Unusual Rate Enhancement in the Hydroxide Ion-Catalyzed

2498

Langmuir 1997, 13, 2498-2503

Unusual Rate Enhancement in the Hydroxide Ion-Catalyzed Hydrolysis of N-Phthaloylglycine in the Presence of Anionic Micelles M. Niyaz Khan Department of Chemistry, Universiti Malaya, 50603 Kuala Lumpur, Malaysia Received July 15, 1996. In Final Form: January 2, 1997X Pseudo-first-order rate constants (kobs) for hydrolysis of ionized N-phthaloylglycine (NPG) at a constant [NaOH] and 30 °C increase nearly 50% with an increase in the total concentration of sodium dodecyl sulfate ([SDS]T) from 0.0 to 0.2 M. The hydrolysis of NPG involves -OH and anionic NPG as the reactants. The increase in [NaBr] from 0.01 to 0.4 0 M increases kobs by nearly 44% at 0.004 M NaOH and 30 °C. Pseudofirst-order rate constants (kobs ) increase by nearly 50% with an increase in [KCl] from 0.0 to 0.1 M at 0.003 and 0.004 M NaOH and 30 °C. The increase in [SDS]T from 0.0 to 0.3 M decreases kobs for hydrolysis of phthalimide at 0.01 M NaOH and 35 °C by nearly 10%. The attempt is also made to explain pseudofirst-order rate constants in terms of interfacial hyroxide ion concentration calculated from the Boltzmann equation using interfacial electrical potential.

Introduction The effects of micelles on reaction rates are usually discussed in terms of the occurrence of parallel chemical reactions in the aqueous pseudophase and the micellar pseudophase.1 The micellized reactions are generally believed to occur in the Stern layer.1 However, a few reports have appeared where the bimolecular reactions have been concluded to occur between micellized and nonmicellized reactants (i.e. cross-border reactions).2 The rates of reactions of several neutral organic compounds with -OH are inhibited by anionic micelles.3 If the micellized reactions occur only in the Stern layer, then on the basis of the energetic considerations, the rate of a bimolecular reaction involving two anionic reactants is expected to be either unaffected (if micellar incorporation of both anionic reactants does not occur) or inhibited (if micellar incorporation does occur with only one anionic reactant which contains a large hydrophobic area) by anionic micelles. Recently, we observed the increase in the rate of hydroxide ion-catalyzed hydrolysis of anionic N-hydroxyphthalimide4 and acetyl salicylate ion5 with an increase in the concentration of sodium dodecyl sulfate (SDS). In the continuation of our search on the effects of anionic micelles on the rates of bimolecular reactions involving anionic reactants, we now report the results on the effects of [SDS] on the rate of hydroxide ion-catalyzed hydrolysis of anionic N-phthaloylglycine. Experimental Section Materials. Reagent grade N-phthaloylglycine (NPG) and sodium dodecyl sulfate (SDS) were obtained from Fluka. All other chemicals used were also of reagent grade commercial products. Distilled water was used throughout. The stock solutions of NPG were prepared in acetonitrile. X

Abstract published in Advance ACS Abstracts, April 1, 1997.

(1) Bunton, C. A. Catal. Rev. Sci. Eng. 1979, 20, 1. (2) Bunton, C. A.; Romsted, L. S.; Savelli, E. J. Am. Chem. Soc. 1979, 101, 1253. Nome, F.; Rubira, A. F.; Franco, C.; Ionescu, G. L. J. Phys. Chem. 1982, 86, 1881. Stadler, D.; Rezende, M. C.; Nome, F. J. Phys. Chem. 1984, 88, 1982. (3) (a) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (b) Bunton, C. A. In Surfactants in Solution; Mittal, K. L., Shah, D. O., Eds.; Plenum Press: New York, 1991; Vol. 11, p 17 and references cited therein. (4) Khan, M. N. Int. J. Chem. Kinet. 1991, 23, 567. (5) Khan, M. N. J. Colloid Interface Sci. 1995, 170, 598.

