Surface Binding of Organophosphates on Silica: Comparing

Article pubs.acs.org/JPCC

Surface Binding of Organophosphates on Silica: Comparing Experiment and Theory DeCarlos E. Taylor,† Keith Runge,*,‡ Marshall G. Cory,§ Douglas S. Burns,§ Joseph L. Vasey,§ John D. Hearn,∥ Kara Griffith,⊥ and Michael V. Henley∥ †

US Army Research Laboratory, Weapons and Materials Research Directorate, RDRL-WML-B, Aberdeen Proving Ground, Maryland 21005, United States ‡ BWD Associates, LLC, 2901 Northwest 54th Avenue, Gainesville, Florida 32653-1819, United States § ENSCO, Inc., 4849 North Wickham Road, Melbourne, Florida 32940, United States ∥ US AFRL RXQL, 139 Barnes Drive, Suite 2, Tyndall AFB, Florida 32403-5323, United States ⊥ UTC, Inc., 139 Barnes Drive, Suite 2, Tyndall AFB, Florida 32403-5323, United States S Supporting Information *

ABSTRACT: A consistent embedding hierarchy is applied to the calculation of binding enthalpies for organophosphate molecules to a silica surface and compared to experiment. The interaction of four probe molecules, dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), diisopropyl fluorophosphate (DFP), and sarin, with the silica surface is examined. Quantum chemical methods are employed to compute binding enthalpies and vibrational spectra for all interactions between probe molecules and silanol sites on the silica surface. Comparison with experimentally measured infrared shifts indicates that the theoretically modeled adsorption sites are similar to those found in experiment. The calculated binding enthalpies agree well with experiment for sarin, ΔHads,443K = −22.0 kcal/mol (calculated) vs −18.8 ± 5.5 kcal/mol (measured, 433 K < Texpt < 453 K), and DIMP, ΔHads,463K = −26.9 kcal/mol (calculated) vs −29.3 ± 0.9 kcal/mol (measured, 453 K < Texpt < 473 K). Agreement with experiment is less good for DMMP, ΔHads,463K = −19.7 kcal/mol (calculated) vs −26.1 ± 1.5 kcal/mol (measured, 453 K < Texpt < 473 K), and DFP, ΔHads,423K = −20.4 kcal/mol (calculated) vs −27.5 ± 3.1 kcal/mol (measured, 413 K < Texpt < 433 K).

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on amorphous silica. As noted by Bermudez2 the interaction is driven by binding to silanol sites (i.e., hydroxylated silicon atoms) on the silica surface. The three general types of binding sites are isolated, geminal, and vicinal silanol sites. Geminal silanol sites are distinguished by having two hydroxyl groups on a single silicon atom, while vicinal sites have single hydroxyl groups on two silicon atoms that share an oxygen atom on the surface. The theoretical approach presented herein applies quantum chemical theory to surface binding while preserving the effects of the full system. Previous quantum chemical calculations have been carried out using clusters as representative of the silica surface. Murashov and Leszczynski3 investigated the interaction of orthosilicic acid with anionic fragments of DMMP using the B3LYP hybrid functional of density functional theory (DFT). Bermudez2 extended the DFT treatment of binding to silica with a much larger cluster study. Using the B3LYP hybrid functional, he studied interactions with DMMP, sarin, and trichlorophosphate and found that the most energetically favorable adsorption complex binds through the oxygen of the PO bond to the silanol site. Kolodziejczyk et al.4 have

INTRODUCTION Understanding the interaction of chemical warfare agents (CWAs) with environmental surfaces presents a challenging scientific problem that is not easily addressed by experimental means. Typically, simulants that mimic physical and/or chemical properties of the target CWA are employed in experiments to reduce associated hazards and costs, but they never fully simulate real environmental fate. Theoretical approaches for investigating molecular-scale phenomena of CWAs are advantageous because they eliminate CWA occupational hazards and they provide fundamental insight that is applicable to a wide range of conditions. One essential component to modeling CWA fate is accurate descriptions of binding to surfaces. This basic scientific understanding for CWA−surface interactions can also be extended to inform the development of improved remediation and protection procedures. We recently developed a consistent embedding approach to the calculation of binding enthalpies, which were compared to experimental values determined by inverse gas chromatography (IGC), with application to binding of small molecules to the silica surface.1 In this contribution, the technique is applied to binding of larger organophosphate (OP) molecules, dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), diisopropyl fluorophosphate (DFP), and sarin (GB), © 2013 American Chemical Society

