Ionic Liquid Gel-Based Containment and Decontamination Coating for

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Ionic Liquid Gel-Based Containment and Decontamination Coating for Blister Agent-Contacted Substrates Bret A. Voss,† Richard D. Noble,*,† and Douglas L. Gin†,‡,* †

Department of Biological and Chemical Engineering, and ‡Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States S Supporting Information *

ABSTRACT: Current methods to contain and decontaminate materials contacted by toxic chemical warfare agents (CWAs) have disadvantages with respect to ease of delivery, portability, and effectiveness on porous substrates. A portable, easy-to-use, spreadable coating that immediately acts as a barrier to contain CWA vapors on contacted substrates and also decontaminates soaked-in CWAs is highly desired. A new type of decontaminating barrier coating for sulfur mustard (i.e., blister agent) CWAs has been developed that is made of (1) a spreadable nonvolatile, fluid matrix based on a room-temperature ionic liquid (RTIL), (2) an organic gelator that acts as a solidifying agent to help the applied coating adhere to and prevent runoff from angled or vertical surfaces, and (3) a polyamine that acts as a reagent to chemically degrade and help draw out adsorbed blister agent. When applied to porous and nonporous substrates contacted with 2-chloroethyl ethyl sulfide (CEES, a mustard agent simulant), this spreadable, soft solid coating was found to act as an effective barrier, blocking 70−90% of the CEES vapor from entering the overhead space compared to uncoated samples. Furthermore, this reactive gel RTIL coating was able to remove (i.e., draw out and degrade) 70−95% of the liquid CEES soaked into porous substrates after 24 h at ambient temperature when applied as a static, single-application coating. Preliminary studies with added dyes and indicators to this coating system have shown that the decontamination process may be followed visually via color changes. KEYWORDS: ionic liquid, gel, coating, blister agent, containment, decontamination, barrier

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INTRODUCTION The ability to effectively contain and decontaminate chemical warfare agent (CWA)-contacted materials and personal equipment in the field is extremely important for maintaining the health and operational effectiveness of military personnel and first-responders. This is especially true when CWA contamination occurs far from a base of operations where traditional CWA decontamination procedures can be implemented.1 Highly toxic CWAs are generally categorized into two classes, depending on their mechanism of action: (1) nerve agents (i.e., reactive phosphorus esters) that block neuroreceptor sites, and (2) blister agents (typically chlorinated thioethers) that alkylate and cross-link tissue and DNA.2−4 CWAs are usually delivered in vapor or aerosol form. Although protective garments can usually shield individuals from direct CWA exposure, CWAs can quickly adsorb into porous substrates such as wood, as well as nonporous but swellable substrates such as rubber and cured paint.5,6 Unfortunately, current CWA decontamination methods have disadvantages when dealing with materials that readily adsorb CWAs.7,8 Although these methods are effective for removing and deactivating surface-bound CWAs, they are not ideal for containing and efficiently removing or degrading soaked-in CWAs.9,10 Residual CWAs can still leach out from the interior of these contaminated materials in vapor and liquid form, causing problems with long-term safety and indirect exposure.11−13 © 2012 American Chemical Society

Traditional methods for decontaminating CWA-contacted materials have involved washing with reactive wash solutions that chemically degrade the CWAs or evaporating the CWAs out with heated forced air.14−16 Current CWA decontamination solutions are based on aqueous strong bases or oxidizing agents.17,18 Although these wash solutions are inexpensive, they are not completely effective unless the contaminated object is immersed in the solutions for long durations, which is not feasible for large items such as vehicles or buildings. In addition, many of these CWA decontamination solutions are heterogeneous in nature (i.e., emulsions) due to limited reagent solubility in the aqueous phase.19 This factor limits their reactivity and effectiveness in penetrating certain types of substrates. Hot air can also be forced into the contamination zone of the materials to drive out any adsorbed CWAs. Although this method works well for porous materials, it takes days or even weeks to reduce the CWA vapor concentration down to safe levels.9 Consequently, there is a need for new, portable, easily applied, and effective materials that allow military personnel and first-responders to rapidly contain and decontaminate CWA-contacted field equipment and materials. Received: December 21, 2011 Revised: February 14, 2012 Published: March 2, 2012 1174

