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Langmuir 2003, 19, 5378-5382
Alkaline Hydrolysis of O,S-Diethyl Phenylphosphonothioate and p-Nitrophenyl Diethyl Phosphate in Latex Dispersions Edward E. Seabolt and Warren T. Ford* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received February 10, 2003. In Final Form: April 25, 2003 Rates of hydrolysis of O,S-diethyl phenylphonsphonothioate (DEPP) and p-nitrophenyl diethyl phosphate (Paraoxon) in 0.1 M aqueous NaOH dispersions of a cross-linked poly(2-ethylhexyl methacrylate) latex containing styrylmethyl(trimethyl)ammonium chloride units were measured by 31P NMR spectroscopy. The reactions followed second-order kinetics to 75% conversion. The rate constants of hydrolysis of both DEPP and Paraoxon were up to six times faster than those in the absence of the latex. Hydrolysis of DEPP gave a 85/15 ratio of products from P-S versus P-O bond breaking in the absence of latex and a 90/10 ratio in the presence of latex. 31P NMR relaxation times and visual observations show that DEPP, Paraoxon, the products of DEPP hydrolysis, and p-nitrophenoxide ion all partition from water into the latex. The diethyl phosphate ion that is produced from Paraoxon partitions into water. The kinetics at these high concentrations of DEPP and Paraoxon do not fit the enzymelike and ion exchange models that have been applied to the kinetics of reactions of lower concentrations of substrates in latex dispersions.
Introduction Reactions of organic compounds in aqueous media can be catalyzed by polymers and colloids such as ion-exchange resins, polymer beads of the types used for peptide synthesis and combinatorial synthesis, polymer latexes, soluble polyelectrolytes, micelles, microemulsions, and bilayer vesicles.1 At one time these media were called enzymelike because (a) the reactions take place in phases separate from the continuous water phase, (b) the sizes of association colloids and synthetic polymers are similar to those of enzymes, and (c) the kinetics can be fit to the Michaelis-Menten model used for enzymes.2,3 Of course synthetic colloids lack the specific structures and the catalytic selectivity of enzymes. Actually, catalysis by polymers and colloids is more closely related to phase transfer catalysis4 by quaternary ammonium and phosphonium ions in aqueous/organic solvent mixtures than to enzymatic catalysis. Polymers and colloids could be catalytic media for decontamination of chemical warfare (CW) agents and other toxic chemicals.5 Ambient temperature conditions have been found for decontamination of the most common CW agents: mustard, organophosphates, and VX (S-2(diisopropylamino)ethyl O-ethyl methylphosphonothioate). The CW agents all have low solubility in water, and their reactions in aqueous media are promoted by micelles, microemulsions, and polymers. However, most successful decontamination conditions are specific for one class of agent. The only conditions known for decontamination of all of them are combustion, hydrolysis with hot alkali, and oxidation by hydrogen peroxide solutions containing activators such as bicarbonate or molybdate.5-7 (1) Ford, W. T.; Miller, P. D. In Functional Polymer Colloids & Microparticles; Arshady, R., Guyot, A., Eds.; Citus Books: London, 2002; Vol. 4, p 171. (2) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (3) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982. (4) Starks, C. M.; Liotta, C. L.; Halpern, M. Phase-Transfer Catalysis; Chapman & Hall, Inc.: New York, 1994. (5) Yang, Y.; Baker, J. A.; Ward, J. R. Chem. Rev. 1992, 92, 1729. (6) Yang, Y.-C. Acc. Chem. Res. 1999, 32, 109.