Received: July 6, 2012 Revised: January 15, 2013 Published: January 23, 2013 2699

dx.doi.org/10.1021/jp306713d | J. Phys. Chem. C 2013, 117, 2699−2708

The Journal of Physical Chemistry C

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Inverse Gas Chromatography. Experimental measurements of the enthalpies of adsorption, ΔHads, for the various OP probes on silica gel were made with IGC, a well-established method for investigating physicochemical properties of heterogeneous interactions.14 Detailed descriptions of IGC experimental methods have been documented,15 so only a brief description of the relevant parameters for these specific measurements is provided here. OP compounds were diluted into a solvent and subsequently injected into a He gas flow (5− 15 sccm, standard cm3/min) using an autoinjector. He gas flow rates were maintained with either an external 100 sccm mass flow controller (MKS Ins.) or the pneumatics of the gas chromatograph (Agilent 6890N or Thermo Trace GC Ultra). In both cases, flow rates were calibrated with a soap film bubble meter. The He flow was passed through a glass tube containing silica gel held in place with plugs of silane-treated glass wool (Supelco), and the elution profile was measured with a flame ionization detector (FID), flame photometric detector (FPD), or a mass spectrometer (MS, Thermo DSQ II). In the linear part of the isotherm, the net retention volume (Vn, volume of gas needed to elute the probe molecule, and in these experiments the James−Martin correction factor was negligible14), is directly proportional to the partition coefficient (K): Vn = KA, in which A is the surface area (or mass) of the stationary phase.16 The van’t Hoff relation yields ΔHads by relating Vg (Vn normalized to the stationary phase mass) to the column temperature (Tc). Equation 1 shows the relationship between Vg and ΔHads in which R is the universal gas constant (8.31415 J/(K mol) or 1.98722 × 10−3 kcal/(K mol)) and C is a constant.

examined DFP adsorption on CaO and MgO surfaces using a cluster approach within DFT. Using temperature-programmed desorption, Henderson et al.5 measured an activation barrier for the desorption of DMMP from silica of ∼16.9 kcal/mol. Two other studies investigated IR shifts for the adsorption of DMMP on silica compared to both gas-phase DMMP and the bare silica surface.6,7 Previously, Ward et al.8 observed the decomposition of DFP and soman on a silica surface which they attributed to surface-assisted hydrolysis. In a more recent experimental study, Bermudez9 used IR spectroscopy to examine the competition between water and DMMP on silica among other surfaces. The observed IR shifts were consistent with his previous theoretical work predicting the formation of an adsorption complex through the oxygen of the PO bond. Quenneville et al.10 used the ReaxFF force field with molecular dynamics to examine the interaction of DMMP with amorphous silica surfaces obtaining a binding energy of −4.7 kcal/mol for a coverage of 4.5 silanols/nm2. Here, we review the theoretical construction of the silica surface from bulk silica and compare it to the experimental surface. Then the binding enthalpies are determined by the consistent embedding procedure and compared with measurements. Finally, we discuss these results and draw conclusions.

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METHODS Amorphous Silica Surface from Molecular Dynamics. The model hydroxylated silica surface was taken from our previous work.1 For the brevity of presentation, the generation of the surface will only be summarized in the following, and specific details can be found in the earlier paper. The silica potential of van Beest et al.,11 which has been shown to provide a good description of the crystalline and amorphous silica structure and properties, was used. To generate the amorphous sample, a 3072-atom crystalline sample was melted at high temperature and quenched to the ambient state following the annealing schedule of Huff et al.12 The atomic radial distribution for the theoretical amorphous silica slab (first and second peak positions at 1.7 and 2.7 Å, respectively) compared well with experimental measurements of silica (1.62 and 2.65 Å). Using this bulk sample, the free surface was prepared by removal of all atoms residing in the upper 6 Å layer (z-axis direction) taking care to maintain the proper stoichiometric ratio of silicon to oxygen. The free surface was then allowed to relax using molecular dynamics (MD) simulation, and defect sites on the relaxed surface were passivated with hydrogen atoms and hydroxyl groups on undercoordinated oxygen and silicon atoms, respectively. The resulting hydroxylated surface contained isolated, geminal, and vicinal silanol clusters which were then extracted from the sample and refined using gas-phase quantum mechanics at the MP2/6-31++G** level of theory. For the MP2 calculations, dangling bonds, resulting from truncation of the clusters from the surface, were passivated using the pseudoatom approach.13 Specifically, terminal oxygen atoms of the truncated clusters were replaced by monovalent fluorine atoms to which a parametrizable effective core potential (ECP) was assigned. The parameters of the pseudoatom ECP were determined in the previous work such that the electrostatic and exchange effects of neighboring SiO4 tetrahedra (present on the extended surface) were properly incorporated into the truncated clusters yielding binding enthalpies representative of those that would be obtained on the infinite surface.