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the RTIL may also afford a faster degradation reaction rate since RTIL as solvents are known to enhance the rates of certain organic reactions involving polar or ionic intermediates.26 Initial studies with glass, wood, rubber, and painted steel substrates contaminated with the mustard agent simulant, 2chloroethyl ethyl sulfide (CEES) (Figure 1a), showed that applying a film of the gel (RTIL + amine) coating over the samples reduces the CEES vapor concentration over the samples by 90% compared to uncoated control samples. In addition, the reactive gel (RTIL + amine) coating is able to extract/react away >98% of the CEES originally applied on the surface of nonadsorbing substrates (e.g., glass) and 70−90% of the original liquid CEES soaked into adsorbing substrates (e.g., wood, rubber, and cured paint) after 24 h of contact at ambient temperature. Control studies with the various components of this reactive composite coating indicate that the RTIL is responsible for a good part of the CEES vapor containment and liquid CEES extraction from the samples; however, the addition of the reactive amine component provides even better CEES vapor containment and extraction results. Preliminary studies have also shown that certain types of RTIL-soluble dyes and indicators can also be added to this new coating system to enable color-based visual monitoring of the blister agent decontamination process.

Herein, we present a new type of spreadable containment and decontamination coating material for blister agents (e.g., sulfur mustard and its analogues) that (1) acts as a nonvolatile barrier to block the transmission of hazardous vapors as soon as it is applied as a coating, and (2) actively removes/ decontaminates soaked-in blister agents from the substrate interior over a period of 1−2 days at ambient temperature. Afterward, this coating can be removed and properly disposed of. This new spreadable coating material is a three-component gel system consisting of (i) a room-temperature ionic liquid (RTIL) (1), (ii) a low-molecular-weight organic gelator (LMOG) (2), and (iii) an organic polyamine (tetraethylenetetramine (TETA) (Figure 1a).20,21 The role of the major

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MATERIALS AND METHODS

Materials. 2-Chloroethyl ethyl sulfide (CEES), triethylenetetraamine (TETA), pararosaniline base, thionin acetate, disperse blue 1, phenolphthalein, and all reagents and solvents used were purchased from Sigma-Aldrich (Milwaukee, WI) and used without further purification. All chemical syntheses were carried out in air, unless otherwise noted. 1-Hexyl-3-methylimidazolium Bis(trifluoromethyl-sulfonimide) (1). 1 was synthesized as previously described in the literature.27 Chemical characterization data obtained for the synthesized compound were consistent with those reported in the literature.27 Aspartame-Based LMOG (2).21 An improved synthesis for 2 was developed and used as follows: p-Toluenesulfonic acid (16.16 g, 84.94 mmol) was mixed with toluene (150 mL) in an 500-mL round-bottom flask equipped with a Dean−Stark trap, and this mixture was heated at reflux for 1 h or until a white suspension formed and little drops appeared in the trap. Aspartame (25.00 g, 84.94 mmol) was then added and the resulting mixture heated at reflux for 5 h. During this reflux time, 2.1 mL of water was collected in the Dean−Stark trap, and the mixture in the flask became viscous. 3,7-Dimethyloctanol (14.19 g, 89.19 mmol) was then added to the reaction mixture. After heating to reflux, the reaction solution became clear light yellow. Aqueous NaOH (3.40 g, 84.9 mmol dissolved in 100 mL of H2O) was then added to the reaction mixture to neutralize the remaining p-toluenesulfonic acid. The solvent was then removed from the resulting white suspension by rotary evaporation, leaving a white solid. Finally, the white solid was suspended in water and filtered (to remove salts) and then stirred in ethyl acetate and filtered (to remove color and impurities), affording pure LMOG 2 as white powder after drying in vacuo. Yield: 22.12 g (65%). Chemical characterization data obtained for the synthesized compound were consistent with those reported in the literature.21 The glass substrate was purchased from Joann’s Fabrics and used without further modification (Hudson, OH). The wood substrate was purchased as a poplar tree dowel and cut to 2 mm thick disks. The rubber substrate disk was cut from a truck tire tube from Sutong CTR (Houston, TX). The painted steel substrate was a cut 316 stainlesssteel plate painted with three coats of flat camouflage spray paint (Rust-Oleum). Instrumentation. All synthesized compounds were confirmed for structure and purity by 1H and 13C NMR spectroscopy using a Bruker Avance-III 300 MHz (for 1H) NMR spectrometer. All quantitative

