Langmuir 1999, 15, 309-313
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Deactivation of Mustard and Nerve Agent Models via Low-Temperature Microemulsions Fredric M. Menger* and Michael J. Rourk Department of Chemistry, Emory University, Atlanta, Georgia 30322 Received July 20, 1998. In Final Form: November 24, 1998 New low-temperature oil-in-water (O/W) type microemulsions that resist freezing and phase separation at -18 °C have been developed. These systems were shown to simultaneously destroy, via oxidative and hydrolytic mechanisms, simulants of three chemical warfare agents. Reactions, monitored at 25 °C by gradient elution high-performance liquid chromatography, took place instantly or over many minutes, depending upon the particular simulant. Neglecting reaction products, the low-temperature microemulsions contained 11 components: propylene glycol, water, base, oxidant/nucleophile, surfactant, cosurfactant, oil, stabilizer, two nerve agent simulants, and a mustard simulant. Only by virtue of self-aggregation does this extraordinarily complex chemical system adopt a useful molecular organization and, in this limited sense, the microemulsion chemistry resembles what happens in a living cell. Substantial practical issues remain: rates for a recalcitrant VX simulant should be increased and overoxidation of the mustard simulant to a sulfone retarded. Nonetheless, the new system demonstrates once again the potential of microemulsions in carrying out useful organic reactions at realistic substrate concentrations in aqueous solvents.
Introduction Nature has endowed humans with a capacity for both selfless acts of compassion and unfathomable acts of cruelty. Among the latter, the deployment of chemical weapons must rank as one of the most horrific. Progress toward chemical weapon control was stimulated by an international agreement, signed by over 100 countries in 1993, stipulating that stockpiled agents are to be destroyed in an “essentially irreversible manner” by the end of the year 2004. In the U.S. alone, about 25 000 tons of agents (primarily mustard and the nerve agents GB and VX) are being incinerated or hydrolytically decomposed. Eliminating stores of chemical weapons will not, however, guarantee peace of mind. Chemical weapons can be readily synthesized using common reagents and only a rudimentary knowledge of chemistry. The 1995 fatalities in a Japanese subway testify to the accessibility of nerve agents among the lunatic fringe. Additional ominous signs beset the world. Thus, new nerve agents (e.g., “novichok5”)1 have appeared on the scene that are five to eight times more toxic than conventional weapons. Fortunately, science (which is inherently amoral)2 can be harnessed to counter those who abuse it. With this thought in mind, many people, including ourselves, are attempting to develop simple and effective decontamination systems, i.e., to fight chemistry with chemistry.3 Our most successful approach to date has made use of microemulsion technology,4,5 and the present article continues this theme. Our attention has focused mainly on chemical detoxification processes in situations where personnel or equipment have become, accidentally or deliberately, exposed to the chemical agents. The following list of specifications reveals the magnitude of the challenge:5 (1) Ember, L. S. Chem. Eng. News 1997, 75, 5 (11), 22. (2) Glenn Seaborg once said, “People must understand that science is inherently neither a potential for good nor for ill. It is a potential to be harnassed by man to do his bidding.”11 (3) Yang, Y.-C.; Baker, J. A.; Ward, J. R. Chem. Rev. 1992, 92, 1729. (4) Menger, F. M.; Elrington, A. R. J. Am. Chem. Soc. 1991, 113, 9621. (5) Menger, F. M.; Park, H. Recl. Trav. Chim. Pays-Bas 1994, 113, 176.
