Influence of Quaternary Amine Organosilane Structure on the

Bio Resource Development, George Mason University, Fairfax, Virginia 22030. Received June 18, 2001. In Final Form: July 31, 2001. A series of quaterna...
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Langmuir 2001, 17, 7085-7092

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Influence of Quaternary Amine Organosilane Structure on the Formation and Adsorption Properties of Surface-Imprinted Silicates Michael A. Markowitz,*,† Gang Deng,‡ Mark C. Burleigh,† Eva M. Wong,† and Bruce P. Gaber† Laboratory for Molecular Interfacial Interactions, Code 6930, Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, D.C. 20375, and Center for Bio Resource Development, George Mason University, Fairfax, Virginia 22030 Received June 18, 2001. In Final Form: July 31, 2001 A series of quaternary ammonium functionalized silicates surface imprinted with the soman hydrolysis product pinacolyl methylphosphonate (PMP) were formed in a one-pot procedure. The formation and adsorption properties of these materials were dependent on the alkyl chain length of the quaternary amine. Brunauer-Emmett-Teller analysis and transmission electron microscopy revealed that silicates formed with added quaternary amine functionalized organosilanes with 10 and 14 carbon hydrocarbon segments (N,N-didecyl-N-methyl-N-(3-trimethoxysilylpropyl)ammonium chloride (DDMAC) and N-tetradecyl-N,N-dimethyl-N-(3-trimethoxysilylpropyl)ammonium chloride (TDDMAC), respectively) formed xerogels with surface areas of 630-712 m2/g and broad pore size distributions. The silicate that formed in the presence of a quaternary ammonium functionalized organosilane with an 18 carbon alkyl chain (N-octadecyl-N,N-dimethyl-N-(3-trimethoxysilylpropyl)ammonium chloride (ODDMAC)) formed a microporous silicate with a lower surface area. Addition of PMP during synthesis did not affect the formation or adsorption properties of the DDMAC or TDDMAC silicates but resulted in an ODDMAC silicate with a higher surface area. Significant differences in the adsorption capacities and equilibrium binding constants of ODDMAC silicates formed with and without PMP were observed. This material demonstrated selectivity for small organophosphonate monoesters over a bulkier organophosphonate monoester, diester, and methyl parathion.

Introduction There is significant interest in developing materials that effectively adsorb and/or degrade organophosphorus compounds because of their use as nerve agents or pesticides.1-17 The phosphotriesterase organophosphorus hydrolase is capable of hydrolyzing a variety of phosphorus * Corresponding author tel, 202-404-6072; fax, 202-767-9594; e-mail, [email protected]. † Naval Research Laboratory. ‡ George Mason University. (1) Yang, Y. C. Acc. Chem. Res. 1999, 32, 109. (2) Yoshida, M.; Uezu, K.; Goto, M.; Furusaki, S. Macromolecules 1999, 32, 1237. (3) Ember, L. Chem. Eng. News 1997, (Sep 15), 26. (4) Kolakowski, J. E.; DeFrank, J. J.; Harvey, S. P.; Szafraniec, L. L.; Beaudry, W. T.; Lai, K.; Wild, J. R. Biocatal. Biotransform. 1997, 17, 297. (5) Yamada, K.; Takahashi, Y.; Yamamura, H.; Araki, S.; Saito, K.; Kawai, M. Chem. Commun. 2000, 1315. (6) Nieuwenhuizen, M. S.; Harteveld, J. L. Sens. Actuators, B 1997, 40, 167. (7) Trettnak, W.; Reininger, F.; Zinterl, E.; Wolfbeis, O. S. Sens. Actuators, B 1993, 11, 87. (8) Cogan, E. B.; Birrell, G. B.; Griffith, O. H. Anal. Biochem. 1999, 271, 29. (9) Rippeth, J. J.; Gibson, T. D.; Hart, J. P.; Hartley, I. C.; Nelson, G. Analyst (Cambridge, U.K.) 1997, 122, 1425. (10) Kanan, S. M.; Tripp, C. P. Langmuir 2001, 17, 2213. (11) Bertilsson, L.; Potje-Kamloth, K.; Liess, H. D.; Liedberg, B. Langmuir 1999, 15, 1128. (12) Koper, O.; Lucas, E.; Klabunde, K. J. J. Appl. Toxicol. 1999, 19, S59. (13) Lucas, E. M.; Klabunde, K. J. Nanostruct. Mater. 1999, 12, 179. (14) Jenkins, A. L.; Yin, R.; Jensen, J. L. Analyst (Cambridge, U.K.) 2001, 126, 798-802. (15) Jenkins, A. L.; Uy, O. M.; Murray, G. M. Anal. Chem. 1999, 71, 373-378. (16) Sasaki, D. Y.; Alam, T. M. Chem. Mater. 2000, 12, 1400. (17) Markowitz, M. A.; Deng, G.; Gaber, B. P. Langmuir 2000, 16, 6148.