Figure 1. (a) Chemical structures of the three components of the decontaminating barrier system (RTIL 1, LMOG 2, and TETA) and the blister agent simulant, CEES. (b) A cross-sectional schematic of the CEES containment and decontamination test apparatus.

component in the system, the RTIL (i.e., a molten organic salt at ambient conditions with negligible vapor pressure), is to provide a very stable fluid medium that can envelop the contamination area, help extract out the CWA via solubilization, and depress the vapor pressure of any CWA underneath or dissolved in the RTIL.22,23 The role of the LMOG is to solidify the RTIL at very low loading levels to form a soft solid with lower vapor diffusivity while retaining spreadability. The formation of a spreadable gel also helps the applied coating adhere onto angled or vertical surfaces and prevents runoff (as in the case of regular liquids). This type of gelling occurs via the LMOG molecules physically bonding with each other (e.g., Hbonding, van der Waals forces, and/or π−π stacking) to create a thermo-reversible, noncovalent network that immobilizes the surrounding liquid RTIL and affords a more mechanically stable material that will not flow under gravity.24,25 Finally, the role of the organic amine is to act as an inexpensive sacrificial nucleophilic agent to chemically react with sulfur mustard and its analogues to degrade/deactivate them via nucleophilic substitution.7 The reaction of the blister agent with the sacrificial amine also creates a negative concentration gradient of the CWA in the area in contact with the gel, thereby helping draw out soaked-in CWA from the substrate interior into the reactive coating layer. The fluid ionic environment provided by 1175

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CEES assays for the vapor barrier and decontamination experiments were performed using a Thermo-Finnigan PolarisQ Ion Trap gas chromatograph−mass spectrometer (GC-MS) system employing a 10 °C/min ramp rate. Preparation of RTIL-Based Spreadable Coatings. The gelled RTIL coatings and control samples were prepared as follows: (i) gelled (RTIL + TETA): 960 μL of 1, 40 μL of TETA, and 40 mg of 2 were mixed in a vial, heated in an Al heating block until homogeneous (130 °C), cooled in an ice bath (0 °C) to form a gel, and allowed to warm up to ambient temperature. (ii) Gelled (RTIL): 1 mL of 1 and 40 mg of 2 were mixed in a vial, heated in an Al heating block until homogeneous (130 °C), cooled in an ice bath (0 °C) to form a gel, and allowed to warm up to ambient temperature. (iii) Liquid (RTIL + TETA): 960 μL of 1 and 40 μL of TETA (no LMOG) were mixed in a vial under ambient conditions. (iv) Liquid pure RTIL: 1 mL of 1 was placed in a vial under ambient conditions. Note: there was no significant difference in the liquid coatings if they were subject to the same thermal profile as the gelled coatings. CEES Vapor Containment Testing and Assay. The selected test substrate (i.e., glass, wood, rubber, or painted steel) was placed in a small Al foil cup and allowed to equilibrate with the ambient conditions (∼19 °C, ∼50% relative humidity) overnight. Liquid CEES (20 μL) was then pipetted directly onto the center of the test substrate in the Al cup and allowed to soak into/equilibrate with the substrate for 1 min under ambient conditions. Then 1.0 mL of the selected RTIL-based coating material was poured or spread over the CEEScontaminated substrate sample (except in the case of the uncoated control samples). Although the same volume of RTIL coating was applied to each contaminated test substrate, the glass and painted steel samples received approximately a 2-mm-thick RTIL coating, whereas the wood and rubber substrates received a 1-mm-thick coating, due to the different dimensions of the substrate coupons.) Next, the Al foil cup containing the treated test substrate was placed in a 140-mL EPA soil sample jar and sealed with a septum (see Figure 1b for a schematic cross-section of this sample testing configuration). After 1−3 h and after 23−25 h of equilibration time at ambient temperature, GC-MS analysis (50 μL dose) was performed on the headspace in the sample jar to determine the concentration of CEES vapor released from the contaminated sample that was able to permeate through the applied coating. Successively diluted stock solutions of CEES in CH2Cl2 (from 5 × 10−5 M to 5 × 10−3 M) were used to establish a calibration curve to quantify the molar amount of CEES detected by the GC-MS. Liquid CEES Decontamination Testing. After the vapor barrier testing described above, the Al foil cup containing the treated test substrate was removed from the sample jar, and the substrate was wiped with a Kim-Wipe to remove all coating and CEES remaining on the surface. The wiped substrate was quickly placed in a scintillation vial with 10 mL of CHCl3 (which is not CEES-reactive) for 24 h at ambient temperature to extract any soaked-in liquid CEES from the interior of the sample. GC-MS analysis (1 μL dose) on the CHCl3 extract was used to determine the amount of CEES extracted from the samples. Calibration of the GC-MS system with standard solutions of known CEES concentration in CHCl3 was employed as before.

