Langmuir 1993,9, 1999-2007
1999
Articles Catalysis of Hydrolysis of p-Nitrophenyl Diphenyl Phosphate by o-Iodosobenzoate in Cationic Latexes and Polyelectrolytes Warren T.Ford' and Hui Yu Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received April 14, 1993. In Final Form: May 20,1993 The o-iodosobenzoate (IBA) catalyzed hydrolysis of p-nitrophenyl diphenyl phosphate (PNPDPP) is greatly acceleratedby cationicpolystyrenelatexesand polyelectrolytes,with a maximum rate enhancement of 3.6 X 104compared with hydrolysis in the absence of both IBA and polymer. The latexes contain varied amounts of (styrylmethy1)trimethylammonium chloride repeat units and are cross-linked by 176 divinylbenzene. The polyelectrolytes contain either (styrylmethy1)trimethylammonium chloride or (styrylmethy1)tri-n-butylphosphoniumchloride repeat units. Half-lives are as short as 10 s with 0.4 mg mL-1 polymer and 4 X 1od M IBA at pH 8 and 25 O C . The rates in latex dispersions increase with increasingconcentrationof ion exchange sites in the water-swollen particles. The rates in polyelectrolytes increasewith increasing lipophilicity of the polymer. The rates in both latexes and polyelectrolytes decrease with increasing concentrations of external electrolytes in the form of either buffer or NaC1. All of the results are qualitatively consistent with an ion exchange model of catalysis in which the IBA competes with chloride ion and with buffer anion for polymer binding sites, and the rates depend primarily on the intrapolymer concentrations of the PNPDPP and the IBA catalyst.
Introduction Most chemical reactions in living organisms, in the environment, and in industry take place under heterogeneous conditions, and are catalyzed by enzymes, by heterogeneous acids and bases, or by metals or metal compounds. However, most chemical reactions in the laboratory are carried out in homogeneous solutions. Consequently our understanding of reactions under heterogeneous conditions is comparatively poor. One approach to bridging the gap between heterogeneous and homogeneous reaction chemistry is the study of reactions in association colloidal systems such as micelles, microemulsions,and bilayer vesiclesthat resemble homogeneous solutions but contain a separate, dynamic phase.1-3 Rates of reaction in these colloids are often faster than those in aqueous solutions, when the reactants are concentrated or the intraaggregate rate constants are higher. With the fundamental goal of understanding chemical reactivity in aqueous colloidal dispersionsand the practical goals of finding methods for the decontamination of water and for organic solvent-free chemical processing, we are investigating reactions in colloidal dispersions consisting of polymer latexes. The latexes are produced by emulsion polymerization, a process long used to manufacture synthetic rubber and polymers for coatings. By use of functional monomers the latexes can be designed with binding sites for a wide range of catalysts. Their typical diameters are 50-500 nm. As aqueous media for the study of fundamental heterogeneous reactionsthey are attractive because of their high surface area, ease of synthesis, and
* To whom correspondence should be addressed. (1) Fender, J. H.; Fender, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (2) Fender, J. H. Membrane Mimetic Chemistry; John Wiley and Sone: New York, 1982. (3) Grlitzel, M.;Kalyanaaundaram, K. Kinetics and Catalysis in MicroheterogeneoueSystems; Marcel Dekker, Inc.: New York, 1991.
ease of control of properties via hydrophile and lipophile content. Latexes are different from surfactant micelles and vesicles. Structurally the latex is a high molecular weight polymer with ionic groups on the particle surface for charge stabilization. The core of a latex particle may be either nonpolar organic polymer, or if the latex has a high percentage of ionic repeat units, a highly hydrated gel as in an ion exchange resin. Dynamically there is exchange only of counterions, neutral solutes, and water with the solution surrounding a latex. In association colloids the noncovalently bound amphiphilic and other molecules dissociate into the surrounding solution and reassociate with lifetimes on the order of 103-10" s for surfactant micelles and hours for vesicle^.^ Most previous research on polymer-supported catalysts has employed large (0.02-0.5 mm) polymer particles.6 For fast catalytic reactions, the large particles usually suffer from diffusionallimitations to the overall rates of reaction.s When polymer colloids are used as catalyst supports, the high surface areas and the short diffusion paths to the particle interiors mean a high probability of little or no diffusionallimitations to reaction rates, and high catalytic activity. Polymer latexes have been used as catalyst supports for the air oxidations of phenols, mercaptans, and alkylaromatic hydrocarbons, for various oxidations of alkenes, and for the decarboxylation of 6-nitrobenzisoxazole-l-carb~xylate.~J In this paper we describe the use of cationic polymer latexes and polyelectrolytes as supports for the o-io(4) Israelachvili,J.Zntermo1ecular& SurjaceForces,2ndd;Academic Preee: San Diego, 1992; pp 346,376. (5) Pittman, C. U., Jr. In Comprehenuiue Organometallic Chemistry; Wilkioson, G., Stone, F. 0. A., Abel, E. W., Eds.; Pergamon: Oxford, 1983; Vol. 8, pp 553-611. (6) Ford, W. T.; Tomoj M.Adu. Polym. Sci. 1984,55,49.
