Study of the Dehydrochlorination of DDT in Basic Media in

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Study of the Dehydrochlorination of DDT in Basic Media in Sulfobetaine Aqueous Micellar Solutions Amalia Rodrı´guez,† Marı´a del Mar Graciani,† Angeles Guinda,‡ Marı´a Mu´n˜oz,† and Marı´a Luisa Moya´*,† Departamento de Quı´mica Fı´sica, Universidad de Sevilla, C/ Profesor Garcı´a Gonza´ lez s/n, 41012 Sevilla, Spain, and Instituto de la Grasa y sus Derivados, Avenida Padre Garcı´a Tejero 4, Sevilla 41012, Spain Received September 17, 1999. In Final Form: December 1, 1999 The reaction of dehydrochlorination of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane, DDT, with hydroxide ions has been studied in aqueous micellar solutions of N-tetradecyl-N,N-dimethyl-3-ammino-1-propanesulfonate, SB3-14, and N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB3-16. A simple expression for the observed rate constant, kobs, based on the pseudophase model, could explain the influence of changes in the surfactant concentration on kobs. The kinetic effects of added electrolytes (NaF, NaCl, NaBr, and NaNO3) on the reaction rate in SB3-14 micellar media have also been studied. They were rationalized by considering the binding of the anions, which come from the added salt, to the sulfobetaine micelles and their competition with the reactive hydroxide ions for the micellar surface. Conductivity measurements have been a helpful tool in the discussion of the kinetic effects of added salts and permitted the estimation of equilibrium constants for the distribution of the anions between the zwitterionic micelles and the aqueous phase.

Introduction Aqueous zwitterionic surfactants have been studied less than ionic and nonionic ones. However, recently they have attracted increasing attention because of their wide applicability and their increased commercial use.1 They have the additional advantage of being especially stable against external variations, particularly ionic strength and temperature.1 Nonetheless, zwitterionic micelles bind ions.2 In the case of sulfobetaine surfactants, radiactive tracer self-diffusion and fluorescence quenching data have shown that anions bind more strongly than cations3-7 to the micelles and that ion binding does not induce any variation of micelle aggregation numbers.3 The binding appears to be essentially of an electrostatic nature (because the charge density due to the cationic ammonium centers is higher than that at the anionic sulfonate centers3,4), although ion specificity cannot be neglected. The charge taken by the micelles remains low. This explains why the micelle aggregation numbers and the phase diagrams of zwitterionic surfactants do not change with the addition of salts. We are interested in the study of kinetic effects caused by added electrolytes to ionic micellar solutions on reaction rates of micelle-modified processes8-10 because these †

Universidad de Sevilla. Instituto de la Grasa y sus Derivados. * To whom all correspondence should be directed. E-mail: [email protected]. ‡

(1) Bluestein, B. R., Hilton, C. L., Eds. Amphoteric Surfactants; Surfactant Sci. Ser. 12; Dekker: New York, 1982. (2) Chevalier, Y.; Kamenka, N.; Chorro, M.; Zana, R. Langmuir 1996, 12, 3225 and references therein. (3) Kamenka, N.; Chevalier, Y.; Zana, R. Langmuir 1995, 11, 3351. (4) Kamenka, N.; Chorro, M.; Chevalier, Y.; Levy, H.; Zana, R. Langmuir 1995, 11, 4234. (5) Chorro, M.; Kamenka, M.; Faucompre´, B.; Partyka, S.; Lindheimer, N.; Zana, R. Colloid Surf. A 1996, 110, 249. (6) Bunton, C. A.; Mhala, M. M.; Moffat, J. R. J. Phys. Chem. 1989, 93, 854. (7) Baptista, M. S.; Politi, M. J. J. Phys. Chem. 1990, 95, 5936. (8) Rodrı´guez, A.; Graciani, M. M.; Moya´, M. L. Langmuir 1996, 12, 4090.