(a) Speed. To be truly effective, the detoxification regime should be completed in less than a minute. (b) Generality. Detoxification should simultaneously encompass both major groups of chemical warfare agents, namely, mustards and phosphorus(V) compounds. (c) Capacity. The detoxification chemistry must operate on realistically high levels of chemical agent. This is an important stipulation because typical concentrations for most physical organic and bioorganic kinetic studies (i.e., 10-5 to 10-3 M substrate) are inappropriate for use in the field. (d) Conditions. Burdensome paraphernalia (e.g., photolysis lamps, heating devices, etc.) should be avoided. Caustic media, such as used in bulk destruction, are valueless in cases where skin, computer boards, and other sensitive surfaces require treatment. Aqueous systems are preferred over more flammable and environmentally hazardous organic solvents. (e) Efficiency. Catalytic processes are, of course, ideal. High molecular weight reagents, reacting in a 1:1 stoichiometry, are not very useful no matter what the velocity. (f) Economics. It hardly needs mentioning that the cost of the detoxification regime must be kept to a minimum. Actually, the above list is incomplete; we needed to extend the “conditions” category to include decontamination at subzero temperatures. This constraint was imposed owing to the possibility of agent release in winter or arctic environments. The problem, therefore, resolved into constructing decon systems composed of oil-in-water microemulsions that did not freeze or phase separate at -18 °C. Developing low-temperature microemulsions for a specific purpose has a decided “applied” touch to it. As mentioned previously,5 applied research teaches basic scientists, like ourselves, a valuable lesson: It is more difficult to solve a problem with external constraints than to solve a problem where any rate, any temperature, any pH, any reaction scale, or any cost will do. The difference here lies in searching for a result that is needed vs obtaining a result that is publishable. This is not to deny both forms of success. But it does seem true that few scientific disciplines remain vibrant for long unless, ultimately, the discipline enhances the welfare of the
10.1021/la980910i CCC: $18.00 © 1999 American Chemical Society Published on Web 12/18/1998
310 Langmuir, Vol. 15, No. 2, 1999 Chart 1
person in the street. Thus, the opportunity and challenge of developing chemistry with an applied component are to be welcomed. Chart 1 lists the five substrates with which we worked: paraoxon, p-nitrophenyl diphenyl phosphate (NPDPP), S-ethyl diphenyl phosphothioate (EDP), half-mustard, and 4-nitrophenethyl ethyl sulfide (NPES). Experimental Section Materials. NPDPP and EDP were prepared and purified by known methods.5 Paraoxon and half-mustard were purchased from Aldrich. NPES and the corresponding sulfoxide and sulfone were synthesized as described below. Peroxide Analysis. The concentration of aqueous hydrogen peroxide was established using a published titrimetric method.6 Solubility. Solubility measurements in microemulsions or PG/water mixtures began by weighing 0.5 mL of the liquid in a small vial. Drops of either paraoxon or half-mustard were then added one at a time, the mixture being gently shaken for several seconds after each drop. Drops were added until the mixture could solubilize no more as judged visually by a persistent cloudiness. There was no intermediate state in this process; the mixtures went from clear to milky with one particular drop. The final added weight of the milky suspension, minus the weight of a single drop, was taken as the solubility of the agent in 0.5 mL of solution. Kinetics. Reaction rates for the hydrolysis of NPDPP and paraoxon were determined spectrophotometrically at 405 nm using a thermostated Varian DMS 200 or 300 spectrophotometer. Pseudo-first-order rate constants were calculated from plots of ln(Abs∞ - Abst) vs time in the usual manner. HPLC. High-performance liquid chromatography (HPLC) work was carried out on a Rainin Dynamax linked to a solvent delivery system (model SD-200) and a UV-vis detector (model UV-1). A Microsorb MV C18 column (4.6 mm i.d. × 10 cm, Rainin 86-200-E3) with an isocratic eluant of 1:1 THF/water was used with individual agents. For multiple agents, we required a Microsorb-MV Phenyl column (4.6 mm i.d. × 10 cm, Rainin 86D00-E3) with a gradient eluant (35% THF/water for 5 min, ramp to 60% THF over 10 min, hold at 60% THF for 5 min). Sample size was 4 µL with a 20 µL dilution loop. In a typical run, 50 µL of 0.1 M agent or agents in THF was added to l mL of microemulsion prepared within 8 h of the experiment. Lowtemperature studies were carried out by cooling the microemulsion in either an ice bath or in Dry Ice/ethylene glycol (kept at roughly -15 °C) for 30 min prior to addition of the substrates; cooling was continued during the HPLC analyses. 4-Nitrophenethyl Ethyl Sulfide (NPES). Ethanethiol (2 mmol, 0.124 g, 0.148 mL was added to a slowly stirring solution of sodium hydroxide (2 mmol, 80 mg) in methanol (10 mL) at room temperature. 4-Nitrophenethyl bromide (2 mmol, 0.46 g) was then dissolved in 10 mL of methanol and added to the stirring solution. After 16 h, the mixture was reduced in vacuo and the residue redissolved in 50 mL of 1:1 diethyl ether/water. The ether layer was dried over magnesium sulfate, filtered, and reduced in vacuo. The residue was then purified by flash chromatograph (10% ethyl actetate/hexanes eluant) to provide 0.30 g of the sulfide as a tan oil (70% yield): 1H NMR (300 MHz, CDCl3) δ 1.21 (t, (6) Vogel, A. I. Textbook of Qualitative Inorganic Analysis: Theory and Practice; Longmans, Green: New York, 1951; p 348.
