Langmuir 1994,10, 1155-1159 1155

Acid Hydrolysis of p-Methoxybenzaldehyde 0-Acyloxime in 1 -Butanol-Modified Micelles of Sodium Dodecyl Sulfate

Danil A. R. Rubio, Din0 Zanette, and Faruk Nome*

Departamento de Qulmica, Universidade Federal de Santa Catarina, 88040-970 Florian6polis, Santa Catarina, Brazil

Clifford A. Bunton

Department of Chemistry, University of California, Santa Barbara, California 93106

Received August 16,1993. In Final Form: December 27,1999

Rates of acid hydrolyses of p-methoxybenzaldehyde 0-acyloximes @-MeOC6H4HC=NOCOR, AMB and OMB, R = CH:, and n-CTH16, respectively) in micelles of 0.05 M sodium dodecyl sulfate (SDS) with 0.05-0.8 M HC1 are reduced by 1-butanol (BuOH). Reaction in aqueous HC1 is slightly slowed by BuOH. Several factors contribute to the inhibition by BuOH in SDS micelles: (i) BuOH reduces the transfer of H+ and AMB from the aqueous to the micellar pseudophase, but OMB is essentially fully micellar-bound; (ii) BuOH increases the volume of the reaction region in the micellar pseudophase and decreases reactant concentrations in that region. In dilute HCl, competition between H+ and Na+ and the overall kinetics fit the pseudophase ion-exchange equation (PIE) and second-order rate constants in the micellar pseudophase are similar to those of AMB in BuOH-H~O mixtures. In moderately concentrated HC1, e.g., 0.5 and 0.7 M, the PIE treatment underpredicts the concentration of H+ at the micellar surface and the overall rate constants. Modifications of the PIE which allow for an increase in the concentration of H+ at the micellar surface with an increase in the total CHC11 fit the data. In one treatment the local concentration of H+ at the micellar surface is assumed to increase linearly with [HCll; in another it follows equations of the form of Langmuir isotherms. The treatmenta are compared.

Introduction The pseudophase model of micellar rate effects treats

micelles and solvent as distinct reaction regions.' The overall rate is the sum of rates in eachregion which depend upon local rate constants and reactant concentrations. The pseudophase ion-exchange (PIE) model provided the first general treatment of micellar enhancements of rates of bimolecular ionic reactions.'I2 The assumptions in this model are that reactive and inert counterions compete at the micellar surface, as on ion-exchange resins, and that fractional micellar ionization, a, is independent of the nature and/or concentration of added ions. Ionic con- centrations a t micellar surfaces can then be calculated by using the mass balance relation and values of the ion- exchange parameter and a.

This treatment correctly predicts relations between overall reaction rates and concentrations of surfactant and reactive and inert counterions and has been extended to reactions mediated by alcohol-modified micelles and microemulsions and in synthetic surfactant vesicles.' I t underpredicts rate constants when the reactive ion con- centration is greater than ca. 0.1 M, and several extensions of the treatment have been used to fit data with moderately concentrated ions.' They have the common feature that they assume that effective concentrations in the micellar reaction region can exceed those calculated from the fractional micellar coverage (O), where j3 = 1 - a.

(i) Counterions in the aqueous pseudophase may "in- vade" the micellar pseudophase so that ionic concentra-

@ Abstract published in Advance ACS Abstracts, March 1,1994. (1) (a) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc.

Chem. Res. 1991,24,357. (b) Bunton, C. A.; Savelli, G. Ado. Phys. Org. Chem. 1986,22,213. (c) Bunton, C. A. In Surfactants in Solution; Mittal, K. L., Shah, D. O., Eds.; Plenum Prees: New York, 1991; Vol. 11, p 17.

(2) (a) Romsted, L. S. In Micellization, Solubilization and Micro- emuleione; Mittal, K. L., Ed.; Plenum Prees: New York, 1977; p 489. (b) Rometed, L. S. In Surfactant in Solution; Mittal, K. L., Lindman, B., Ede.; Plenum Preea: New York, 1984, Vol. 2, p 1015. (c) Quina, F. H.; Chaimovich, H. J. Phys. Chem. 1979,83,1844.

