Vapor Interface: A

Article pubs.acs.org/JPCC

Interfacial Structure of Toluene at an Ionic Liquid/Vapor Interface: A Molecular Dynamics Simulation Investigation Francois Delaunay,† Alfredo-Santiago Rodriguez-Castillo,‡ Annabelle Couvert,‡ Abdeltif Amrane,‡ Pierre-Francois Biard,‡ Anthony Szymczyk,‡ Patrice Malfreyt,¶ and Aziz Ghoufi*,† ‡

Institut des Sciences Chimiques de Rennes, CNRS, UMR 6226, Université de Rennes 1-ENSCR, 263 Avenue du Général Leclerc, 35042 Rennes, France † Institut de Physique de Rennes, IPR, UMR CNRS 6251, 263 Avenue du Général Leclerc, 35042 Rennes, France ¶ Institut de Chimie de Clermont-Ferrand, ICCF, UMR-CNRS 6296, Clermont Universit, Universit Blaise Pascal24 Avenue des Landais, 63170 Aubire, France ABSTRACT: Recently, it has been highlighted that Volatile Organic Compounds (VOCs) could be removed through the coupling of an absorption step in a solvent followed by biodegradation mainly at the liquid/liquid (solvent/water) interface. Among the solvents fulfilling the required characteristics (non toxicity and no biodegradability, high affinity for VOCs, solvent regeneration, good mass transfer, ...), octyl isoquinolium bis(trifluoromethyl)sulfonimide ionic liquid (IL), [octiq+][Tf2N−], appears especially promising. The first step of the process consists in the absorption of the VOC contained in the air to be treated by the IL as the VOC vapor contacts the IL. In this work, we report molecular dynamics simulations of {IL + toluene}/vapor and IL/toluene vapor interfaces to elucidate the physical phenomena ruling the interfacial adsorption of toluene and its absorption by the IL. We first predicted a high affinity between [octiq+][Tf2N−] and toluene, in agreement with experimental data. Moreover, we evidenced an enhancement of the interfacial toluene density, which allowed us improving the understanding of the interfacial capture and degradation of toluene.

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INTRODUCTION In the recent past, legislation on air quality in developed countries has significantly progressed, especially by imposing limitations of their emissions to relevant activities. A large market is thus booming, backed by strong dynamic research aimed at finding new methods of air treatment. Among the targeted compounds, volatile organic compounds (VOCs) are a major concern, due to both their odorous character and their toxicity, and are now subject to regulation. In this family of compounds, some are hydrophobic and can only be removed by means of specific process, such as adsorption, thermal, or catalytic oxidation, or in some cases conventional bioprocesses1−3 (biofilters, bioscrubbers, and biotrickling filters). A new technology promises to remove VOCs by absorbing them into a regenerative room temperature ionic liquid (RTIL), implemented in a bioprocess4−6 involving two successive stages: a gas−liquid absorption column, in which the liquid phase is an ionic liquid (IL), and a multiphase bioreactor involving activated sludge. The high absorption capacity of ILs for a wide number of pollutants, such as phenolic compounds, organic acids, and hydrocarbons, is well-known.7 However, to the best of our knowledge, the literature dealing with ILs potential for the absorption of hydrophobic odorous compounds and VOC biodegradation remains very scarce, showing thus the innovative nature and the novelty of this bioapproach. In 2012, Darracq et al. demonstrated the feasibility of a hybrid process coupling absorption and biodegradation to © 2015 American Chemical Society

remove hydrophobic VOCs such as toluene or dimethyldisulfide8 absorbed in the silicone oil. This process is based on a series of operations allowing one (i) to absorb the target VOCs in an organic liquid (step 1: absorption); then (ii) to regenerate the scrubbing solution by biodegradation (step 2: two-phase partitioning bioreactor (TPPB) containing the organic phase loaded with the absorbed VOCs and an aqueous phase containing the microorganisms); and finally, (iii) to separate the liquid phases in order to reuse the organic phase at the inlet of the whole process. Thus, TPPB is based on the addition of a nonaqueous liquid phase (NAP) offering a high affinity for the target pollutants to be removed9 in order to improve pollutant mass transfer from the gaseous to the liquid phase, and hence to improve the subsequent biodegradation kinetics. In most cases, pure microorganism strains, a microbial consortium, or activated sludge are implemented in this kind of reactor. As a consequence, among the only two classes of solvents fulfilling the required characteristics, namely silicone oils and ILs, these latter appear especially promising because of their attractive properties. Indeed, owing to the possibility of fine-tuning their physicochemical properties, they can be advantageously implemented as NAP in place of silicone oil5 in view of process optimization, on the one hand, and to improve the understanding of the mass transfer phenomena of the targets Received: March 3, 2015 Revised: April 16, 2015 Published: April 16, 2015 9966