Letters J ) 7 Hz, 3H), 2.51 (q, J ) 7 Hz), 2.78 (t, J ) 8 Hz), 2.96 (t, J ) 8 Hz, 2H), 7.33 (d, J ) 8 Hz, 2H), 8.10 (d, J ) 8 Hz, 2H); 13C NMR (75 MHz, CDCl3) δ 14.83, 26.31, 32.59, 36.00, 123.75, 129.53, 146.74, 148.45. Anal. Calcd for C10H13NO2S (211.29): C, 56.85; H, 6.20; N, 6.63; S, 15.17. Found: C, 57.09; H, 6.25; N, 6.61; S, 15.25. 4-Nitrophenethyl Ethyl Sulfoxide. Into 15 mL of methanol was dissolved pNPES (2 mmol, 0.42 g) with stirring. Hydrogen peroxide (30%) (2 mmol, 0.24 mL) was added, and the mixture was stirred for 96 h. The mixture was then reduced in vacuo and purified by flash chromatography (7.5% MeOH/CHCl3) to provide 0.34 g of pure sulfoxide (75% yield): 1H NMR (300 MHz, CDCl3) δ 1.36 (t, J ) 7 Hz, 3H), 2.76 (q, J ) 7 Hz, 2H), 2.94 (m, 2H), 3.25 (t, J ) 8 Hz, 2H), 7.45 (d, J ) 8 Hz, 2H), 8.19 (d, J ) 8 Hz, 2H); 13C NMR (75 MHz, CDCl ) δ 7.08, 28.92, 46.31, 52.38, 124.24, 3 129.77, 146.89, 147.18. Anal. Calcd for C10H13NO3S (227.29): C, 52.85; H, 5.77; N, 6.17; S, 14.08. Found: C, 52.93; H, 5.79; N, 6.12; S, 13.98. 4-Nitrophenethyl Ethyl Sulfone. To pNPES (0.52 g, 2.5 mmol) dissolved in 10 mL of methanol was added 30% hydrogen peroxide (1 mL, ∼9 mmol) and 1 drop of glacial acetic acid as a catalyst. The mixture was stirred at room temperature for 24 h, and then m-chloroperbenzoic acid (0.43 g, 2.5 mmol) was added. Stirring was continued for another 24 h. One hundred milliliters of 0.05 M potassium carbonate was added, and the mixture was extracted with chloroform (3 × 50 mL). The chloroform was dried over magnesium sulfate and reduced. The resultant residue was purified by flash chromatography (20% THF/CHCl3) to afford 0.45 g of the pure sulfone (74% yield): 1H NMR (300 MHz, CDCl3) δ 1.40 (t, J ) 8 Hz, 3H), 2.99 (q, J ) 8 Hz, 2H), 3.26 (s, 4H), 7.42 (d, J ) 8 Hz, 4H); 13C NMR (75 MHz, CDCl3) δ 6.81, 27.68, 48.05, 52.46, 124.27, 129.61, 145.53, 147.28. Anal. Calcd for C10H13NO4S (243.29): C, 49.37; H, 5.39; N, 5.76; S, 13.15. Found: C, 49.38; H, 5.41; N, 5.70; S, 13.04.