0743-7463/94/2410-1155$04.50JQ

tions in this region increase with increasing total ionic concentration.314

(ii) Ionic distribution is written in terms of equations that have the form of Langmuir isotherms, and with increasing ionic concentration in the aqueous phase, the counterion concentration at the micellar pseudophase will increase to a value corresponding to j3 = 1.5

(iii) Ionic distributions are calculated by solving the Poisson-Boltzmann equation (PBE) in the appropriate symmetry with allowance for non-Coulombic-nonspecific and ion-specific micelle-ion interactions.6

Each of these methods has been used to fit rate data with moderately concentrated ionic reagents in aqueous micelles. We are interested in rate effects of alcohol- modified micelles, so we have compared the predictive powers of methods i and ii. We did not use the PBE treatment, method iii, because it requires knowledge of the micellar structure, in particular of the radius and charge density, whose values are uncertain for alcohol-modified micelles.

Anionic micelles of sodium dodecyl sulfate (SDS) increase rates of hydrogen ion catalyzed hydrolyses of 0-acyloximes, and with >0.1 M HC1 the PIE model with constant a underpredicts the reaction rates. The sub- strates were p-methoxybenzaldehyde 0-acetyloxime and 0-octanoyloxime (AMB and OMB, R = CH3 and n-C.IHl5, respectively). Hydrolysis follows theA~c2 mechanism with rate-limiting attack of water on the protonated substrate.7

(3) He, Z. M.; Loughlin, J. A.; Romsted, L. S . Bull. Soc. Chil. Quim. 1990,35,43.

(4) Ferreira, L. C. M.; Zucco, C.; Zanette, D.; Nome, F. J. Phye. Chem. 1992,96, 9058.

(5) (a)Bunton,C.A.;Gan,L.H.;Moffatt,J.R.;Rometed,L.S.;Savelli, G. J. Phys. Chem. 1981,85,4118. (b) Gennani, R.; Ponti, P.; Savelli, G.; Spreti, N.; Bunton, C. A.; Moffatt, J. R. J. Chem. SOC., Perkin %ne. 2 1989, 401. (c) Rodenas, E.; Vera, S. J. Phys. Chem. 1986,89,513.

(6) (a) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1986,90,538. (b) Bunton, C. A.; Moffatt, J. R. Langmuir 1992,8, 2131.

0 1994 American Chemical Society

1994 Rubio et al. 1156 Langmuir, Vol. 10, No. 4,

p-MeOCeH4HC=NOCOR + H+

1.. OH I

The 0-acetyloxime, AMB, is not strongly micellar- bound, so even at high [SDS] there is a significant reaction in the aqueous pseudophase, but the 0-octanoyloxime is almost completely insoluble in water, so its reaction is almost wholly in the micellar pseudophase. The PIE model, with a constant value of a, fits the rate data for reactions in aqueous SDS with dilute HC1, but for [HClI > 0.1 M, rate constants are underpredicted and an additional term was added to fit the data.49538

We followed reactions in 0.05 M SDS with and without added BuOH and varying [HCll. Structural parameters needed for fitting of the kinetic data, e.g., a, the critical micelle concentration, cmc, and the binding constant of BuOH to SDS micelles, have been m e a s ~ r e d . ~ Binding constants of the 0-acyloximes to SDS micelles were measured as a function of [BuOHl , as was the rate constant of acid-catalyzed hydrolysis of AMB in water. The 0-octanoyloxime, OMB, is too insoluble for its hydrolysis to be followed, even in aqueous BuOH.

Experimental Section Materials. Purification of SDS has been described,8" the

cmc was 8.0 X 109 M, and there was no minimum in the plots of surface tension against log [SDS]. Preparations of AMB and OMB have been described.* Reaction solutions were made up in deionized, distilled water.

Kinetics. Hydrolyses were followed at 25.0 OC in a Shimadzu UV 210 spectrophotometer by monitoring absorbance at 275 nm for up to 90% reaction. Rate constants, k,h, s-l, are the means of 2-4 determinations.

Data Simulation. Both methods of data fitting require values of the binding constant, K,, of AMB and OMB, second-order rate constants in BuOH-HaO, and values of the cmc and a of SDS micelles modified by BuOH. These latter values have been determined.9 Electrolytes decrease the cmc, and we assume that the effect of HC1 is the same as that of NaCl and that the relative effeds are unaffected by addition of BuOH. On this basis we reduce the cmc in the absence of electrolyte by the following factors: 0.05 M HC1, 3.6; 0.1 M HC1, 5.7; 0.5 M HCl, 25; 0.7 M HCl, 52. In practice the cmc can be neglected in the more acidic solutions.

