Competition between Aggregation and Hydrolysis in the Reaction of

Jun 22, 2006 - ... Nacional de Córdoba, Ciudad Universitaria, 5000, Córdoba, Argentina. Langmuir , 2006, 22 (15), pp 6523–6530. DOI: 10.1021/la060...
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Competition between Aggregation and Hydrolysis in the Reaction of Arylperfluorooctanoates in Micellar Solutions Mariana A. Ferna´ndez and Rita H. de Rossi* Instituto de InVestigaciones en Fı´sico-Quı´mica de Co´ rdoba (INFIQC), Facultad de Ciencias Quı´micas, Departamento de Quı´mica Orga´ nica, UniVersidad Nacional de Co´ rdoba, Ciudad UniVersitaria, 5000, Co´ rdoba, Argentina The rate of hydrolysis of phenyl and p-nitrophenyl perfluorooctanoate (2a and 2b) was measured in water and in the presence of different cationic (dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, cetyltrimethylammonium bromide), anionic (sodium dodecyl sulfate (SDS) and perfluorooctanoate (PFO)) and neutral (Brij-35) surfactants. In water solution, the formation of phenol from 2a and p-nitro phenol from 2b takes place through two kinetic processes, both of which are much slower than the expected rate of hydrolysis for the monomeric compounds in water. The two kinetic processes are attributed to a coupling of the rates of hydrolysis and aggregation of the substrates. In the presence of charged surfactants at concentrations below the respective critical micellar concentration (cmc), two relaxation times are also observed. These are of the same order of magnitude as the substrates alone in the case of SDS, but faster for the cationic surfactants. At some concentration above the cmc, all the surfactants, except for PFO, showed a clean pseudo-first-order behavior attributed to the hydrolysis of the substrate incorporated into the micellar phase. In cationic micelles, the rates for 2a are slower and those for 2b are faster than the value expected for the monomer in water. The difference in behavior is attributed to the location of the substrates in the micellar phase and to the charge distribution in the transition state of the reactions. It is shown that the reactions in the micellar phase are catalyzed by the buffer PO4H2-/PO4H2-. The reactions in SDS micelles are faster than those in water but slower than the estimated value for the monomer in water. The rate of the reactions in the presence of nonionic surfactant has values between those in cationic and anionic surfactants, that is, the rates are kcationic > knonionic > kanionic. The behavior of 2a and 2b in water and in micellar solutions indicates that the substrates form aggregates in water at a rate that competes with the rate of hydrolysis.

Introduction The replacement of a hydrogen atom with fluorine in an organic molecule significantly alters its thermal, chemical, and biological characteristics. Perfluorinated amphiphiles have exceptional properties, such as low solubility in water and in polar and nonpolar organic solvents, high density, fluidity, compressibility, and high dielectric constants. These unique properties of perfluorinated materials are useful for different applications. Thus, they are used in blood substitute formulations,1 oxygen2 and drug delivery systems,3 adhesive formulations, cleaners, herbicides, and cosmetics,4-6 as well as in fire-fighting foams.7 Perfluorinated surfactants exhibit more surface-tension-lowering ability and chemical resistance than their corresponding hydrogenated ones. In aqueous solutions, the critical micellar concentration (cmc) of perfluorinated surfactants is considerably lower than that of the corresponding hydrocarbon compounds. This is because perfluorinated chains are highly hydrophobic, having a higher tendency to form aggregates than do hydrogenated chains with the same number of carbons.8 If these chains have some functionality able to react, the reactivity in the aggregated state decreases compared to that in the free solution; that is, when the aggregation increases, the reactivity decreases.9,10 * Corresponding author. E-mail: [email protected]. (1) Riess, J. G. Artif. Cells, Blood Substitutes Inmobilization Biotechnol. 1994, 22, 215. (2) Riess, J. G. J. Fluorine Chem. 2002, 114, 119. (3) Patel, N.; Marlow, M.; Lawrence, M. J. J. Colloid Interface Sci. 2003, 258, 345. (4) Baran, J. R., Jr.; Decker, E. L.; Wilcox, H. N. J. Dispersion Sci. Technol. 2002, 23, 23. (5) Halpern, D. J. Fluorine Chem. 2002, 118, 47. (6) Kraft, M. P. Cosmet. Sci. Technol. Ser. 1998, 19, 195. (7) Pabon, M.; Corpart J. M. J. Fluorine Chem. 2002, 114, 149. (8) Mukerjee, P. Colloids Surf., A 1994, 84, 1. (9) Jiang X.-K.; Hui, Y.-Z.; Fan, W.-Q. J. Am. Chem. Soc. 1984, 106, 3839. (10) Jiang, X.-K. Acc. Chem. Res. 1988, 21, 362.

A remarkable property of perfluorinated amphiphiles is that the process of aggregation is very slow compared with the similar process for hydrogenated compounds. The lifetime of monomers in conventional hydrocarbon surfactant micelles is generally on the order of 1 ms to 1 µs.11 Spherical micelles of lithium perfluorooctylsulfonate have been characterized by a monomer lifetime of 4.2 × 10-6 s. It was also reported that fluorocarbonhydrocarbon hybrid surfactants have unusually long lifetimes.12 The slowness of the aggregation process was also recognized for other perfluorinated compounds.13 We clearly established in a previous publication that compound 1 interacts with perfluorononanoic acid micelles in a two-step process. One of the steps takes place in about 1.2 min, while the other takes 5 min.14

We also reported that compound 1 is deaggregated by hydrocarbon surfactants at concentrations higher than their cmc in a two-step process. The first of these is faster and involves a dimer formed by 1 and the surfactant monomer, while the (11) Thurn, T.; Couderc-Azouani, S.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 2003, 19, 4363 and references therein. (12) Kondo, Y.; Miyazawa, H.; Sakai, H.; Abe, M.; Yoshino, N. J. Am. Chem. Soc. 2002, 124, 6516. (13) Eastoe, J.; Dalton, J. S.; Downer, A.; Jones, G.; Clarke, D. Langmuir 1998, 14, 1937. (14) Granados, A. M.; Fidelio, G. D.; de Rossi, R. H. Langmuir 1997, 13, 4079.

