Effects of Polymer and Salt Concentrations on Ketal Acid Hydrolysis in

Jan 15, 1997 - The acid-catalyzed hydrolysis of 2-(methoxyphenyl)-1,3-dioxolane (p-MPD) and di-n-butyl benzaldehyde acetal (BBA) has been studied in ...
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Langmuir 1997, 13, 659-665

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Effects of Polymer and Salt Concentrations on Ketal Acid Hydrolysis in Solutions of Sodium Dodecyl Sulfate and Poly(vinyl pyrrolidone) or Poly(ethylene oxide) Dino Zanette,* Sandro Jose´ Froehner, Edson Minatti, and A ˆ ngelo Adolfo Ruzza Departamento de Quı´mica, Universidade Federal de Santa Catarina, SC, 88040-900, Brazil Received June 6, 1996. In Final Form: October 7, 1996X The acid-catalyzed hydrolysis of 2-(methoxyphenyl)-1,3-dioxolane (p-MPD) and di-n-butyl benzaldehyde acetal (BBA) has been studied in solutions containing poly(ethylene oxide) (PEO) or poly(vinyl pyrrolidone) (PVP) and sodium dodecyl sulfate (SDS). First-order rate constant-[SDS] profiles were obtained at 0.010 and 0.105 M PEO or PVP, and both polymers strongly inhibit the reaction, to an extent depending on the polymer and SDS concentrations. Added NaCl also decreases the rate, and the behavior is similar to that with SDS micelles. The inhibition induced by increasing polymer concentration was interpreted by assuming decreases in the interfacial H+ concentration. This conclusion is supported by values of pH apparent obtained with the pH indicator pyridine-2-azo-p-dimethylaniline (PADA) at the same experimental kinetic conditions. Qualitatively, the results are interpreted in terms of the pseudophase ion exchange (PPIE) model applied to bimolecular reactions.

1. Introduction The interaction between ionic surfactant and watersoluble polymer that produces aggregates with charged interfaces with properties similar to ionic micelles has been examined.1,2 Recently we investigated the ketal acidcatalyzed hydrolysis in polymer-surfactant solutions and found that the pseudophase ion-exchange (PPIE) formalism, originally applied to ionic micelles,3-9 can be adapted to this system to fit the rate-surfactant concentration profiles.10 We also estimated the degree of ionization (R) from the ratio of slopes of plots of conductivity against sodium dodecyl sulfate concentrations [SDS] by applying the Evans equation for poly(ethylene oxide)-sodium dodecyl sulfate (PEO-SDS)10 and poly(vinyl pyrrolidone)sodium dodecyl sulfate (PVP-SDS)11 complexes. We found R values of 0.41 ( 0.01 and 0.38 ( 0.01, respectively, slightly higher than those found for SDS micelles by using different techniques.12 A comparison of kinetic results led us to conclude that SDS micelles are better catalysts than PEO-SDS or PVP-SDS complexes. Nevertheless, intriguing questions concerning the mechanism of interaction and regarding the nature of the driving forces inducing the binding of the surfactant on the polymer surfaces still exist. Several references describe the different forces inducing the surfactant binding, and the hydrophobic character of the polymer X Abstract published in Advance ACS Abstracts, January 15, 1997.

(1) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press, Inc.: Boca Raton, FL, 1993; p 123. (2) Witte, F. M.; Engberts, J. B. F. N. J. Org. Chem. 1987, 52, 4767. (3) Berezin, I. V.; Martinek, K.; Yatsimirski, A. K. Russ. Chem. Rev. (Engl. Transl.) 1973, 42, 487. (4) Romsted, L. S. Ph.D. Thesis, Indiana University, 1975. (5) Quina, F. H.; Chaimovich, H. J. Phys. Chem. 1979, 83, 1844. (6) Romsted, S. R.; Zanette, D. J. Phys. Chem 1988, 92, 4690. (7) Re, Z. M.; O’Connor, P. J.; Romsted L. S.; Zanette, D. J. Phys. Chem. 1989, 93, 4219. (8) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (9) Ruzza, A. A.; Walter, M. R. K.; Nome, F.; Zanette, D. J. Phys. Chem. 1992, 96, 1463. (10) Ruzza, A. A.; Froenher, S. J.; Minatti, E.; Nome, F.; Zanette, D. J. Phys. Chem. 1994, 98, 12361. (11) Zanette, D.; Ruzza, A. A.; Froenher, S. J.; Minatti, E. Colloids Surf. 1996, 108, 91. (12) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant System; National Bureau of Standards: Washington, DC, 1971.