Our research on catalysis in heterogeneous media and on the decontamination of simulants of CW agents has focused on polymer latexes functionalized with quaternary ammonium ions.8-13 The latexes are swollen polymer gels such as ion-exchange resins and the resins used in peptide synthesis. However, with diameters of 50-500 nm the latexes are 100-1000 times smaller in diameter. The kinetics of reactions in latex dispersions can be fit with ion-exchange or enzymelike models as in Scheme 1, where substrate S having total concentration [S]t partitions between the water phase and the particle phase with equilibrium constant KS and reacts with hydroxide ion or another reagent in each phase with rate constants k2W in water and k2L in the particles to form products P.9,11 The observed rate of reaction is the sum of the rates in the two phases. The rate in the colloidal dispersion is greater than the rate in water alone when k2L[S]L[OH-] > k2W[S]W[OH-]. Scheme 1 can be expanded to more complex reactions by use of the appropriate rate equation and inclusion of an equilibrium constant for each additional catalyst and reagent that appears in the rate equation. Quaternary ammonium ion latexes are highly efficient media for reactions of anionic and uncharged organic species. The rate of o-iodosobenzoate (IBA) catalyzed hydrolysis of p-nitrophenyl diphenyl phosphate (PNPDPP) increases up to 600 times,8,9 the rate of decarboxylation of 6-nitrobenzisoxazole-1-carboxylate increases up to 10 000 times,12,14 and the rate of hydrolysis of p-nitrophenyl carboxylates increases up to 16 times.11,12 The kinetics of all of these reactions fit ion-exchange models such as Scheme 1. In general, favorable partitioning of the reactants into the latex phase, where their concentrations are much higher than the bulk concentrations, is the major cause of the increase of rates of the bimolecular (7) Wagner, G. W.; Yang, Y.-C. Ind. Eng. Chem. Res. 2002, 41, 1925. (8) Ford, W. T.; Yu, H. Langmuir 1993, 9, 1999. (9) Lee, J.-J.; Ford, W. T. J. Am. Chem. Soc. 1994, 116, 3753. (10) Ford, W. T. React. Funct. Polym. 1997, 33, 147. (11) Miller, P. D.; Spivey, H. O.; Copeland, S. L.; Sanders, R.; Woodruff, A.; Gearhart, D.; Ford, W. T. Langmuir 2000, 16, 108. (12) Miller, P. D.; Ford, W. T. Langmuir 2000, 16, 592. (13) Ford, W. T. React. Funct. Polym. 2001, 48, 3. (14) Lee, J. J.; Ford, W. T. J. Org. Chem. 1993, 58, 4070.
10.1021/la030051k CCC: $25.00 © 2003 American Chemical Society Published on Web 05/24/2003
Hydrolysis of Organophosphates in Latex Dispersions Scheme 1
Scheme 2
Langmuir, Vol. 19, No. 13, 2003 5379 Scheme 3
if latexes affect the distribution of products from P-S and P-O bond breaking. Hydrolysis of Paraoxon proceeds as shown in Scheme 3 to give p-nitrophenoxide ion (6) and diethyl phosphate ion (7). Paraoxon was chosen for study because its hydrolysis kinetics can be followed both by UV and by 31P NMR to determine changes in the kinetics over a wide range of concentrations. Experimental Section
reactions. The intraparticle bimolecular rate constants increase by factors of no more than ten. Previously the kinetics of these latex-catalyzed reactions have been measured by UV-visible absorption of the nitroaromatic product ion at substrate concentrations of 0.01-0.1 mM and e1 mg of particles per milliliter of dispersion. However, practical decontamination methods must be efficient at high as well as low concentrations of toxic agents. The kinetics at low concentrations may not apply at higher concentrations. If the amount of substrate is much larger than the amount of particles, only a small fraction of the substrate at one time can be in the particle phase, where the rate is faster, and the particles may cause little increase of rate. Therefore, to test the activity of latex catalysts at much higher substrate and particle concentrations, we have measured the kinetics of hydrolysis of the organophosphates O,S-diethyl phenylphosphonothioate (DEPP, 1) and diethyl p-nitrophenyl phosphate (Paraoxon, 2, a common insecticide) in 0.1 M NaOH by 31P NMR spectroscopy. DEPP is a simulant of the nerve agent VX (3). The rates of hydrolysis of DEPP
and Paraoxon are accelerated at high concentrations (0.025 M), but not as much as at the lower concentrations when the kinetics are followed by UV-visible spectroscopy. Hydrolysis of DEPP in aqueous NaOH proceeds as shown in Scheme 2 with about 86% P-S and 16% P-O bond breaking to give O-ethyl phenylphosphonate ion (4) and S-ethyl phenylphosphonate ion (5), respectively.6 VX hydrolyzes to give a similar 78/22 distribution of products from P-S and P-O bond breaking.15 Because the product of P-O cleavage of VX is still highly toxic, oxidative methods are required to detoxify VX. The thiolate ions, which are produced by P-S cleavage of both DEPP and VX in basic solutions, react with dioxygen to form disulfides.6 DEPP was chosen for this study to determine (15) Yang, Y. C.; Szafraniec, L. L.; Beaudry, W. T.; Rohrbaugh, D. K. J. Am. Chem. Soc. 1990, 112, 6621.