(7) Ford,W.T.;Badley,R.D.;Chandran,R.S.;Babu,S.H.;Haseauein, M.;Srinivaaan, S.;Turk, H.; Yu, H.; Zhu,W. ACS Symp. Ser. 1992,492,
423 and references therein. (8) Lee, J.-J.; Ford, W. T.J . Org. Chem., in preee.
0 1993 American Chemical Society
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2000 Langmuir, Vol. 9, No. 8, 1993
dosobenzoate ion (IBA) catalyzed hydrolysis of p-nitrophenyl diphenyl phosphate (PNPDPP), shown in eq 1.
Table I. Latex Compositions and Sizes N+, N+, TEM DLS latex mol % mmol g1 R,," nm Rh,b nm RB,' nm Vm+/Vhd Q2 Q5 Q25
This reaction, first reported by Moss and co-workers? shows spectacular catalytic activity in micelles%l2and high activity also in microemulsions1%l6and other colloidaland polymeric media.1e1g Polymer latexeslg and (CTA)Cl (cetyltrimethylammonium chloride) micelleslo are by far the most active supports previously reported for this IBAcatalyzed reaction, giving shortest half-lives of hydrolysis of about 10s at room temperature with IBA concentrations of 1 1 P M and very small amounts of dispersed particles and surfactants. 0
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Results Latexes and Polyelectrolytes. The latexes were synthesized by emulsion copolymerization of varied amounts of styrene and vinylbenzylchloride (VBC),1wt% divinylbenzene (DVB), and 0.6 wt% (vinylbenzy1)trimethylammonium chloride (N+ monomer) by the shot growth technique, in which a batch of monomer is polymerized to 90-9595 conversion, a second shot of monomers, containing a larger amount of the charged monomer, is added, and the polymerization is completed.20 Copolymer formed initially from the N+ monomer functioned as an emulsifier to control the size and size distribution of the latexes. A constant amount of the N+ monomer in the first shot, with varied proportions of VBC and styrene, gave a series of copolymer latexes having diameters of about 150 nm each. After quaternization of the VBC repeat units with trimethylamine, the latexes contained from 1to 60 mol 95 quaternary ammonium ion repeat units (structure 1) as reported in Table I. The numbers of the samples refer to the weight percent of VBC used in the copolymerization. Polyelectrolytes with the same compositions as the two highest ion content latexes were synthesized by solution polymerization and quaternization. The molecular weights of the copolymer precursors of the polyelectrolytes were determined by size exclusion chromatography and are reported in Table 11. ~
~~~
(9) Moss, R. A,;Alwis, K. W.; Bizzigotti, G. 0.J.Am. Chem.Soc. 1983, 105,681. (10) Moas,R.A.; Alwis,K. W.;Shin, J.-S.J.Am. Chem. SOC.1984,106, 2651. (11) Moss, R. A.; Kim, K. Y.;Swarup, S.J.Am. Chem. SOC.1986,108, 788. (12) Hammond, P. S.;Forster,J. S.; Lieske, C. N.; Durst, H. D. J.Am. Chem. Sac. 1989,111,7860. (13) Mackay, R. A.; Longo, F. R.; Knier, B. L.; Durst, H. D. J.Phys. Chem. 1987,91,861. (14) Burnside, B. A.; Knier, B. L.; Mackay, R. A.; Durst, H. D.; Longo, F. R. J. Phye. Chem. 1988,92,4505. (15) Knier, B. L.;Durst, H. D.; Burnside, B. A,; Mackay,R. A.; Longo, F. R. J. Solution Chem. 1988, 17, 77. (16) Moss, R. A.; Chung, Y.-C.; Durst, H. D.; Hovanec, J. W. J. Chem. SOC.,Perkin Tram. 1 1989, 1350. (17) M m , R. A.; Chung, Y.4.J. Org. Chem. 1990,55, 2064. (18) Moss,R. A.; Chung, Y.-C.hngmuir 1990,6, 1614. (19) Ford, W. T.; Yu,H. Langmuir 1991, 7, 615. (20) Ford, W. T.; Yu,H.; Lee, J.-J.;El-Hamshary, H. Langmuir 1993, 9, in press.