The substrate binding constants, K,, of the 0-acyloximes to micellized SDS were determined by the solubility method.lOJ1 Concentrations were monitored by absorbance changes at 270 nm.

Results Values of K, for the binding of AMB to SDS decrease

slightly with addition of BuOH; however, the binding of the more hydrophobic substrate, OMB, is essentially quantitative even at the highest BuOH concentrations (Table 1).

Table 2 contains the second-order rate constants for the acid-catalyzed hydrolysis of AMB as a function of [BuOH]. The values were calculated from linear plots of

(7) Brady, 0. I.; Miller, J. J. Chem. SOC. 1960, 1234. (8) Gonsalves, M.; Probta, S.; Rezende, M. C.; Nome, F.; Zucco, C.;

(9) Danil, A. R. R.; Zanette, D.; Nome, F.; Bunton, C. A., preceding

(10) Bunton, C. A,; Ohmenzetter, K.; Sepulveda, L. J. Phys. Chem.

(11) Bertoncini, C. R. A.; Neves, M. de F. S.; Nome, F.; Bunton, C. A.

Zanette, D. J. Phys. Chem. 1985,89, 1127.

paper in this ieeue.

1977,81,2000.

Langmuir 1993,9, 1274.

Table 1. Effect of BuOH on the Substrate Binding

[BuOHI,M 0 0.11 0.33 0.55 0.76 K,, M-'

Constant to SDS.

72 (13 OOO) 61 (11 OOO) 48 (6300) 33 (3900) 39 (3200) a At 25.0 O C for AMB. Values in parentheses are for OMB.

Table 2. Effect of BuOH on the Acid Hydrolysis of AMB in Aqueous HCl.

[BuOH],M 0 0.109 0.219 0.437 0.655 0.764 0.874 109k;, M-8-1 3.25 3.67 3.59 3.64 3.26 3.45 3.26

a At 25.0 "C.

the first-order rate constant versus [HClI in the 0-0.8 M range (not shown).

First-order rate constants for the hydrolysis reactions of both AMB (Table 3) and OMB (Table 4) decrease monotonically on addition of BuOH to 0.05 M SDS, irrespective of the acid concentration. The rate decrease is similar to that observed for the reaction of methyl naphthalene-2-sulfonate with B r in cetyltrimethylam- monium and cetyltriethylammonium bromides in the presence of BuOH.ll

Discussion

The first-order rate constant for the overall reaction, koh, is given by eq 1. The concentration of micellized SDS is the total concentration less the critical micelle concentration, cmc, under kinetic conditions. The second-

(1) ky[H+w] + krK8([SDSl - cmC)[H+~]l

1 + K8( [SDSI - cmc) k o h =

order rate constant ky, M-l s-l, relates to reaction in the aqueous pseudophase, and k r is the second-order rate constant with respect to [H+M]I which is the local concentration of H+ as molarity a t the micellar surface. Quantities in brackets without a subscript 1 are molarities in terms of the total solution volume, and subscripts W and M denote aqueous and micellar pseudophases, re- spectively.

In method i the local concentration, [H+M]~, is that estimated by the PIE with values of a and cmc in the presence of BuOH plus the molarity [H+wl in water, with allowance for dilution in the micellar pseudophase by BuOH. In eq 2, VM is the partial molar volume (M-l) of

[H+MIR + [H+,] (2) [H+M1l = ([SDSI - cmc) V ,

the reaction region in the micellar pseudophase and R is given by

[SDSI - cmc ([SDS] - cmc + [BUOHM]) R = (3)

The estimated value of the ion-exchange constant between H+ and Na+ is 1 9 0

(4)

Le., micelles do not discriminate between H+ and Na+,lO and therefore [H+M] is given by

[HCll([SDS] - cmc)(l - a) [HClI + [SDSI (5) [H+M] =

Acid Hydrolysis of p-Methoxybenzaldehyde 0- Acyloximes Langmuir, Vol. 10, No. 4, 1994 1157

Table 3. Effect of BuOH on the First-Order Rate Constants for the Hydrolysis of AMB. lo%& s-1

[BuOHl, M [HCll 0.05 M [HCll 0.10 M [HCll 0.20 M [HCII 0.30 M [HClI 0.50 M [HClI 0.70 M [HCl] 0.80 M 0.00 0.89 1.19 0.109 0.70 1.16 0.219 0.57 1.02 0.328 0.42 0.77 0.437 0.35 0.65 0.546 0.28 0.55 0.665 0.764 0.20 0.53 0.874 0.18 0.47

4 [SDS] = 0.050 M, at 25.0 "C.

