes the tetradecyl (myristyl) amine oxide residue. MATERIALS AND METHODS

1

Materials. Preparation and purification of the surfactants and reagents have been described (3, 4, 9). Reactions were carried out in redistilled, deionized, and CO2-free water. Critical micelle concentrations were measured by surface tension in water (pH ;5.8; 1.41 3 1024 M and 5.44 3 1025 M for DMMAO and DPMAO, respectively) and in aqueous basic and

To whom correspondence should be addressed at Dipartimento di Chimica, Universita` di Perugia, Via Elce di Sotto, 8, 06100 Perugia, Italy. 179

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TABLE 1 Reaction of MeONs in Acidic Solutions of Amine Oxidesa HBr b

a b

CH3SO3H b

H2SO4b

102 [Amine oxide], M

DMMAO

DPMAO

DMMAO

DPMAO

DPMAO

0 0.2 1 3

0.19 3.5 4.6 4.9

0.19 4.7 6.3 6.2

0.13 0.097 0.076 0.075

0.13 0.091 0.071 0.071

0.13 0.17 0.22 0.24

Values of 104 k obs, s21, at 25.0°C. 0.05 M.

acidic solutions (0.1 N NaOH, 1.11 3 1024 M and 4.84 3 1025 M; 0.1 M HBr, 1.17 3 1024 M and 5.92 3 1025 M; 0.1 M HCl, 1.60 3 1024 M and 1.05 3 1024 M; 0.05 M H2SO4, 1.22 3 1024 M and 9.35 3 1025 M for DMMAO and DPMAO, respectively). Kinetics. Reactions were followed at 25.0°C in Shimadzu double-beam or HP diode array spectrophotometers as described (9). The concentration of MeONs was 1024 M, and it was added in MeCN so that the reaction solution contained 1 vol% MeCN.

Reaction in acidic solutions. Reactions of the amine oxide, water, and added nucleophiles with MeONs will be affected by protonation of the head group, and their contributions must be separated. Amine oxides are protonated by strong acids, and the pK a of the micellized conjugate acid is ca. 5 (5). Therefore in acid they behave like other cationic micelles by accelerating the reaction of Br2 with MeONs in solutions of HBr (Table 1, Fig. 1). Under these conditions nucleophilic attack by the

amine oxide is suppressed, but there is probably a minor contribution of reaction with water, which can be neglected, and reaction with Br2 is all important. An increase in head group bulk increases k obs, the same as for reaction in other cationic micelles, but with fully bound substrate overall reaction with Br2 is somewhat slower than in micelles of quaternary ammonium ions (10). Protonation of the amine oxide by acids with weakly nucleophilic anions, e.g., methane sulfonic acid, decreases k obs, but dilute H2SO4 slightly increases it (Table 1), possibly because there is a minor contribution of reaction with SO22 4 in the cationic micelles. Reactions in basic solutions. Under these conditions we have to consider the reactions of the amine oxide, as well as those of other nucleophiles. Nonionic amine oxides increase k obs in solutions of NaBr (Table 2, Fig. 2). Values of k cobs are corrected for the contributions of reactions with H2O and the amine oxides themselves that are given in Table 2. There is a significant contribution from SN2 reactions of the amine oxides with MeONs that is larger with DMMAO than with DPMAO, where steric hindrance slows reaction (Scheme 1). However Br2 is not excluded from the amine oxide micelles, although

FIG. 1. First-order rate constants of reaction of MeONs with Br2 (0.05 M HBr) at 25.0°C in aqueous amine oxides: DMMAO (■); DPMAO (Œ). Solid lines are theoretical.

FIG. 2. Corrected first-order rate constants of reaction of MeONs with Br2 (0.05 M NaBr) at 25.0°C in solutions of amine oxides in aqueous carbonate buffer (pH 9.6): DMMAO (■); DPMAO (Œ). Solid lines are theoretical.

RESULTS

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EFFECTS OF AMINE OXIDES ON SN2 REACTIONS

TABLE 2 Reaction of MeONs in Basic Solutions of Amine Oxidesa Buffer b

NaBr c

NaOH d

102 [Amine oxide], M

DMMAO

DPMAO

DMMAO

DPMAO

DMMAO

DPMAO

0 0.2 1 3

0.15 0.40 0.60 0.65

0.15 0.18 0.21 0.22

0.19 0.91 1.5 1.6

0.19 0.71 1.4 1.4

0.73 0.88 1.1 1.2

0.73 0.67 0.62 0.72

Values of 104 k obs, s21, at 25.0°C. 0.025 M carbonate buffer, pH 9.6. c 0.05 M in 0.025 M carbonate buffer. d 0.05 M. a b

these are formally neutral, but rates of reactions of bound MeONs with Br2 are lower than those in zwitterionic, sulfobetaine micelles (11) and considerably lower than those in cationic micelles (Table 3).

reaction media (2) and k obs depends on the distribution of reagents and rate constants in each pseudophase (Eq. [1]).