Information). TETA was selected as the reactive component because CEES is known to react with amine-based nucleophiles, in particular, because sulfur mustard reacts with guanine and other biological amines.28 In addition, TETA is relatively inexpensive and is a relatively nonvolatile organic solid at ambient conditions. Although stronger nucleophiles and bases could also be used, our premise was to keep the coating relatively nonreactive to most substrates and relatively nontoxic in the event of dermal contact. The CEES-reactive gelled (RTIL + amine) coating system was prepared by mixing 960 μL of RTIL 1 with 40 μL of TETA and 40 mg of LMOG 2, such that a total of 1 mL of coating mixture was formed for subsequent application onto each CEES-contaminated test substrate coupon. On the basis of information found in the Defense Threat Reduction Agency CWA Testing 2007 Source Document,29 glass (nonporous), wood disks (porous), tire rubber (swellable), and painted steel (dense but with a swellable top layer) were chosen as the test substrates in order to mimic the range of materials often contacted by CWAs. The CEES containment and decontamination testing apparatus is shown in Figure 1b. This laboratory-scale apparatus and the associated testing procedure described in Materials and Methods were developed to be inexpensive and allow for high-throughput, small-scale laboratory testing, while providing a quantitative assay on both CEES vapor barrier effectiveness, and liquid CEES desorption capability. The final data obtained from these procedures include (1) the ppm (parts per million) of CEES vapor in the headspace above the coated, contaminated samples (which is a direct measurement of the vapor barrier effectiveness of the applied coatings) and (2) the molar amount of CEES remaining within the treated substrates (which is a direct measurement of soaked-in liquid CEES decontamination and desorption effectiveness). Following the testing procedures described, the CEES vapor containment and liquid CEES desorption performance results for the reactive gel (RTIL + TETA) coating on each of the four substrate materials are summarized in bar graph form in Figures 2 and 3, respectively. For comparison, coatings consisting of (a) pure RTIL 1 [(i.e., no LMOG or TETA) (1 mL)], (b) nonreactive gelled RTIL [(i.e., 1 + 40 mg LMOG 2 (i.e., no amine) (1 mL)], and (c) liquid reactive RTIL + TETA [(i.e., 960 μL RTIL 1 + 40 μL TETA (i.e., no gelator) (1 mL)] were also tested using the same procedures and apparatus. Uncoated, CEES-contaminated test substrates were also used as baseline controls to evaluate the effectiveness of all the applied coatings. As can be seen in Figure 2, all of the RTIL-based coating formulations substantially reduce the concentration of CEES vapor in the overhead space above the CEES-wetted substrates compared to the uncoated control samples. On average, the coated glass, wood, and painted steel substrate samples had approximately 10 times less CEES vapor in the headspace compared to the uncoated control samples, whereas the coated rubber substrate samples had approximately 5 times less (Figure 2). In general, the gelled RTIL coatings performed better than nongelled (i.e., liquid) RTIL coatings. This is mostly likely due to the gelled RTIL (soft semisolid) coatings having a lower diffusion coefficient compared to the less dense and more mobile liquid RTIL-based coatings. As expected, the reactive coatings containing TETA as an active reactant for CEES degradation performed better than the RTIL coatings without TETA, across all substrates. Of particular interest, the gelled (RTIL + TETA) system is able to quickly reduce the

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RESULTS AND DISCUSSION In order to demonstrate proof-of-concept for this approach, an initial RTIL, LMOG, and amine were chosen to prepare a reactive gel RTIL coating system for initial testing. 1-Hexyl-3methylimidazolium bis(trifluoro-methylsulfonyl)imide (1) was selected as the RTIL after preliminary studies showed that CEES is completely miscible with this RTIL. This RTIL also has the advantage of being commercially available and easily and cheaply synthesized. The aspartame-derived compound 2 was chosen as the LMOG because it has previously been shown to gel many different types of RTILs.21 We also determined that 2 can also form a stable gel (>6 months) with varying mixtures of 1 and CEES, (i.e., from 1:0 to 0:1, mol RTIL:mol CEES), making it ideal for this application (see Supporting 1176