Results and Discussion Our past experience with microemulsions as reaction media should be described briefly. In 1990 we reported the oxidative deactivation of “half-mustard” using a microemulsion composed (in weight percent) of 3% cyclohexane in 82% water stabilized by 5% sodium dodecyl sulfate (SDS) as a surfactant and 10% 1-butanol as a cosurfactant.4 Common household bleach (5% aqueous hypochlorite) served as the oxidant. Conversion of the sulfide into a sulfoxide was completed in less than 15 s. The general approach was extended 3 years later to include microemulsions that simultaneously destroyed simulants of both mustard and nerve agents.5 For example, the nerve agent simulant p-nitrophenyl diphenyl phosphate (NPDPP) was hydrolyzed with a half-life of 1.4 s in a microemulsion composed of 4% hexadecane, 54% water, 18% cetyltrimethylammonium chloride (CTAC), and 24% tert-butyl alcohol containing dilute hypochlorite. The method is rapid, cheap, and mild, and no special equipment is required. From a basic science standpoint, it is noteworthy that self-organization (a subject of intense current interest) converted a complex and seemingly intractable eight-component mixture into a useful system. Low-temperature micremulsions were recently prepared using mixtures of propylene glycol (PG) and water as the continuous phase. Screening such systems invariably produced solid phases at -18 °C when CTAC or SDS was used as a surfactant. Since ionic surfactants were, clearly, unsuitable for low-temperature work, we resorted to nonionic surfactants, i.e., long-chain polyoxyethylene ethers. Without going into detail, one of the first promising systems which remained liquid at -18 °C had the following composition: 2.5 g of polyoxyethylene-10 lauryl ether (C12E10) in 50 mL of 1:1 propylene glycol/water. Although a separate cosurfactant was not added (C12E10 itself being an alcohol), the system could solubilize 2.5 mL of oil (nheptane). In the absence of surfactant, heptane did not
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Langmuir, Vol. 15, No. 2, 1999 311 Table 1. Oil Solubilization Capacity and Low-Temperature Stability of Various O/W Microemulsions
sample
PG (mL)a
H2O (mL)
C12E10 (g)b
1-hexanol (mL)
oil (mL)c
stability at -18 °C
1 2 3 4 5 6 7 8 9 10 11 12 13 14
25 25 25 19 19 12.5 12.5 12.5 12.5 12.5 19 19 12.5 12.5
25 25 25 6 6 12.5 12.5 12.5 12.5 6 6 6 12.5f 12.5g
2.5 2.5 2.5 1.3 2.5 2.5 5 5 1.3e 1.3e 2.6e 2.6e 5 5
2 4 6 2 2 2 2 2.5 3 3 6 6 2.5 2.5
2 9 9 4 4 4 4 5 3 3 3 5 5 5
homogend phase sep. phase sep. phase sep. gel phase sep. homogen. homogen. phase sep. phase sep. phase sep. homogen. homogen. homogen.
a PG ) propylene glycol. b C E 12 10 dC12H25(OCH2CH2)10OH. Maximum volume of n-heptane soluble in system. d Homogen. ) a clear single phase. e Surfactant ) mL of C12E4 rather than g of C12E10. f Water replaced by 0.01 M phosphate buffer, pH ) 10.0. g Water replaced by 0.01 M phosphate buffer, pH ) 11.7.