DOI: 10.1021/acs.jpcc.5b02081 J. Phys. Chem. C 2015, 119, 9966−9972

Article

The Journal of Physical Chemistry C

Figure 1. Illustration of the [octiq+][Tf2N−] ionic liquid and toluene, and the protocol followed to model the {IL+toluene}/vapor system (blue, red, and cyan colors correspond to the anion, cation, and toluene, respectively).

generic nonpolarizable force fields, such as AMBER and OPLS, are able to well reproduce the interfacial properties of ILs.12 All MD simulations were carried out with the DL_POLY package,13 using a combination of the velocity-Verlet coupled with the SHAKE-RATTLE algorithms14 and the Nose−Hoover thermostat15,16 with a relaxation time of τt = 0.5 ps. Periodic boundary conditions were applied in the three directions. MD simulations were performed at T = 400 K. This temperature was selected to allow the dipolar relaxation of the ionic liquid. MD simulations were performed using a time step of 0.002 ps. The electrostatic interactions were truncated at 12 Å and calculated by using the Ewald sum with a precision of 10−6. Short range interactions were modeled by using the Lennard− Jones potential and a cutoff of 12 Å. The statistical errors for the calculated proper ties were estimated using the block average method. The first step in the modeling of the Liquid− Vapor (LV) interface consisted in modeling a liquid phase in the isobaric−isothermal (NpT) statistical ensemble at p = 1 bar with a rectangular box of dimensions Lx = Ly = 44.5 Å and Lz = 90 Å with 288 ion pairs. Five toluene concentrations were investigated, corresponding to a number of toluene molecules Nt = 5, 10, 20, 50, and 100. The different systems were differentiated according to the surface molar concentration of toluene, cS = 0.0, 2.4, 5.5, 9.3, 12.9, 20.6 μmol m−2. MD simulations of the liquid phase were carried out by means of the

VOCs between the different phases (air, aqueous, and organic) on the other hand. To achieve this goal, understanding the phenomena at liquid−vapor and liquid−liquid interfaces is essential, since that would help to optimize the absorption and biodegradation steps. That requires a molecular level understanding of the interfacial properties and then the use of molecular simulations such as Molecular Dynamics (MD) simulations, which can provide a relevant vision of the interfacial structure. Indeed, VOCs are absorbed in the ILs, while the biodegradation of the VOC by the IL is performed by anchoring bacteria at the VOC+IL/water interface. The potential of these applications cannot be fully exploited without molecular-level insight into the surface structure and properties and a rationalization of IL/VOC interactions, which constitutes the main aim of the present work. Let us note that to the best of our knowledge, no work on the IL+toluene/vapor interface has been reported yet.

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COMPUTATIONAL METHOD The IL represented in Figure 1 corresponds to the ion pair of the octyl isoquinolium [octiq + ] cation and the bis(trifluoromethyl)sulfonimide [Tf2N−] anion. This IL was described using the nonpolarizable AMBER99 force field,10 while toluene was modeled by means of the nonpolarizable OPLS force field.11 Recently, Lisal et al. have shown that 9967


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Figure 2. (a) Center-of-mass density profiles of the total [octiq+][Tf2N−] ionic liquid along the z axis as a function of the surface molar concentrations (indicated in the legend) at 400 K. (b) Liquid density of the [octiq+][Tf2N−] ionic liquid as a function of the surface concentration of toluene (cS).