ester bonds, and its efficacy in detoxifying pesticides and chemical warfare agents has been demonstrated.4,18,19 Nonenzymatic methods of cleaving phosphoester bonds include hydrolysis of phosphodiester bonds catalyzed by a peptide-sheet‚Zn complex5 and metal oxide-catalyzed decomposition of adsorbed organophosphates.12-14 Various methods of adsorbing and detecting organophosphorus compounds in the environment have also been investigated.6-11,14-17 Because of the capability of forming stable, robust materials with molecular selectivity for a wide variety of compounds, molecular imprinting has become an attractive method for the formation of sensor components and selective catalysts.14,15,20-28 In molecular imprinting, the target molecule is used as the template for the formation of the molecular recognition site and is combined with metal oxide or polymer precursors, some of which can form complementary binding interactions with the imprint (18) Dumas, D. P.; Durst, H. D.; Landis, W. G.; Raushel, F. M.; Wild, J. R. Arch. Biochem. Biophys. 1990, 277, 155. (19) Rastogi, V. K.; DeFrank, J. J.; Cheng, T.; Wild, J. R. Biochem. Biophys. Res. Commun. 1997, 241, 294. (20) Lahav, M.; Kharitonov, A. B.; Katz, O.; Kunitake, T.; Willner, I. Anal. Chem. 2001, 73, 720. (21) Makote, R.; Collinson, M. M. Chem. Mater. 1998, 10, 2440. (22) Piletsky, S. A.; Piletskaya, E. V.; Panasyuk, T. L.; El’skaya, A. V.; Levi, R.; Karube, I.; Wulff, G. Macromolecules 1998, 31, 2137. (23) Turkewitsch, P.; Wandelt, B.; Darling, G. D.; Powell, W. S. Anal. Chem. 1998, 70, 2025. (24) Schweitz, L.; Andersson, L. I.; Nilsson, S. Anal. Chem. 1997, 69, 1179. (25) Markowitz, M. A.; Kust, P. R.; Deng, G.; Schoen, P. E.; Dordick, J. S.; Clark, D. S.; Gaber, B. P. Langmuir 2000, 16, 1759. (26) Markowitz, M. A.; Kust, P. R.; Klaehn, J.; Deng, G.; Gaber, B. P. Anal. Chim. Acta 2001, 435, 177. (27) Katz, A.; Davis, M. E. Nature 2000, 403, 286. (28) Wulff, G.; Gross, T.; Schonfeld, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 1962.

10.1021/la010904d CCC: $20.00 © 2001 American Chemical Society Published on Web 10/02/2001

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the longest hydrocarbon plus nitrogen segment of N-octadecyl-N,N-dimethyl-N-(3-trimethoxysilylpropyl)ammonium chloride (ODDMAC) is 22 atoms long, this chain would likely be in a coiled conformation. The influence of these differences in alkyl chain properties on the formation and adsorption properties of the surface-imprinted and nonimprinted silicates formed with these added organosilanes was examined. Particle morphology and surface area were examined using transmission electron microscopy (TEM) and nitrogen gas adsorption-desorption isotherms. Adsorption capacities and equilibrium adsorption constants for PMP association with the silicates were calculated from adsorption isotherms. Figure 1. Structures of quaternary amine functionalized organosilanes.