kinetic studies give information about the binding of ions to the ionic micellar aggregates. Given that anions are bound more strongly to sulfobetaine zwitterionic micelles than cations, the reaction of dehydrochlorination of 1,1,1trichloro-2,2-bis(p-chlorophenyl)ethane, DDT, with hydroxide ions is a good choice to investigate the competitive binding of different anions to sulfobetaine zwitterionic micelles. This process was studied in N-tetradecyl-N,Ndimethyl-3-ammonio-1-propanesulfonate, SB3-14, aqueous micellar solutions in the presence and/or absence of different concentrations of various electrolytes, NaF, NaCl, NaBr, and NaNO3. Conductivity measurements were used to aid in the interpretation of the kinetic data and permit the estimation of the binding parameters of the different anions to the zwitterionic micelles. All experiments were done at 298.2 K. Experimental Section Materials. 1,1,1-Trichloro-2,2-bis(p-chlorophenyl)ethane, DDT, was purchased from Aldrich. Aqueous solutions of sodium hydroxide (Merck) were prepared, and hydroxide ion concentrations were determined by titration. All electrolytes were obtained from Merck. N-Tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB3-14, and N-hexadecyl-N,N-dimethyl-3-ammonio1-propanesulfonate, SB3-16, were from Fluka and used as received. The critical micelle concentrations of the aqueous solutions of these two surfactants were obtained by surface tension measurements and were in good agreement with literature values (see below). Water was obtained from a Millipore Milli-Q water system, its conductivity being less than 10-6 S cm-1. Surface Tension Measurements. Surface tension was measured by using a Lauda tensiometer (Lauda-Ko¨nishofen, Germany). The tensiometer was connected to a water-flow thermostat maintained at 298.2 ( 0.1 K. Prior to each measurement the plate was heated briefly until glowing by holding it above a Bunsen burner. The vessel was cleaned by using chromic (9) Mu´n˜oz-Perez, M.; Rodrı´guez, A.; Graciani, M. M.; Mozo, J. D.; Moya´, M. L. Langmuir 1998, 14, 3524. (10) Mu´n˜oz, M.; Rodrı´guez, A.; Graciani, M. M.; Moya´, M. L. Langmuir 1999, 15, 2254.

10.1021/la991229z CCC: $19.00 © 2000 American Chemical Society Published on Web 03/03/2000

Dehydrochlorination of DDT

Langmuir, Vol. 16, No. 7, 2000 3183 Table 1. Critical Micelle Concentrations, cmc, of SB3-14 and SB3-16 Aqueous Solutions in the Absence and/or Presence of Various Species; T ) 298.2 K surfactant SB3-14

SB3-16

Figure 1. Dependence of the observed rate constant, kobs/s-1, for the reaction DDT + OH- on surfactant concentration in SB3-14 and SB3-16 aqueous micellar solutions. [NaOH] ) 0.1 mol dm-3. T ) 298.2 K. sulfuric acid, boiled for a prolonged period in distilled water, and then flamed with a Bunsen burner before use. The precision in the measurements was (1 mN m-1. Conductivity Measurements. Conductivity was measured with a Crison MicroCM 2201 conductometer connected to a water flow thermostat maintained at 298.2 ( 0.1 K. Kinetics. Rates of dehydrochlorination of DDT in the presence of hydroxide ions were determined by using the same method described previously.9,10 Because of the low solubility of the pesticide in water,11 the DDT solutions were prepared in acetonitrile (Merck). The percentage of acetonitrile in the reaction mixture was always 2 vol %. Rate constants were reproducible within 4%.