1.52 1.42 1.30 1.04 0.95 0.89 0.80 0.70 0.61

1.70 1.45 1.37 1.06 0.98 0.89 0.81 0.66 0.67

1.96 1.71 1.55 1.45 1.40 1.26 1.16 1.04 0.96

2.33 2.26 2.09

1.77 1.77 1.50 1.43 1.44

Table 4. Effect of BuOH on the First-Order Rate Constants for the Hydrolysis of OMB.

2.69 2.51 2.35 2.15 2.14 1.96 1.84 1.81 1.61

0.00 0.85 1.15 0.109 0.67 0.86 0.219 0.57 0.75 0.328 0.45 0.60 0.437 0.35 0.46 0.546 0.26 0.39 0.665 0.19 0.26 0.764 0.22

[SDS] = 0.050 M, at 25.0 "C.

~~ ~

1.43 1.14 1.04 0.87 0.76 0.63 0.48 0.42

~~ ~

109kob, s-1

[BuOHl, M [HClI 0.05 M [HClI 0.10 M [HClIO.20 M CHC110.50 M [HCII 0.80 M [HCll 1.00 M [HCl] 1.50 M 1.92 2.32 2.49 3.74 1.67 1.97 2.10 3.03 1.37 1.71 1.84 2.74 1.23 1.40 1.44 2.15 1.05 1.22 1.34 1.74 0.98 1.06 1.29 1.69 0.75 0.89 1.05 0.66 0.80

Table 5. Rate Constants Calculated for the Hydrolyses of AMB and OMB with the Modified PIE and Langmuir

Models

modified PIE model Langmuir model [HCll,M AMB OMB AMB OMB

0.05 0.10 0.20 0.30 0.50 0.70 0.80 1.00 1.50

6.1 5.0 7.9 5.0 7.7 5.5 6.2 6.0 6.0 7.0 8.0 6.0

6.1 7.5

5.8 4.0 5.5 3.8 5.8 4.5 5.8 7.0 5.5 7.5 7.8 5.8

7.0 8.8

Variations of hob with [HCU and [BuOHl were fitted to eqs 1-3 and 5, with values of K, and k: for AMB taken from Tables 1 and 2, respectively; values of a and cmc were taken from ref 9. In fitting the data, we initially took VM = 0.25 M-' and assume that it is unaffected by BuOH. This value is that frequently used in fitting rate data in aqueous SDS.*

The rate data for hydrolysis of AMB were fitted with basically a single value of k: = (7.0 1.0) X 10-4 M-' 8, as shown in Table 5. Since the values of K8 are relatively low and the second-order rate constant in the bulk solvent is ca. 5-fold greater than that in the micellar phase, reaction in the aqueous pseudophase makes a significant contri- bution to the overall reaction. Reaction of OMB is almost wholly in the micellar pseudophase, and in fitting the data by using eqs 1-5, we take k: (Table 2) as for reaction of AMB, although the contribution of this reaction is negligible, since the incorporation of OMB into the micellar phase is quantitative (see Kavalues in Table 1). The values for the second-order rate constant in the micellar phase are also given in Table 5. As can be seen, k: increases from 5.0 X 10-4 ([HClI = 0.05 M) to 7.5 X 10-4 M-l a-1 ([HClI = 1.5 M).

We can rationalize eq 2 on the assumption that if a micelle did not interact with H+, ita concentration in the

interfacial region at the micellar surface would be [H+wl. The first term in eq 2 is the incremental concentration due to attraction of H+ to the anionic surface, a calculated by the modified PIE model, including a as a variable.' In dilute H+, this first term is dominant and the contribution from the [H+w] is not needed in order to fit the data.