molecule. This process creates molecular recognition sites that are specific for the target molecule’s chemical functionality and spatial orientation. Molecular imprinting techniques have been used to form polymers and metal oxides that selectively adsorb organophosphonates.14-17 To have adsorption/active sites that are easily accessed by the organophosphonate compounds, surface-imprinted silicates functionalized with a variety of amines have been synthesized and characterized.16,17 We are interested in developing surface-imprinted silicates as sensor components. In addition, identifying those elements of imprinting and materials synthesis that impart the largest differential in adsorption capacity and selectivity between imprinted and nonimprinted silicates is an important goal in this effort. Recently, it was demonstrated that imprinted materials formed with quaternary ammonium-, dihydroimidazole-, and pyridine-terminated organosilanes selectively adsorb pinacolyl methylphosphonate (PMP), the hydrolysis product of the nerve agent soman. The dihydroimidazole and pyridine functionalized silicates provide the largest differential in adsorption capacity between imprinted and nonimprinted materials.17 However, the surface-imprinted silicate formed with the quaternary ammonium terminated organosilane had the largest overall adsorption capacity. Consequently, we decided to investigate the effects of quaternary amine structure on the adsorption properties of surface-imprinted silicates in an attempt to optimize the adsorption capacity of the imprinted material relative to the nonimprinted material. Silicate surfaces were imprinted during particle formation by adding a mixture of PMP and an organosilane to a water-in-oil microemulsion formed from polyoxyethylene (5) nonylphenyl ether (NP-5), cyclohexane, ammoniated ethanol, and water. Imprinted and nonimprinted silicates were formed with the quaternary amine functionalized organosilanes shown in Figure 1. These organosilanes were chosen because of the increasing chain length of their hydrocarbon segments. Beyond a certain length (∼20 carbons), alkyl chains are believed to collapse upon themselves and assume a coiled rather than extended conformation.29 The longest segment of each of the organosilanes will be the total of the propyl, nitrogen, and longest alkyl chain components. For N,N-didecyl-Nmethyl-N-(3-trimethoxysilylpropyl)ammonium chloride (DDMAC) and N-tetradecyl-N,N-dimethyl-N-(3-trimethoxysilylpropyl)ammonium chloride (TDDMAC), the longest segments are 14 and 18 atoms long and would therefore be expected to be in an extended conformation. Because (29) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley: New York, 1989; p 108.

Experimental Section Materials. Unless otherwise noted, all reagent grade chemicals were used as received, and water was deionized to 18 MΩ cm. NP-5 (IgepalCO-520), ethanol, 2-propanol (anhydrous), glacial acetic acid, L-ascorbic acid, ammonium heptamolybdate tetrahydrate, magnesium nitrate hexahydrate, sulfuric acid, hydrochloric acid, tetraethyl orthosilicate (TEOS), and PMP were obtained from Aldrich Chemical Co. (Milwaukee, WI). TDDMAC, DDMAC, and ODDMAC were purchased from Gelest Inc. (Tullytown, PA). A saturated solution of ammonia in ethanol was prepared by passing ammonia gas into denatured ethanol at 20 °C for 5-6 h. UV/visible spectroscopy was performed with a Beckman DU-650 spectrophotometer. Silica Particle Synthesis. Silica particles were prepared by stirring a mixture of 37 mL of saturated ammonia solution, 25 mL of ethanol, 0.5 g of IgepalCO-520, 13.5 mL of cyclohexane, and 1.44 mL of water for 30 min at room temperature and then adding 3.6 mL of TEOS. Imprinted silica particles were prepared by mixing the functionalized silanes and the imprinting molecule PMP in ethanol solution before the addition of TEOS. The mixtures were then stirred overnight at room temperature. The resulting dispersions were separated by centrifugation and then washed sequentially with the following solvents: 20 wt % water in ethanol (5×, 12 mL), acetic acid/ethanol/water (3/3/4, v/v/v) (5×, 12 mL), 20 wt % water in ethanol (5×, 12 mL), and ethanol (5×, 12 mL). The particles were then dried over vacuum for over 10 h at room temperature. With this method, we obtained 1.00 g of silica particles. Particle morphologies were examined using an Hitachi H-8100 transmission electron microscope operating at 200 kV. Samples were prepared by dispersing the particles in ethanol, ultrasonicating the particles for 15 min, and then placing a drop of the suspension onto a carbon-coated copper grid for 30 s. The excess liquid was removed by wicking, and the specimens were then allowed to dry by evaporation in air. Elemental analysis (EA) of each silicate was conducted by Oneida Research Services. Nitrogen Gas Adsorption-Desorption Isotherms. Gas adsorption experiments were performed using a Micromeritics ASAP 2010 instrument.30 Nitrogen gas was used as the analysis adsorptive, and the isotherms were measured at cryogenic temperature (77.4 K). Brunauer-Emmett-Teller (BET) surface areas were calculated from the linear part of the adsorption isotherms (i.e., from P/Po ) 0.06 to 0.2) according to IUPAC recommendations.31 Adsorption Isotherm Measurements. One milliliter of PMP in anhydrous 2-propanol (0.1-20 mM) was added to a microcentrifuge tube containing 15 mg of silica particles. The mixture was shaken vigorously on a vortex mixer for 60 min. After the mixture was centrifuged, an aliquot of the clear supernatant (5-700 µL) was taken and transferred into a glass test tube to determine the phosphorus content. Phosphorus analysis of supernatants taken as functions of vortex times and substrate concentrations confirmed that adsorption equilibrium was established within 20 min. The phosphorus analysis experiments were carried out as follows: 100 µL of 10 wt % Mg(NO3)2 was added into test tubes (30) Webb, P. A.; Orr, C. Analytical Methods in Fine Particle Technology; Micromeritics Instrument Corporation: Norcross, GA, 1997, Chapter 3. (31) Sing, K. S. W.; Evertt, D. H.; Haul, R. A. W.; Moscou, L.; Pierrotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603.