Results and Discussion Figure 1 shows the dependence of the observed rate constant for the reaction DDT + OH- on [SB3-14] and on [SB3-16], at [NaOH] ) 0.1 mol dm-3. The value of kobs increases with increasing surfactant concentration at low [surfactant], and plateaus were reached at 0.01 and 10-3 mol dm-3 for SB3-14 and SB3-16, respectively. This behavior is described by the treatment of first-order reactions based on pseudophase models,12 which in the case of SB3-14 reads

kobs )

kw′ + km′Ks[SB3-14n] 1 + Ks[SB3-14n]

(1)

Here kw′ and km′ are pseudo-first-order rate constants in the aqueous and micellar pseudophases, respectively, Ks is the equilibrium binding constant of the DDT molecules to the zwitterionic micelles, and [SB3-14n] is the concentration of micellized surfactant, equal to the total surfactant concentration minus the critical micelle concentration, cmc. The low solubility of the DDT in water prevented determining kw′, and eq 2 (kw′ = 0) was used to estimate km′ and Ks.

kobs )

km′Ks[SB3-14n] 1 + Ks[SB3-14n]

(2)

The solubility of DDT in water is close to 1 ppb,11 but 4 (11) Bowman, M. V.; Acree, F.; Corbett, M. K. J. Agric. Food Chem. 1960, 8, 406. (12) Bunton, C. A.; Mhala, M. M.; Moffat, J. R. J. Org. Chem. 1987, 52, 3832.

species present in the surfactant solution none 2% acetonitrile [NaOH] ) 0.1 mol dm-3 [NaOH] ) 0.1 mol dm-3 and [NaF] ) 0.1 mol dm-3 [NaOH] ) 0.1 mol dm-3 and [NaCl] ) 0.1 mol dm-3 [NaOH] ) 0.1 mol dm-3 and [NaBr] ) 0.1 mol dm-3 [NaOH] ) 0.1 mol dm-3 and [NaNO3] ) 0.1 mol dm-3 none 2% acetonitrile [NaOH] ) 0.1 mol dm-3

cmc/mol dm-3 2.77 × 10-4 2.70 × 10-4 2.10 × 10-4 2.65 × 10-5 2.56 × 10-5 1.95 × 10-5 1.70 × 10-5 2.80 × 10-5 2.83 × 10-5 1.95 × 10-5

Table 2. Adjustable Parameters Obtained from the Fittings of the Experimental Kinetic Data (Figures 1 and 2) by Using Eq 2; T ) 298.2 K micellar reaction medium SB3-14 no added salt [NaF] ) 0.1 mol dm-3 [NaCl] ) 0.1 mol dm-3 [NaBr] ) 0.1 mol dm-3 [NaNO3] ) 0.1 mol dm-3 SB3-16 no added salt

10-4KS, mol-1 dm3

103km′, s-1

1.2 ( 0.1 1.18 ( 0.09 1.30 ( 0.06 1.09 ( 0.06 1.1 ( 0.1

3.70 ( 0.06 2.96 ( 0.04 2.07 ( 0.02 1.16 ( 0.01 0.95 ( 0.09

0.96 ( 0.08

3.30 ( 0.05

× 10-5 mol dm-3 dissolved in SB3-14 and SB3-16 aqueous micellar solutions, indicating Ks is large. At high surfactant concentration 1 , Ks[SB3-14n] and kobs ) km′ (eq 2), consistent with the plateau observed for SB3-14 and SB316. It is worth noting that when the kinetics were studied in the presence of hydroxide ion concentrations ranging from 0.05 to 0.4 mol dm-3, kobs reaches the plateau at the same surfactant concentration for all the [OH-] used. To estimate Ks using eq 2, the cmc must be known under the reaction conditions. These values were obtained by tension surface measurements and are listed in Table 1. Note that the presence of the small amount of acetonitrile does not affect the cmc of the zwitterionic surfactants. The solid lines in Figure 1 are obtained by fitting the experimental kinetic data using eq 2, and the values of km′ and Ks are listed in Table 2. Figure 2 shows values of kobs obtained by varying the surfactant concentration when a salt concentration is 0.1 mol dm-3. The four salts, NaF, NaCl, NaBr, and NaNO3, do not change the dependence of kobs on the surfactant concentration (although they affect its value), and the solid lines in Figure 2 were determined by fitting the kinetic data in the presence of added salts using eq 2. Table 2 also includes the values of km′ and Ks. The equilibrium binding constants of the DDT molecules to the zwitterionic micelles are the same, within experimental error, in the presence and in the absence of the different salts. Because the hydrophobicity inside the sulfobetaine micelles would be similar in the absence and/ or the presence of the different salts, obtaining similar values for Ks is reasonable. Note that the value of the equilibrium binding constants obtained in this work are of the same magnitude as those found in nonionic micellar solutions of DDT, e.g., Ks ) 2.8 × 104 mol-1 dm3 in Triton X-100 micellar solutions.13 (13) Mu´n˜oz, M.; Graciani, M. M.; Rodrı´guez, A.; Moya´, M. L. Langmuir 1999, 15, 7876.