S0743-7463(96)00691-9 CCC: $14.00

Kinetic Measurements. The rates of alkaline hydrolysis of NPG were studied spectrophotometrically by monitoring the disappearance of reactant, NPG, at 300 nm. The details of the kinetic procedure and data analysis are described elsewhere.6

Results and Discussion (a) Effect of [SDS]T on Alkaline Hydrolysis of Phthalimide (PTH). A few kinetic runs were carried out within the [SDS]T (total concentration of SDS) range 0.0-0.3 M at 0.01 M NaOH and 35 °C. The pseudo-firstorder rate constants, kobs, are summarized in Table 1. The increase in [SDS]T from 0.0 to 0.3 M decreased kobs by ≈10% (Table 1). The rate of alkaline hydrolysis of phthalimide has been found to be independent of [-OH] within its range 0.0050.050 M.7 Under such experimental conditions, the reactants are either SH and -OH or S- and H2O, where SH and S- represent nonionized and ionized PTH, respectively. It has been concluded elsewhere4 that the rate of pH-independent hydrolysis of PTH involves SH and -OH as the reactants. Thus, under the present experimental conditions the rate of hydrolysis of PTH involves SH and -OH as the reactants. The insignificant effect of [SDS] on kobs indicates the kinetically insignificant SDS micellar incorporation of SH and -OH. However, the activity of water and the concentrations of SH and -OH in the Gouy-Chapman layer or at the junctural region of the Gouy-Chapman and Stern layers may not be significantly different from their respective concentrations in the aqueous pseudophase. The Gouy-Chapman layer differs significantly from the aqueous pseudophase in terms of ionic strength because nearly 30% counterions (Na+) remain in the Gouy-Chapman layer. Since the rate of pH-independent hydrolysis of PTH is almost insensitive to the change in [NaCl] within its range 0.02.5 M,8 the present observations on the effect of [SDS]T on kobs do not exclude the possibility of the occurrence of micellar-mediated reaction in the Gouy-Chapman layer or at the junctural region of the Gouy-Chapman and Stern layers. The assumption that the SDS micellar binding affinity of SH is extremely weak seems to be inconceivable for the reason that the SDS micellar binding constants of (6) Khan, M. N. J. Chem. Soc., Perkin Trans. 2 1989, 199. (7) Khan, M. N. Int. J. Chem. Kinet. 1987, 19, 143. Khan, M. N. J. Pharm. Biomed. Anal. 1989, 17, 685. (8) Khan, M. N.; Sumaila, M. B. U.; Mohammad, A. M. J. Chem. Res. (S) 1991, 233; (M) 1991, 2301.

© 1997 American Chemical Society

Hydrolysis of N-Phthaloylglycine

Langmuir, Vol. 13, No. 9, 1997 2499

Table 1. Effect of [SDS]T on Pseudo-First-Order Rate Constants (kobs) for Hydrolytic Cleavage of Phthalimide at 0.01 M NaOH and 35 °Ca [SDS]Tb/M

103 kobs/s-1

0.0 0.02 0.04 0.06 0.08 0.10 0.15 0.20 0.30

2.30 ( 0.04c 2.27 ( 0.04 2.22 ( 0.03 2.22 ( 0.04 2.34 ( 0.02 2.16 ( 0.03 2.32 ( 0.02 2.13 ( 0.04 2.01 ( 0.05

Table 2. Effect of [NaBr] on Pseudo-First-Order Rate Constants (kobs) for Hydrolysis of N-Phthaloylglycine (NPG) at 0.004 M NaOH and 30 °Ca 103kobs/s-1

103kcalcdb/s-1

0.01 0.05 0.10 0.20 0.30 0.40

20.0 ( 0.1c 22.0 ( 0.2 24.0 ( 0.2 26.6 ( 0.1 27.0 ( 0.2 28.8 ( 0.2

20.0 22.1 24.0 26.2 27.6 28.5

nonionized and ionized methyl salicylate (a molecule almost similar to PTH in terms of hydrophobic area) turned out to be 100 and 5.7 M-1 , respectively, at 30 °C.9 (b) Effect of [NaBr] on Alkaline Hydrolysis of N-Phthaloylglycine (NPG). In order to discover the effect of [NaBr] on the rate of alkaline hydrolysis of NPG, a few kinetic runs were carried out within the [NaBr] range 0.01-0.40 M at 0.004 M NaOH and 30 °C. Pseudofirst-order rate constants, kobs, as summarized in Table 2, showed a nonlinear increase with an increase in [NaBr]. These observed data cannot be explained in terms of the Debye-Huckel limiting law because the salt concentrations attained are much higher than the limiting concentration above which the Debye-Huckel limiting law is no longer valid.10 The observed data were found to fit reasonably well to eq 1

k0 + ksKA[NaBr] 1 + KA[NaBr]