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Table 1. Incorporation of Quaternary Amine Groups into Silicates

a

silicatea

added organosilane (mol %)

wt % C

wt % H

wt % N

organosilane incorporation (mol %)

plain silicate plain silicate-PMP imprinteda TDDMAC TDDMAC-PMP imprinteda DDMAC DDMAC-PMP imprinteda ODDMAC ODDMAC-PMP imprinteda

0 0 4.75 4.75 4.75 4.75 4.75 4.75

3.00 2.64 14.95 13.40 18.94 18.70 15.48 16.85

1.25 0.90 3.26 2.03 3.74 3.70 3.53 3.55

0.00 0.00 0.72 0.74 0.87 0.77 0.78 0.75

0.00 0.00 3.81 3.79 4.88 4.30 4.17 4.08

Silicates surface imprinted with 20 mol % (total silica) of PMP.

containing the supernatant. The test tubes were heated thoroughly over a propane torch to give a white residue. To this residue was added 0.5 mL of 0.1 N hydrochloric acid, and the tubes were shaken vigorously on a vortex mixer and then heated in a boiling water bath (100 °C) for 30 min. Next, 1.7 mL of colorimetric reagent containing one part of 10 wt % of l-ascorbic acid in water and six parts of 0.5 wt % of ammonium heptamolybdate tetrahydrate in 1.0 N sulfuric acid was added. The test tubes were incubated at 45 °C for 30 min and cooled to room temperature, and the absorbance was measured at 820 nm. The phosphorus content was calculated based on the absorbance at 820 nm using a calibration curve determined with standard KH2PO4 solutions. Each data point was calculated based on the average of three trials.

Results and Discussion Influence of Quaternary Amine Organosilane Structure on Silicate Physical Properties. EA of each of the imprinted and nonimprinted materials was used for quantitative determination of the quaternary amine functional groups in each silicate (Table 1). With the exception of the DDMAC silicate, all products contained less than the 4.75 mol % of initially added organosilane. This result is consistent with previous reports for incorporation of functional groups into mesoporous silica using base-catalyzed co-condensation of TEOS with trialkoxy organosilanes and has been attributed to the different rates of hydrolysis for the matrix precursor and functionalized silanes.32 The N/Si molar ratio calculated from the EA was similar for each organosilane used. The N,Ndidecyl-N-methyl-N-propylammonium group was incorporated more efficiently than the other two ammonium groups. The presence of PMP in the condensation mixture did not have a major effect on the amount of incorporated ammonium ion. Nitrogen gas sorption isotherms and corresponding pore size distributions (insets) of imprinted and nonimprinted silicates formed in the presence of added DDMAC, TDDMAC, and ODDMAC are shown in Figures 2-4, respectively. The N,N-didecyl-N-methyl-N-propylammonium functionalized silicates (DDMAC) exhibited type II isotherms characteristic of silica xerogels with broad pore size distributions (Figure 2). The desorption branch of the DDMAC silicate formed in the presence of PMP showed a broader hysteresis than that observed for the nonimprinted silicate. This is indicative of bottlenecked pore openings and relatively more disordered pore structures in the imprinted DDMAC silicate. The N-tetradecyl-N,Ndimethyl-N-propylammonium functionalized silicates (TDDMAC) both exhibited type II isotherms with similar broad pore size distributions (Figure 3). In contrast to the DDMAC and TDDMAC silicates, the N-octadecyl-N,Ndimethyl-N-propylammonium functionalized silicates (ODDMAC) exhibited type I isotherms with large de(32) Fowler, C. E.; Burkett, S. L.; Mann, S. Chem. Commun. 1997, 1769.