Figure 1. Absorbance (405 nm) vs time (min) plots for the hydrolysis of paraoxon (5 × 10-5 M) at 25.0 °C in the operational microemulsion (see text for components).
c
dissolve in the glycol/water. Structural work on the mixtures remains to be carried out, and until this is completed the term “microemulsion” must be used cautiously. Suffice it to say that we define a successful “microemulsion” as an aqueous PG solution of surfactant (with or without cosurfactant) that can solubilize substantial amounts of oil at low temperatures. According to this designation, for example, a mixture of 1:1 propylene glycol/water containing 2.5 g of C12E28 was unsuccessful because it solidifies at -18 °C and was, therefore, not further investigated. Our search for suitable microemulsions was expanded to include (a) adding various types and amounts of cosurfactant (i.e., low molecular weight alcohols) to the PG/water + surfactant mixtures, (b) determining the maximum oil-solubilizing capacity of the resulting systems, and (c) testing the systems for stability at -18 °C. Sample data, using 1-hexanol as the cosurfactant, are given in Table 1. As is seen in the table, successful microemulsions were prepared with both C12E10 (samples 1, 7, 8, 13, and 14) and C12E4 (sample 12). All but one of these solubilize between 12 and 20% oil at -18 °C, an amount sufficient for many practical purposes including dissolving and destroying environmentally realistic levels of water-insoluble chemical warfare (CW) agents. Note that the oil solubilization capacity of the microemulsion far exceeds that obtainable by solutions of spherical micelles.7 The continuous phase in the first 12 entries of Table 1 consists of water admixed with PG. But we anticipated that the microemulsions eventually developed for lowtemperature decon purposes would require ionic components in the aqueous region. For this reason the stabilities of microemulsions possessing 0.01 M phosphate buffer instead of water were examined. Entries 13 and 14 show that homogeneous systems can be obtained at -18 °C even in the presence of salts. As already mentioned, our previous microemulsions contained hypochlorite as the chemically reactive component. Although hypochlorite is an inexpensive and potent nucleophile/oxidant, we felt it would be advantageous to have a chlorine-free regime. Thus, hypochlorite was replaced with peroxide anion, a less active but more (7) Myers, D. Surfactant Science and Technology; VCH: New York, 1988; pp 153-172.
environmentally and surface-friendly reagent. Ultimately, we arrived at the following formulation for most of our decon studies: 7.5 mL of PG, 2.0 mL of l.0 N NaOH, 0.5 mL of 30% H2O2, 1.1 mL of C12E4, 2.5 mL of 1-hexanol, and 1.2 mL of n-heptane. To prevent effervescence caused by peroxide decomposition, 40 mg of EDTA was included in order to chelate adventitious catalytic metals. The apparent pH of the microemulsion was 11.6. Henceforth, the system will be referred to as the “operational” microemulsion. If, say, two chemical agents are added to the operational microemulsion, then it will consist of 10 components! Although this may seem extraordinarily complex, in actual fact the components adopt a relatively simple organization (assuming the presence of a conventional microemulsion): The continuous phase of PG/water contains peroxide and hydroxide along with the EDTA; the n-heptane forms droplets that solubilize the water-insoluble agents; and at the droplet interface one finds the C12E4 and the 1-hexanol. Naturally, certain components might distribute themselves between more than one locus (e.g., the 1-hexanol could exist both at the droplet interface and in the continuous medium). All in all, self-assembly has converted a seemingly intractable mixture into a potentially useful chemical reactor. Addition of NPDPP (an oft-used nerve agent simulant8) to the operational microemulsion (ME) led to a hydrolysis rate too fast to measure by conventional means. This is significant because NPDPP is inherently less reactive than the actual nerve agents. Paraoxon (a G-series nerve agent simulant9) in the operational ME had a half-life of about 15 s (Figure 1). For the sake of convenience, the kinetics were carried out with only 5 × 10-5 M paraoxon, but the capacity to solublize and hydrolyze substrate far exceeds this value. When the pH was lowered from 11.6 to 10.2 (by adding to the ME 2.0 mL of 0.1 N NaOH instead of 1.0 N NaOH), then the half-life increased to about 9 min. It is important to realize that a related non-microemulsion also managed to hydrolyze paraoxon with a halflife of about 15 s. This non-ME system consisted of the microemulsion components minus the surfactant, cosurfactant, and oil (i.e., 7.5 mL of PG, 2.0 mL of 1.0 N of NaOH, 0.5 mL of 30% H2O2, and 40 mg of EDTA; pH ) 11.6). Despite the rapid kinetics of the non-ME system, it has little general utility owing to its inability to solubilize “real world” levels of oil and highly water-insoluble agents (8) Moss, R. A.; Kim, K. Y.; Swary, S. J. Am. Chem. Soc. 1986, 108, 788. (9) Lavey, B. J.; Janda, K. D. J. Org. Chem. 1996, 61, 7633.