Materials. Latexes were prepared as described before.12 DEPP (provided by the U.S. Army Edgewood Chemical Biological Center) and Paraoxon (Aldrich, containing e10% p-nitrophenol) were used without further purification. A 0.1443 M DEPP stock solution was prepared from 0.1329 g of DEPP in 0.40 mL of acetonitrile diluted to 4.00 mL with water. A 0.2100 M Paraoxon stock solution was prepared from 0.1650 g of Paraoxon in 0.30 mL of acetonitrile diluted to 3.00 mL with water. Water was purified with a Barnstead E-pure 3-module system. 31P NMR Measurements. Spectra were acquired at 161.9 MHz from samples in 5 mm tubes with 80% H3PO4/20% D2O as an external reference using a Varian UnityInova 400 MHz spectrometer. For DEPP spectra in latexes typical conditions were 90° pulse width, 0.4-0.8 s acquisition time, 3-6 s relaxation delay, and 128 acquisitions per spectrum. The Fourier transformations used exponential line broadening of 5-15 Hz of signals that were originally 50-150 Hz wide at half-height. For Paraoxon spectra line broadening of 5-10 Hz was applied to 32 acquisitions per spectrum. WALTZ gated 1H decoupling was applied only during acquisition of data. Kinetic experiments were started by adding a nitrogen-purged stock solution of NaOH/latex to the substrate stock solution in the NMR tube to give a total volume of 1.0 mL and shaking vigorously before putting the tube in the probe. The temperature in the probe was controlled to (0.2 °C. Analysis of Kinetics. Observed second-order rate constants were calculated statistically via nonlinear least squares minimization of the NMR data using a Levenberg-Marquardt algorithm from numerical recipes in C.16 The program inputs were concentration, time, and standard deviation (weighting factor) of each experimental data point. Concentrations of substrate and products were determined from the 31P NMR peak heights for DEPP and from integrated areas for Paraoxon. A weight of 1.0 was applied initially to all data points, and then the weighting factors were rescaled using eq 1 with the computed χ2 value to give a best estimate of the standard deviation.
σbest ) σused[χ2/DF]0.5
(1)
σbest is the best estimate of the standard deviation of the data, σused is the weight used initially (1.0), χ2 is the squared sum of deviations between experimental and fitted data points, and DF is the number of degrees of freedom. Observed second-order rate constants were computed using data up to 75% conversion of substrate. The program accepts five initial conditions from the user: initial substrate concentration, initial hydroxide concentration, number of moles of hydroxide consumed per mole of substrate consumed, an estimate of the rate constant, and number of iterations. Typically 5-10 iterations were needed to reach (16) Press, W. H.; Teukolsky, S. A.; Vetterling, W. T.; Flannery, B. P. Numerical Recipes in C. The Art of Scientific Computing, 2nd ed.; Cambridge University Press: New York, 1997.
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Seabolt and Ford
Table 1. Properties of the Latex Catalyst mmol N+Cl- g-1 mol % N+Cl- a Dnb (nm) Dzc (nm)
Table 2. Compositions of DEPP Solutions after Hydrolysis
1.22 24 135 200
a Percent of repeat units. b Number average diameter from transmission electron microscopy. c Hydrodynamic diameter from dynamic light scattering.