Figure 2. Nitrogen gas sorption isotherm and pore size distribution of (a) nonimprinted DDMAC and (b) PMPimprinted DDMAC.

sorption hysteresis characteristic of materials that are primarily microporous with bottlenecked pore openings (Figure 4). The pore size distributions of these materials indicate that the pore diameter maxima are 25 Å or less. BET surface areas and pore volumes calculated from the nitrogen gas sorption isotherms are presented in Table 2. The DDMAC and TDDMAC silicates have similar surface areas and large pore volumes and the presence or absence of PMP during silicate formation did not have a major effect on the formation properties of these materials. In stark contrast, strikingly lower surface areas and pore volumes were obtained for the silicates formed with added ODDMAC. In addition, formation in the presence of PMP did significantly affect the physical properties of the

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Figure 3. Nitrogen gas sorption isotherm and pore size distribution of (a) nonimprinted TDDMAC and (b) PMPimprinted TDDMAC.

Figure 4. Nitrogen gas sorption isotherm and pore size distribution of (a) nonimprinted ODDMAC and (b) PMPimprinted ODDMAC.

resulting material, producing a silicate with 80% larger surface area and 73% greater pore volume. The morphologies of the silicates formed with added TDDMAC and ODDMAC in the presence of PMP were examined using TEM. TEM micrographs of the TDDMAC silicate (Figure 5a) and ODDMAC silicate (Figure 5b) reveal the major effect that the increase of four methylene units had on the materials formed using ODDMAC. The absence of PMP during silicate synthesis did not affect the morphology of the TDDMAC or ODDMAC silicates observed by TEM. The TDDMAC silicate morphology is characteristic of xerogels formed by the fusion of nanoparticles that arise from a gel network of collapsed polymer clusters formed during the initial stages of base-catalyzed alkoxysilane condensation. The morphology of the ODDMAC silicate is more characteristic of higher density particles formed as a result of the collapse of the gel network. The morphology of the TDDMAC silicate shown in Figure 5a is conducive to the formation of textural porosity.33 The fusion of small nanoparticles results in the formation of pores between them. These pores are much larger than the internal pores, and this explains the macroporosity (Figure 3a) and large total pore volumes associated with the TDDMAC silicate relative to the ODDMAC silicate. The TEM micrograph of the ODDMAC

Table 2. Surface Area and Porosity Data for Imprinted and Nonimprinted Silicates

(33) Zhang, W.; Pauly, T. R.; Pinnavaia, T. J. Chem. Mater. 1997, 9, 2491.

silicatea TDDMAC TDDMAC-PMP imprintedb DDMAC DDMAC-PMP imprintedb ODDMAC ODDMAC-PMP imprintedb

BET surface adsorption total pore area (m2 g-1) volume (cm3 g-1) 712.27 658.10 633.03 637.14 135.29 243.18

1.035 0.973 1.225 0.985 0.071 0.123

a Silicates synthesized with 4.75 mol % (total silica) added organosilane. b Silicates surface imprinted with 20 mol % (total silica) of PMP.

silicate (Figure 5b) also shows that particles are fused together, but the interparticle spaces are too large and open ended to result in dinitrogen condensation at 77 K, hence the lower total pore volumes of the ODDMAC materials. Because the surface chemistry of the initially formed nanoparticles controls the eventual morphology and corresponding porosity of the resulting silicates,34,35 the differences in the observed physical properties between the TDDMAC and the ODDMAC materials must involve differences in the structure of the added organosilanes. (34) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (35) Brinker, C.; Scherer, G. W. Sol-Gel Science; Academic Publishers: San Diego, CA, 1990.

Influence of Quaternary Amine Organosilane Structure

Figure 5. Transmission electron micrographs (200 kV) of (a) PMP-imprinted TDDMAC and (b) PMP-imprinted ODDMAC.

Examples of the effects of ammonium surfactant chain length on mesophase formation and the subsequent morphology of mesoporous silicates have been reported.36 In that report, quaternary ammonium surfactants with chain lengths g20 produced substantially lower curvature phases than quaternary ammonium surfactants with chain lengths less than 20. The explanation given for the effect of chain length on phase morphology was that the longer surfactant chains coiled thereby reducing their effective chain length.29 This led to an increase in the interface surface bending energy of the aggregated chains resulting in the formation of lamellar rather than hexagonal phases. Therefore, a possible explanation for the differences in formation properties of the ODDMAC and TDDMAC silicates may similarly be found in the increased chain length of the ODDMAC quaternary amine. Adding the propyl and octadecyl segments of this quaternary amine gives a chain comprised of 21 carbons. As just discussed, surfactants with chains >20 are believed to adopt coiled rather than straight chain conformations. Thus, the effective chain length and hydrocarbon exposure of the ODDMAC ammonium group at the nanoparticle surface would be significantly lower than that of the TDDMAC ammonium group. Consequently, silicate nano(36) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (37) Scherer, G. W. In Better Ceramics Through Chemistry III; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; Materials Research Society: Pittsburgh, PA, 1988; pp 179-186.