10.1021/la060550w CCC: $33.50 © 2006 American Chemical Society Published on Web 06/22/2006

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slower process refers to insertion of the dimer into the micellar pseudophase.15 In a study of the hydrolysis of ester 2a there was a clear indication that the aggregation and hydrolysis reactions occur on a similar time scale;16 therefore, we undertook a study of the hydrolysis reaction of compounds 2a and 2b to measure the rate of aggregation in competition with the rate of hydrolysis and to determine how these rates are affected by the presence of hydrocarbon-derived surfactants. We choose detergents with different head, anionic (sodium dodecyl sulfate (SDS)), cationic (dodecyltrimethylammonium bromide (DTABr); dodecyltrimethylammonium chloride, (DTACl); cetyltrimethylammonium bromide, (CTABr)), and neutral polyoxyethylene(23)lauryl ether (Brij-35). We also determined the effect of perfluorooctanoate micelles on the rate of hydrolysis of compounds 2a and 2b to compare the effect of a perfluorinated surfactant with that of hydrocarbon-derived compounds. It turns out that all hydrocarbonderived detergents are able to associate with the substrate at a rate faster than the rate of self-aggregation of the substrate, and, well above the cmc, the hydrolysis takes place in the micellar pseudophase at a rate considerably faster than the rate measured in the absence of the micelles. It is important to stress that, to the best of our knowledge, this is the first report in which the dynamic processes related to the aggregation compete with a chemical reaction of the substrate on a time scale on the order of minutes.

Results The rate of the hydrolysis of esters 2 in water at pH ) 6.00 and constant ionic strength (0.3 M) was followed by UV-visible absorption spectroscopy measuring the change in absorption at two wavelengths. These were 269.6 and 242.0 nm for substrate 2a and 319.0 and 260.0 nm for 2b. The longer wavelength corresponds to the maximum absorption of the product, namely, phenol and p-nitrophenol for 2a and 2b, respectively. In most cases, the total change in optical density measured at this wavelength was larger than that measured at the shortest one, and, consequently, the values of the rate constants obtained were more accurate. Therefore we have selected the data obtained at this wavelength to calculate the rate data reported in this paper. However, it is important to remark that the analysis of the data obtained at the shortest wavelength led to the same general conclusions. In the reactions of 2b, a well-defined isosbestic point is observed (Figure 1A), but the absorbance versus time data fit to a double exponential equation. This observation implies that at least two kinetic processes were taking place.17 On the other hand, the spectra of 2a did not show a well-defined isosbestic point under all conditions (Figure 1B is representative); this is a behavior typical of reactions that take place in more than one step.17 Except for the reaction of 2a at 1.02 × 10-5 M (Figure 2A) all the other absorbance versus time data for 2a and 2b fit to a double exponential equation (Figure 2B is representative). The (15) Ferna´ndez, M. A.; Granados, A. M.; El Seoud, O.; de Rossi, R. H. Langmuir 2002, 18, 8786. (16) Ferna´ndez, M. A.; de Rossi, R. H. J. Org. Chem. 2003, 68, 6887. (17) Bernasconi, C. F. Relaxation Kinetics; Academic Press: New York, 1976; p 142.

Figure 1. Hydrolysis of (A) 2b (7.73 × 10-5 M; total time elapsed: 30 min) and (B) 2a (4.92 × 10-5 M; total time elapsed: 7.2 h) at 25.1 ( 0.1 °C, pH ) 6.00, [Buffer] ) 0.096 M (Na2HPO4/NaH2PO4), and µ ) 0.3 M (NaCl); solvent contains MeCN ) 3.8%.

values of the two rate constants obtained for 2a and 2b at different substrate concentrations are summarized in Table 1. It can be seen that there is a clear tendency for the rate constants to diminish when the concentration rises, and this behavior is characteristic for compounds that self-aggregate in solution.10 The rate constants shown in Table 1 for 2a are slightly different from the values reported previously,16 but the ionic strength used here is 0.3 M, while, in a previous work, it was 0.2 M. It is well-known that the rate of the processes involving the association of surfactants is strongly dependent on the ionic strength.18 The rate constant ratios for 2a and 2b are 9-20 for k2 and 3-5 for k1. These rate ratios can be compared with the ratio of the rate constants for the alkaline hydrolysis of p-nitrophenylacetate (570 M-1 min-1) and phenyl acetate (76 M-1 min-1), which is 7.5.19 In the presence of surfactants, the kinetic behavior of the system changed when the concentration of the surfactants rose. At low surfactant concentrations with DTACl, DTABr and CTABr, both 2a and 2b gave the hydrolysis product at a rate characterized by two time domains. The behavior was similar to that shown in the absence of surfactants, although the two rate constants (18) Molina-Bolivar, J. A.; Aguiar, J.; Carnero Ruiz, C. J. Phys. Chem. B 2002, 106, 870. (19) Kirsch, J. F.; Jencks, W. P. J. Am. Chem. Soc. 1964, 86, 837.

Hydrolysis in Arylperfluorooctanoate Reactions

Figure 2. Absorbance at 269.6 nm vs time for the reaction of 2a at 25.1 ( 0.1 °C, pH ) 6.00, [Buffer] ) 0.096 M (Na2HPO4/NaH2PO4), and µ ) 0.3 M (NaCl); solvent contains MeCN ) 3.8%. (A) [2a] ) 1.03 × 10-5 M. The line was drawn using the following equation: A ) A∞ + a1e-k1t. (B) [2a] ) 2.49 × 10-5 M. The line was drawn using the following equation: A ) A∞ + a1e-k1t + a2e-k2t.