convergence. After execution the program returns least squares minimized parameters, a complete statistical analysis, and graphs. Assuming that attack of hydroxide ion on the substrate is rate limiting, the rate law is
d[P]/dt ) -d[S]/dt ) k2obs[S][OH-]
(2)
where the concentrations [P] for products and [S] for reactant are based on total volume of reaction mixture, and k2obs is the observed rate constant in M-1 s-1. In the presence of latex the rate law is
d[P]/dt ) -d[S]/dt ) k2W[S]W[HO-]W + k2L[S]L[HO-]L
(3)
where the concentrations and the rate constants k2W and k2L refer to the water and latex phases. Integration of eq 2 gives eqs 4 and 5
[S]t ) [S]∞ + [S]0 exp(-k2obs[HO-]tt)
(4)
[P]t ) -[S]0 exp(-k2obs[HO-]tt) + [P]∞
(5)
where the subscripts 0, t, and ∞ refer to reaction times. [HO-]t was calculated from eq 6,
[HO-]t ) [HO-]0 - n([S]0 - [S]t)
(6)
in which [HO-]0 ) 0.100 M and n is the number of moles of hydroxide per mole of substrate consumed in the reaction. The value of n was calculated from the product distributions. For DEPP hydrolyses, n in the absence of latex was 1.85, and n in the presence of latex was 1.90. For Paraoxon hydrolyses n was always 2.0.
Results Latex. A copolymer was made by shot growth emulsion copolymerization of 2-ethylhexyl methacrylate with 25 wt % vinylbenzyl chloride (an m/p mixture), 1.2 wt % divinylbenzene (an m/p mixture), and 1.3 wt % vinylbenzyl(trimethyl)ammonium chloride (a m/p mixture).12 Treatment of the copolymer with trimethylamine quaternized the vinylbenzyl chloride units to give the active catalyst. The particles swell to 3.2 times their dry volume both in water and in 0.1 M NaOH, according to measurements of swollen diameters by dynamic light scattering and dry diameters by transmission electron microscopy. The swollen particle sizes did not vary significantly between 23.5 and 10 °C. The particles are nearly monodisperse with a ratio of z-average diameter to number average diameter of 1 s. These relaxation data indicate that fast spin-spin relaxation is responsible for line broadening, although there could also be a contribution from heterogeneity of chemical shifts. In contrast, 7 in a latex dispersion had T2 ) 0.2 s, which was g40 times longer than the T2 values of 1, 2, and 5. The longer T2 and narrower line width of 7 correspond to much faster rotational diffusion. Therefore, the diethyl phosphonate ion was almost all in the aqueous phase, not in the gel phase of the latex where rotational diffusion is restricted. Because only one peak was observed for each species, the reactant molecules and product ions must exchange between the aqueous phase and the latex phase rapidly on the NMR time scale. Hydrolysis of DEPP. Solutions of DEPP and its ionic products (Scheme 2) have 31P NMR signals at 55.1 ppm (DEPP), 36.8 ppm (4), and 18.9 ppm (5) in 0.1 M NaOH and 53.7, 31.3, and 16.8 ppm, respectively, in latex dispersions. P-O bond breaking consumes 1 mol of NaOH per mol of DEPP, whereas P-S bond breaking consumes 2 mol of NaOH per mol of DEPP. Product distributions from latex-promoted hydrolysis in 0.1 M NaOH and from control experiments are reported in Table 2. In the absence of NaOH, hydrolysis was negligible during the times of the kinetic experiments. In 0.1 M NaOH with no latex, hydrolysis over long time converts the product 4 to the phenylphosphonate dianion. The P-S/P-O product ratio was 85/15 in NaOH with no latex and 90/10 in NaOH with latex. The method for determination of observed second-order rate constants is in the Experimental Section. The product distributions in Table 2 were input parameters for the calculations. A typical graph of experimental concentrations of DEPP and product 5 and least-squares fits to the data is shown in Figure 1. Table 3 reports rate constants in the absence of latex and at six different concentrations of latex in the reaction mixtures. At the highest concentration of latex used (25.5 mg mL-1), the rate constant for hydrolysis of DEPP at an initial concentration of 5.8 mg mL-1 was six times faster than that in the absence of latex. Hydrolysis of Paraoxon. Dispersions of Paraoxon and its products in latexes have 31P NMR signals at -6.9 ppm (Paraoxon) and 1.0 ppm (7). A typical graph of experi-
Hydrolysis of Organophosphates in Latex Dispersions
Langmuir, Vol. 19, No. 13, 2003 5381 Table 4. Observed Rate Constants for Hydrolysis of Paraoxon at 20 °C [N+] (M) 0.0000 0.0168 0.0223 0.0313 0.0413
Paraoxona
7a
N/A 12.0 14.2 23.9 39.7
6.71c 14.4 16.4 27.5 42.0
k2obs/k2wb 2.15 2.44 4.10 6.26
a Rate constants in units of 10-3 M-1 s-1 are the average of at least two trials from decreasing Paraoxon peak areas and increasing 7 peak areas. b Values calculated from the Paraoxon column. [Paraoxon]0 ) 0.026 M; [HO-]0 ) 0.1 M. c From UV measurement of increasing concentration of p-nitrophenoxide ion.