103kobsb/ s-1

103kcalcdc/ s-1

20 30 35 40 45 50

10.2 ( 0.1f 12.7 ( 0.1 15.3 ( 0.4 18.1 ( 0.2

10.8 12.9 15.1 17.2

103kobsd/ s-1

103kcalcdc/ s-1

7.62 ( 0.07e 15.4 ( 0.1

9.31 15.5

24.1 ( 0.3

21.7

30.2 ( 0.2

27.9

[NPG]0 ) 5 × 10-4 M; the aqueous reaction mixture for each kinetic run contained 2.5% (v/v) CH3CN. b Ionic strength was not kept constant. c Calculated from eq 2 with k0 ) 0 and the kOH value mentioned in the text. d Ionic strength was kept constant at 0.1 M by KCl. e Error limits are standard deviations. f Error limits are average deviations.

(c) Effect of [NaOH] on Aqueous Cleavage of Anionic NPG. The rate of hydrolysis of NPG was studied under the [NaOH] range 0.0030-0.0045 M at 30 °C in mixed aqueous solvent containing 2.5% (v/v) CH3CN. Pseudo-first-order rate constants, kobs, revealed a linear increase with an increase in [NaOH] (Table 3) and were treated with eq 2

kobs ) k0 + kOH[-OH]cor

a [NPG] ) 4 × 10-4 M; the aqueous reaction mixture for each 0 kinetic run contained 2.0% (v/v) CH3CN. b Calculated from eq 1 using calculated parameters, k0, ks, and KA, as summarized in the text. c Error limits are standard deviations.

kobs )

104[NaOH]/ M

a

a [Phthalimide] ) 5 × 10-4 M; the aqueous reaction mixture for 0 each kinetic run contained 2.5% (v/v) CH3CN. b Total concentration of SDS. c Error limits are standard deviations.

[NaBr]/M

Table 3. Effect of [NaOH] on Pseudo-First-Order Rate Constants (kobs) for the Cleavage of NPG at 30 °Ca

(1)

where k0, ks, and KA represent unknown parameters. Pseudo-first-order rate constants, kobs, obtained for alkaline hydrolysis of ionized N-hydroxyphthalimide4 and acetyl salicylate ion11 at different [salt] were found to fit to an equation similar to eq 1 where KA represents the association constant for the hydrated loose ion-pair formation between cations of the salts and anionic substrates (such as Na+ and ionized NPG in the present study). The nonlinear least squares calculated values of k0, ks, and KA are (19.3 ( 0.5) × 10-3, (32.9 ( 1.5) × 10-3, and 5.2 ( 1.8 M-1, respectively. The fitting of the observed data to eq 1 is evident from the calculated values of the rate constants (kcalcd) as summarized in Table 2. (9) Khan, M. N. J. Phys. Org. Chem. 1996, 9, 295. (10) Atkins, P. W. Physical Chemistry; Oxford University Press: Oxford, U.K., 1978; p 324. (11) Khan, M .N.; Gleen, P. C.; Arifin, Z. Ind. J. Chem. 1996, 35A, 758.

(2)