S0743-7463(96)00557-4 CCC: $14.00

increases the surfactant binding.2,13 The hydrophobic character of the surfactant also plays a role in the binding. Recently, Kwak et al.14 reported on the binding of alkylbenzenesulfonates to various hydrophobically modified poly(acrylamide)s with different hydrophobicities. They concluded that surfactant binding occurs only on the hydrophobic side chains of the polymer. They emphasize that there is no evidence for surfactant binding to the unmodified poly(acrylamide). Also the binding was influenced by the structure of the surfactant and the degree of hydrophobic substitution. In some cases the hydrophobic character seems to be important, but this does not explain why cationic surfactants bind only weakly to PEO.2,15 A more recent model to explain the nature of the driving force for binding was suggested by Dubin et al.16,17 for PEO-SDS and PEO-lithium dodecyl sulfate complexes. They assume that counterions play a role in this interaction, by simultaneously coordinating with the polymer oxygens and by being electrostatically bound to the SDS aggregates. It has already been proposed by Schwuger15 that the interaction between SDS and PEO should be partly electrical in nature. That is, the oxygen atom of the ether linkage could become partially cationic and, as a consequence, bind a sulfate ion. The author found that the effectiveness of binding depended on pH. On the other hand, it has been established that cations can associate with PEO with different affinities. Lissi et al.18 have shown that lithium chloride does not interact measurably with PEO, but significant binding was verified for sodium, potassium, rubidium, and cesium chlorides. The results also are consistent with an association creating a net positive potential that attracts anions electrostatically. Studies of effects of sodium and lithium counterion on the interaction of dodecyl sulfate with PEO lead one to (13) Petit, F.; Auderbert, R.; Iliopoulos, I. Colloid Polym. Sci. 1995, 273, 777. (14) Effing, J. J.; Mclennan, I. J.; van Os, N. M.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 12397. (15) Schwuger, M. J. J. Colloid Interface Sci. 1973, 43, 491. (16) Dubin, P. L.; Gruber, J. H.; Xia, J.; Zhang, H. J. Colloid Interface Sci. 1992, 148, 35. (17) Xia, J.; Dubin, P. L.; Kim, Y. J. Phys. Chem. 1992, 96, 6805. (18) Sartori, R.; Sepulveda, L.; Quina, F.; Lissi, E.; Abuin, E. Macromolecules 1990, 23, 3878.

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Scheme 1

conclude that it is stronger in the presence of sodium than lithium ion.19 Recently, Bowers et al.20 proposed a conformation in the gas phase for poly(ethylene oxide) polymers converted to cations by sodium ions. Modeling of PEO with nine ethylene oxide monomers with molecular mechanics methods indicates that the lowest energy structure has Na+ solvated by the polymer chain with seven oxygen atoms. The authors also emphasized the importance of metal ions or H+ as cations in determining gas phase conformation of biomacromolecules such as proteins and peptides. A primary focus of this work is the determination of effects of ionic aggregates in solutions of PEO and PVP containing SDS on the rates of acid-catalyzed hydrolysis of 2-(p-methoxyphenyl)-1,3-dioxolane (p-MPD) and di-nbutyl benzaldehyde acetal (BBA) (Scheme 1). Also, the extent of the effect of added salt on the rate was investigated, and the results obtained suggest that these polymer-surfactant complexes behave similarly to ionic micelles. However, an extensive inhibition promoted by increasing polymer concentration was observed, which constitutes the central theme of this discussion. Finally, the dependence of interfacial proton concentration on polymer concentration was examined by using a pH indicator, and the apparent pH values obtained are consistent with the rate inhibition. The model proposed suggests a dilution of interfacial H+ because of its adsorption on the polymer surface. Qualitatively, the results are interpreted by applying the concepts of the pseudophase ion exchange (PPIE) model of bimolecular reactions.5,8,10 2. Experimental Section 2.1. Materials. PEO and PVP, with weight-average molecular weights 10 000 and 40 000, respectively, were obtained from Aldrich and were used as received. For all polymer solutions, concentrations are given as molarity on a monomer basis (moles of monomer per liter of solution). The stock solutions of polymers were routinely prepared with magnetic stirring for at least 12 h. SDS (Sigma, 99%) was used without purification. The critical aggregation concentration determined by a surface tension vs [SDS] plot (square brackets, here and thoughout the text, indicate molarity) agrees with literature values,12 and the profile shows no minimum. Distilled water was demineralized in a Millipore Milli-Q Water system. Pyridine-2-azo-p-dimethylaniline, PADA (Sigma), was used without further purification. The substrates 2-(p-methoxyphenyl)-1,3-dioxolane (p-MPD) and di-n-butylbenzaldehyde acetal (BBA) are as in our previous investigations.10,21 2.2. Methods. Hydrolyses were performed by adding 5 µL of ketal stock solution (0.002 M in dry acetonitrile) to 2.5 mL of 0.02 M succinic buffer solution, pH 5.80, 25.0 °C. Formation of the products, p-methoxybenzaldehyde and benzaldehyde, was followed spectrophotometrically at 284 and 252 nm, respectively, by using a Hewlett-Packard 8453A diode array spectrophotometer. The rate constants were estimated by using HP 89532K (19) Abuin, E.; Lissi, E.; Quina, F.; Paredes, S. Bol. Soc. Chil. Chim. 1995, 40, 65. (20) von Helden, G.; Wyttenbach, T.; Bowers M. T. Science 1995, 267, 1483.