calculated are different in the presence or absence of the surfactant (compare data in Tables 1 with those in Tables 2 and 3). At high surfactant concentrations, only one kinetic process is observed. It is important to note that the values of the cmc for the surfactants shown in Tables 2 and 3 were taken from the literature, and they correspond to the values in water without any added salt. It is well-known that the cmc decreases at high salt concentrations,20 so the cmc values for the surfactants used in this work, at an ionic strength of 0.3 M, should be lower than the reported values. At high surfactant concentrations, that is, well above the cmc value of the corresponding surfactant, only one kinetic process was observed. This was evident from two main observations: (i) the absorbance versus time data fit to a single-exponential equation, and (ii) there were no differences in the rate constants measured at different wavelengths.17 The results for the observed rate constants for the hydrolysis of substrates 2a and 2b at a constant substrate concentration (∼2.5 × 10-5 M) in the presence of hydrocarbon surfactants are shown in Tables 2 and 3, respectively. The effect of the addition of perfluorooctanoic acid, 3, on the reactivity of compounds 2 is different for 2a and 2b (Table 4). In the case of compound 2a, two kinetic processes are observed at all concentrations of 3 used. The rate of the faster process (k1) is slightly more rapid than the one observed for the substrate alone, while the rate of the slower process (k2) is similar to that observed for the substrate alone. On the other hand, compound 2b shows only one kinetic process at all concentrations of 3 used. The time constant for this process is somewhere between the values of the two kinetic processes observed for the substrate alone. We did some reactions of 2b in the presence of 3 in a stopped flow spectrophotometer to determine whether there was a fast process that might be undetectable in a conventional spectrophotometer, but, even on the shorter possible time scale of 5 ms, we could not see any measurable process faster than the one reported in Table 4.

Discussion The rate constants for the hydrolysis reaction of substrates 2a and 2b in water varied with the substrate concentration and were (20) Myers, D. Surfactant Science and Technology; VCH Publishers: New York, 1992; p 119.

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remarkably slower than expected in comparison with the corresponding values for the shorter derivatives (five carbons or less).16 The expected values for the rate of hydrolysis of 2a and 2b can be estimated considering the rate constants reported in the literature for related esters. It was found that the relative rate constants for phenyl acetate and phenyl propionate is 2.1, while the phenyl propionate/phenyl butanoate rate ratio is 1.4.23 Similar rate ratios were observed for phenyl perfluoroacetate/phenyl perfluoropropionate and for phenyl perfluoropropionate/phenyl perfluorobutanoate.16 The relative rate constant for the basic hydrolysis of p-nitrophenyl acetate and p-nitrophenyl octanoate is 1.8.24 Assuming a similar rate ratio for phenyl perfluoroacetate (k ) 2.06 s-1 at pH ) 6.00)16 and 2a, we can estimate that the rate constant value for phenyl perfluorooctanoate in water should be around 1 s-1. Additionally, the rate constant reported for this compound at pH 6.00 in the presence of β-cyclodextrin (0.07 s-1)16 is probably close to the rate of neutral hydrolysis in water and must be taken as the lower limit of the rate constant for the neutral hydrolysis of 2a. It can be seen in Table 1 that the faster rate constant measured for this compound (0.19 × 10-2 s-1) is about 40 times lower than that value. To estimate the rate constant for the neutral hydrolysis of 2b, we can use the value of βlg ) -0.49 measured for aryl trifluoroacetates25 and the known values of the pKa of phenol and p-nitrophenol.26 Using 0.07 s-1 as the value for the rate of 2a, we estimated that the rate constant for 2b should be around 1.7 s-1. To have an experimental value for this rate constant, we determined the rate in the presence of cyclodextrin. In the presence of β-cyclodextrin at concentrations higher than 9.3 × 10-4 M, the decay of the absorbance of 2b or the increase of the absorbance at the wavelength maximum of p-nitrophenol is monoexponential. The observed rate constant, namely, 5 s-1, probably corresponds to the rate of hydrolysis of the substrate complexed with β-cyclodextrin, which is expected to be similar to that in water for the monomeric substrate.27 In Table 1, we can see that the faster rate constant measured for 2b is 0.01 s-1, which is well below the expected value for the hydrolysis of monomeric 2b. Therefore, we suggest that, for both substrates, 2a and 2b, the reaction can be represented as shown in Scheme 1, where S represents the substrate, Sn is the aggregated substrate, and P means the products. It should be noticed that Scheme 1 is an oversimplification of the real system since, in the real solution, one should consider species such as Sn-1, Sn-2, and so on.28 What is remarkable in this system is that the rate of hydrolysis of the ester competes with the rate of self-aggregation; therefore, these two phenomena must take place on similar time scales. If this were not the case, as it is for hydrocarbon-derived esters, only one kinetic process should be observed, that is, that corresponding to the hydrolysis of the aggregated substrate. The slow rate of aggregation appears to be a characteristic of perfluorocarbon compounds. Autoag(21) Mukerjee, K.; Mysels, K. Critical Micelle Concentrations of Aqueous Surfactant Systems; Series NSRDS-NBS 36; U.S. National Bureau of Standards: Washington, DC, 1971. (22) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1976, 80, 2468. (23) Tee, O. S.; Mazza, C.; Du, X. X. J. Org. Chem. 1990, 55, 3603. (24) Bonora, G. M.; Fornasier, R.; Scrimin, P.; Tonellato, U. J. Chem. Soc., Perkin Trans. 2 1985, 367. (25) Ferna´ndez, M.; de Rossi, R. H. J. Org. Chem. 1999, 64, 6000. (26) CumulatiVe Series Index for CRC Handbook of Biochemistry and Molecular Biology, 3rd ed; CRC Press: Cleveland, OH, 1977; p 314. (27) The rate constant for the hydrolysis of 2b (2.5 × 10-4 M) slightly decreases as the cyclodextrin concentration increases. The observed rate constant at a β-cyclodextrin concentration of 2.3 × 10-3M is 4.8 s-1, and at 4.8 × 10-3 M it is 4.0 s-1. (28) We have evidence that the substrate is not in its monomeric form, even in pure acetonitrile. So, when the solution of 2a or 2b is added to water, the substrate is aggregated, and the change in solvent changes the state of aggregation.