ln

Vg Tc

=

ΔHabs +C RTc

(1)

IGC measurements were made for the conditions listed in Table 1. For DMMP, neat and solvent-mediated injections Table 1. IGC Experimental Conditions OP

solvent(s)

Tc [°C]

DMMP DIMP DFP GB

ethyl acetate neat ethyl acetate hexanes 2-propanol

180−200 180−200 140−160 160−180

were performed to determine potential artifacts arising from the use of solvents. Neat liquid DMMP was injected using the autoinjector, and DMMP vapor was injected by filling a sample loop (90 μL) with DMMP/He and subsequently injecting the loop contents using a six-port valve. DMMP loadings for sample loop injections were determined from the integrated FID signal by generating a calibration curve from liquid injections of DMMP/ethyl acetate on a silica column. In all experiments, Tc was run in a random order to prevent any systematic errors from affecting ΔHads, and the column was conditioned after each injection at 200 °C to drive off remaining OP mass. Molar OP/SiOH ratios were determined from the mass of OP compound injected and the silica gel mass in the column. From our previous work,1 the SiOH surface density is 2.43 nm−2, of which 41% (±11%) are H-bonded (i.e., vicinal sites). The total SiOH density is a little lower than the results of Zhuralev (4.9 nm−2).17 Total moles of SiOH in the IGC column was calculated from our measured SiOH density, the 2700



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silica gel mass, and the surface area of the silica gel (product literature). Infrared Spectroscopy. Transmission Fourier transform infrared (FTIR) spectra of DMMP, DIMP, and DFP molecules adsorbed onto silica samples were measured using a Nicolet Magna FTIR Spectrometer 750. Fumed silica was heated to 100 or 500 °C to remove physisorbed water and vicinal silanols, respectively. Pellets of fumed silica were then pressed to make an opaque-to-slightly translucent pellet that was held in place with a Teflon spacer in a glass gas cell sealed with zinc selenide windows pressed against Buna rubber o-rings. OP compounds were introduced by heating a small quantity (∼1 μL) inside a borosilicate glass tube with a small flow of Ar. The resulting OP vapor and Ar mixture were directed to the silica pellet under an Ar atmosphere and at room temperature. The gas cell was then closed and placed in the FTIR spectrometer with a N2 purge. Spectra are averages of 24 scans or more recorded at 4 cm−1 resolution. FTIR spectra for sarin were collected using a diamond ATR accessory (Apecac Golden Gate ATR MKII or A225/QI Platinum ATR) and a Thermo Nicolet Avatar 380 or Bruker Tensor 27 FTIR. Powder and/or liquid samples were placed onto the ATR surface, and typical spectral acquisition parameters were 64 or 256 scans at 4 or 1 cm−1 resolution, Mertz phase correction, and Happs−Genzel apodization. Following the liquid spectra collection, fumed silica (10−30 mg, degassed at 150 °C) was placed onto the liquid droplet until incipient wetness occurred. A spectrum was subsequently gathered, and a further liquid sample (50−100 μL) was deposited onto the incipient wet powder and a spectrum collected. Fumed silica was added again (∼10−30 mg) to the wet slurry to incipient wetness and another spectrum collected. Chemicals and Gases. All chemicals were used without further purification unless indicated otherwise: dimethyl methylphosphonate (DMMP, 97%, Aldrich), diisopropyl methylphosphonate (DIMP, 96%, Alpha Aesar), diisopropyl fluorophosphate (DFP, 98%, CALBIOCHEM), isopropylmethyl phosphonofluoridate for IGC measurements (sarin (GB), 0.159% (w/w) in 2-propanol, CASARM (ID#: SIN# 072811002, prepared from GB-U-5045-CTF, Vial-142-ZDC)), and sarin for IR measurements (95.1 ± 0.4%, non-CASRM GB-S0264-CTF-N) with the major impurity of tributylamine (4.1%). Silica gel (Davisil grades 636 and 646 with surface areas of 480 and 300 m2/g, respectively) was purchased from Sigma-Aldrich. Fumed silica (CAB-O-SIL) was purchased from Eager Plastics, Inc. Gases (He, N2, and H2, UHP grade) were purchased from Airgas. Compressed air was made and purified in house with an Aadco pure air generator model 737.