where [-OH]cor ) [NaOH] - [NPG]0 with [NPG]0 representing the initial concentration of NPG. The linear least squares calculated values of k0 and kOH are (-3.0 ( 0.3) × 10-3 s-1 and 5.2 ( 0.1 M-1 s-1, respectively. The negative value of k0 is chemically and physically meaningless and may be considered as the consequence of the insignificant contribution of the k0 term compared with the kOH[-OH] term under the experimental conditions imposed. The reported value of k0 for N-hydroxyphthalimide, NHPH, is 2 × 10-6 s-1 at pH 6-7 and 30 °C.4 The value of k0 for ionized NPG is expected to be significantly smaller than that for NHPH because the reactivity toward water of NHPH is apparently much higher than that of ionized NPG. Thus, the value of k0 for ionized NPG should be smaller than 10-6 s-1. Even if we assume that k0 ) 1 × 10-6 s-1, then the maximum contribution of k0 (expected at the lowest [NaOH], i.e. at [-OH] ) 0.0030 M) is only ≈0.01%. This shows that k0 may be neglected compared with kOH[-OH] in eq 2. The value of kOH calculated from eq 2 with k0 ) 0.0 is 4.3 ( 0.2 M-1 s-1. A few kinetic runs were also carried out within the [NaOH] range of 0.002-0.005 M at a constant ionic strength of 0.1 M (by KCl) and 30 °C. The treatment of observed data (Table 3) to eq 2 with k0 sets to zero resulted in kOH ) 6.2 ( 0.8 M-1 s-1. The larger value of kOH ()6.2 M-1 s-1) at 0.1 M ionic strength compared with kOH ()4.3 M-1 s-1) at an ionic strength e 4.5 × 10-3 M may be attributed to the salt effect. The value of kOH ()6.2 M-1 s-1) may be compared with the reported value of kOH ()11.8 M-1 s-1) at 0.6 M ionic strength (by KCl) and 30 °C.12 (d) Effect of [SDS] on Alkaline Hydrolysis of Ionized NPG. A series of kinetic runs was carried out within the total sodium dodecyl sulfate concentration, [SDS]T, range 0.0-0.2 M at a constant [NaOH] and 30 °C. Pseudo-first-order rate constants, kobs, are shown graphically in Figure 1. It is evident from Figure 1 that the rate constants, kobs, show a larger increase with increase in [SDS]T at low [SDS]T compared to that at high [SDS]T. It is generally believed that the micellar-mediated reactions occur in the Stern layer. Hydroxide ion exhibits (12) Hoagland, P. D.; Fox, S. W. J. Am. Chem. Soc. 1967, 89, 1389.

2500 Langmuir, Vol. 13, No. 9, 1997

Figure 1. Plots showing the dependence of kobs upon SDS micelle concentration ([Dn]) for hydrolysis of ionized NPG at 30 °C and 0.0030 M NaOH (0), 0.0035 M NaOH (3), 0.0040 M NaOH (O), and 0.0045 M NaOH (4). Solid lines are drawn through the least squares calculated points from eq 5 using the parameters ks and KA′ listed in Table 4.

extremely weak SDS micellar binding affinity because it is a highly hydrophilic anion. In terms of purely electrostatic interaction effect, the rate of a bimolecular reaction involving -OH and an anionic substrate as the reactants must be either unaffected by the increasing concentration of SDS (if the anionic substrate is not hydrophobic enough to have significant SDS micellar binding affinity) or decreased with an increase in [SDS]T (if the anionic substrate is hydrophobic enough to have significant SDS micellar binding affinity). In view of these speculative facts, the increase in kobs with an increase in [SDS]T, although modest but real, seems to be unusual and surprising. The increase in [SDS]T from 0.0 to 0.2 M caused a 47% increase in kobs at 0.004 M NaOH. Similarly, the increase in [NaBr] from 0.0 to 0.2 M increased kobs by 52% at 0.004 M NaOH. Although the effects of [NaBr] and [SDS]T on kobs appear to be similar, the effect of [SDS]T on kobs cannot be attributed to the salt effect on rate of reaction taking place only in the aqueous pseudophase because, at a constant [NaOH], the increase in [SDS]T cannot increase [Na+] in the aqueous pseudophase. The concentration of sodium ions in the aqueous pseudophase must be less than or equal to the sum of the concentration of added NaOH and the critical micelle concentration (cmc) of SDS. A skeptic might think that the [Na+] in the aqueous pseudophase may be larger than the sum of the [NaOH] and the cmc of SDS because of the ionization of micelles. But it should be noted that most of the ionized counterions (Na+) must remain in the vicinity (i.e. Gouy-Chapman layer)3b of the micellar surface. In terms of electrostatic interaction, the ionized counterions (Na+) cannot be expected to enter deep into the aqueous pseudophase. However, the effects of [SDS] on the rate of hydroxide ion-catalyzed hydrolysis of NPG may be attributed to the