Figure 1. Effect of [SDS] on the spectral absorbance of PADA in 0.020 M succinate buffer, pH 5.80. From the bottom to the top the absorbance spectra refer to [SDS]: 6.76; 8.60; 10.43; 12.60; 13.95; 15.67; 17.35; 19.00; 20.63; 22.22; 23.00; 26.00; 27.96; and 33.33 mM. kinetic software. Standard deviations for log(A∞ - A) vs time were less than 10-5. pH measurements were made on a Beckman model Φ71. Conductivity titration was carried out in 0.02 M succinate buffer, pH 5.80, 25.0 °C, in a water-jacketed flow dilution cell, with an Analion conductivity meter Model C-701. The data were stored in a microcomputer, and the software permitted us to contain up to 2000 points in the conductivity vs [SDS] plot. The first and second conductivity vs [SDS] breakpoints are determined by using a standard linear regression routine. The apparent interfacial pH (pHapp) of the solutions containing SDS and mixtures of SDS and PVP, at 0.02 M succinate buffer, pH 5.80, was estimated by using the pH indicator PADA according to eq 1. Amax and Amin represent the absorbances of the fully protonated and deprotonated species, respectively,

[AH+]/[A] ) (A - Amin)/(Amax - A)

(1)

pKa ) pHapp + log([AH+]/[A])

(2)

and A is the absorbance at different concentrations of PVP or SDS. Therefore, two set of pHapp measurements were made: (i) at PVP constant, 0.010 and 0.105 M, varying the SDS concentration, and (ii) at SDS constant, 0.005, 0.035, and 0.100 M, varying the PVP concentration. Absorbance values were measured spectrophotometrically at 552 nm, λmax of the protonated species, by using a HP UV 8452-A diode array spectrophotometer. The pHapp values were calculated from eq 2 with a pKa of PADA ) 4.50.6 A typical set of absorption spectra in the presence of 0.010 M PVP is shown in Figure 1. The effect of added PVP (i) on the absorbance of PADA in solution containing a constant weighed amount of SDS dissolved in succinate buffer was determined from solutions prepared in a 10 mL volumetric flask; then, a known volume of 0.1 M PVP stock solution, also dissolved in succinate buffer, and 25 µL of 0.002 M PADA in CH3CN (spectrophotometric grate) were added, and the final volume was completed with succinate buffer. The final concentrations of PADA were 5 × 10-6 M. This procedure was repeated for the effect of added SDS (ii) on the absorbance of PADA. The spectra of protonated (Amax) and neutral (Amin) forms were measured in 0.020 M succinate buffer, pH ) 3.50, and in 0.020 M borate buffer, pH 9.80, respectively, both containing 0.020 M SDS. These solutions were prepared by using the same above procedure.