Figure 1. Experimental FTIR transmission spectrum of CAB-O-SIL fumed silica pellets with (black lines) and without (red lines) adsorbed DFP, for which the silica was heated to (A) 500 °C or (B) 100 °C prior to pressing the pellets. Insets show difference spectra with increasing loadings of DFP (black < red < green < blue; higher loadings exhibit larger Δ absorbance at 3400 cm−1).

attributed to bulk silanols that are perturbed by their interparticle environment. These bulk silanols are typically inaccessible to adsorbents. The broad transition centered near 3550 cm−1 in Figure 1B is attributed to H-bonded silanols (vicinal silanols).24 Heating the silica to 500 °C removed vicinal silanols in Figure 1A. Upon adsorption of DFP, the intensity of the isolated silanol transition decreased for both silica treatments indicating that DFP adsorbs to these silanols. These decreases are clear in the difference spectra (see Figure 1 insets) as sharp negative peaks, which exhibit a larger decrease for larger DFP loading. The broad transition centered near 3380 cm−1 in Figure 1A appears with DFP exposure and corresponds to νSiO−H H-bonded to adsorbed DFP molecules. The three other transitions that appear between 3020 and 2850 cm−1 are C−H stretches from adsorbed DFP. In Figure 1A, the assignment of the broad transition centered near 3380 cm−1 is assigned to isolated silanols H-bonded to DFP since there are no vicinal silanols. Data in Figure 1B, however, were obtained for silica possessing both isolated and vicinal silanols (geminal silanols cannot be distinguished with IR spectroscopy), and due to increased degrees of freedom and spectral broadening, H-bonding interactions between the adsorbate (DFP) and these two types of silanol sites cannot be separated from one another. From the observed decrease in the transition at 3746 cm−1, we safely conclude that DFP adsorbs to isolated silanols. In addition, the transition centered near 3380 cm−1 is much less symmetric than what was observed for DFP adsorbed to silica heated to 500 °C (see insets). This asymmetry, in which the difference peak falls off more rapidly at higher wavenumbers, is consistent with a decrease in the transition intensity of vicinal silanols, so we conclude that DFP adsorbs to vicinal silanols as well.

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RESULTS Comparison of Theoretical and Experimental Surfaces. We previously demonstrated1 the quality of the theoretical silica surface; the radial distribution function (RDF), the coordination numbers of the silicon and oxygen, the infrared spectrum, and the surface hydroxyl concentration of the theoretically generated surface compared favorably with experimentally observed and published data. FTIR Signatures. Transmission FTIR spectra of fumed silica pellets heated to 500 and 100 °C with and without adsorbed DFP are presented in Figure 1 (difference spectra for increasing DFP loadings are shown in the insets). The sharp transition at 3746 cm−1 corresponds to isolated silanols, and the weak feature centered between 3670 and 3650 cm−1 is 2701