650 675
0.55 1.67 16.9 34.1 60.4
0.05 0.12 1.37 2.42 3.50
73.8 77.3 88.2 85.2 91.4
79.4 85.6 109 125 168
79.6 85.8 109 137 190
1.25 1.35 2.17 4.1 8.0
0 Weight-average radius [ ( W i i e / Z R , W 3 , where Ri is the radius measured on a TEM negative]. Hydrodynamic radius in pure water from dynamic light scattering. Hydrodynamic radius in 5 mM TAPS buffer at pH 8.0. Swelling ratio [Rhb3/RW9,where R, is the size calculated from the TEMR, of the precursor copolymer latex, weight gain by quaternization, and deneities of copolymer and quaternized latexes].
Table 11. Characterization of the Polyelectrolytes sample N+ or P+, mmol g-1 M," Mw/Mn PE50N PE75N PE44P a
2.69 3.46 2.06
7 100 6 700 16 OOO
1.96 2.15
Molecular weights before quaternization.
A polyelectrolyte containing (styrylmethyl)tri-n-buty1)phosphonium chloride repeat units (structure 2) was investigated too. The ion contents of the latexes and the polyelectrolytes were measured with a chloride-selective electrode, and the particle sizes of the latexes were determined by transmission electron microscopy (TEM) and dynamic light scattering.20p21
The number-average and weight-average radii, R, and R,, in Table I from TEM of the quaternized latexes are reliable for determination of polydispersity, but the values for the Q50 and Q75 samples are inaccurate because of distortion of the high ion content gel particles into nonspherical shapes on the TEM grid.20 The swelling ratios of the particles, V,J V b ,were calculated from TEM measurements of the radii of the copolymer latexes before quaternization, the gain in mass of the particles due to quaternization, and the densities of the copolymer and the quatemized particles.2o The hydrodynamic radii from dynamic light scattering were calculated from the StokesEinstein equation assuming spherical nondraining particles. The hydrodynamic radii of the higher ion content particles, Q50 and Q75, decrease substantially with increasing electrolyte content of the aqueous phase, due to the polyelectrolyte-like character of the highly swollen gel particles. This phenomenon is reported and discussed in detail elsewhere.,l Since most of the kinetic experiments in this work were carried out in 5 mM TAPS buffer a t pH 8.0, the hydrodynamic radii and the swelling ratios under those conditions are reported in Table I. Kinetic Experiments. Rates of hydrolysisof PNPDPP at 25.0 OC were measured by the appearance of the spectrophotometric absorbance of p-nitrophenoxide ion at 402 nm in latex dispersions. Figure 1 shows typical absorbance vs time data and first-order kinetic plots of (21)Ackerson,B. J.; Davis, K.; Dubin, P.; Ford, W. T.; Yu, H. Manuacript in preparation.
Catalysis of Hydrolysis of PNPDPP
Langmuir, Vol. 9, No. 8, 1993 2001 80 1
/
A 0
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IO
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1
2
3
4
5
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Figure 2. Dependence of k o w on the IBA concentration in 0.2 mg mL-l Q50 latex.
(min)
1
Table 111. Dependence of h~on Polymer Composition polymer mg mL-l N+ or P+ units, mol % DH@ km*.M-18-1 ~~
62 Q5 650 Q25
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Figure 1. (A) Absorbance of p-nitrophenoxide ion vs time from 10 cM PNPDPP,0.2 mg mL-lQ50 latex, and (a) 4.0 NMIBA or (b) 50 cM IBA. (B)First-order kinetic plots of the data of part A.