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Table 1. Observed Rate Constants for the Hydrolysis of 2a and 2b as a Function of Substrate Concentrationa [2a] (10-5 M)b

k1 (10-2 s-1)

k2 (10-2 s-1)

[2b] (10-5 M)c

k1 (10-2 s-1)

k2 (10-2 s-1)

1.03d 2.49 3.77 4.92 7.46

0.19 ( 0.03 0.16 ( 0.04 0.11 ( 0.02 0.072 ( 0.005

0.0136 ( 0.0004 0.0091 ( 0.0004 0.0073 ( 0.0006 0.0070 ( 0.0007 0.0070 ( 0.0008

1.08 2.47 3.45 3.86 7.73

1.03 ( 0.02 0.89 ( 0.01 0.469 ( 0.003 0.37 ( 0.03 0.26 ( 0.02

0.191 ( 0.008 0.212 ( 0.004 0.118 ( 0.002 0.11 ( 0.02 0.06 ( 0.02

a T ) 25.1 ( 0.1 °C; µ ) 0.3 M (NaCl); solvent contains MeCN ) 3.8%; pH ) 6.00; [Buffer] ) 0.096 M (Na2HPO4/NaH2PO4). Unless otherwise noted, the values of the rate constants were obtained by fitting the absorbance vs time data to the following equation: A ) A∞ + a1e-k1t + a2e-k2t. b λ ) 269.6 nm. c λ ) 319 nm. d The data fits a single-exponential equation.

Table 2. Effect of Hydrocarbon Surfactants on the Deaggregation and Hydrolysis of 2aa [Surf] (10-2 M) 0.05 0.10 0.50 0.80 1.00 1.50 2.00 2.50 2.90 3.80 4.80

DTACl (0.019 M)b k1 (10-2 s-1) k2 (10-2 s-1) 0.38 ( 0.01 0.43 ( 0.04 0.31 ( 0.03

0.19 ( 0.01 1.54 ( 0.04 1.84 ( 0.04 2.57 ( 0.01 2.9 ( 0.3 3.5 ( 0.2 4.23 ( 0.01 4.2 ( 0.2 4.3 ( 0.4 4.1 ( 0.3

DTABr (0.015 M)b k1 (10-2 s-1) k2 (10-2 s-1) 0.079 ( 0.001

0.28 ( 0.02

0.15 ( 0.03

0.89 ( 0.09

0.23 ( 0.06

1.19 ( 0.03 1.37 ( 0.07 2.02 ( 0.01 2.0 ( 0.1 1.8 ( 0.2 2.3 ( 0.2

CTABr (0.0009M)b k2 (10-2s-1) 1.5 ( 0.2 2.3 ( 0.1 3.2 ( 0.3

SDS (0.008 M)b k1 (10-2 s-1) k2 (10-2 s-1)

0.067 ( 0.007

3.2 ( 0.4 1.9 ( 0.1 1.9 ( 0.1

0.0108 ( 0.0004 0.0099 ( 0.0005 0.0100 ( 0.0004

1.8 ( 0.2

Brij-35 (1 × 10-4 M)b,c k2 (10-2 s-1) 0.116 ( 0.001 0.208 ( 0.002 0.200 ( 0.005

0.0098 ( 0.0006

0.199 ( 0.004

0.0105 ( 0.0001

0.201 ( 0.002

[2a] ) 2.55 × M; T ) 25.1 ( 0.1 °C; µ ) 0.3 M (NaCl); solvent contains MeCN ) 3.8%; pH ) 6.00; [Buffer] ) 0.096 M (Na2HPO4/ NaH2PO4); λ ) 269.6 nm. Each value corresponds to the average of at least two determinations, and the errors are standard deviations. If only one value of rate constant is provided, it means that the data fit to a single-exponential equation. When two values are provided, they were obtained by fitting the absorbance vs time data to the following equation: A ) A∞ + a1e-k1t + a2e-k2t. b Values within brackets correspond to the cmc of the surfactant (ref 21). c Reference 55. a

10-5

Table 3. Effect of Hydrocarbon Surfactants on the Deaggregation and Hydrolysis of 2ba DTACl (0.019 M)b [Surf] (10-2 M) 0.20 0.50 0.67 0.78 0.98 1.50 2.00 2.50 3.00 4.00 4.80

DTABr (0.015 M)b

k1 (10-2 s-1)

k2 (10-2 s-1)

k1 (10-2 s-1)

k2 (10-2 s-1)

670 ( 70

180 ( 50

550 ( 90

170 ( 30

650 (60 720 (80 730 ( 80 820 ( 20

170 ( 50 230 ( 40 140 ( 50 190 ( 90

620 ( 60

220 ( 50

630 ( 60

310 ( 70

910 ( 1160 ( 40 1130 ( 60 40d

CTABr (0.0009 M)b k1 (10-2 s-1)

k2 (10-2 s-1)

790 ( 50

250 ( 40 900 ( 100 1100 ( 100 1050 ( 90 980 ( 30 850 ( 30 920 ( 30 800 ( 100 770 ( 20 790 ( 30 770 ( 10

500 ( 40 530 ( 30 530 ( 30

SDS (0.008 M)b k1 (10-2 s-1)

k2 (10-2 s-1)

1.31(0.01

0.234 ( 0.008

Brij-35 (1 × 10-4 M)b,c k2 (10-2 s-1) 49 ( 4 50 ( 4

2.34 ( 0.01 2.33 ( 0.01

53 ( 3

2.52 ( 0.01

50 ( 3 55 ( 2

[2b] ) 2.47 × 10-5 M; T ) (25.1 ( 0.1) °C; µ ) 0.3 M (NaCl); solvent contains MeCN ) 3.8%; pH ) 6.00; [Buffer] ) 0.096 M (Na2HPO4/ NaH2PO4); λ ) 319 nm. If only one value of rate constant is provided, it means that the data fit to a single-exponential equation. When two values are provided, they were obtained by fitting the absorbance vs time data to the following equation: A ) A∞ + a1e-k1t + a2e-k2t. Each value corresponds to the average of at least 10 determinations, and the errors are standard deviations. b Values within brackets correspond to the cmc of the surfactant (ref 21). c Reference 55. d µ ) 0.335 M a

gregation of perfluorinated compounds at relatively low concentrations is a well-known phenomenon, and even short chain compounds such as sodium heptafluorobutyrate self-assemble in aqueous solution.29 The best known relationship between perfluorocarbons and their hydrogenated analogues refers to the cmc. The cmc of a perfluorinated surfactant is approximately equal to that of a hydrocarbon surfactant with a hydrocarbon chain 1.5 times longer than the fluorocarbon chain.30 It is important to note that neither k1 nor k2 represents rate constants of elemental reactions. They are a combination of several (29) Gonza´lez-Pe´rez, A.; Ruso, J. M.; Prieto, G.; Sarmiento, F. Colloids Surf., A 2004, 249, 41. (30) Blanco, E.; Gonzalez Perez, A.; Ruso, J. M.; Pedrido, R.; Prieto, G.; Sarmiento, F. J. Colloid Interface Sci. 2005, 288, 247.