In method ii distributions of H+ (and Na+) are written in the form of Langmuir isotherms as in eq 6, with no invasion of H+w.5 The ion-exchange parameter of unity

(eq 4) indicates that the micelle does not discriminate between H+ and Na+, i.e., K'H = K'N~, eq 6 reduces to

K', = [H+M]

[H+wl([SDSl - cmc - CH+,l)(l+ [SDSl/[HCl])

(7)

and H+M is given by eq 8. Values of kr/VM can be calculated from the rate data and values of K'H which vary with [BuOHl.

We follow the approach applied to the effect of BuOH on values of K' in solutions of cetyltrimethylammonium bromide and chloride" and relate K'H to CY by eq 9. This

K', - - (9) (1 - 8)((1- B)([SDSI - cmc) + cmc]

equation is slightly different from that used earlier where the low cmc allowed a simplification in the treatment."

The data are fitted with eqs 1 and 7-9 in terms of values of kF/VM so that hF varies inversely with the assumed value of VM.

1158 Langmuir, Vol. 10, No. 4,1994 Rubio et al.

simple treatment is inadequate when the solution contains only very hydrophilic ions or high ionic concentrat i~n.~*~J~

Micellar rate data with concentrations of ionic reagents greater than 0.1 M are fitted by models that allow ionic concentrations in the micellar pseudophase to exceed values calculated by the PIE treatment. In the present work we fit variations of kob for BuOH-modified micelles of SDS over a wide range of [HClI with either the modified PIE model, which includes CY as a variable and allows for ninvasionn of ions in the aqueous phase, eq 2, or that based on the assumption that ionic distributions between water and micelles follow Langmuir isotherms (eq 6).

Equation 2 predicts that [H~llwil l always increase with increasing [H+w], whereas eq 6 predicts that the micellar surface will become saturated with hydrogen ions at high [H+wI. However, concentrated electrolytes have major effects upon micellar structure, so the kinetic effects that we ascribe to increasing values of [ H M ~ at high [HCD may be caused by changes in the properties of the reaction medium at the micellar surface, e.g., a decrease in VM, or in the ability of micellar-bound water molecules to hydrate hydrogen ions, and will increase reaction rates a t the micellar surface. It is important toremark that the modest increase in k; with [HClI is general for acid-catalyzed hydrolyses, so k; may not be constant, as assumed in the usual pseudophase models.lP2

Despite these reservations we believe that equations such as 2 and 6 that allow ionic concentrations at micellar surfaces to exceed the limits set by PIE are useful in predicting kinetic behavior in micellar systems. They also eliminate the strict distinction between the aqueous and micellar pseudophases that is implicit in the pseudophase model in its simplest form.

All quantitative treatments of rates of nonsolvolytic bimolecular reactions in colloidal assemblies involve assumptions regarding the volume of the reaction region, or the appropriate measure of concentration in that region. The use of molarity as a measure of concentration in homogeneous solutions is justified on the basis of con- venience rather than of logic, but comparison of k: and k; requires that we define molarity at the micelle-water surface, as in eqs 2 and 8.

Equation 2 includes VM, but the fitting is not very sensitive to its value. We can fit the rate data equally well with &of 0.14 or 0.25 M-1, provided that we change values of k;. In data fitting with eqs 6-9, we estimate k;/ V,. We assume that VM is unaffected by incorpora- tion of BuOH into the micellar pseudophase;ls then values of k? are only slightly affected by changes in [BuOHl and [HCl] up to 0.5 M, and the two methods give similar values for a given VM.

Kinetic data for reactions of H+ in anionic, and of halide ions in cationic, micelles with added BuOH can be fitted on the assumption that VM does not change, probably because the dilution in the micellar pseudophase by BuOH is largely corrected by the R term (eq 3)."J3J4

There is no indication that incorporation of the hydrogen ion in an SDS micelle decreases its protonating power, i.e., micellized dodecylsulfuric acid appears to be disso- ciated even though anionic micelles decrease dissociation of weak acids.% Consistently anionic and cationic micelles have similar effects on second-order rate constants of acid

0 0.2 0.4 0.6 0.8 [I-Butanol] , M

Figure 1. Effect of increasing butanol concentration on the first- order rate constant for hydrolysis of p-methoxybenzaldehyde 0-acetyloxime (Am) at 0.05 M SDS and in the presence of 0.05 M (A), 0.30 M (o), and 0.80 M (0) HC1, at 25.0 OC. Solid and dashed lines were calculated using the Langmuir and modified PIE models, respectively.