3. Results and Discussion Effect of [Polymer] on the kobs-[SDS] Profiles. Figure 2 shows biphasic profiles for the effect of [SDS] on the observed first-order rate constant (kobs) for acid hydrolysis of BBA in the presence of 0.01 and 0.105 M PEO, at 0.02 M succinic buffer, pH 5.80. At lower [PEO] the profile exhibits a maximum rate at ca. 0.025 M SDS, which is typical of micellar-catalyzed bimolecular reactions,4,5,8-11,21and it is similar to that in the absence

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Figure 3. Specific conductance vs [SDS] in 0.020 M succinate buffer, pH 5.80, in the presence of 0.105 M PEO, at 25.0 °C. The inset magnifies the first conductivity breakpoint (cac). Figure 2. Effect of [SDS] on kobs for acid-catalyzed hydrolysis of BBA in 0.020 M succinate buffer, pH 5.80, in the presence of (O) 0.010 M and (0) 0.10 M PEO, at 25.0 °C.

of PEO at pH 6.00.21 In the earlier work the kinetic data were treated by applying the pseudophase ion-exchange (PPIE) model of bimolecular reactions, and the best kinetic fit was obtained with a binding constant of 1500 M-1. This value and maximum rate of kobs, at ca. 0.012 M SDS, are consistent with observations on other moderately hydrophobic substrates.4,5,8,9,11,21 A comparison, in the absence21 and in the presence of 0.010 M PEO (Figure 2), makes clear the similarity of both profiles. Nevertheless, a relevant observation is the remarkable inhibition of the reaction observed in 0.105 M PEO, as compared to the profile for a lower concentration, and there is a complete change in the shape of the kinetic profile. This behavior has been seen for the acid hydrolysis of the hydrophobic ketal p-(nonoxyphenyl)-1,3-dioxolane and has been used to demonstrate polymer-surfactant interactions under kinetic conditions.11 Figure 2 also illustrates that the discontinuities observed in the kobs-[SDS] profile in 0.105 M PEO are consistent with those in the conductivity-[SDS] plot. Indeed, in the experimental conditions of Figure 2, the profile of Figure 3 exhibits two breakpoints, which correspond to the critical aggregation concentration (cac) and to the polymer saturation point (psp) with values of 2.10 and 39.0 mM, respectively. These values agree with those reported22 taking into account that, at this buffer concentration and pH, the total sodium concentration is equivalent to 30 mM. The cac has been interpreted as the onset of cooperative adsorption of surfactant on the polymer.1,2,15-17,23 The second breakpoint marks the saturation of polymer, and above this [SDS] regular SDS micelles are forming,1,2,15,23 although the psp cannot always be attributed exclusively to saturation of the polymer. Recently, salt effects on the psp and cac for the PEOSDS system suggest that a real psp is observed only in [PEO] higher than 0.05 M. At low concentrations ([PEO] ) 5 × 10-4 M) we found that the second breakpoint corresponded to the critical micellar concentration of SDS.22 Nevertheless, the two breakpoints found by conductivity are in agreement with the discontinuities (21) Froehner, S. J.; Nome, F.; Zanette, D.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1996, 673. (22) Minatti, E.; Zanette, D. Colloids Surf. 1996, 113, 237.

Figure 4. Effect of [SDS] kobs for acid-catalyzed hydrolysis of BBA in 0.020 M succinate buffer, pH 5.80, in the presence of (O) 0.010 M and (0) 0.105 M PVP, at 25.0 °C. The inset magnifies the kobs-[SDS] profile, at low surfactant concentration, for 0.105 M PVP.

observed in the kinetic profile at 0.105 M PEO (Figure 2), showing that changes in the composition of the PEOSDS system also affect the rate of BBA hydrolysis. Two water-soluble polymers have been extensively studied in the last three decades: PEO and PVP. Both systems give similar results. Indeed, in Figure 4 are shown the kobs-[SDS] profiles for the PVP-SDS system at the same conditions, [PVP] and buffer, as shown in Figure 2. The inset of Figure 4 magnifies changes in kobs in the first kinetic breakpoint region, which agree with the value of the breakpoint obtained by conductivity (Figure 3). Comments about the similarity of the profiles are trivial, but, from a kinetic point of view, since several factors exist leading to analogous results, it appears that the properties of the two systems are similar. We have already made this same conclusion. For instance, by using conductivity, we have estimated degrees of ionization of 0.4110 and 0.3811 for PEO-SDS and PVP-SDS complexes, respectively, which are similar but higher than for SDS

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to that in its absence. However, at 0.105 M PEO, kobs decreases only slightly and the values are close to the value in bulk water. These observations are explained in terms of the binding constant of p-MPD. The pseudophase model describes the substrate distribution in each pseudophase by a binding constant (Ks) defined in eq 4: Ks