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immediately adsorbs into the wood substrate when applied. As can be seen in Figure 3 (left), after 24 h, the uncoated wood coupon released 30 mol % of the CEES applied into the CHCl3 extraction solvent. The other 70% of the CEES applied most likely reacted with the nitrogen-based or other nucleophilic compounds present in the wood matrix. The gelled (RTIL + TETA) decontaminating coating reduces the amount of unreacted CEES in the wood sample down to 6 mol % of the total amount of CEES initially applied to the wood sample (Figure 3). This result indicates that the gelled (RTIL + TETA) coating reduces adsorbed CEES contamination by a factor of 5 in the wood substrate. The rubber sample is similar to tire rubber, which is a swellable, dense, hydrophobic, cross-linked polymer material. Liquid CEES slowly adsorbs into the rubber sample after application. As can be seen in Figure 3 (middle), after 24 h, the uncoated, CEES-contaminated rubber sample released 90 mol % (almost all) of the applied CEES into the CHCl3 extraction solvent. This almost-complete extraction of CEES out of the rubber indicates that the CHCl3 solvent desorption method works well. This also shows how CWA-contaminated rubber is a potential long-term hazard, slowly releasing unreacted CWAs. However, when the gelled (RTIL + TETA) coating is applied to the contaminated rubber coupon, CEES contamination is reduced to 25 mol % of the original applied CEES amount (as determined by CHCl3 extraction of the sample after RTIL coating removal). Lastly, the painted steel was used to mimic the surface of painted vehicles, demonstrating how this decontamination coating system will work on paint coatings. The uncoated CEES-contaminated painted coupon released 40 mol % of the applied CEES after 24 h (Figure 3, right). The other 60% of the applied CEES probably reacted irreversibly with nucleophilic moieties within the paint coating, similar to the wood coupon. When a decontaminating gelled (RTIL + TETA) coating is applied, less than 1.5 mol % of the original applied CEES remains in the painted steel sample after 24 h. This result represents a >20 times reduction in the amount of CEES remaining compared to an uncoated sample. As in the vapor containment experiments, comparative CEES decontamination studies employing pure liquid RTIL, gelled RTIL (no TETA), liquid (RTIL +TETA), and gelled (RTIL + TETA) coatings were also performed on the four substrates, in order to determine the relative contributions of the components to decontamination coating performance. As can be seen in Figure 4 (left), the wood sample responded well to all treatments, with a 65−85% reduction of liquid CEES remaining in the sample as compared to the uncoated control. The liquid RTIL coatings (with and without TETA) performed better than their respective gelled analogues, most likely due to better penetration of the more fluid liquid coatings into the macroporous wood structure. Also, the addition of TETA significantly increased CEES decontamination capability when applied on the wood substrate. The rubber sample (Figure 4 (middle)) showed a similar decrease in residual liquid CEES of 70−85% as compared to an uncoated control sample. Once again, the addition of TETA increases the CEES decontamination effectiveness of the coating. The most remarkable response to coating treatment can be seen in the painted steel substrate (Figure 4 (right)). All of the applied RTIL coatings reduced the amount of remaining liquid CEES in the painted steel substrate by 95−98% as compared to an uncoated sample.

Figure 2. Relative molar ratio of CEES vapor in the headspace above the coated samples compared to that above uncoated, CEEScontaminated control samples after 22−24 h at room temperature. (The overhead CEES vapor levels from the uncoated CEES-contacted substrates have all been assigned a relative ratio of 1.0.) Lower values are better, indicating less CEES has diffused through the coating and volatilized into the headspace above the samples.

overhead CEES vapor concentration to ≤10% of that of untreated samples for all of the contaminated substrate types. These results demonstrate that these RTIL-based coatings (and especially the reactive gel RTIL systems containing amine) act as effective vapor barriers for mustard-type compounds. As can be seen in Figure 3, all of the RTIL coatings also significantly reduce the amount of liquid CEES contained