4 2.0 ' 9

I 01 . . . ) . . . I ~ . " ' " " J

~

0 02 04 06 08 [I-Butanol] , M

Figure 2. Effect of increasing butanolconcentrationon the first- order rate constant for hydrolysis of p-methoxybenzddehyde 0-octanoyloxime (OMB) at 0.05 M SDS and in the presence of 0.05 M (A), 0.20 M (a), and 0.80 M (0) HCl, at 25.0 O C . Solid and dashed lines are calculated using the Langmuir and modified PIE models, respectively.

The fits to the experimental data, with both the modified PIE and Langmuir models, are shown in Figures 1 and 2 for AMB and OMB, respectively. Values of k; are also given in Table 5. Values of k; calculated by using eq 7 increase slightly in high [HClI, similarly to the results calculated for OMB using the modified PIE model. This increase in rate constant in the micellar phase shows nicely the predictive power of both models, since at [HClI > 0.8 M the first-order rate constant in the bulk solvent shows a clear upward curvature?

Values of kr /k ; are in the range of 4-9, and as in many micellar-mediated ionic reactions, second-order rate con- stants are lower in the micelles than in water and rate enhancements are due wholly to increased reactant concentrations a t micelle-water interfaces.192

In the original PIE model, counterion concentrations a t the water-micelle interface are limited by the value of the fractional micellar neutralization, p.112 The molarity in the reaction region at the interface is given by B~VM, so with = 0.8 and since VM is in the range of 0.14-0.37 M-l, the predicted molarity is in the range of 2.2-5.5 M. This

~ ~~~

(12) (a) Nome, F.; Rubira, A. F.; Franco, C.; Ionescu, L. G. J. Phya. Chem. 1982,86,1881. (b) Stadler,E.; Zanette,D.;Rezende,M. C.; Nome, F. J. Phys. Chem. 1984,88, 1892.

(13) Bertoncini, C. R. A.; Nome, F.; Cerichelli, G.; Bunton, C. A. J. Phys. Chem. 1990,94,6875.

(14) Chandhuri, A.; Rometed, L. S.J.Am. Chem. SOC. 1991,113,5062.

Acid Hydrolysis of p-Methoxybenzaldehyde O-Acyloximes

hydrolyses of acetals and dioxolanes in the micellar pseudophases.8

On the basis of consideration of electrostatic interac- tions, concentration gradients of both co-ions and coun- terions between micelles and water decrease with increas- ing ionic concentration in water. Inclusion of a term for [H+wl in eq 2 is, therefore, consistent with physical descriptions of micelle-ion interactions.16 Treatments based on classical electrostatics, e.g., with solution of the Poisson-Boltzmann equation,8J6 lead to similar conclu- sions, but calculations depend upon the shape and size of the micelle, which may not be known, and the simpler models have the advantage of being size-independent.'

The various pseudophase treatments of micellar effects upon rates and equilibria of thermal reactions are based

(15) Gunnarsson, G.; Joneson, B.; Wennerstrom, H. J. Phye. Chem.

(16) Ortaga, F.; Rodenae, E. J. Phys. Chem. 1987,91,837. 1980,84,3114.

Langmuir, Vol. 10, No. 4, 1894 1159

on different assumptions and approximations, but they lead to similar values of k:, at least for dilute ionic reactants. Modifications of the simpler treatments, e.g., of the PIE, which fit data in more concentrated electrolyte. give values of k: similar to those estimated in dilute solution. However, these values depend upon the assumed value of V#7and on the assumption that it is independent of the nature and concentrations of reactants and chem- ically inert solutes.

Aoknowledgment. Support of this work by CNPq (Conselho Nacional de Desenvolvimento Cientffico e Tecnol6gic0, Brazil), FINEP (Financiadora de Estudos e Projetos), and the National Science Foundation, Organic Chemical Dynamics and International Programs, is grate- fully acknowledged.

(17) Hicks, J. R.; Reineborongh, V. C. A u t . J. Chem. 1982,36, 15.