Sw + Cd y\z Sm

(4)

where Cd represents the stoichiometric concentration of surfactant in the micellar pseudophase related to the total amount concentration of surfactant (Ct) by

Cd ) Ct - cmc

Figure 5. Effect of added NaCl on kobs for the acid-catalyzed hydrolysis of p-MPD in 0.020 M succinate buffer, pH 5.80, in the absence (O) and in the presence of 0.010 M (0), 0.035 M (4), and 0.105 M (b) PEO, at 25.0 °C. The inset shows the kobs[PEO] profiles given from this figure in 0.010 M NaCl (O) and from ref 25 (0) (comments, see the text; dashed lines are extrapolated).

micelles.4,12 The values of aggregation numbers of 35 ( 5 and 28 ( 5 for PEO-SDS and PVP-SDS complexes, respectively, determined by a fluorescent method, also supporte this argument.24 Salt Effect on the Hydrolysis of p-MPD. The rates of bimolecular reactions between neutral organic substrates with a reactive counterion such as H+ in solutions of anionic surfactants are drastically affected by adding salt.5,8 In the PPIE model, the surface is treated as a selective ion exchanger, where Na+ counterions, for example, compete with the reactive H+. In this way, the H+ and Na+ distributions are governed by ion-exchange of the competitive species between water (w) and at the micellar surface (m), eq 3: KH/Na

Hw+ + Nam+ y\z Naw+ + Hm+

(3)

where the constant KH/Na, the ion-exchange constant, depends on the binding selectivity of each species to the surface. We found the same behavior for salt effects on rates of ketal acid hydrolysis in solutions containing PEO-SDS complexes. Figure 5 shows the effect of [NaCl] on kobs for acid hydrolysis of p-MPD at fixed 0.010 M SDS and at various [PEO]. For comparison, we also show the kobs[NaCl] profile in the absence of PEO. As [NaCl] increases, kobs values decrease exponentially. As for ionic micelles, these salt-induced rate decreases are completely consistent with typical salt effects predicted by the PPIE model. Added Na+ displaces H+ from the micelle surface and decreases the interfacial concentration of the ionic reactant.4-8 We note also, at low PEO concentration ([PEO] ) 0.010 M), that the effect of added salt exhibits a profile similar (23) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36.

(5)

A Ks value of 52 M-1 has been estimated,10 characteristic of a moderately hydrophilic substrate; therefore, hydrolysis of p-MPD occurs only partially in the micellar pseudophase because, and from eqs 4 and 5, p-MPD is about 59% in water, considering the cac ) 0.0021 M (Figure 3) and 0.01 M SDS (experimental [SDS] of Figure 5). A comparison with salt effects on the acid hydrolysis of a very hydrophobic substrate, 2-(p-nonoxyphenyl)-1,3-dioxolane, at the same [SDS], illustrates how isotherm shapes are related to respective Ks values of the substrates.25 The original reason for choosing this SDS concentration was the fact that, in these conditions, only PEO-SDS complexes may exist in solution and to minimize the effect of regular SDS micelles, which can be present, especially at lower polymer concentrations. This matter has been extensively debated,22 and there is evidence of SDS micelles and PEO-SDS complexes coexisting at [PEO] below 0.05 M, but this behavior depends on the experimental conditions. Salts decrease the cac and cmc and they can increase psp, depending on [PEO].22 In this context, the kobs-[NaCl] profile at 0.01 M PEO, whose shape is similar to that obtained in the absence of PEO (Figure 5), is controlled by SDS micelles, while in 0.105 M PEO it may not be. We emphasize again the inhibition promoted by increasing polymer concentration, shown in Figures 2 and 4, and also shown in Figure 5 at lower [NaCl]. For example, at 0.01 M NaCl, the ratios of the rate constant in the absence and in the presence of 0.01 and 0.035 M PEO are ca. 2.4 and 4.3, respectively. The polymer concentration has an important effect on the rate constant and on the shape of the isotherms; therefore, interpretation of the differences must be the central theme of discussion. Effects of Polymer Concentration on kobs of BBA Acid Hydrolysis. Figure 6A shows in detail the effect of [polymer] on the ketal acid hydrolysis of BBA, at different fixed concentrations of SDS. Our purpose is first to systematically quantify the effect of [PVP] on the rate and, second, to show the significant role that surfactant concentration is playing in this phenomenon. Indeed, at 0.005 M SDS, just above cac, adding PVP decreases kobs sharply and it becomes constant above 0.050 M PVP. At this [SDS], kobs values are similar to those shown in Figure 4 at 0.105 M PVP. At high SDS concentration, 0.100 M, the kobs-[PVP] profile exhibits a different behavior, and the rate changes only slightly in all ranges of [PVP] examined. Finally, at 0.035 M SDS it is intermediate between the above profiles, and SDS micelles play a role in “buffering” the system, therefore minimizing the effect of [polymer] on the ketal acid hydrolyses. Effect of [PVP] and [SDS] on the Apparent pH (pHapp) of PADA. To investigate the local interfacial (24) Lissi, E. A.; Abuin, E. J. Colloid Interface Sci. 1985, 105, 1.