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Table 2. Solubility in a Microemulsion and 3:1 Propylene Glycol/Water (24 °C) solubilizate
microemulsiona
3:1 PG/water
paraoxon half-mustard
470 mg/0.5 mL 75 mg/0.5 mL
215 mg/0.5 mL b
a Operational microemulsion minus the NaOH/H O . b Too low 2 2 to measure readily.
(“real world” being defined as 0.1 M or higher). Table 2, listing solubilities of paraoxon and half-mustard in the operational ME (without NaOH/H2O2) and in 3:1 PG/ water, drives home the point. Paraoxon is twice as soluble in the ME as in the continuous phase.10 More strikingly, the half-mustard has an ME solubility of 75 mg/0.5 mL (corresponding to 1.2 M), whereas half-mustard solubility in 3:1 PG/water is too low for practical purposes. The ability of low-temperature microemulsions to rapidly hydrolyze two nerve agent models (NPDPP and paraoxon) propelled us to the next step: evaluation of the microemulsion as a universal chemical warfare decon system using nerve agent and mustard simulants in combination.11 HPLC was selected as the main analytical tool in this work for several reasons: (a) HPLC afforded us a simple means for investigating mixtures at higher (and thus more realistic) substrate concentrations. (b) Once suitable HPLC conditions are established, one can readily observe the progress of several simultaneous reactions. (c) Mustard and its simulants are sulfides that can be oxidatively detoxified by conversion into sulfoxides. Further oxidation leads to sulfones that are toxic (although not nearly as virulent as the original sulfide). Thus, it is advantageous to develop a regime that can stop at the sulfoxide stage of oxidation. HPLC allows the easy monitoring of sulfide disappearance concurrently with the production of sulfoxide/sulfone. Initially, various substrates were examined individually beginning with EDP (a simulant for the nerve agent VX; see Chart 1). Using HPLC conditions described in the Experimental Section, we could observe an EDP peak isolated from the other components of the operational microemulsion. Periodic monitoring of the EDP peak showed complete hydrolysis after 8 min at 25 °C. Deactivation of EDP required 90 min at 0 °C, a reaction time that is slower than desirable for use in the field. On the other hand, according to HPLC analysis of NPDPP hydrolysis (which allowed direct observation of the NPDPP disappearance as well as the appearance of the products, diphenyl phosphate and nitrophenol), NPDPP reacted instantly within the operational ME. Thus, HPLC and spectrophotometry are in agreement on the remarkable speed of the NPDPP decontamination. Mustard simulant NPES (Chart 1) and its sulfoxide and sulfone oxidation products, obtained by synthesis, gave three distinct HPLC peaks. It required at least 40 min at 25 °C for the NPES peak to disappear upon adding the NPES (5 mM) to the operational ME. Less than 10% of the product was sulfone. Since the NPES was detectable in the operational ME at 0 °C even after 90 min, oxidations (10) Williams, E. F. Ind. Eng. Chem. 1951, 43, 950. (11) For previous studies of reactions in microemulsions, see: Bunton, C. A.; de Buzzaccarini, F. J. Phys. Chem. 1982, 86, 5010. Bunton, C. A.; de Buzzaccarini, F.; Hamed, F. H. J. Org. Chem. 1983, 48, 2461. Blandamer, M. J.; Burgess, J.; Clark, B. J. Chem. Soc., Chem. Commun. 1983, 659. Martin, C. A.; McCrann, P. M.; Ward, M. D.; Angelos, G. H.; Jaeger, D. A. J. Org. Chem. 1984, 49, 4392. Mackay, R. A.; Longo, F. R.; Knier, B. L.; Durst, H. D. J. Phys. Chem. 1987, 91, 861. Erra, P.; Solans, C.; Azemar, N.; Parra, J. L.; Clausse, M.; Touraud, D. Prog. Colloid Polym. Sci. 1987, 73, 150. Garlick, S. M.; Durst, H. D.; Mackay, R. A.; Haddaway, K. G.; Longo, F. R. J. Colloid Interface Sci. 1990, 135, 508. Holmberg, K. Surfactant Sci. Ser. 1997, 66, 69.