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without the ability to separate isolated and vicinal silanols, we take this average as that for the OP−vicinal H-bonding transition energy. Silica conditioned at 500 °C provides a means of removing vicinal silanols and only investigating isolated silanols, and closer inspection of the 3380 cm−1 transition in Figure 2B shows a slight red shift at higher DFP loadings. The positions of peak maximum for DFP as well as DMMP and DIMP adsorbed on silica heated to 500 °C are shown in Figure 2C, and the red shift at higher OP loading is clear. Fractional loading was calculated from the change in intensity of the isolated silanol transition (3746 cm−1). We are interested in OP−silica interactions at zero coverage, so these apparent trends were fit to a line and extrapolated to zero to determine the transition energy. OP Vibrational Analysis. We use the vibrational analysis to confirm that the theoretical clusters are representative of those seen in experiment. The theoretical vibrational analysis results in single frequencies computed from the normal-mode analysis. If the computed frequencies are within the range of the experimental peaks, we conclude that the theoretical clusters are representative. As noted in our previous work,1 two concerns that are commonly addressed in matching IR spectra to calculated frequencies include (1) the use of scaling factors (for MP2 we use 0.9518) and (2) the role of anharmonicity. These two concerns were not included in the frequencies shown in Figure 3, however tabulated frequencies and frequency shifts are scaled. Figure 3 compares the experimental IR spectra of DMMP, DIMP, and sarin with the MP2/6-31++G** calculated IR spectrum. The gas-phase data for DMMP and DIMP were taken from NIST,19 and the spectrum for liquid GB was obtained at Edgewood Chemical and Biological Center (ECBC). Table 2 summarizes our measured and computed IR data for gas-phase DMMP and the corresponding assignments. Similar analyses were performed for the other OP probes. Adsorption IR Shifts. The adsorption of the various OP compounds causes shifts in the vibrational frequencies (IR spectra) that are indicative of the type of silanol binding site occupied (νSiO−H). To assess the quality of the truncated model, with respect to the experimental system, we compared the magnitude and direction of these shifts. Optimized structures describing the interaction of DMMP, DIMP, DFP, and sarin with each of the three silanol sites were completed at the MP2/6-31++G** level of theory using Gaussian0321 and are presented in Figure 4. The vibrational frequencies of the silanol SiO−H stretch (νSiO−H) were tabulated from experiment and from MP2/6-31++G** optimized structures for both the bare surface and the probe−surface interaction. The difference between the νSiO−H of the bare surface and νSiO−H of the probebound surface is the shift (ΔνSiO−H). Shifts in the stretching mode (ΔνSiO−H) were tabulated for comparison and analysis. MP2 calculated vibrational frequencies were scaled by 0.95 as outlined on the Computational Chemistry Comparison and Benchmark Database maintained by NIST.20 Depending on the orientation of the OP compound, in the case of the geminal silanol, there may be an interacting and a noninteracting OH with respect to the OP compound (see GB in Figure 4). For the OP compounds investigated here, the binding is found to occur primarily through the PO···H−OSi linkage. Adsorption Vibrational Analysis. Changes in the IR spectrum associated with the SiO−H stretch (νSiO−H) that

The H-bonded SiO−H transition energies for silica conditioned at 100 and 500 °C do not appear to change with increasing DFP loading (see insets to Figure 1). Figure 2A

Figure 2. Normalized difference spectra of DFP adsorbed to silica heated to (A) 100 °C and (B) 500 °C. DFP loadings from lowest to highest are black, red, blue, and green. (C) Peak position as a function of OP loading on silica heated to 500 °C. Lines are linear fits to the data and are extrapolated to 0. Fractional loading was calculated as 1 − Sads/S0, for which Sads and S0 are the absorbances at 3743 cm−1 with and without adsorbed OP, respectively.

confirms this for DFP adsorbed to silica conditioned at 100 °C, which shows the normalized difference spectra and no shift in the peak position. DMMP and DIMP adsorption to silica conditioned at 100 °C also exhibited no shift in the peak position of the H-bonded SiO−H transition (see Supporting Information). Thus, for the silica conditioned at 100 °C, we averaged all the measurements to obtain the peak position, and 2702



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Table 2. Assignment of IR Bands for Gas-Phase DMMP

a

this work

Kanan6

MP2

assignment

3003 2962 2929 2858 NRa NR NR 1315 1276 1186 1050 914 815

3006 2958 2917 2859 1471 1459 1420 1319 1275 1190 1075/1049 914 816

2973−3102 CH3 stretching

νa(CH3), ν(CH3O) νa(CH3O) νa(CH3), ν(CH3O) νs(CH3O) δa(P−CH3) δs(O−CH3) δs(P−CH3) δa(P−CH3) ν(PO) ρ(O−CH3) ν(O−CH3) ρ(P−CH3) ν(P−O−(C))

1315−1476 CH3 bending

1198 1158 1022−1043 C−O stretch 792 673

Not resolved.