Khan

normal salt effect on kobs for the reaction occurring only in the aqueous psedophase provided (i) the degree of ionization (R) of SDS micelles is nearly 1.0 and (ii) all the ionized counterions (Na+) enter deep into the aqueous pseudophase where the nonmicellar mediated reaction occurs. But it is impossible to believe that R ≈ 1.01,3b,13 (the reported R value is 0.2-0.31,3b,13) and all the ionized counterions (Na+) reach to the reaction site of the occurrence of the aqueous-pseudophase mediated reaction.3b The alkaline aqueous cleavage of NPG involves -OH and anionic NPG as the reactants. Since hydroxide ion is highly hydrophilic, the development of a significant concentration of -OH in the Stern layer cannot be expected because of the energetically unfavorable electrostatic interaction between SDS micellar headgroups and micellized -OH. Thus, if the micellar mediated reaction occurs in the Stern layer, then the increase in [SDS]T must decrease kobs. The most probable reaction region for the micellarmediated reaction is the Gouy-Chapman layer or the junctural region of the Gouy-Chapman and Stern layers where the loosely bound counterions (Na+) exist. The loosely bound counterions in the Gouy-Chapman layer are the result of the so-called ionization of the micellar headgroups. The concentrations of water (H2OM) and hydroxide ions (-OHM) in the Gouy-Chapman layer may be similar to their corresponding concentrations in the aqueous pseudophase. But the concentration of anionic NPG molecules (S-) in the micellar region depends upon the micellar location of S-. The following two possibilities may be considered for the micellar location of S-. (i) If the micellized S- molecules (S-M) are located at the junctural region of the Stern and Gouy-Chapman layers, then [S-M] * [S-W], where [S-W] represents the concentration of anionic NPG in the aqueous psedophase. Under such conditions, the micellar-mediated reaction occurs between S-M and -OH (in the Gouy-Chapman layer). Such a reaction is regarded as a cross-border reaction, and there are reports on the occurrence of such reactions.2 Thus, under such conditions, the alkaline aqueous cleavage of NPG in the presence of micellized SDS (Dn) may be expressed by

Scheme 1 KS

S-W + Dn y\z S-M k′W

S-W + -OH 98 P k′M

S-M + -OH 98 P where W and M represent the aqueous pseudophase and the micellar pseudophase, respectively. The observed rate law, rate ) kobs[NPG]T where [NPG]T ) [S-W ] + [S-M], and Scheme 1 can lead to eq 3

kobs )

kW + kMKS[Dn] 1 + KS[Dn]

(3)

where [Dn] ) [SDS]T - cmc (cmc represents the critical micelle concentration of SDS), kW ) k′W[-OH]Tcor, kM ) k′M[-OH]Tcor, and [-OHW]cor ≈ [-OH]Tcor with [-OH]Tcor ) [-OH]T - [NPG]T ([-OH]T represents the total concentration of hydroxide ions). (13) Cutler, S. G.; Meares, P.; Hall, D. G. J. Chem. Soc., Faraday Trans. 1, 1978, 74, 1758.

Hydrolysis of N-Phthaloylglycine

Langmuir, Vol. 13, No. 9, 1997 2501 Table 4. Empirical Parameters Calculated from Eq 5a

[NaOH]/M

103kwb/s-1

103kwc/s-1

0.1e

0.4f

0.0030

10.2 (

0.0035

12.7 ( 0.1

0.0040

15.3 ( 0.4

0.0045

18.1 ( 0.2

10.8 ( 10.8 14.9 ( 0.4 14.9 18.0 ( 0.4 18.0 19.6 ( 0.4 19.6

KA′/M-1

103ks/s-1 1.9f

18.7 ( 20.4 ( 3.2 24.4 ( 1.7 28.8 ( 5.1 32.0 ( 3.1 35.4 ( 5.2 31.4 ( 0.9 34.3 ( 2.7

3.3f

7.6 ( 4.9 ( 2.6 6.3 ( 2.0 3.0 ( 1.6 5.6 ( 2.1 3.6 ( 1.6 11.0 ( 1.7 6.0 ( 1.9

103(cmc)/M

108Σdi2 d/s-1

9.5 0.0 13.0 0.0 9.0 0.0 13.0 0.0

87.82 99.01 44.86 68.42 114.8 117.6 44.15 121.9

a [NPG] ) 5 × 10-4 M; 30 °C; the aqueous reaction mixture for each kinetic run contained 2.5% (v/v) CH CN. b Average values of k 0 3 obs (3 to 5) obtained in the absence of SDS micelles ([SDS]T range 0.0-0.008 M). c Intercept of the plot of kobs versus [SDS]T within the [SDS]T d range of 0.02-0.10 M. di ) kobsi - kcalci where kobsi and kcalci represent the observed and calculated rate constants at the ith concentration of SDS. e Error limits are average deviations. f Error limits are standard deviations.