the same data. The kinetic plots showed no deviation from first-order behavior over 4-5 half-lives. Despite light scattering by the particles, so little of the latexes was required to effect catalysis that standard absorbance measurements were possible. For example, 0.4mg mL-l Q50 latex had a turbidity (log[lo/ll) of 1.392 at 402 nm. The turbidity was canceled by use of a blank sample of the latex in the reference beam of the spectrophotometer. At still higher particle concentrations, the kinetic data were somewhat noisy due to so little light reaching the detector, and 0.8 mg mL-l was the highest concentration at which kinetics were measured. Since salts often cause coagulation of colloidal particles, the effects of buffers and salts on colloidal stability were studied using the Q50 particles. Phosphate and borate buffers, which are most commonly used in the pH 8-10 range, could not be used in this work. Sodium phosphate buffer (5 mM), which exists mainly as HP04%at pH 8,caused precipitation of the latexes. Borate inhibited the PNPDPP hydrolysis, probably due to some unidentified reaction with the IBA catalyst.22 Zwitterionic buffers at concentrations up to 0.1 M caused no coagulation detectable by turbidity changes of 0.1 mg mL-l Q50 latex. No coagulation was observed in 0.1 M NaC1, and coagulation of highly dilute Q50 and Q75 particles was slow enough in 1.0M NaCl that the radii could be measured by light scattering. The substrate, PNPDPP, is insoluble in water, and was added to reaction mixtures in a small volume of acetonitrile. Since an overall concentration of PNPDPP of more than 1.5 X 10-6 M in the presence of the least amount of particles studied, 0.01 mg mL-l, gave increased turbidity due to precipitating PNPDPP, a concentration of 1.0 X 10-6 M was chosen for all kinetic experiments. Binding of 1.0 X lV M PNPDPP to 0.02 mg mL-l Q50 particles in the absence of IBA was measured by mixing the (22)Beaudry, W.L.; Ward,J. R. (U.S.Army CRDEC). Personal
communication.
Q50 Q50 Q50 Q75 675 PE5ON PE75N PE75N PE44P
0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.4 0.2 0.4 0.4 0.2 0.4
0.55 1.7 16.9 34.1 34.1 34.1 34.1 60.4 60.4 39.3 61.0 61.0 44.3
8.0 8.0 8.0 8.0 8.0 8.5 9.0 8.0 8.0 8.0 8.0 8.0 8.0
167 184 1722 1359 1323 1078 929 622 578 848 185 128 2050
TAPS,5.0 mM.
PNPDPP into the dispersion followed quickly by ultrafiltration. UV-vis spectrophotometric analysis of the aqueous filtrate showed no PNPDPP and an amount of p-nitrophenoxide equal to 5% of the amount of PNPDPP charged, and the acetonitrile solution from extraction of the particles collected on the filter showed 80% and 83% of the original PNPDPP in duplicate experiments. Attempts to carry out control experiments with higher concentrations of particles resulted in faster hydrolysis of the PNPDPP. We conclude that, within limits of experimental error, all of the PNPDPP was bound to the particles. For each amount and type of latex studied, rate constants for pseudo-first-order hydrolysis of PNPDPP were measured at four to six IBA concentrations. Plots of k o vs~ [IBAI were linear, as shown in Figure 2,and the secondorder rate constants, kIBA, were calculated as the leastsquares slopes of the plots. The ~ I B Avalues for all of the latexes and polyelectrolytes are reported in Table 111. Measurements of the hydrolysis rates in 2.5-10.0 mM NaOH in the absence of IBA and polymer showed that and gave a secondthe reaction was first-order in [OH-], order rate constant for reaction of PNPDPP with hydroxide ion of 0.398 M-l s-l. Extrapolation of that data to pH 8, assuming that the uncatalyzed reaction is due only to hydroxide ion, gives a pseudo-first-order rate constant for uncatalyzed hydrolysis of 3.98 X 10-7 s-l, and a rate enhancement in the presence of IBA and 0.4 mg mL-l Q25 particles of 3.6 X 104. The rate enhancement of the hydroxide reaction a t pH 11 due to particles in the absence of IBA was 10.5. Effects of Experimental Parameters on Rates of Hydrolysis of PNPDPP. Most of the kinetic experiments were performed in 5 mM TAPS buffer solution at pH 8.0. The effects of varied amounts of latex were studied using the Q50 and Q75 particles. Values Of kIBA increased to a maximum and then decreasedwith increasing amounts of particles as shown in Figure 3. The rate constants depend greatly on the composition of the latex, as shown
Ford and Yu
2002 Langmuir, Vol. 9, No. 8,1993
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Figure 6. Dependence of k m on the amount of PE75N polyelectrolyte.
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