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Figure 7. Effect of added SDS on pHapp of PADA in 0.020 M succinate buffer, pH 5.80, containing (O) 0.010 M and (0) 0.105 M PVP, at 25.0 °C.

Figure 6. (A) Effect of added PVP on kobs for the acid-catalyzed hydrolysis of BBA in 0.020 M succinate buffer, pH 5.80, in the presence of (0) 0.005 M, (4) 0.035 M, and (O) 0.100 M SDS, at 25.0 °C. (B) Effect of added PVP on pHapp of PADA in 0.020 succinate buffer, pH 5.80, at the following [SDS], at 25.0 °C: (0) 0.005 M, (4) 0.035 M, and (O) 0.100 M SDS. Lines are drawn to aid the eye.

pH of the aggregates in solutions containing SDS and PVP, we use the indicator PADA. This pH indicator was first introduced as an interfacial probe by James and Robinson.26 It is a convenient because the pKa in water is about 4.50,6,26 and changes in absorption spectra are observed within the pH range where the p-MPD and BBA suffer acid hydrolysis. It was also employed to probe the interfacial properties of micelles of sodium decyl phosphate monoanion at several pH and surfactant concentrations over a large [NaCl] range.6,7 In these works the experimental controls showed that both protonated and neutral forms should be fully incorporated above the cmc of the surfactant. Here, in order to use eq 1, we also assume that both forms of PADA are completely bound; therefore, they are sensing the variations of local interfacial pH. As evidence, a set of absorption spectra at different [SDS], and in a low PVP concentration, is shown in Figure 1. The sharp isosbestic point observed is good evidence that changes in the absorption spectra are caused only by changes in the microenvironment where the probe resides. Absence of a sharp isosbestic point would indicate changes in binding of PADA because the absorption spectra of both protonated and neutral forms in water should differ. Absorbance spectra are sensitive to polarity; that is λmax of the neutral form bound to SDS micelles differs from that in water by 15 nm and of the protonated form by 2 nm.6 (25) Zanette, D.; Minatti, E.; Ruzza, A. A.; Yunes, S. F.; Froehner, S. J. Atual. Fis. Chem. Org., in press.

In Figure 6B are shown pHapp, estimated by using eqs 1 and 2, as a function of PVP concentration at 0.005, 0.030, and 0.100 M SDS, at the same experimental conditions shown in Figure 6A. It is interesting to note that at 0.005 M SDS the profile exhibits a rapid increase in pHapp, going to a plateau whose shape is qualitatively similar to that in 0.005 M SDS (Figure 6A). Thus, the rapid decrease in the rate constant shown in Figure 6A corresponds to a rapid increase in interfacial pH (Figure 6B). Figure 7 shows the pHapp-[SDS] profiles obtained at 0.010 and 0.105 M PVP, at the same experimental conditions shown in Figure 4. In both profiles the probe senses a greater increase of interfacial H+ at low surfactant concentrations, and it is more pronounced at lower polymer concentrations. Nevertheless, the more relevant finding is that at 0.105 M PVP the probe senses a less acidic environment than at 0.010 M PVP. This behavior is in agreement with the kobs-[SDS] profiles shown in Figure 2 for PEO and in Figure 4 for PVP. This fact supports that the strong reduction of the reaction rate is due to a decrease in the “local” H+ concentration. Ketal Hydrolyses Inhibition According to the PPIE Model. A reasonable explanation for the effects of both [NaCl]25 and [polymer]10,11,25 on the rate constant of ketal hydrolyses can be attributed to changes in the interfacial pH of the micellar aggregates where the reactions are occurring. For a bimolecular acid-catalyzed reaction between a neutral organic substrate (S) and H+ in the presence of polymer, one can apply the concepts of the PPIE model by assuming that the reactants are in dynamic equilibrium between the aqueous, micellar, and polymer-SDS complex pseudophases. The observed rate is the sum of the rates in each pseudophase, and the overall rate has been defined by10