(ii) If the micellized anionic NPG molecules (S-M) are located in the Gouy-Chapman layer where [S-W] ≡ [S-M] ) [NPG]T, then the micellar-mediated reaction occurs in the micellar region which is not different from the aqueous pseudophase in terms of the concentrations of -OH and anionic NPG. Under such conditions, the alkaline aqueous cleavage of NPG in the presence of micellized SDS may be shown by Scheme 2

Scheme 2 KA

S- + Na+ y\z S-‚Na+ k′w

S- + -OH 98 P k′s

S-‚Na+ + -OH 98 P where S-‚Na+ represents the association complex formed from S- and Na+. The observed rate law, rate ) kobs[NPG]T, where [NPG]T ) [S-] + [S-‚Na+], and Scheme 2 can lead to eq 4

kobs )

kw + ksKA[Na+] 1 + KA[Na+]

(4)

where kw ) k′w[-OH]Tcor, ks ) k′s[-OH]Tcor, and [Na+] ) cmc + [Na+]GC with [Na+]GC representing the concentration of Na+ ions in the Gouy-Chapman layer. Since the volume of the Gouy-Chapman layer is difficult to ascertain, the actual concentration of Na+ ions in the Gouy-Chapman layer cannot be determined with reliability. However, [Na+]GC should be proportional to [Dn], and hence [Na+]GC ) δ[Dn], where δ is the proportionality constant. Furthermore, since [Na+]GC . cmc, the actual concentration of Na+ ions in the Gouy-Chapman layer must be larger than the cmc and hence [Na+] = [Na+]GC ) δ[Dn]. In terms of these considerations, eq 4 is reduced to eq 5

kobs )

kw + ksKA′[Dn] 1 + KA′[Dn]

(5)

where KA′ ) δKA. Equation 5 is similar to eq 3 with kw ) kW, kM ) ks, and KS ) KA′. The observed data were treated with eq 5 with kw as a known parameter and ks and KA′ as unknown parameters. Equation 5 predicts that kw ) kobs at [Dn] ) 0 (i.e. in the absence of SDS micelles). But this is true only if the pH of the aqueous pseudophase remains unchanged with the presence and absence of SDS micelles. The pH of the aqueous pseudophase has been found to be larger in the presence than in the absence of SDS micelles at a constant [NaOH] ([-OH] < 0.01 M) and 2 × 10-4 M phenyl

salicylate.9 These observations are the consequence of the SDS micellar exclusion of -OH ions due to energetically unfavorable electrostatic interaction between the micellar headgroups and hydroxide ions. It has also been observed that, at a constant [NaOH], the increase in [SDS]T from 0.04 to 0.20 M does not change the pH of the micellar solution. The intercept of the linear plot of kobs versus [SDS]T ([SDS]T range 0.02-0.10 M) was considered as a more appropriate kw for use in eq 5. The cmc values at different [NaOH] were calculated from the observed data (kobs versus [Dn] at [SDS]T g 0.02 M; note that kobs values turned out to be independent of [SDS]T within the range of 0.0-0.008 M) using an iterative technique described elsewhere.14 These calculated cmc values are summarized in Table 4. The average cmc value of (11 ( 2) × 10-3 M may be compared with the reported value of 8 × 10-3 M (in 100% water solvent).15 The unknown parameters, ks and KA′, were calculated from eq 5 using the nonlinear least squares technique with known values of kw and cmc as listed in Table 4. The calculated values of ks and KA′ at different [NaOH] are summarized in Table 4. The fitting of the observed data to eq 5 is evident from the standard deviations associated with ks and KA′ as well as from the plots of Figure 1, where solid lines are drawn through the calculated points. The unknown parameters, ks and KA′, were also calculated from eq 5 using known values of kw with cmc ) 0. Under such data treatment, although the values of ks remain essentially unchanged while KA′ values are reduced, the residual errors between kobs and kcalcd remain almost the same compared with the data treatment where cmc * 0 (Table 4). Scheme 2 predicts that ks should increase linearly with an increase in [-OH]. The rate constants, ks, revealed a reasonably good fit to eq 2 with k0 ) 0, and least squares calculated kOH ) 8.2 ( 0.7 M-1 s-1 at cmc * 0 and kOH ) 9.1 ( 0.9 M-1 s-1 at cmc ) 0. These calculated values of kOH may be compared with the kOH ()6.2 ( 0.8 M-1 s-1) obtained at 0.1 M ionic strength (by KCl) in the absence of SDS micelles. The values of KA′ at different [NaOH] (Table 4) are not significantly different from the KA ()5.2 M-1) obtained from eq 1 for NaBr. However, the present data are not sufficient to differentiate between whether the listed values of KA′ in Table 4 represent KS of Scheme 1 or KA of Scheme 2. The kinetically observed value of KS for ionized phenyl salicylate (PS-) with SDS micelles is 5.916 and 3.017 M-1 . But KS for ionized methyl salicylate (MS-) with SDS micelles could not be kinetically detected.17 The specific kinetic probe used in these studies could detect the SDS micellar binding of substrate only (14) Khan, M. N.; Arifin, Z. Langmuir 1996, 12, 261. Khan, M. N.; Arifin, Z. J. Chem. Res. (S) 1995, 132. (15) Bunton, C. A.; Savelli, C. Adv. Phys. Org. Chem. 1986, 22, 213. (16) Khan, M. N. J. Chem. Soc., Perkin Trans. 2 1990, 445. (17) Khan, M. N. J. Mol. Catal. 1995, 102, 93.