k obs 5

The behavior of OH2 in amine oxide micelles is very different from that of Br2. In 0.05 M OH2 k obs initially decreases as MeONs is taken up by the micelles and partially protected from OH2 which remains largely in the aqueous pseudophase, but then k obs increases modestly due to the reaction of the amine oxide with micellar-bound MeONs (Fig. 3, Table 2). A similar, but smaller, inhibition of nucleophilic attack by OH2 is seen with sulfobetaine micelles (Table 3), but reaction of Br2 is accelerated, although much less so than by cetyltrimethylammonium bromide (CTABr). Quantitative treatment. We fit the data for the various reactions in micellized amine oxides by using a pseudophase model in which water and micelles are treated as distinct

k9W 1 k9MK S@D n# . 1 1 K S@D n#

[1]

In Eq. [1] k9W and k9M are first-order rate constants with respect to MeONs in the aqueous and micellar pseudophases, respectively, and K S is the association constant of MeONs to micellized surfactant (detergent, Dn) whose concentration is the total less that of the monomer. Conventionally the monomer concentration is taken as the critical micelle concentration, cmc, under kinetic conditions. Reactions with added nucleophiles k9W and k9M depend upon local concentrations of the nucleophile, Nu, which in the micellar pseudophase is unambiguously defined as a mole ratio, i.e., k9M 5 k M[NuM]/[Dn].

[2]

TABLE 3 Reaction of MeONs in Aqueous Surfactantsa

NaBr,b 0.05 M NaBr,c 0.1 M NaOH,d 0.05 M NaOH,c 0.1 M

Water

SB3-14, 0.05 M

CTAX, 0.04 M

0.038 0.076 0.41 0.83

— 1.96 — 0.73

8.85 (X 5 Br) — 11.0 (X 5 OH) —

Values of 104 k obs, s21, at 25.0°C. Ref. (9). c Ref. (11), SB3-14 is tetradecyldimethylammonium propanesulfonate. d Ref. (10). a b

FIG. 3. Corrected first-order rate constants of reaction of MeONs with OH2 (0.05 M NaOH) at 25.0°C in aqueous amine oxides: DMMAO (■); DPMAO (Œ).

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TABLE 4 Fitting Parameters for Reaction of MeONs with Bromide Ionsa Surfactant DMMAOb DPMAOb DMMAOH1c DPMAOH1c MyTABr CTABr d CTBABr d SB3-14e

[Nu], M Br2, Br2, Br2, Br2, Br2, Br2, Br2, Br2,

0.05 0.05 0.05 0.05 0.05 0.05 0.1 0.1

K9Nu (M21)

104 k M (s21)

21 21 104 k m s ) 2 (M

k rel

K S (M21)

1.8 1.5 20 15 475 475 155 4.3

12 18 12 18 9.5 9.6 25 7.0

1.7 2.5 1.7 2.5 1.3 1.3 3.5 0.98

2.1 3.1 2.1 3.1 1.7 1.7 4.4 1.2

500 500 500 500 1000 1000 1000 1000

k W 5 8 3 10 25 s21 for Br2 and k rel 5 k m 2 /k W. In basic solution. c In acidic solution. d Ref. (9). e Ref. (11). a b

In the aqueous pseudophase, with a weakly bound anionic nucleophile, k9W 5 k W[NuW] < k W[NuT].

[3]

Subscripts W, M, and T denote concentrations in the aqueous and micellar pseudophases and in the total volume, respectively, and quantities in square brackets are in terms of total solution volume. The second-order rate constant, k M, (Eq. [2]), is written with concentration as a mole ratio in the micellar pseudophase. In favorable cases distributions of ionic reagents between pseudophases can be determined experimentally, (1, 12, 13) but they are usually calculated from distribution models. Here we use Eq. [4] with the form of a Langmuir isotherm that has been applied to a variety of micellar-mediated ionic reactions (14). K9Nu 5

[NuM] . [NuW]([Dn] 2 [NuM])

[4]

The simulation of rate-surfactant profiles followed earlier methods, but the derived parameters are least reliable with weakly bound ions (9, 10). In this situation data can be fitted by a combination of values of K9Nu and k M and an increase in one decreases the other. The data for reaction with Br2 can be fitted quantitatively (Figs. 1 and 2), but we did not attempt to fit the data for reaction of OH2 because of the large contribution of reaction with the nonionic amine oxides, and OH2 is a relatively ineffective nucleophile in amine oxide micelles (Fig. 3), probably because of its weak affinity for these micelles. From variations of kobs with [DMMAO] and [DPMAO] in the absence of added nucleophile we estimate KS ' 500 M21 and we use this value in all our fits. Except in dilute surfactant fits are insensitive to the value of KS.