molecules were systematically higher than those measured, and correspondingly, the magnitudes of the theoretical ΔνSiO−Hs were consistently less than measured shifts. The agreement improved for the larger OP compounds, and for DFP, the theoretical ΔνSiO−H fell within the broad transition of that measured. The theoretically predicted νSiO−H for vicinal silanols and vicinal silanols with adsorbed OP molecules were higher than those measured. However, ΔνSiO−H for vicinal silanols showed very good agreement between theory and experiment. In no case did the theoretically predicted ΔνSiO−H systematically err relative to those measured, and all but two predicted values (+160 and +164 cm−1 for SiOH−DIMP) fell within the broad measured transitions. We conclude that the surface−adsorbate clusters are similar, though not identical, to the surface adsorption configurations encountered in the experimental system. Sarin−silica FTIR spectra were collected with an ATR accessory instead of in a transmission setup. Unfortunately, silica spectra did not exhibit the characteristic SiO−H transitions, so sarin’s perturbation on these vibrational transitions could not be resolved. While the theoretical predictions for adsorbed sarin are shown in Table 3, we rely on the enthalpy measurements and predictions in the following section to evaluate the theory. Comparison of Heats of Sorption and Binding Enthalpies. Quantum chemical calculations were performed using Gaussian0321 and GAMESS22 which are ab initio quantum chemistry software packages that solve the timeindependent Schrödinger equation with both correlated-wave function and density functional techniques. We have used the codes in parallel on up to 256 processors, with good efficiency, on the IBM P6 and Linux clusters, for the calculation of energies, analytic gradients, and both analytic and numerical Hessians. As in our previous work,1 calculation of minimum energy configurations and vibrational frequencies was done using the MP2/6-31++G** level of theory. The geometry of each adsorbate−silanol cluster was optimized with fixed pseudoatoms to mimic the constrained silica framework of the bulk system. The binding enthalpy of each adsorbate was computed as

Figure 3. Theoretical stick spectra (along the bottom axis) computed at the MP2 level of theory overlaying measured spectra of gas-phase DMMP and DIMP19 and liquid sarin (this work).

result from the interaction of each OP adsorbate with the surface were investigated as was the PO stretch, and Table 3 summarizes the experimental and theoretical results. Consistent with studying H-bonded vibrations, the features of the FTIR spectrum in Figure 1 are broad, with the exception of the sharp transition at 3746 cm −1 . Importantly, theoretical and experimental νisolated for SiO−H agree very well (only a 4 cm−1 difference). The isolated SiO−H vibration was the sharpest experimentally measured transition, so it provides the best peak to which theory can be accurately evaluated. All other transition frequencies in Table 3 arise from H-bonded SiOH and are therefore experimentally broad. The theoretically predicted νSiO−Hs for isolated silanols with adsorbed OP

H = Eelec + Evib + Etrans + Erot + PV 2703

(2)



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Figure 4. MP2/6-31++G** optimized structures of DMMP, DIMP, DFP, and GB interacting with the isolated, vicinal (H-bonding), and geminal silanol sites. Atoms are hydrogen (white), carbon (blue), oxygen (red), phosphorus (violet), silicon (yellow), fluorine (green), and fluorine with a parametrized ECP (black).

Table 3. Comparison of Vibrational Transitions (νSiO−H and ΔνSiO−H) for Silanol−Adsorbate Systemsa silica system −1

ν [cm ]

experiment theory (MP2/6-31G**)

Δν [cm−1]

isolated vicinal

geminal a

isolated vicinal isolated vicinal geminal experiment theory experiment theory

SiOH

SiOH−DMMP

SiOH−DIMP

SiOH−DFP

SiOH−GB

3746 3530 3742 3658/3730 3732/3736

3320 3325 3696 3436/3492 3461b/ 3569c −426 −46 −205 −222/−238 −294/−166 −271/−167 −275/−163

3269 3286 3609 3473/3606 3454/3896 −477 −133 −244 −185/−124 −257/−52 −278/160 −282/164

3393 3383 3505 3502/3538 3480/3748 −353 −238 −147 −156/−192 −228/−120 −252/12 −256/16

not resolved

theory

3323 3477/3513 3482/3695 −419 −181/−217 −253/−145 −250/−41 −254/−37

Transitions for silanol−adsorbate complexes are not distinguishable. bInteraction with P−O−C. cInteraction with PO.

in which H is the enthalpy; Eelec is the MP2 electronic energy including the zero-point energy, but neglecting basis set superposition error;23 and Evib, Etrans, and Erot are the

vibrational, translational, and rotational enthalpy contributions. The pseudoatoms have fixed positions during optimization and are assigned an infinite mass to remove their contribution to 2704