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Figure 2. Dependence of the observed rate constant, kobs/s-1, for the reaction DDT + OH- on surfactant concentration in SB3-14 aqueous micellar solutions, in the presence of added electrolytes. [NaOH] ) 0.1 mol dm-3 and [salt] ) 0.1 mol dm-3. T ) 298.2 K.

Figure 2 shows that kobs and km′ values (Table 2) depend on the nature of the salt, following the trend kobs(no salt) > kobs(NaF) > kobs(NaCl) > kobs(NaBr) > kobs(NaNO3). The same trend in kobs has been observed for the reaction DDT + OH- in cationic micellar solutions of TTAB and CTAB, for a given electrolyte and surfactant concentration.9 In these cases the specific salt effects were explained on the basis of competition between the reactive OH- ions and the inert anions from the added electrolyte at the interfacial region. To rationalize the kinetic effects of added electrolytes, eq 2 was written as follows:

kobs )

k2mKs[SB3-14n][OH-]m 1 + Ks[SB3-14n]

(3)

where k2m is the second-order rate constant in the micellar pseudophase and [OH-]m is the hydroxide ions concentration in the reaction region at the micelle-water interface. This local molar concentration is related to that written in terms of total solution volume in the form14 -

[OH ]m )

[OHm] [SB3-14n]Vm

(4)

where Vm is the partial molar volume of the reaction region and [OHm] is the analytical hydroxide ion concentration. Comparison of the eqs 2 and 3 shows that km′ ) k2m[OH-]m. Assuming that k2m has similar values in the zwitterionic micellar solutions in the absence and in the presence of added salts, the differences in the km′ values listed in Table 2 will be due to the differences in local hydroxide ion concentration. As mentioned in the Introduction, anions are known to bind to zwitterionic micelles. Comparison of the observed rate constant values for the reaction DDT + OH- in ionic, nonionic, and zwitterionic micellar solutions can give information about the possibility of hydroxide ions binding to SB3-14 micelles. In all these micellar solutions, reaction takes place only in the micellar pseudophase, and kobs is determined by the local hydroxide ion concentration. The trend observed is kobs(cationic (14) (a) Blasko´, A.; Bunton, C. A.; Foroudian, H. J. J. Colloid Interface Sci. 1995, 175, 122. (b) Romsted, L. S.; Bunton, C. A.; Yao, J. Curr. Opin. Colloid Interface Sci. 1997, 2, 622.