We corrected kobs for reactions with water in acidic solutions, and with water and amine oxide in basic solutions, and fits are based on corrected values, kcobs. The correction is very small in acidic solutions where protonation suppresses nucleophilicity of the head group, but it is significant in nonacidic solutions (Tables 1 and 2). DISCUSSION

The differences in the effects of both zwitterionic and nonionic amine oxide micelles on the overall rates of reactions of OH2 and Br2 illustrate the major role of dispersive forces in micelle–ion interactions (1, 14 –17). Interaction of a very hydrophilic ion such as OH2 with cationic micelles is governed largely by coulombic forces, and the interaction of OH2 with sulfobetaine micelles depends largely upon differences in charge densities of the cationic and anionic regions in the micelle–water interface (4a, 18). The interaction has been treated quantitatively in these terms, and it should be much weaker in amine oxide micelles with adjacent charges than in sulfobetaine micelles where charges are separated by a tether group. As a result there is very little contribution of reaction with OH2 in amine oxide micelles (Fig. 3). Coulombic or charge-dipole forces are important in interactions of Br2 with cationic and zwitterionic micelles, but the polarizability and relatively weak hydration of this ion means that dispersive forces are very important. As a result Br2 binds strongly to cationic micelles (1, 10, 13–16) but less strongly to both sulfobetaine and amine oxide micelles. Protonation of micellized amine oxides introduces an attractive, coulombic term in their interactions with Br2 and values of K9Nu for Br2 increase (Table 4). However, they are lower than those estimated for cationic trialkylammonium micelles, showing that replacement of alkyl by OH at the cationic center sharply decreases the affinity for Br2. Quaternary ammonium and bromide ions are not strongly hydrated (17) so hydrophobic forces should increase their mutual affinity, but a proton-

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EFFECTS OF AMINE OXIDES ON SN2 REACTIONS

TABLE 5 Inhibition of Reaction of MeONs with Water by Trialkylammonium, Sulfobetaine, and Protonated Amine Oxide Micellesa 102 [Surfactant], M

CTAMs

CTPAMs

SB3-14b

SBBu3-14b

DMMAO

DPMAO

0.3 1 3 5

1.1 1.0 0.96 0.95

0.74c 0.73 0.71 0.71

0.81 0.71 0.69 0.71d

0.85 0.77 0.77 0.77

0.097 0.076 0.075 —

0.091 0.071 0.071 —

Values of 105 k obs, s21, at 25.0°C. Ref. (11). c 0.005 M. d 0.1 M. a b

ated amine oxide should readily hydrogen bond to water which decreases its affinity for Br2. However, the SN2 reaction of water with MeONs is inhibited to similar extents by trialkylammonium, sulfobetaine, and protonated amine oxide micelles, indicating that they have similar effects on the reactivity of water at the reaction center (Table 5). The second-order rate constant, k M, for reaction of Br2 is not very sensitive to changes in the charge or structure of micellar head groups, although it is mildly increased by an increase in head group bulk, as found for a wide range of reactions in ionic and zwitterionic micelles (Table 4 and Refs. (9) and (10)). Therefore variations of k obs with surfactant structure are controlled largely by affinities of the micellar head groups for Br2 and not by its reactivity at the micellar surface. This behavior contrasts with that for spontaneous reactions such as decarboxylation or dephosphorylation where reactivity at the micellar surface is sensitive to structures of the head groups (3b, 19). However, a decrease in the length of the hydrophobic tail from C16H33 in CTABr to C14H29 in MyTABr does not affect the reactivity of micellar-bound Br2 with MeONs and 104 k M is in the range 8.5 to 9.5 s21 for both surfactants over a range of added Br2 (9). Values of k M in Table 4 are calculated with concentration as a mole ratio and those of k W are calculated conventionally in terms of concentration as a molarity in water. We can compare the two sets of constants by estimating the local molarity of Br2 in the micelle–water interface, which requires selection of a value for the molar volume, V M, of this region. The secondorder rate constant, k m 2 , in terms of local molarity is given by (1) k m2 5 k MV M.

[5]

Values of V M generally used in fitting rate data are in the range 0.14 to 0.37 M21 and based on the lower value of V M k m 2 for reaction of Br2 are larger than k W by factors of ca. 2.1 to 3.1 (k rel, Table 4), and the difference would be greater if we selected a higher value of V M. There is considerable uncertainty in the fits for reactions in the absence of HBr because when K9Nu is small fitting is subject to uncertainty. The differ-

ences in k m 2 /k W for micelles with different head groups may represent differences in V M rather than reactivity. In addition the composition of the reaction region, the micelle–water interface, is probably not uniform (20), so the estimates of local molarity, or k m 2 (Eq. [5]), are based on a model that involves a number of unproven assumptions (18). This limitation does not apply to Romsted’s trapping method (13) and it is encouraging that his values of local molarity are in reasonable agreement with those from pseudophase models where comparison can be made. All these observations show that interactions of weakly hydrophilic ions with surfaces of association colloids are sensitive to noncoulombic forces and ions can bind readily to formally neutral colloids that thereby gain electrical charge. CONCLUSIONS

In basic media micellized amine oxides react nucleophilically with bound MeONs and modestly affect its reaction with Br2. Protonation suppresses nucleophilicity of the amine oxide, but increases its affinity for Br2. Contributions of these various reactions depend on the head group bulk and, in some cases, can be separated in terms of quantitative pseudophase models of micellar rate effects. ACKNOWLEDGMENTS Support of this work by CNR, Progetto Finalizzato Chimica Fine II, Rome; MURST, Rome; and the U.S. Army Office of Research is gratefully acknowledged.

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