Figure 3. Total mole fraction of liquid CEES remaining in the uncoated control and gelled (RTIL + TETA) samples compared to the initial amount of CEES applied (after 22−24 h at room temperature. Lower values are better, indicating that more soaked-in liquid CEES has been removed/decontaminated from the substrates into the RTIL coatings.

within the substrates after treatment, compared to the uncoated samples (as determined by CHCl3 extraction of soaked-in CEES from the treated samples). Glass was used as a nonporous control substrate in order to determine if wiping the coupon also effectively removed the surface CEES. As expected, less than 0.5 mol % of the CEES applied to the glass substrates in each treatment was detected in the wiped coupons, indicating almost all surface CEES can be removed by simple wiping of dense samples. The wood sample was used to test a common building material with a macroporous structure. Liquid CEES 1177

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Figure 5. Mole fraction of CEES remaining in coated substrates compared to the original amount of applied CEES, as a function of coating residence time for a single static coating application. The rate at which the values approach zero indicates a faster CEES decontamination rate.

Figure 4. Relative molar ratio of liquid CEES remaining in the coated samples compared to that in uncoated, CEES-contaminated control samples after 22−24 h at room temperature. (The amounts of liquid CEES in the uncoated CEES-contacted substrates have all been assigned a relative ratio of 1.0.).

the Cl atom on CEES by an amine-functionalized dye, resulting in changing the electronic and light absorption properties of the dye; and (2) CEES reaction with the available basic TETA, resulting in a pH change in the coating mixture that can activate a conventional pH indicator. The first approach was tested by incorporating a small amount of an amine-functionalized organic dye molecule (e.g., pararosaniline base, thionine acetate, or disperse blue 1) to gelled RTIL model mixtures (no TETA) and exposing the mixtures to liquid CEES. As can be seen in Figures 6a−c, the gelled RTIL + amino-dye mixtures all undergo noticeable color changes after 110 min at ambient temperature (i.e., pararosaniline base (red → purple), thionin acetate (purple → blue), disperse blue 1 (blue → green)). UV−visible and 1H NMR studies on these systems indicate that CEES reacts with the amine groups on these aromatic dyes, causing a change in their π electronic energy levels and light absorption profiles (see Supporting Information). The second approach was tested by adding a small amount of phenolphthalein pH indicator to a gelled (RTIL + TETA) coating mixture, in order to show via color change when the sacrificial amine (TETA) has been depleted, and the coating is no longer effective for decontamination. As can be seen in Figure 6d, an initial reactive gelled (RTIL + TETA) mixture containing phenolphthalein is pink, indicating a net basic environment. When this mixture was exposed to liquid CEES, it became colorless after 110 min, indicating the near complete consumption of the basic TETA by the added CEES. As CEES reacts with TETA and reduces the number of active basic (i.e., amine) sites, the phenolphthalein will cyclize and shift color.30 These preliminary results show that the addition of RTILsoluble, reactive organic dyes and pH indicators can be used for simple visual detection/reaction monitoring in these new blister agent containment and decontamination coatings. In addition, very preliminary trials with some of these dye-containing gelled (RTIL + amine) mixtures on CEES-contaminated painted substrates have shown that this monitoring approach is promising (see Supporting Information). The addition of organic dyes to RTILs has been used in dye-sensitized solar cell research;31 however, the use of dyes and pH indicators in RTILs for chemical detection appears to be unprecedented.