micellar solutions)9,15 > kobs(zwitterionic micellar solutions) > kobs(nonionic micellar solutions)13 > kobs(anionic micellar solutions).13 The fact that the rate of the reaction in zwitterionic micelles is 4 or 5 times faster than in nonionic micelles indicates that the hydroxide ion concentration at the reaction site is larger in sulfobetaine micelles probably because of the binding of OH- ions to the zwitterionic micelles. Figure 1 shows that the dependence of kobs on N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB316, concentration is similar to that on [SB3-14]. We have not discussed the difference between the km′ values corresponding to SB3-14 and SB3-16 micellar solutions because it is too small. These data were also fitted by using eq 2, and the best fit parameters are listed in Table 2. The cmc values corresponding to the SB3-16 micellar solutions are listed in Table 1. In SB3-16 solutions km′ ) 3.3 × 10-3 s-1, which is about 10 times smaller than the km′ value in hexadecyltrimethylamonium hydroxide, CTAOH, micelles, ca. 4.2 × 10-2 s-1.16,17 If we assume that the OH- ion concentration at a surface of a cationic micelle would be ca. 4 mol dm-3 6 and that the second-order rate constants in CTAOH and SB3-16 micelles are similar, the local concentration of hydroxide ions at the surface of the zwitterionic micelles would be about 0.32 mol dm-3, close to 3 times greater than the analytical concentration, 0.10 mol dm-3. This could explain why the observed rate constant values for the reaction DDT + OH- in zwitterionic micelles are higher than those in nonionic micellar solutions. These results indicate, but do not prove, that hydroxide ions bind to the sulfobetaine micelles since the location of the OH- ions and the substrate could be different in the CTAOH and zwitterionic micelles, and therefore, the k2m rate constants did not have to be the same. The differences in km′ values listed in Table 2 can be explained by considering the relative binding of the various anions. Added anions displace hydroxide ions from the micellar surface and decrease km′. The size of this effect will be related to the affinities of the different anions to the SB3-14 micelles. Conductivity measurements were done to compare the affinities of the different anions toward SB3-14 micelles. Figure 3 shows the specific conductance at several amounts of [SB3-14] in aqueous solutions of different electrolytes. In all cases, addition of zwitterionic surfactant to a solution 5 × 10-3 mol dm-3 of salt decreases the specific conductance in the sequence F- < Cl- < Br- < NO3-. On this basis, the stronger the anion binds, the more effective it desplaces OH-, and the local hydroxide ion concentration and km′ should decrease in the same order; therefore, one would expect km′(F-) < km′(Cl-) < km′(Br-) < km′(NO3-), as was found. To obtain quantitative estimations of the selectivities of sulfobetaine micelles toward different anions, we express an ion distribution as3,18-20

KX′ )

[Xm] [Xw]([SB3-14n] - [Xm])

(5)

(15) Nome, F.; Rubira, A. F.; Franco, C.; Ionescu, L. G. J. Phys. Chem. 1982, 86, 1881. (16) Stadler, E.; Zanette, D.; Rezende, M. C.; Nome, F. J. Phys. Chem. 1984, 88, 1892. (17) Otero, C.; Rodenas, E. J. Phys. Chem. 1986, 90, 5771. (18) Rodenas, E.; Vera, S. J. Phys. Chem. 1985, 89, 513. (19) Bacaloglu, R.; Bunton, C. A.; Ortega, F. J. Phys. Chem. 1989, 93, 1497. (20) Bonan, C.; Germani, R.; Ponti, P. P.; Savelli, G.; Cerichelli, G.; Bacaloglu, R.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5331.

Dehydrochlorination of DDT

Figure 3. Plots of the variations of the specific conductance, κ/µS cm-1, against the SB3-14 concentration added to the aqueous salt solutions of various electrolytes. [salt] ) 5 × 10-3 mol dm-3. T ) 298.2 K.

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Figure 4. Dependence of the observed rate constant, kobs/s-1, for the reaction DDT + OH- on added electrolyte concentration concentration in SB3-14 aqueous micellar solutions, for various salts. [NaOH] ) 0.1 mol dm-3 and [SB3-14] ) 1 × 10-3 mol dm-3. T ) 298.2 K.

In this expression all the concentrations are expressed as moles per liter of total solution volume, [Xw] and [Xm] being the anion concentration in the aqueous and micellar pseudophases, respectively. This equation has been shown to fit conductivity data well for added salts in zwitterionic micelles4 and has permitted the estimation of KX′ values for different anions. The effect of sulfobetaines upon conductivities of added electrolytes does not give information about micelle-hydroxide ion interactions because the high mobility of these ions and their low binding to cationic surfaces make the method insensitive.21 To fit the conductivity data shown in Figure 3, we assume that anion binding is more important than cation binding. Following the derivation of the equations done in ref 21, the specific conductance of the aqueous electrolyte solutions in the presence of SB3-14 micelles can be expressed as