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slow removal of surface silanols during the measurements at the high column temperatures (180−200 °C) required for desorbing DMMP, which has been documented elsewhere.24 This systematic drift also affected ΔH measurements during which the column temperature was varied in order. To circumvent this, column temperature was varied in a random order, producing consistent and reproducible results. Figure 5B shows the effect of column loading and injection method on ΔH for DMMP−silica. Closed symbols represent injections in which neat DMMP was used (i.e., there was no solvent coinjected with DMMP), and there is no systematic difference between these and the solvent-mediated injections. This indicates that solvent-mediated injections do not affect the observed ΔH measurement. We remind the reader that the sample loop injection data were placed on the x axis using the DMMP signal, and the broad width of the elution profiles can cause significant error with this placement. However, even if the sample loop data are off by a factor of 2 on the x axis, the conclusion that the solvent does not interfere with the measurement would still be valid. As an interlaboratory comparison, the open circle in Figure 4B shows the result from the ECBC experimental setup (used for acquiring GB results), and this measurement agrees well with those obtained using our setup. Finally, in Figure 5B the magnitude of ΔH decreased at higher column loadings, which is expected because higher loadings increase the frequency of DMMP−DMMP interactions on the surface and ΔHvap for DMMP is −11 kcal/mol at 180 °C.25 At DMMP molar loadings less than 0.2% of the total SiOH in the column, ΔH is constant, so we conclude this represents the linear part of the isotherm. ΔH results for DIMP adsorption on silica gel are also shown in Figure 4B for which the measurements are constant below DIMP/SiOH = 0.0015. DMMP and DIMP are inert on the hydrated silica surface under the conditions of these experiments (i.e., they do not react with surface silanols), but DFP and GB are much more reactive, both possessing a hydrolyzable P−F bond. To ensure that measured elution profiles were not degradation products, a mass spectrometer was used for detection of DFP (mass spectrometer detection was unavailable for GB experiments). The two primary ion fragments from electron impact ionization of DFP are m/z = 101 and 127, and their elution profile is shown in Figure 6 for two different column temperatures. Only slight asymmetry was observed in their elution, suggesting the

the mass-weighted Hessian when vibrational analysis is carried out. For comparison of theoretically derived values of binding enthalpy to experimental measurement, we assume that isolated, geminal, and vicinal silanol sites occur in roughly equal proportions. It is further assumed that there are no other defect types, pores, or strained regions, which contribute appreciably to the binding enthalpy. The selected silanol sites are taken to be representative of all such sites of each type, which appears to be justified by previous work with small molecules.1 Finally, assuming that the specific heat at constant pressure is independent of the silanol site, eq 3 is used to determine the ensemble-averaged enthalpies. ⟨⟨H ⟩⟩ =

∑i pH e−Hi / kT i i ∑i e−Hi / kT

(3)

In eq 3, the summation is over silanol site types, while pi is the population of each type, which is assumed to be equal for the system studies. The temperature used is chosen to be the midpoint of the experimental temperature range. ΔH for DMMP adsorption on silica was investigated as a function of column loading and injection method, and Figure 5 shows the results. The duplicate runs in panel A show good linear fits (R2 > 0.98) to eq 1, but the second run is systematically lower than the first. This drift was attributed to

Figure 5. (A) Dependence of retention volume on column temperature for DMMP on silica gel. Lines are linear fits to the data according to eq 1, from which the slopes are used to determine ΔH. (B) ΔH measurements as a function of column loading for the indicated injection method (DMMP) and DIMP/ethyl acetate. Error bars represent the uncertainty on the slope of the linear fit.

Figure 6. Elution profiles for DFP fragment ions at column temperatures of 150 (black) and 140 (gray) °C. Inset shows mass spectrum at the peak of the DFP elution profile. 2705