elemental reactions. To find the relationship between the observed values of k1 and k2 and the elemental steps of the mechanism, it would be necessary to know the detailed mechanism of the reactions shown in Scheme 1. This is not possible with the available data. Comparing the data for k1 and k2 of the two esters, we can see that the k1 values for 2a are similar to the k2 values for 2b, while 2a the ratio for the two other rate constants, namely, k2b 1 /k2 , ranges from 37 to 98. It is known that the rates of kinetic processes related to the dissociation of hydrocarbon-derived surfactants are about the same for compounds with the same alkyl chain and different headgroups.31 Therefore, we suggest that the process characterized by the observed rate constants k1 in 2a and k2 in

Hydrolysis in Arylperfluorooctanoate Reactions

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Table 4. Observed Rate Constants for the Hydrolysis of Substrates 2 in the Presence of 3a 2ac [3] (10-2 M)b

k1 (10-2 s-1)

k2 (10-2 s-1)

0.50 0.70 1.00 1.20 1.50 1.86 2.00

0.68 ( 0.30 0.36 ( 0.09 1.4 ( 0.4 1.5 ( 0.2 1.7 ( 0.2

0.0098 ( 0.0001 0.0089 ( 0.0007 0.0076 ( 0.0002 0.0068 ( 0.0002 0.0072 ( 0.0001

Scheme 2. Schematic Representation of the Reaction of Substrates 2 in Water in the Presence of Surfactants

2bd k2 (10-2 s-1) 0.552 ( 0.001 0.510 ( 0.001 0.4305 ( 0.0006 0.446 ( 0.001 0.4482( 0.0008 0.408 ( 0.006

[2a] ) 2.55 × 10-5 M; [2b] ) 2.47 × 10-5 M; T ) (25.1 ( 0.1) °C; µ ) 0.3 M (NaCl); solvent contains MeCN ) 3.8%; pH ) 6.00; [Buffer] ) 0.096 M (Na2HPO4/NaH2PO4). b cmc (perfluorooctanoic acid) ) 9 × 10-3 M (ref 22). c λ ) 269.6 nm. The values were obtained by fitting the absorbance vs time data to the following equation: A) A∞ + a1e-k1t + a2e-k2t. d λ ) 319 nm. The data fit to a single-exponential equation. a

Scheme 1. Schematic Representation of the Processes Taking Place When Substrates 2 Are Dissolved in Water Solutiona

a S represents substrate 2a or 2b, S is the aggregated substrate, n and P represents the phenolate for 2a and the p-nitrophenolate for 2b.

2b is mainly associated with the aggregation phenomena, while the other process has a major contribution from the hydrolysis rate constant, thus it mainly reflects the different rates of hydrolysis of the two compounds. The kinetics of the hydrolysis of 2 in the presence of DTABr, DTACl, and SDS surfactants also shows two kinetic processes at low surfactant concentrations but only one at higher concentrations. Additionally, under the latter conditions, the rate constants measured at different wavelengths give the same value. This is a clear indication17 that, under those conditions, only one kinetic process is being measured, and this probably corresponds to the neutral hydrolysis of the substrate associated with the surfactant micelles.32 The concentration of surfactant required to completely suppress the biphasic kinetic behavior is higher for 2b than it is for 2a (Tables 2 and 3). For instance, in the case of 2a, one kinetic process is observed at a DTACl concentration of 0.01 M and a DTABr concentration of 0.02 M, while, for 2b, concentrations higher than 0.02 M are required to observe the same effect. These results may indicate that the affinity of the surfactants for 2a is higher than that for 2b. The relationship between the lipophilicity of a compound and the association constant (Km) with micelles is well documented, and it has been shown that there is a good correlation between Km and the Hanch π parameter of the substituents in reactions of substituted phenyl acetates.33 The π parameter34 for the NO2 group with reference to benzene is -0.28,35 so 2b is expected to be more hydrophilic than 2a; however, the values of log P calculated for the two compounds (31) Clint, J. H. Surfactant Aggregation; Chapman and Hall: New York, 1992; p 101. (32) The cmc values were taken from the literature, and they correspond to the values in water without any added salt. In this work, the ionic strength is 0.3 M, thus the cmc should be lower. This means that in order to observe one kinetic process, the concentration of the surfactant must be somewhat above the cmc, but how much above depends on the relative rates of the different processes involved (see Scheme 2). (33) Pirinccioglu, N.; Zaman, F.; Williams, A. J. Org. Chem. 2000, 65, 2537.