2502 Langmuir, Vol. 13, No. 9, 1997

Khan

if the micellized substrate existed in the Stern layer where the water concentration ([H2OM]) was significantly smaller than the water concentration ([H2OW]) in the aqueous pseudophase. The use of a probe which involved pH measurement coupled with UV spectrophotometric determination yielded KS for PS- and MS- as 4.0 and 5.7 M-1, respectively.9 If we assume that the KA′ values (Table 4) stand for KS, and that the micellized ionized NPG (SM-) molecules are lying in the Stern layer where [H2OM] < [H2OW], then under such conditions, kobs cannot be expected to increase with an increase in [SDS]T. But, if we assume that SM- molecules are lying in the Gouy-Chapman layer where [H2OM] ) [H2OW], then kobs should be independent of [SDS]T because, under such conditions, kW ≈ kM (because [-OHW] ≈ [-OHGC], where [-OHGC] represents the hydroxide ion concentration in the Gouy-Chapman layer). Thus, it appears that the KA′ values of Table 4 stand for KA of Scheme 2. An alternative way to treat the data presented is to say that reaction of -OH and ionized NPG occurs in the Stern layer of the micelle and that this is influenced by the local concentration of -OH in this area.18 The local interfacial hydroxide ion concentration, [-OHi], may be obtained from eq 619

[-OHi ] ) [-OHW] exp(RΨ)

(6)

where [-OHW] represents the hydroxide ion concentration in bulk water, Ψ is the interfacial electrical potential, and R ) F/RT with F as Faraday’s constant, R as the gas constant, and T as temperature. In terms of eq 6, pseudofirst-order rate constants, kobs, at different [SDS]T can reflect

kobs ) k′ exp(RΨ)

(7)

where k′ is the pseudo-first-order rate constant in the absence of an electrostatic field around the micelle. The values of Ψ in SDS at different [Na+] were obtained from ref 19. Pseudo-first-order rate constants, kobs, obtained at a constant [-OH] and within the [SDS]T range 0.02-0.20 M were tested with eq 7. The plots of ln kobs versus Ψ are shown in Figure 2. The intercept, ln k′, and slope, R, of these plots were calculated from the linearized form of eq 7 using the linear least squares technique. The calculated values of ln k′ and R at different [NaOH] are summarized in Table 5. It is interesting to note that although the plots of ln kobs versus Ψ at different [NaOH] appear to be reasonably good linear plots, the calculated values of R are nearly 6-fold smaller than the theoretical or predicted value of R {)F/RT ) 37.7 × 10-3 (mV)-1} at 35 °C. If we restrict the R value at 37.7 × 10-3 (mV)-1, then the pseudo-firstorder rate constants, kobs, no longer fit to eq 7. Furthermore, the calculated values of k′ are nearly 2.5-fold larger than pseudo-first-order rate constants, kW, obtained under similar experimental conditions in the absence of SDS micelles (Table 5). This is difficult to believe for the reason that, in the absence of an electrostatic field around a micelle (i.e. a nonionic micelle), [-OHi] cannot be expected to be larger than [-OHW] even in the presence of an inert salt such as NaCl because hydroxide ion is highly hydrophilic. The dielectric constant of the micellar surface is shown to be nearly 32,19 and the decrease in dielectric constant of the aqueous reaction medium containing protic organic solvent decreases the rate of reaction between (18) I thank one of the reviewers for suggesting this data treatment. (19) Fernandez, M. S.; Fromherz, P. J. Phys. Chem. 1977, 81, 1755.