rate ) k2w[S]w[H+]w + k2m[S]m[H+]m + k2p[S]p[H+]p (6) where the subscripts, w, m, and p denote the aqueous, micellar, and polymer-surfactant complex pseudophases. k2w, k2m, and k2p are, respectively, the second-order rate constants for the reaction occurring in each pseudophase. The terms in eq 6 are taken to be “local” proton and substrate concentrations in each pseudophase. The decreases in the rate promoted by increasing [polymer] or

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[salt] can be attributed to changes in “local” concentrations of substrate and H+. Firstly, we discard the possibility of changes in Ks to be the principle factor that justifies the reductions of the rates shown in Figure 2 and in Figures 4-6. We also have observed this kind of inhibition for hydrophobic substrates. Qualitatively, effects are similar and result in similar profiles. Indeed, comparisons of the effects of increasing [PEO] on kobs of p-MPD and of a very hydrophobic substrate, 2-(p-nonoxyphenyl)-1,3-dioxolane (pNPD),25 both at 0.01 M NaCl, support this argument (see inset of Figure 5). We note that the inhibition effect exerted by [PEO] is greater at low polymer concentrations. Several other explanations of the [polymer] effects on the rate can be considered. There is an increase of the reaction volume within the polymer-SDS complexes with increasing polymer concentration, whose defined unit is molar volume (liters/mole) of reactive region. Usually for SDS micelles in the PPIE model the assumed value is 0.25 M-1, which is approximately equal to the total molar volume of SDS micelles.6,8 There are questions regarding the effective reaction volume as a function of polymer concentration, and there are uncertainties about the mechanisms of polymer-surfactant interactions. For instance, Kwak et al.27 reported on the solubilization capacity of SDS aggregates for PEO and found a stoichiometric composition of approximately 1.9 PEO monomers per dodecyl sulfate ion. They also concluded that PEO units are predominantly solubilized within the aggregates. On the other hand, Cabane and Duplessix28 have concluded from neutron scattering that the polymer in water is weakly absorbed at the (hydrocarbon/water) interface. We note the discrepancy between these conclusions. However, there is some evidence to support the assumed constancy of the molar volume within a certain range of polymer concentration: (i) The onset of binding (cac) is independent of [PEO] or [PVP],1,11,15,23 but if such important properties of the complex as the degree of ionization (R) and the aggregation number should be affected, then the cac should also be changed since it is related to the thermodynamic stability of the aggregate.1,16,19 An increase in the molar volume should be related to changes in the above properties and, therefore, by variation in the cac. (ii) The degree of ionization of the PEO-SDS complexes is constant within a certain [PEO] range.10,11,22 We have estimated R from the ratios of the slopes of specific conductance by assuming that, in the cac-psp concentration ranges, only a single type of aggregate was developing, on the basis of the linearity of these plots, which is, therefore, more evidence for constancy in the pseudophase reaction volume. The analogy between our system and alcohol-modified SDS micelles supports the above considerations. Adding 1-butanol to SDS affects rates of acid hydrolyses of p-methoxybenzaldehyde O-acyloximes, and the effects are explained by an increase in the volume of the reaction region in the micellar pseudophase and a decrease in reactant concentration in that region. These conclusions were supported by observed changes in the degree of ionization of the SDS micelles as a function of 1-butanol concentration.29,30 (26) James, A. D.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1979, 74, 10. (27) Gao, Z.; Wasylishen, R. E.; Kwak, J. C. T. J. Phys. Chem. 1991, 95, 462. (28) (a) Cabane, B.; Duplessix, R. J. Phys. Chem. 1982, 43, 1529. (b) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (29) Rubio, D. A. R.; Zanette, D.; Nome, F.; Bunton, C. A. Langmuir 1994, 10, 1155.