are the same within experimental error (namely, 6.38 and 6.4236 for 2a and 2b, respectively), indicating that the lipophilicity of both compounds are about the same. It should be kept in mind that the interaction of solutes with micelles does not depend only on the lipophilicity of the substrate, but that there are many other factors influencing the association. The polarizability of substrate 2b (29.39 × 10-24 cm3) is higher than that of 2a (26.8 × 10-24 cm3),37 and this could be a factor favoring the interaction of 2a with the micelles. It is known that polarizability is one important factor in solvent-solute interactions.38,39 The results of the kinetic study in the presence of surfactants may be interpreted as shown in Scheme 2. At surfactant concentrations (D) smaller than the cmc, there are no micelles (M), so the species in the upper right corner of the scheme is not formed; therefore, the observed reactions are related to the self-association of the substrate (kag) competing with association with the detergent (kD). For the cationic surfactants, the rate of hydrolysis of the substrate associated with the detergent (khSD in Scheme 2) seems to be faster than that corresponding to the aggregated substrate, since the rates of the two kinetic processes measured are faster than those measured for the substrate alone. This is especially true for substrate 2b (see Table 3). The formation of premicellar complexes was observed in the reaction of 2,4-dinitrophenyl acetate and octanoate in the presence of surfactants.40 All cationic surfactants produce an acceleration of the hydrolysis rate constants compared with that of the substrate in water. However, the effect is more marked for 2b than it is for 2a. The rate increases and then decreases, and this effect has been explained by the ionic exchange in the surface of the micelle. 41 In the cationic micelles, the rate constant for the hydrolysis of 2b is faster than the value expected for the reaction of the monomer in water solution. Taking the value of the rate constant at the higher detergent concentration and the estimated value for the hydrolysis of the compound in water (∼5 s-1), we calculated that the rate ratio in the micellar phase and in water is 2.3, 1, and 1.54 for DTACl, DTABr, and CTABr, respectively. On the other hand, similar calculations for 2a gave 0.6, 0.3, and 0.26. (34) Fujita, T.; Iwasa, J.; Hansch, C. J. Am. Chem. Soc. 1964, 86, 5175. (35) Hansch, C.; Leo, A.; Unger, S. H.; Kim, K. H.; Nikaitani, D.; Lien, E. J. J. Med. Chem. 1973, 16, 1207. (36) The log P values refer to the partition of the substrate in octanol/water and were obtained from the molecular properties given in Chemdraw Ultra, version 8.0. (37) This value was taken from the molecular properties provided by Chemsketch, version 8.0. (38) Reichardt, C. SolVent and SolVent Effects in Organic Chemistry, 3rd ed.; VCH Publishers: Weinheim, Germany, 2004. (39) Kubinyi, H. QSAR: Hansch Analysis and Related Approaches; Mannhold, R., Krogsgaard-Larsen, P., Timmerman, H., Eds.; Methods and Principles in Medicinal Chemistry; VCH Publishers: Weinheim, Germany, 1993; Vol. 1, p 23. (40) Marconi, D. M.; Frescura, V. L.; Zanette, D.; Nome, F.; Bunton, C. J. Phys. Chem. 1994, 98, 12415. (41) Dwars, T.; Paetzold, E.; Oehme, G. Angew. Chem., Int. Ed. 2005, 44, 7174.

6528 Langmuir, Vol. 22, No. 15, 2006

The effect of the surfactants on the observed rate may be the result of a combination of factors such as (i) the localization of the substrate in the micelle, (ii) the nature of the transition state for the reaction, and (iii) the concentration of ions on the surface of the micelle, which may affect the polarity of the Stern layer or may act as a catalyst to the reaction. In the following paragraphs, we will discuss each of the points mentioned above. (i) Because of the higher polarizability and lower lipophilicity of the substituent in substrate 2b, it is probably located in a more aqueous region of the interface, while 2a is more deeply included in the core of the micelle. This factor may contribute to decrease the rate of 2a compared with that of 2b in the micellar phase. Micelles inhibited most of the neutral hydrolysis of alkyl or acyl centers, with the exception of acyl chlorides, having strong electron withdrawing substituents.42-44 The micellar inhibitions are qualitatively consistent with the interfacial region, being less polar and “less aqueous” than water.45,46 One of the factors that may influence the rate of neutral hydrolysis is the polarity of the micellar surface region of the different aggregates. It is known that the hydrolysis of esters is faster in a higher polar solution.47,48 On the basis of ET parameters, DTABr (53.7) appears to be slightly more polar than CTABr (53.4).49 According to the estimate of Mukerjee et al.,50  is approximately 36 on the micellar surface. Thus, micelles behave as a submicroscopic solvent, with less ability than water to interact with a polar transition state. The relative rate of the neutral hydrolysis of phenyl chloroformate in water and in a CTABr micelle is 3.2, and the inhibition has been attributed to the decrease in solvent polarity at the interface. The fact that the inhibition is not large indicates that the reaction occurs in a relatively wet region, that is, the hydrolysis takes place in the bulk micelle interfacial region.49 (ii) It was suggested that the transition state for the neutral hydrolysis of aryl trifluoroacetates25 is as depicted in the schematic of compound 4, where B is any base, including water. A similar transition state was suggested for other reactions involving the addition of water to the carbonyl group.51 In such a transition state, there is an increase in the negative charge of the organic moiety as the reaction proceeds, and proof of that is the fact that the reactivity of trifluoroacetate derivatives is significantly higher than that of acetate derivatives.

(42) Al-Lohedan, H.; Bunton, C. A.; Mhala, M. M. J. Am. Chem. Soc. 1982, 104, 6654. (43) Possidonio, S.; Siviero, F.; El Seoud, O. A. J. Phys. Org. Chem. 1999, 12, 325. (44) Brinchi, L.; Di Profio, P.; Micheli, F.; Germani, R.; Savelli, G.; Bunton, C. A. Eur. J. Org. Chem. 2001, 1115. (45) Novaki, L. P.; El Seoud, O. A. Phys. Chem. Chem. Phys. 1999, 1, 1957. (46) Menger, F. M.; Yoshinaga, H.; Venkatasubban, K. S.; Das, A. R. J. Org. Chem. 1981, 46, 415. (47) Ferna´ndez, M. A.; de Rossi, R. H. J. Org. Chem. 1997, 62, 7554. (48) Neuvonen, H. J. Chem. Soc., Perkin Trans. 2 1986, 1141. (49) Rodriguez, A.; Graciani, M. D.; Mun˜oz, M.; Moya, M. L. Langmuir 2003, 19, 7206. (50) Mukerjee, P.; Ray, A. J. Phys. Chem. 1966, 70, 2144. (51) Jencks, W. P. Chem. Soc. ReV. 1981, 10, 345.