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particle surfaces with extended tetradecyl hydrocarbon chains would have lower surface energy than nanoparticle surfaces with coiled octadecyl hydrocarbon chains. Lowering the surface energy of the nanoparticles within the gel network reduces the rate of gel collapse and allows particle cross-linking to occur.37 On the basis of this analysis, the added TDDMAC would lower the nanoparticle surface area sufficiently to allow xerogel formation but the added ODDMAC would behave as a shorter chain hydrocarbon on the nanoparticle surface with corresponding higher surface energy leading to faster collapse of the gel network and higher density particles. The addition of PMP during ODDMAC silicate formation resulted in a material with significantly larger surface area and porosity as compared to the ODDMAC silicate formed without PMP. It is not clear why PMP had this effect. It should be noted that the silicates formed with and without PMP both gave type I nitrogen gas sorption isotherms and that the silicates had similar morphologies. The binding of PMP to the ammonium ion may result in a partial uncoiling of the octadecyl hydrocarbon chain thereby lowering the relative nanoparticle surface area, resulting in a silicate with higher surface area and porosity. Influence of Organosilane Structure and Imprinting on the Adsorption Properties of SurfaceImprinted Quaternary Ammonium Functionalized Silicates. Having established the effect of hydrocarbon chain length of the organosilanes on silicate surface area and porosity, we wanted to examine whether the possible differences in hydrocarbon chain conformation arising from the differences in chain length played a significant role in PMP adsorption by the imprinted and nonimprinted quaternary ammonium functionalized silicates. The influence of quaternary ammonium ion structure on the adsorption of PMP in 2-propanol was investigated using adsorption isotherms. A previous study of the effects of a variety of added organosilanes on PMP adsorption by silica particles has shown that adsorption equilibria were reached within 20 min.17 Therefore, PMP was equilibrated with each silicate used in this study for 60 min prior to centrifugation of the adsorption mixture and phosphate analysis of the supernatant. Isotherms for PMP adsorption to the imprinted and nonimprinted ODDMAC, TDDMAC, and DDMAC silicates as functions of silicate mass and surface area are shown in Figures 6-8, respectively. Isotherms of the nonimprinted silicates were fitted to the single-component Langmuir equation, As ) ΑmaxKc/(1 + Kc), where As is the concentration of adsorbed PMP, Amax is the maximum adsorption density, c is the equilibrium concentration of PMP in solution, and K is the equilibrium constant for the adsorption of PMP to the silicate. Isotherms of the surface-imprinted silicates were fitted with the single- and double-component (As ) Amax1K1c/(1 + K1c) + Amax2K2c/(1 + K2c)) Langmuir equations with the single-component equation giving the best fit. The isotherms were analyzed as functions of the amount of particles used in each experiment and particle surface areas (Table 3). For the DDMAC and TDDMAC silicates, the addition of PMP during silicate formation did not have a significant impact on either the total number of adsorption sites formed or the equilibrium adsorption constant. In contrast, a major difference in both adsorption capacity and equilibrium adsorption constant of imprinted and nonimprinted ODDMAC silicates was observed. The number of adsorption sites formed as a function of silicate mass and the equilibrium adsorption constant of the ODDMAC silicate formed with added PMP were similar to that for the corresponding TDDMAC silicate. However,

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Figure 6. Langmuir adsorption isotherms of PMP-imprinted (R2 ) 0.9812) and nonimprinted DDMAC silicate.

Figure 7. Langmuir adsorption isotherms of PMP-imprinted (R2 ) 0.9945) and nonimprinted TDDMAC silicate.