Figure 2. An HPLC chromatogram of NPDPP, EDP, and pNFES (5 mM each) in a microemulsion composed of propylene glycol, water, C12E4, 1-hexanol, and n-heptane (see text). Reaction products (sulfoxide, sulfone, and p-nitrophenol) have been deliberately added to microemulsion. Monitoring wavelength ) 254 nm; time scale ) 0-20 min.
Figure 3. HPLC of three-agent mixture (see Figure 2) after reaction at 25 °C for 5 min in the operational microemulsion. Monitoring wavelength ) 254 nm; time scale ) 0-18 min.
were not attempted at -15 °C. With these results in hand, it was now possible to examine the reactivity of three chemical warfare simulants (EDP, NPDPP, and NEPS) in combination with each other. Gradient elution HPLC provided the means with which to analyze the complex mixtures (see Experimental Section). An HPLC chromatogram showing excellent resolution of the three agents, plus some of their hydrolysis and oxidation products, is presented in Figure 2. The experiments with combined agents involved adding an aliquot of stock solution containing the agent mixture to a microemulsion such that the final concentration of each agent was 5 mM. A second variation entailed a modest change in the formulation of the operational ME: 7.5 mL of PG, 2.4 mL of 30% H2O2, 0.1 mL of 12 N NaOH, 1.1 mL of C12E4, 2.5 mL of 1-hexanol, 1.2 mL of n-heptane, and 40 mg of EDTA. It was eventually discovered, in our continuous screening program, that this ME has two advantages over the previous one: (a) reactions are somewhat faster and (b) the apparent pH is only 9.8. After 5 min at 25 °C, no trace of either NPDPP or NPES remained, whereas about 20% of EDP was still unreacted (Figure 3). Approximately half of the sulfide in pNEPS had been overoxidized to the sulfone. Remarkably, these hydrolysis/oxidations rates are faster than those observed with the individual agents as if some sort of synergistic effect was taking place. Control runs (in which two of the three agents were examined together) indicate that the presence of NPDPP promotes the pNPES oxidation. A likely explanation is that a perphosphate intermediate, drawn below, contributes to the oxidative process, but this speculation has not been substantiated experimentally. In summary, we have developed new low-temperature systems, assumed to be conventional O/W microemulsions, that resist freezing and phase separation at -18 °C. These
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systems were shown to simultaneously destroy, via oxidative and hydrolytic mechanisms, simulants of three chemical warfare agents. Reactions, monitored at 25 °C by gradient elution HPLC, took place instantaneously or over several minutes depending upon the particular simulant. Decon rates would be slower at -18 °C, but these were not measured directly. Neglecting reaction products, the low-temperature microemulsions contained 11 components: propylene glycol, water, base, oxidant/ nuclcophile, surfactant, cosurfactant, oil, stabilizer, two nerve agent simulants, and a mustard simulant. Only by virtue of self-aggregation does this extraordinarily complex chemical system adopt a useful molecular organization,
Langmuir, Vol. 15, No. 2, 1999 313
and, in this sense, the microemulsion chemistry models what happens in a living cell. Practical problems remain: Rates for the more reluctant VX simulant should be increased especially if the system is to be used under subfreezing conditions. Nonetheless, the low-temperature microemulsions demonstrate the potential of colloid chemistry in carrying out organic reactions in polar solvents. Acknowledgment. This work was supported by the Army Research Office. LA980910I