Ferna´ ndez and de Rossi Table 5. Effect of Buffer Concentration on the Rate of Hydrolysis of 2a and 2b at a Constant CTABr Concentrationa [Buffer] (M)b

-2 -1 c k2a s ) obs (10

-2 -1 d k2b s ) obs (10

0.010 0.025 0.050 0.073 0.096 0e 0f

0.4647 ( 0.0008 0.667 ( 0.002 1.206 ( 0.006 1.763 ( 0.004 3.2 ( 0.4 0.23 ( 0.02 0.164 ( 0.006

110 ( 8 190 ( 20 350 ( 20 600 ( 30 1050 ( 90 57 ( 5 59 ( 2

T ) (25.1 ( 0.1) °C; solvent contains MeCN ) 3.8%. b pH ) 6.00; buffer of Na2HPO4/NaH2PO4; µ ) 0.3 M (NaCl) unless otherwise noted. c [2a] ) 2.55 × 10-5 M; [CTABr] ) 0.0096 M; λ ) 269.6 nm. d [2b] ) 2.47 × 10-5 M; [CTABr] ) 0.0077 M; λ ) 319 nm. e [HCl] ) 1 × 10-3 M instead of buffer, without ionic strength control. f [HCl] ) 1 × 10-3 M instead of buffer, µ ) 0.3 M (NaCl). a

In the transition state for the hydrolysis of acyl compounds, a negative charge builds up on the organic moiety and interacts unfavorably with the anionic headgroup of a SDS micelle but favorably with a cationic headgroup. The negative charge on the organic moiety in transition state 4 is a factor that should favor its interaction with the positive head of cationic micelles and thereby stabilize the transition state. Substituents change the relative extents of bond making and bond breaking in the hydrolysis where water is the nucleophile. This fact affects the relative reactivity in water and in micelles of compounds with different substituents. For example, the hydrolysis of 4-nitrophenylchloroformate and nitrobenzoyl chlorides is accelerated by cationic micelles, although the hydrolysis of the parent compound is inhibited.44 These results are similar to what is reported here for 2a and 2b. (iii) As mentioned above, the transition state for the hydrolysis of 2 involves a base, which may be water or another base. Under the conditions of our study (Tables 2 and 3), we have PO4H2-/ PO4H2- as a buffer.52,53 These two ions can exchange with the ions at the interface, and it is expected that the fraction of micelles having PO4H2- as a counterion decreases as the concentration of surfactant increases. The equilibrium constant for the exchange of Br-/PO4H2- is 0.15.54 Catalysis by PO4H2- may be responsible for at least part of the increase in rate. To establish the importance of the concentration of phosphate, we determine the rate constant for the hydrolysis of 2a and 2b in the presence of constant concentrations of CTABr and at variable concentrations of buffer. The data are summarized in Table 5 and clearly show that the rate of the reaction is strongly dependent on the phosphate concentration. Additionally, the maximum rate measured for 2a and 2b is higher for DTACl than it is for CTABr, consistent with the higher Cl-/PO4H2- exchange equilibrium constant (KCl/PO4H2) 0.78).54 At concentrations where only one kinetic process is observed with SDS as the surfactant (>0.01 M), the value of the observed rate constant is significantly smaller than the value expected for the substrate in water. Considering the estimated rate constants for the neutral hydrolysis of monomeric 2a and 2b, that is, 0.07 and 5 s-1, respectively (see above), the ratio of the values of the rate constants in water and in the SDS micelles can be calculated as 670 and 198, respectively. These differences in rate ratios are consistent with the localization of 2a in a less aqueous region, as discussed above. The reactivity in the micellar pseudophase (52) In CTACl micelles, it was shown that the pH of a PO4H2-/PO4 H2- buffer changes significantly with the concentration of CTACl (ref 53). We did not observe similar changes under the conditions of our study. (53) Quarti, N.; Blagoeva, I. B.; El Seoud, O. A.; Ruasse, M.-F. J. Phys. Org. Chem. 2001, 14, 823. (54) Bartet, D.; Gamboa, C.; Sepulveda, L. J. Phys. Chem. 1980, 84, 272.

Hydrolysis in Arylperfluorooctanoate Reactions

Langmuir, Vol. 22, No. 15, 2006 6529

Scheme 3. Schematic Representation of the Reactions of Substrates 2 in the Presence of 3

Considering that the relationship Kd > 1 probably holds, eq 2 simplifies to eq 3:

kobs 2 )

should be related to the availability of water in the interfacial region and also to interactions with the ionic headgroups in the transition state. In the presence of Brij-35, a neutral detergent, only one kinetic process is observed, even at very low concentrations.55 The phenomenon is not due to a medium effect because the addition of triethylene glycol to substrate 2a in an equivalent concentration to 0.05 M of Brij-35 (in terms of ethylene oxide units) has no effect on either the shape of the absorbance versus time curves or the hydrolysis rate constant. The only experimental observation under these conditions was that the solution becomes cloudy after 23 h. The strong association of Brij-35 with 1 was attributed to interactions between the -Cδ+-Fδ- dipoles of the amide and the corresponding -Cδ+-Oδ- of the oxyethylene units of the surfactant.15,56 Similar types of interactions may take place with substrates 2a and 2b. The observed rate constant values for the hydrolysis when the substrates are deaggregated have the following order for hydrocarbon surfactants: kcationic-surfactants > knonionic-surfactant > kanionic-surfactants. This behavior is in agreement with results reported in the literature for other compounds. For instance, spontaneous hydrolysis of carboxylic anhydrides, diaryl carbonates, and aryl chloroformates is faster in cationic micelles than it is in anionic micelles, regardless of the nature of the counterion in the cationic micelle.42,57 In the presence of 3, the kinetic behavior of 2a and 2b is different since 2a shows two kinetic processes at all concentrations of 3, below and above the cmc, but 2b shows only one kinetic process, even at the concentrations used. This behavior may be interpreted in terms of Scheme 3. The formation of a dimer between 2 and 3 is favored over the self-aggregation of the substrate because 3 is more than 80 times more concentrated than 2. Assuming that the faster process for 2a corresponds to the formation of the dimer (2:3) (kd), which is in competition with the hydrolysis (kho), the observed rate constant is given by eq 1. The plot (not shown) of k1 (Table 3) versus [3] is reasonably linear, with a slope of 1.08 M-1 s-1 and an intercept indistinguishable from zero within experimental error: h kobs 1 ) kd[3] + ko