Figure 2. Plots of ln kobs versus interfacial electrical potential, Ψ, of SDS micelles for hydrolysis of ionized NPG at 30 °C and 0.0030 M NaOH (O), 0.0035 M NaOH (4), 0.0040 M NaOH (0), and 0.0045 M NaOH (3). Solid lines are drawn through the least squares calculated points from the linear form of eq 7 using the parameters ln k′ and R listed in Table 5. Table 5. Calculated Values of ln k′ and r from the Linear Form of Eq 7a [NaOH]/M

-ln k′/s-1

103Rb/(mV)-1

(k′/kW)c

0.0030 0.0035 0.0040 0.0045

3.59 ( 0.10d 3.41 ( 0.06 3.14 ( 0.05 3.04 ( 0.06

6.61 ( 0.88d 5.81 ( 0.55 6.23 ( 0.449 6.30 ( 0.58

2.7 2.6 2.8 2.6

a [NPG] ) 5 × 10-4 M; 30 °C; the aqueous reaction mixture for 0 each kinetic run contained 2.5% (v/v) CH3CN; Ψ varies from -131 to -85 mV with an increase in [Na+] from 0.02 to 0.20 M. b The predicted value of R is 37.7 × 10-3 (mV)-1 at 35 °C. c The values of kW are summarized in Table 4. d Error limits are standard deviations.

ionized NPG and -OH.20 The rate of reaction between ionized NPG and -OH in aqueous organic solvents was found to increase with an increase in the aprotic organic cosolvent at its content >50%, (v/v).20 But this rate increase is not due to an increase in the apparent concentration of hyroxide ions due to an increase in the content of aprotic organic cosolvent in mixed aqueous solvents. It may be noted that kobs at 70% (v/v) CHC3CN and 1,4-dioxan < kobs at 0% (v/v) CH3CN and 1,4-dioxan at 0.004 M NaOH.20 Thus, it seems that k′ should be e kW. Pseudo-first-order rate constants (kobs) for hydrolysis of phthalimide (PTH) at 0.01 M NaOH decreased slightly (∼8%) while kobs for hydrolysis of ionized N-phthaloylglycine (NPG) at different [NaOH] (range 0.0030-0.0045 M) increased nearly 50% with an increase in [SDS]T from 0.0 to 0.2 M. The characteristic difference in the rate of alkaline hydrolysis of PTH and NPG in the presence of SDS may be explained in terms of structural aspects of (20) Khan, M. N. J. Org. Chem. 1996, 61, 8063.

Hydrolysis of N-Phthaloylglycine

the reactants PTH and NPG. As described earlier, nonionized phthalimide (SH) and -OH are involved as

Langmuir, Vol. 13, No. 9, 1997 2503

because the ion-pair formation between nonionized PTH (SH) and Na+ may be highly unlikely. Thus, the presence and absence of ion-pair formation in the alkaline hydrolysis of NPG and PTH, respectively, explain the observed effects of [SDS]T on kobs for NPG and PTH. Conclusion

reactants in the hydrolysis of PTH at 0.01 M NaOH. But ionized N-phthaloylglycine (NPG or S-) and -OH act as the reactants in the alkaline hydrolysis of NPG. The rate of alkaline hydrolysis of NPG increased with an increase in [Na+] at a constant [NaOH] due to ion-pair formation between NPG (or S-) and Na+ as shown in Scheme 2. But the rate of hydrolysis of PTH at 0.01 M NaOH was found to be independent of [Na+] within the range 0.0-2.5 M8

The increase in pseudo-first-order rate constants (kobs) for alkaline hydrolysis of ionized NPG with an increase in [SDS]T at a constant [NaOH] has led to the conclusion that the micellar-mediated reaction occurs in the GouyChapman layer or at the junctural region of the GouyChapman and Stern layers, where [H2OM] ≈ [H2OW] and [-OHM] ≈ [-OHW]. The micellar-mediated reaction involves the collapse of the ion-pair complex formed between ionized NPG and Na+ in the rate-determining step. Acknowledgment. This work was supported by a research grant (F408/96) from Universiti Malaya. LA9606916