both of these values calculated for the ODDMAC silicate prepared without added PMP were significantly lower than those obtained for the corresponding TDDMAC material. Indeed, at low surface coverage the difference between the number of sites formed in the silicates prepared with and without added PMP is nearly equivalent to the total number of sites formed in the material prepared with added PMP. Furthermore, the equilibrium adsorption constant of the silicate prepared without PMP is similar in magnitude to that calculated from the isotherm for the adsorption of PMP to nonfunctionalized silica particles, suggesting that the PMP is not coming into contact with the quaternary amine at all (Table 3).17 As with the differences in surface area and porosity between the TDDMAC and the ODDMAC silicates, the explanation for the observed differences in adsorption behavior may result from differences in the hydrocarbon chain conformations of the two quaternary ammonium groups. For the ODDMAC silicate prepared without added PMP, coiling of the octadecyl hydrocarbon segment of the N-octadecyl-N,N-dimethyl-N-propyl quaternary amine could cause it to fold back upon the amine thereby blocking access of the PMP to the ammonium ion. Complementary electrostatic interaction of PMP with the ammonium ion during silicate formation may uncoil the octadecyl chain sufficiently to allow ready access of PMP to the adsorption site of the resulting material. Analysis of the adsorption isotherms as a function of silicate surface area provides a clear indication of the effects of adding PMP during silicate formation on the PMP adsorption capacity of the ODDMAC silicate. By

examining the data in this manner, the effects of differences in surface area between ODDMAC silicates prepared with and without added PMP are normalized and the intrinsic effect on adsorption of adding PMP during silicate synthesis may be determined. The increased adsorption capacity for the silicate formed with added PMP indicates that more PMP adsorption sites were formed in the surface of this material. The addition of PMP during synthesis did not significantly affect incorporation of the quaternary amines into the silicate. Therefore, it seems likely that the additional adsorption sites resulted from the interaction between PMP and the quaternary amine during particle synthesis. As discussed in the section on silicate physical properties, the complementary electrostatic interaction of PMP with the ammonium ion may have resulted in a change in the organosilane alkyl chain conformation during silicate formation. Consequently, the quaternary ammonium ions at the silicate surface became more accessible to the PMP. This is reflected in the higher equilibrium adsorption constant that is characteristic of adsorption constants observed for other quaternary ammonium functionalized silica surfaces. Selectivity of Surface-Imprinted ODDMAC Silicate. Throughout the previous discussion, we have referred to the silicates formed with added PMP as being PMP imprinted. This inference was based on our previous results with PMP-imprinted silicates formed with different organosilanes that demonstrated selectivity for PMP over other organophosphonates. To determine the effects of ODDMAC on imprinting, the affinity of different organophosphonates for the PMP surface-imprinted ODDMAC

Influence of Quaternary Amine Organosilane Structure

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Figure 9. Structures of organophosphorus compounds.

Figure 8. Langmuir adsorption isotherms of PMP-imprinted (R2 ) 0.9836) and nonimprinted ODDMAC silicate. Table 3. Maximum Adsorption Densities, Amax, and Equilibrium Adsorption Constants, K, of PMP Adsorption onto Imprinted and Nonimprinted Silicates silicatea

Amax (µmol/g)

Amax (µmol/m2)

K (mM-1)

TDDMAC TDDMAC-PMP imprintedb DDMAC DDMAC-PMP imprintedb ODDMAC ODDMAC-PMP imprintedb plain nonimprintedc plain imprintedb,c

301.56 ( 42.20 312.60 ( 33.43

0.42 ( 0.06 0.40 ( 0.05

8.37 ( 0.25 9.35 ( 0.70

323.81 ( 41.72 390.82 ( 49.09

0.52 ( 0.07 0.61 ( 0.08

5.27 ( 0.74 3.29 ( 0.47

87.80 ( 6.58 243.31 ( 30.01

0.65 ( 0.05 1.00 ( 0.06

0.71 ( 0.06 11.62 ( 1.51

4.17 ( 0.23 6.33 ( 0.68

0.27 ( 0.02 1.05 ( 0.11

0.18 ( 0.02 0.39 ( 0.09

a Silicates synthesized with 4.75 mol % (total silica) added organosilane. b Silicates surface imprinted with 20 mol % (total silica) of PMP. c No organosilane added during particle formation.

silicate was examined. Specifically, the adsorption of ethyl methylphosphonate (EMP), the sarin hydrolysis product isopropyl methylphosphonate (IPMP), PMP, 2,6-dinitro4-trifluoromethylphenyl butylphosphonate (BDNTFPP), diethyl methylphosphonate (DEMP), and methyl parathion (MPTH) adsorbed from separate 2-propanol solutions onto nonimprinted surfaces and surfaces imprinted with PMP was determined. The structures of these compounds are shown in Figure 9. The amounts (wt %) of each organophosphonate adsorbed from solution are given in Table 4. The structurally similar EMP, IPMP, and PMP compounds were adsorbed by both the imprinted and the nonimprinted silicates to the same extent. The imprinted sites did not demonstrate enhanced affinity for