(1)

For the second kinetic process, we consider the dimer to be in fast equilibrium with the monomeric species, and the observed rate constant is given by eq 2:

kobs 2 )

kho + Kdkhd[3] 1 + Kd[3]

(2)

(55) The value of the cmc of Brij-35 obtained from different sources is very different, namely, 0.005 (ref 21) and 9 × 10-5 M (cited by Cortona, M. N.; Vettorazzi, N. R.; Silber, J. J.; Sereno; L. E. J. Electroanal. Chem. 1999, 470, 157). We determined a cmc of 1 × 10-4 M using Merocyanine 540. (56) Esumi, K. Colloids Surf., A 1994, 84, 49. (57) Bunton, C. A.; Ljunggren, S. J. Chem. Soc., Perkin Trans. 2 1984, 355.

kho Kd[3]

+ khd

(3)

A plot (not shown) according to eq 3 gives an intercept of 5 × 10-5 s-1 and a slope of 3.4 × 10-7 s-1 with a correlation coefficient of 0.95. The numbers obtained from the correlations must be treated with caution because Scheme 3 represents an oversimplification of this complex system,28 and therefore the equations derived for that scheme are only approximate. However, the fact that the concentration dependence of the observed rate constant is as predicted indicates that Scheme 3 closely represents the behavior of this system. In the case of compound 2b, the value of kd and k-d are expected to be similar to those for 2a, but kho and khd should be significantly faster. The ratio ({(kho)2b}/{(kho)2a}) is estimated from the data mentioned earlier in this paper as 71, and the ratio of the corresponding rate constants in SDS micelles is 240; so, in Scheme 3, it may be that khd . k-d, and only one kinetic process is observed. In conclusion, we have shown that compounds 2a and 2b are strongly aggregated in water and that the rate of aggregation occurs on a time scale similar to that of the rate of hydrolysis of the substrates, which marks a big difference from the behavior of hydrocarbon-derived surfactants. Additionally, it is also shown that the association of 2a and 2b with cationic or neutral hydrocarbon surfactants is faster than the self-association of the substrate. At high surfactant concentrations, that is, well above the cmc, the association of 2a and 2b with the micellized surfactants is faster than the rate of hydrolysis, and the reaction takes place only in the micellar phase. The behavior of the substrates in the micelles is similar to what was observed for the neutral hydrolysis of other compounds. Experimental Section The aqueous solutions were made from water purified in a Millipore apparatus. Acetonitrile (HPLC grade) was used as received. The substrates were prepared by the reaction of perfluorooctanoic acid chloride with the corresponding phenol following literature methods.58 The products were obtained after distillation of the remaining acid chloride and phenol and were characterized by IR and mass spectrometry. The analysis for phenyl perfluorooctanoate was previously reported.16 p-Nitrophenyl perfluorooctanoate: IR (KBr, cm-1): 1798, 1597, 1542, 1350, 1259, 1213, 1153, 866, 674. MS m/z (rel intensity): 535 (M+, 33), 169 (31), 131 (23), 122 (37), 119 (38), 95 (20), 75 (17), 69 (100). The purity was also controlled comparing the spectrum of a completely hydrolyzed solution with that of a solution of the corresponding phenol under the same conditions. The surfactants were purchased from Aldrich, Fluka, Merck, and the Japan Halon Co. for 3. The ionic surfactants were recrystallized from ethanol or an ethanol-acetone mixture. The pH measurements were done in a pH meter at a controlled temperature and calibrated with buffers prepared in the laboratory according to the literature.59 The pH was adjusted to 6.00, and the buffer used was PO4H2-/PO4H2-. The buffer concentration was 0.096 M. All surfactant solutions in buffer were prepared at least 24 h before use to ensure that the aggregation equilibrium was reached. (58) Clark, R. F.; Simons, J. H. J. Am. Chem. Soc. 1953, 75, 6305. (59) Analytical Chemistry, The Working Tools; Strouts, C. R. N., Gilgilan, J. H., Wilson, H. N., Eds.; Oxford University Press: London, 1958; pp 228-233.

6530 Langmuir, Vol. 22, No. 15, 2006 The reactions were followed by measuring the change in absorbance with time for the appearance of phenol at 269.6 nm or p-nitrophenol at 319 nm. The reactions of 2b in the presence of DTACl, DTABr, and CTABr were carried out in an Applied Photophysics SF 18MV apparatus with unequal mixing. The substrate was dissolved in dry acetonitrile and placed in the small syringe (0.1 mL). The larger syringe (2.5 mL) was filled with a water solution containing all the other ingredients. The reactions with the other surfactants or without them were measured in a conventional UVvisible spectrophotometer (Shimadzu UV-2101PC). For these kinetic runs, 0.6 mL of a stock solution of 2a in acetonitrile was injected into a 5 cm optical pass length quartz cuvette containing 15 mL of a water solution with all the other ingredients. The runs of 2b were prepared by injecting the substrate into acetonitrile (0.12 mL) in conventional quartz cuvettes (1 cm optical pass length) containing

Ferna´ ndez and de Rossi 3 mL of the aqueous solution. In all cases, the total acetonitrile concentration was 3.8%, and the reactions were carried out at 25.1 ( 0.1 °C with a constant ionic strength (µ) of 0.3 M using NaCl as the compensating electrolyte.

Acknowledgment. This research was supported in part by the Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), the Agencia Nacional de Ciencia y Tecnologı´a (FONCyT), the Agencia Co´rdoba Ciencia, SECyT (Universidad Nacional de Co´rdoba), and the Fundacio´n Antorchas. We thank Pablo Sales for carrying out some of the experiments reported in Table 1. LA060550W