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Nose−Hoover barostat15,16 using a relaxation time of τp = 1.0 ps. After 20 ns of equilibration, the LV interface was built by adding two vacuum slabs of 75 Å on both sides of the IL along the z direction (see Figure 1). From this new configuration, an equilibration of 20 ns in the canonical ensemble was carried out, leading to the formation of two interfaces, followed by a production phase of 40 ns. In Figure 1, we report the initial and final configurations of the {toluene−[octiq+][Tf2N−] mixture}/ vapor interface. The calculation of the partial charges of both the cation and the anion of the IL was carried out from DMol3.17 A combination of the Becke exchange plus the Lee− Yang−Parr correlation functional and all-electron core potentials was used. Additionally, the double-ξ numerical polarization (DNP) basis set was adopted to account for the dtype into heavier atoms and p-type polarization into hydrogens atoms. This basis is similar to the 6- 31G(d,p) Gaussian-type basis set. In the simplest electrostatic model, atom centered charges were derived to reproduce the molecular electrostatic potential (ESP charges) using the fitting procedure implemented. We chose this basis in order to be in line with the original parametrization of the AMBER force field. The partial charges correspond to the ESP charges, such that the atomic centered charges best reproduce the electrostatic potential.

RESULTS AND DISCUSSION Description of the Interfacial Region. To elucidate the interfacial structure of the {IL+toluene}/vapor system, the center-of-mass density profiles were managed along the normal to the interface, i.e., the z direction. We report in Figure 2a the center-of-mass density profiles of the [octiq+][Tf2N−] ionic liquid as a function of the toluene surface concentration (cS). The fact that the density profiles are symmetric indicates that the equilibration was well performed at each point in the simulation box. Figure 2a shows a local density enhancement at both interfaces, which decreases as a function of cS. Let us note that these interfacial layers do not propagate into the bulk phase. The absence of ordering at 400 K was also evidenced by A. Sanmartin et al.18 Additionally, we observe that the density of the IL in the bulk phase decreases as a function of cS. By averaging the density between z = −20 Å and z = 20 Å (we can approximate that this region corresponds to the bulk phase) we report in Figure 2b the density of the liquid phase as a function of the surface concentration in toluene. As displayed in Figure 2b, the density of the pure IL (cS = 0.0 μmol m−2) is about 1351 kg m−3, which corresponds to a deviation of 9% with respect to the experimental value. Moreover, Figure 2b shows a decrease in the density of the IL as a function of cS due to the increase in the amount of toluene absorbed in the liquid phase. For completeness, we report in Figure 3 the atomic density profile of atoms belonging to the cation and the anion of the IL for three toluene concentrations. Given the symmetry between both interfaces (see Figure 2a), the profiles have been reported by only one interface. The meaning of the different atom labels is given in Figure 1 (Ct and Ca atoms correspond to the alkyl chain and the benzene rings of [octiq+], respectively). Whatever the concentration, the density profiles show an outermost peak for both Ct and fluoride atoms (F) located in the vapor phase. This result is in good agreement with the work of Lisal et al.12 who found a similar atomic density profile for the 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl) imide ionic liquid in contact with hexane. Additionally, the position of the interfacial peaks seems to be independent of the toluene surface concentration. The structures of both the anions and cations of the IL close to the interface were investigated by

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DENSITY MEASUREMENT Ionic Liquids density was measured using an Anton Paar DMA 350 Ex vibrating tube densimeter (Graz, Austria), thermostated at different temperatures. The instrument’s accuracy is 10−3 g· cm−3 and 0.2 °C with a repeatability of 5.10−4 g·cm−3 and 0.1 °C. Density measurements are obtained by warming and cooling two different samples of the same ionic liquid then injected into the apparatus. Values are noted when the temperature varies 1 °C and until each sample is at room temperature. One sample was first warmed (maximum at 40 °C) then injected, the other one was cooled (minimum at 10 °C). Each sample was analyzed twice. The densimeter’s calibration was performed at atmospheric pressure using methanol (Sigma-Aldrich, ≥99.9%) and ethanol absolute (TechniSolv, ≥ 99.5%). The densities of alcohols are in satisfactory agreement with the literature values. 9968