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Figure 8. Scheme of a partial cross section of the region near the interface of the micelle-like aggregates, PEO-SDS or PVPSDS complexes, showing the micellar and aqueous pseudophases separated by the so-called Stern layer (firstly designated to micelles). The Stern layer contains sulfate head groups, sodium and proton counterions, micellar-bound substrate, and segments of polymer chain. The aqueous phase contains SDS monomer, proton, sodium, the various forms of succinate buffer, the free substrate, and segments of polymer chain. The bold type representation of H+ emphasizes the protons adsorbed to the polymer chain and probably contribute to the “dilution” of the [H+] interface concentration (see the text).

In this context, the inhibition induced by increasing polymer concentration, we explain assuming displacement of H+ to the polymer surface region, or to water. On the other hand, the decrease of kobs (Figure 6A) might arise because added polymer displaces H+ from the “local” micellar surface where the substrate is located and, consequently, reduces the interfacial proton concentration. Similar behavior of hydrogen ions absorbed to polymer has been documented for several other ionic species. Lissi et al.18 found that alkali-metal cations bind to PEO and follow the sequence lithium , sodium < rubidium ≈ potassium ≈ cesium. Conversely, Bowers et al.20 noted the interaction of PEO with Na+ and emphasized the importance of determining conformations of biomacromolecules by using matrix-assisted laser desorption ionization (MALDI) in conjunction with ion chromatography (IC). Recently, Dubin et al.16,17 suggested that counterions play an important role in surfactant-polymer interactions by coordinating the complex double layer with the nonionic polymer to form a “pseudopolycation”. Finally, Schwuger15 has observed that the binding depends on pH. Despite the many techniques applied to characterize the nature of the driving forces controlling the binding of solutes, controversies still exist and the detailed mechanisms are not well-understood. Usually, interactions conveniently are attributed to hydrophobic forces, however, generally the effect is due to a mixture of intricate forces, hydrophilic, hydrophobic, and ionic interactions, involving segments of the polymer, the tail and head group of the surfactant, and the type of counterion. For instance, the conductivity vs sodium decyl phosphate (NaDeP) concentration plots in the presence of PEO do not exhibit the classical profile,31 although mixtures of NaDeP and (30) Rubio, D. A. R.; Zanette, D.; Nome, F.; Bunton, C. A. Langmuir 1994, 10, 1151.

Ketal Acid Hydrolysis

SDS follow an ideal behavior21 and the kinetic effects on the ketal acid hydrolyses are similar to those of SDS micelles.9,21 These results indicate that the surfactant head group plays a decisive role in the interaction because the counterion and tail are the same. Finally, Figure 8 depicts a scheme emphasizing the interface region of PVP-SDS or PEO-SDS complexes in which the ketal acid-catalyzed hydrolysis occurs. We note that the distribution of the polymer segment chains in the surrounding interface is in agreement with that of the suggested model by Cabane.28b For the PEO-SDS system, he stated that the polymer is wrapped around the mixed micelle and some of the monomers of the polymer are directly adsorbed at the hydrocarbon/water interface. For each surfactant monomer, there are at least 3.3 and at the most 4.1 monomers associated with the interface, either directly adsorbed or indirectly adsorbed (loops), and the monomers of the polymer interact with hydrated methylene groups of the surfactant. Therefore, assuming that the proton is “diluted” in the reaction region of the aggregates because it is trapped on the polymer surface, the scheme in Figure 8 is an adapted model to explain the inhibition of the ketal acid-catalyzed hydrolysis observed in this work. We note that the parts

Langmuir, Vol. 13, No. 4, 1997 665

of segment chains, located in the hydrated methylene groups of the surfactant, are the clearest evidence to explain the “dilution” phenomenon. Morever, PEO-SDS and PVP-SDS complexes as aggregates are similar and behave like SDS micelles in acid hydrolyses. Salt effects and the catalyses observed here in the presence of the polymers are similar to many results with SDS micelles,21,32 apart from the inhibition caused by the polymers, which should be important in acid-catalyzed hydrolysis but in all probability not in reactions between neutral reactants. Acknowledgment. We are grateful to CNPq (Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico, Brazil), FINEP, and PADCT for the financial support of this work. We thank Professor Clifford A. Bunton, University of California, for the collaboration and revision of the manuscript. LA960557L (31) Lima, C. F.; Nome, F.; Zanette, D. J. Colloid Interface Sci., in press. (32) Ruzza, A. A.; Nome, F.; Zanette, D.; Romsted, L. Langmuir 1995, 11, 2393.