the bulkier BDNTFPP or the organophosphonate diester DEMP. Some additional affinity of MPTH for the imprinted silicate was observed. In this case, the increase in affinity of this compound for the imprinted silicate over that observed for the nonimprinted silicate is quite small. The results obtained for the adsorption of DEMP and MPTH by the ODDMAC silicates are consistent with results obtained previously with a surface-imprinted silicate formed using added PMP and N,N,N-trimethylN-(3-trimethoxysilylpropyl) ammonium chloride.17 The low levels of DEMP and MPTH can be attributed to the lack of electrostatic interaction between these compounds and the ammonium ions of the ODDMAC silicates. Despite the apparent selectivity of the ODDMAC silicate formed with added PMP for the smaller organophosphonates over the bulkier BDNTFPP, it is not clear that this preference is due solely to imprinting. The uncertainty arises from the lower level of adsorption of BDNTFPP by the nonimprinted ODDMAC silicate relative to that observed for adsorption of the smaller organophosphonates. This lower level of BDNTFPP adsorption by the nonimprinted silicate indicates that there may be adverse steric interactions between the BDNTFPP and the alkyl chain of ODDMAC that limit access of BDNTFPP to the surface quaternary ammonium ions. Similarly, the lack of enhanced affinity of BNTFPP for the ODDMAC silicate formed with added PMP might be attributable to the blockage of the adsorption sites by the octadecyl chain of the incorporated organosilane. On the other hand, it should be noted that addition of PMP during ODDMAC silicate formation significantly increased both the PMP adsorption capacity and the K value obtained for this silicate and suggested that the alkyl chain did not block access of PMP to the surface ammonium ions. If access of BNTFPP to the surface ammonium ions by the octadecyl chain is not blocked, then the lack of enhanced affinity of the imprinted ODDMAC silicate would be attributed to the inability of BNTFPP to fit into the molecular recognition site formed during silicate preparation. Thus, the observed selectivity of the imprinted ODDMAC silicate for smaller organophosphonates over BNTFPP appears to be due to a combination of imprinting and steric factors that might block effective access of BDNTFPP to the surface ammonium ions.

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Markowitz et al.

Table 4. Percentage of Organophosphorus Compounds Adsorbed by N-Octadecyl-N,N-dimethyl-N-propyl Ammonium Ion Functionalized Silicatesa,b

c

organophosphonate

amount organophosphonate adsorbed by nonimprinted silicate (wt %)

amount organophosphonate adsorbed by surface-imprinted silicatec (wt %)

EMP IPMP PMP BDNTFPP DEMP MPTH

20.37 ( 0.34 13.48 ( 0.52 19.37 ( 2.70 3.75 ( 3.11 1.61 ( 1.37 0.95 ( 2.56

34.50 ( 1.01 34.41 ( 1.31 38.10 ( 1.80 7.17 ( 1.67 1.17 ( 0.16 4.72 ( 0.63

a Percentages calculated as a function of silicate surface area. b Silicates synthesized with 4.75 mol % (total silica) added organosilane. Silicates surface imprinted with 20 mol % (total silica) of PMP.

Conclusions The influence of organosilane structure on the formation and adsorption properties of the silicates formed with and without added PMP was examined. The differences in adsorption capacity and equilibrium adsorption constant observed between the ammonium ion functionalized ODDMAC silicates prepared with and without PMP are significantly larger than those previously observed for amine functionalized surface-imprinted silicates. Analysis of nitrogen gas sorption isotherms and transmission electron micrographs of each silicate revealed that silicates formed using added TDDMAC and DDMAC formed xerogels with higher surface areas and pore volumes than silicates formed with added ODDMAC. These differences were attributed to the possible coiling of the octadecyl hydrocarbon segment of ODDMAC, resulting in precursor gel network nanoparticles with higher surface energy, leading to faster collapse of the network and the formation of denser silicates. Similarly, the larger differential between the adsorption properties of the ODDMAC

silicates prepared with and without added PMP was thought to arise from the blocking of PMP access to the ammonium ion in the nonimprinted material by the coiled octadecyl chain. The ODDMAC silicate formed with added PMP displayed equivalent affinity for the similarly structured EMP, IPMP, and PMP organophosphonates, but the surface-imprinted sites did not recognize the bulkier organophosphonate monoester, the organophosphonate diester, or MPTH. Acknowledgment. M.C.B. and E.M.W. are NRC/NRL Postdoctoral Research Associates. We thank the Joint Service CB Defense Technology Base Program for providing funding for this research. Additional funding was provided by the Office of Naval Research through a Naval Research Laboratory Accelerated Research Initiative. LA010904D