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strongest. DIMP binds most strongly followed by sarin and then DFP and DMMP. For the other two binding sites, Table 4 shows that DMMP, DFP, and sarin bind more strongly to geminal sites than isolated silanol sites. Qualitatively, these trends are intuitive since isolated sites offer only a single H atom for H-bonding, whereas the OP compounds can interact with two OH groups on geminal and vicinal sites. It is instructive to note that the additional binding enthalpy observed for vicinal sites compared to isolated ones is about 4−7 kcal/mol which is consistent with an additional H-bonding interaction. The strength of the isolated binding enthalpy, however, indicates that there is an interaction with the surface that is more complex than a single H-bonding interaction which is captured by the consistent embedding framework. However, experimental results (based on average interaction) cannot corroborate these theoretical results. The experimental binding enthalpy for DMMP adsorbed on silica was measured in the temperature range 453−473 K, temperatures higher than those used in our previous work. This is indicative of the fact that the binding of DMMP to the silica surface is stronger than the small molecules previously studied. The ensemble-averaged theoretical binding enthalpy, determined at 463 K (the midpoint of the experimental range), is −19.7 kcal/mol, which correlates to a binding energy of 16.0 kcal/mol, in good agreement with the previously measured binding energy of 16.9 kcal/mol.5 The enthalpy of adsorption measured with IGC is −26.1 ± 1.5 kcal/mol, which is much larger than the calculated binding enthalpy. Comparison to the previous measurement of DMMP binding to silica is difficult because monolayer and multilayer coverages of DMMP were used. The highest coverage employed in these measurements was DMMP/SiOH (mol/mol) = 0.04 (much less than a monolayer). At the higher coverage, the adsorption enthalpy of DMMP on silica approached −21 kcal/mol (see Figure 5), which correlates to a binding energy of −17.3 kcal/molin good agreement with the binding energy reported by Henderson et al.5 Hence, the origin of the disagreement between experimentally and theoretically determined binding enthalpies cannot be resolved simply by reference to previous work. In contrast to DMMP, the measured binding enthalpy for DIMP shows much better agreement with the ensembleaveraged value. The experimental binding enthalpy, −29.3 ± 0.9 kcal/mol, is in good agreement with the theoretical value of −26.9 kcal/mol, though not within the experimental error bars, which represent the standard deviation of the measurements and do not account for any systematic errors. The column temperature (as with DMMP) was high, suggesting that the removal of vicinal silanols and the generation of two-membered rings and/or undercoordinated oxygen atoms, which starts at temperatures higher than ≈420 K, is not a likely explanation for the discrepancy with DMMP. This is further corroborated by the comparison for DFP for which experimental measurements were performed at much lower column temperatures (413−433 K). If high column temperatures caused a systematic error in the experimental measurements, we would expect better agreement with theory for DFP than those OP adsorption measurements at higher column temperatures; however, the theoretical and experimental results show worse agreement. In this case, DFP shows a larger disagreement between experiment and theory where experiment finds a much larger binding enthalpy of −27.5 ± 3.5 kcal/mol compared to the ensemble averaged value of −20.4 kcal/mol.

measurements were made near the linear part of the isotherm, and this is corroborated by the molar DFP/SiOH ratio (0.005). The inset to Figure 5 shows the mass spectrum at the peak of the DFP elution with no observable degradation products coeluting with DFP (other than residual hexane). Further, no mass spectral evidence was found to indicate that degradation products eluted at other times. We concluded from these observations that under the conditions and time scale of these experiments DFP interacted with the silica surface through reversible mechanisms, and because GB is chemically similar to DFP, we extrapolate this conclusion to GB. ΔH measurements for DFP and GB are shown in Figure 7 as a function of column loading. Only a narrow range of DFP was

Figure 7. ΔH measurements for DFP and GB as a function of column loading. Hexanes and 2-propanol were used as diluting solvents for DFP and GB, respectively. Error bars represent the uncertainty on the slope of the linear fit of the data to eq 1.

investigated, but based on the comment above and the DMMP results, these measurements were made at or near the linear part of the isotherm. GB measurements exhibit considerable scatter, but no systematic trend was observed over the injection loadings investigated. In addition, these measurements were made at even lower column loadings than the other OP compounds in this study. Average results for adsorption measurements are disclosed in Table 4. Table 4 summarizes the theoretical binding enthalpies for the four OP adsorbates on vicinal, isolated, and geminal silanol sites. For all adsorbates, OP binding to vicinal sites is the Table 4. Summary of Binding Enthalpies (kcal/mol) from Theory and Experiment system isolated silanol vicinal geminal ensemble average temperature [K] experimentala OP/SiOH (mol/mol) exptl temp range [K]

DMMP

DIMP

DFP

sarin

−15.7 −20.0 −18.8 −19.7

−19.8 −26.9 −15.2 −26.9

−16.5 −20.5 −18.3 −20.4

−17.5 −22.1 −18.8 −22.0

463

463

423

443

−26.1 (±1.5)

Surface Binding of Organophosphates on Silica: Comparing

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