κ)

qFR[SB3-14n] 9.11 × 1014F

+ -3 + 10-3[Xw]ΛX + 10 [Naw]ΛNa (6)

Here F is the Faraday constant (esu mol-1), q is the elementary charge (4.8 × 1010 esu), R is the fractional charge of the micelle equal to [Xm]/[SB3-14n]. F is the friction coefficient given by the Stokes approximation F ) 6πηRh, η being the macroscopic viscosity of the dilute electrolyte solutions, which can be approximated to that of water ()0.89 cP at 298.2 K), and Rh the micellar hydrodinamic radius equal to 27 Å for SB3-14 micelles. + + [Xw] ) [XT ] - [Xm] and [Naw] ) [NaT ]. Λ is the equivalent conductance at infinite dilution of the different ions which can be taken from the literature,22 and F ) 96 500 Cul mol-1. The numerical factors in eq 6 are introduced to express κ in the CGS system. The first term on the righthand side of eq 6 takes into account that despite the SB314 micelles being neutral, in the presence of salts a fraction of the anions bind to the micelles, and the resulting micelles are negatively charged. Therefore, this term accounts for the micelles’ contribution.23 The solid lines in Figure 3 were fitted to the experimental specific conductance data by getting the following (21) Di Profio, P.; Germani, R.; Savelli, G.; Cerichelli, G.; Chiarini, M.; Mancini, G.; Bunton, C. A.; Gillitt, N. D. Langmuir 1998, 14, 2662. (22) Robinson, R.; Stokes, R. H. Electrolyte Solutions; Butterworths: London, 1959. (23) (a) Evans, H. C. J. Chem. Soc. 1956, 579. (b) Eicke, H. F.; Denss, A. Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; p 699.

Figure 5. Plot of the observed rate constant, kobs/s-1, for the reaction DDT + OH- against the hydroxide ions concentration in SB3-14 aqueous micellar solutions. [SB3-14] ) 1 × 10-3 mol dm-3. T ) 298.2 K.

KX′ values for the anions: KX′(F-) ) 1.3 mol-1 dm3, KX′(Cl-) ) 2.0 mol-1 dm3, KX′(Br-) ) 4.3 mol-1 dm3, and KX′(NO3-) ) 7.1 mol-1 dm3. The equilibrium constant obtained for bromide ions is in good agreement with previous results.21 Figure 4 shows the variations of kobs with added electrolyte at constant [SB3-14]. The values of kobs decrease with added salt, and this decrease is larger for anions with higher KX′ values. A larger value of KX′ means a stronger binding of the X- anion to the sulfobetaine micelles and a more effective displacement of reactive hydroxide ions from the micellar surface and, therefore, a larger decrease in kobs. Kinetic results for SN2 reactions in sulfobetaine micelles show that the dependence of binding of fluoride and hydroxide ions to the micellar aggregates is similar.2,12 Similar results were obtained in cationic micellar solutions of hexadecyltrimethylammonium bromide, CTAB.24 On these bases, we set KOH′ = KF′ ) 1.3 mol-1 dm3. This assumption of weak binding by hydroxide ions to sulfobetaine micelles is supported by a linear dependence of kobs on hydroxide concentration (Figure 5) without sign of saturation observed for other more polarizable anions. (24) Bartet, D.; Gamboa, C.; Sepulveda, L. J. Phys. Chem. 1980, 84, 272.

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We conclude that the reaction DDT + OH- takes place in the micellar pseudophase of aqueous sulfobetaine micellar solutions. The influence of changes in the surfactant concentration on the observed rate constant is consistent with the pseudophase model. The effects of added salts on kobs can be rationalized by considering the affinity of the anions for the sulfobetaine micelles and their competition with the reactive hydroxide anions for

Rodrı´guez et al.

the micellar surface. Conductivity measurements are helpful for interpreting the kinetic data. Acknowledgment. This work was financed by D.G.C.Y.T. (Grant PB98-1110) and Consejerı´a de Educacio´n y Ciencia de la Junta de Andalucı´a (FQM-274). LA991229Z