Again, adding sacrificial amine to the coating formula produced a significantly better decontaminating barrier. From the aforementioned vapor containment and decontamination studies, it can be seen that the RTIL component provides the greatest single contribution with respect to blockage of CEES vapor transport and extraction of soaked-in liquid CEES from contaminated samples. While TETA causes a substantial but lower-level increase in CEES vapor barrier effectiveness compared to the RTIL alone, the main purpose of the added TETA is to help draw out and decontaminate soakedin liquid CEES via chemical reaction to form a less toxic CEES−ammonium salt adduct. RTIL coatings without a sacrificial amine component may function as an effective decontaminating barrier, but the CWA extracted into the RTIL would still be efficacious. In addition to a 24-h application study, a longer-term 96-h application study of the gelled (RTIL + TETA) coating was performed on CEES-contaminated wood, rubber, and painted steel substrates in order to determine the length of time needed for a single, static (nonagitated) coating to deplete the soakedin liquid CEES down to minimum levels. In all cases, the CEES vapor in the headspace over the contaminated samples slowly decreased every 24 h in an exponentially decaying fashion (see Supporting Information). This is most likely due to the TETA chemically decontaminating (via nucleophilic attack on CEES28) and reducing the total quantity of CEES in the sample and headspace above. The CEES extracted from the coupons also decreased every 24 h in a roughly exponential manner (Figure 5). The CEES extracted from the painted steel sample decreased to 1% of the original applied CEES amount within 24 h of application with the gelled (RTIL + TETA) coating. The wood substrate was also decontaminated quickly, and the CEES was reduced to 1% of the originally applied amount within 96 h under ambient conditions. Finally, the amount of CEES in the rubber substrate was reduced down to approximately 5% of the original applied amount within 96 h. Preliminary experiments involving addition of colorimetric indicators also suggested that it may be possible to visually detect the presence of CEES and monitor the extent of decontamination in the gelled (RTIL + TETA) coatings. These initial studies utilized two mechanisms by which CEES can likely chemically interact with an added dye or indicator to produce a visual color change: (1) nucleophilic substitution of 1178

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TETA)-based coatings and full testing of the dye- and indicator-containing coatings on CEES-contaminated test substrates. In addition, tests with this coating system on substrate samples contacted with live blister agent (i.e., actual sulfur mustard) are planned. This latter work will be performed in collaboration with and at U.S. Army testing laboratories.

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ASSOCIATED CONTENT

S Supporting Information *

Structures of the added dyes and pH indicator, UV−visible absorption spectra of gelled RTIL mixtures containing the amine-functionalized dyes and gelled (RTIL + TETA) mixtures containing phenolphthalein before and after CEES exposure, 1 H NMR spectra of the amine-functionalized dyes and phenolphthalein + TETA before and after reaction with added CEES, and pictures showing color changes of some dye-containing gelled (RTIL + TETA) coatings applied to CEES-contaminated painted Al samples in preliminary trials (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Figure 6. From left to right: (a−c) Color changes as a function of time after CEES exposure for three gelled RTIL mixtures (no TETA) containing primary amine-functionalized organic dyes ((a) pararosaniline base, (b) disperse blue 1, (c) thionin acetate) (compositions: 1.0 mL 1 + 40 mg 2 + 2 mg dye). (d) Color change as a function of time after CEES exposure for a gelled RTIL + TETA) mixture containing the pH indicator, phenolphthalein (composition: 1.0 mL 1 + 40 mg 2 + 40 μL TETA + 6 mg phenolphthalein). When CEES (200 μL) is pipetted on top of these gel mixtures (time = 0), it slowly diffuses through the samples and triggers a chemical reaction in the added dyes and pH indicator, leading to color changes. For the three aminefunctionalized dyes, CEES covalently binds with their primary amine groups to change the electronics and absorption wavelength, indicating the presence of CEES (see Supporting Information). For the pH indicator, CEES reacts with/consumes the nucleophilic and basic TETA (which normally turns phenolphthalein pink, if present), causing a loss of color and indicating depletion of the amine reagent needed for decontamination.

*E-mail: [email protected]; [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Primary funding for this research from the U.S. Defense Threat Reduction Agency (Grant HDTRA1-08-1-0028) is gratefully acknowledged.

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SUMMARY In summary, a new type of spreadable, nonvolatile coating system based on a gelled RTIL containing an organic amine has been prepared and evaluated as an effective containment and decontaminating coating for variety of common materials contacted by mustard-type CWAs. When applied to CEEScontaminated samples, this new reactive, nonvolatile gel coating has been shown to act as an effective CEES vapor barrier to greatly limit the amount of CEES released in the overhead space above the samples. After the coating has been applied and removed, it was shown to be able remove most of the CEES from within macroporous samples (i.e., wood), from swellable dense samples (i.e., rubber), and from painted materials (i.e., painted steel). This coating can also be modified to include a number of color indicators that can visually signal the presence of mustard agent or when the coating is no longer effective for decontamination and may need a second coat. This new spreadable decontaminating barrier system is the first step in developing a new, easy-to-apply product that could be used by the military and first-responders to treat mustard-agentcontaminated materials and equipment in the field. Future research plans include optimization of these gelled (RTIL + 1179

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