Figure 1. Plot of experimental and fitted data for the disappearance of substrate and the appearance of product 5 (labeled PS for P-S cleavage) using 0.025 M DEPP (5.8 mg mL-1), 0.1 M NaOH, and 7.5 mg mL-1 latex catalyst at 10 °C. Exp ) experimental data. Fit ) best fit of the experimental data.
an intense yellow color in the retained latex and a colorless filtrate. Therefore, almost all of the yellow p-nitrophenoxide ion (6) was retained in the latex. Because of its low solubility, the rate constant of hydrolysis of Paraoxon in 0.1 M NaOH with no latex was measured by UV-visible absorbance of the p-nitrophenoxide ion (6). The pseudo-first-order rate constant of 6.71 × 10-4 s-1 corresponds to a second-order rate constant of 6.71 × 10-3 s-1, as reported in Table 4. Rate constants in the presence of latex were up to six times higher than those with no latex. Discussion
Figure 2. Plot of experimental and fitted data for the disappearance of substrate and the appearance of products using 0.026 M Paraoxon (6.8 mg mL-1), 0.1 M NaOH, and 19.5 mg mL-1 latex catalyst at 20 °C. Exp ) experimental data. Fit ) best fit of experimental data. Table 3. Observed Rate Constants for Hydrolysis of DEPP at 10 °C [N+] (M)
DEPPa
5a
k2obs/k2wb
0.0000 0.0073 0.0123 0.0168 0.0223 0.0313 0.0413
2.79 3.84 4.54 5.07 8.44 9.62 16.4
2.76 3.83 4.54 5.12 8.46 9.63 16.6
1.38 1.63 1.82 3.03 3.45 5.87
a Rate constants in units of 10-3 M-1 s-1 are the average of at least three trials from decreasing DEPP peak heights and increasing 5 peak heights. b Values calculated from DEPP column. [DEPP]0 ) 0.025 M; [HO-]0 ) 0.1 M.
mental and best fit concentration versus time data is shown in Figure 2. Because of the low solubility of Paraoxon in water, at least 10.5 mg mL-1 latex was needed to dissolve 7.2 mg mL-1 Paraoxon. Therefore, almost all of the Paraoxon was bound in the latex. Filtration of the product mixture through a 0.1 µm porous membrane left
Rates of alkaline hydrolysis of DEPP and Paraoxon are increased by the quaternary ammonium ion latex. The rate in the particle phase must be faster than the rate in the water phase. The observation that latex was needed to dissolve Paraoxon into the aqueous dispersions, and the broad NMR peaks of DEPP and Paraoxon prove that the concentrations of DEPP and Paraoxon in the latex phase are much higher than those in the aqueous phase. The concentration of hydroxide ion in the latex phase is probably also higher than that in the aqueous phase. Complete ion exchange of hydroxide for chloride would give an intraparticle concentration of hydroxide ion equal to that of the quaternary ammonium ions, which is 0.4 M. Since the intraparticle rate constants for alkaline hydrolyses of p-nitrophenyl carboxylates in latexes differ little from the rate constants in water,11 there is probably little difference in rate constants in the latex phase versus water also for hydrolysis of DEPP and Paraoxon. Thus, the overall rate increases are due to high concentrations of the organic substrates in the latex, with possibly a significant contribution also from higher intraparticle concentration of hydroxide ion. Our previous investigations of hydrolysis of p-nitrophenyl carboxylates in latexes were carried out by UVvisible detection of