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calculating the angular distribution of the angle between the surface normal vector and a vector characteristic of anions and cations. For anions, this vector was defined between the two sulfur atoms, while it was defined between the first and the last carbon atoms of the alkyl chain for cations. For cations and anions, the maximum probability was found around 7° and 165°, respectively. These findings suggest an orthogonal orientation with respect to the surface for both anions and cations. These results are in line with the previous atomic density profiles and the interfacial orientations of a few ionic liquids reported in the literature.12,18,19 The density profile of toluene was reported as a function of cS in Figure 4a. Two characteristic regions were observed: (i) an absorbed phase in the liquid phase; and (ii) an adsorbed phase in the interfacial boundary. Figure 4a,b clearly shows that a significant amount of toluene remains confined in the liquid phase, while only a small amount is transferred into the vapor phase. This gives evidence for the high affinity between the [octiq+][Tf2N−] IL and toluene. Unlike what was observed for the IL density, Figure 4a shows that the interfacial density of toluene increases as a function of the absorbed toluene amount. At the same time, the density of toluene in the vapor phase increases (see Figure 4b), while the evaporation of IL was not observed. Interestingly, Figure 5 shows that toluene molecules in the interfacial region are located in the vapor side of the interface (i.e., beyond the IL). Moreover, although the profile of cation density seems to be dependent on the toluene concentration, the density profile of the anion is almost independent of cS. The formation of a hydrophobic microscopic thin film at the LV interface was already evidenced in the literature.20,21 Usually, modeling of a liquid−vapor interface was performed by considering a liquid phase surrounded by two vapor phases.21,22 However, in this study, a toluene−IL mixture was considered and the evaporation process was modeled by adding two vacuum boxes on the both sides of the liquid. Thus, for the first time we show the formation of an interfacial thin layer by starting from an initial configuration with toluene embedded in the IL matrix. This behavior depicts that the interfacial enhancement in toluene density is thermodynamically favorable. To ensure this result, we further modeled the IL/toluene vapor system following the protocol

Figure 3. Atomic density profiles of [octiq+] and [Tf2N−] for thee surface concentrations between z = 0 Å and z = −60 Å at 400 K. The meaning of the different atom labels is given in Figure 1.

Figure 4. (a) Center-of-mass density profiles of the toluene as a function of cS (indicated in the legend) at 400 K. (b) Enlargement of the density profiles in the vapor phase (between z = −100 Å and z = −60 Å). 9969


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Figure 5. Center-of-mass density profiles of [octiq+], [Tf2N−], and toluene for various cS along the z direction at 400 K. Density profiles of the toluene are to be read on the right axis. cS = 0.0 (a), 2.4 (b), 5.5 (c), 9.3 (d), 12.9 (e), and 20.6 (f) μmol m−2.

described in refs 12, 20, and 21 (this will be discussed later). A similar interfacial enhancement in density of the toluene was found. This highlights that the interfacial thin film formation is independent of the initial configuration. The local increase in density close to the interface could be imputed to the most favorable toluene−toluene hydrophobic interactions. Indeed, as discussed by Chandler, this interfacial configuration allows maximizing toluene−toluene interactions.23 To check it, both toluene−toluene and IL−toluene interactions were investigated through the inspection of 2D radial distributions functions (RDF).18 We report in Figure 6 the 2D RDF between the hydrophobic parts of the IL (Ca and Ct) and the carbon atoms of the benzene rings (CA) and the methyl groups (CT) of toluene in the liquid phase and the interfacial boundary. Figure 6 shows that the interactions between the carbons atoms of the benzene rings are privileged. Indeed, in both the liquid phase and the interfacial regions, the maximum of the RDF of the CA−Ca interactions was located at 4.4 Å, while the maximum of the RDF of the CT−Ca and of the CT−Ct pairs are around 5.1 Å. Moreover, Figure 6 depicts that the interactions between methyl group of toluene (CT) are more favorable than the IL− toluene interactions. Indeed, a typical distance of 3.9 Å was found. Thus, the interfacial location of toluene allows maximizing the toluene−toluene hydrophobic interactions. Therefore, this enhancement of the interfacial density of toluene could favor the interfacial capture of this VOC. Furthermore, the absorption of the toluene into the IL is rather controlled by the hydrophobic IL−toluene interactions than the presence of microstructure in the IL leading to a porosity. Indeed, we performed an additional simulation by

Figure 6. 2D radial distribution functions (RDF) between carbon atoms of toluene (CT and CA) and carbon atoms of the ionic liquid (Ct and Ca) at 400 K. The meaning of the different atom labels is given in Figure 1. The solid and dotted lines represent the RDF in the liquid phase and in the interfacial regions, respectively. The empty symbols represent the RDF between toluene molecules. RDFs were computed for cS = 20.6 μmol m−2.

contacting the IL and a vapor phase containing water molecules, and no absorption of water into the IL was observed. This confirms that the absorption process is ruled by the favorable hydrophobic IL−toluene interactions. The energy of toluene−toluene and toluene−IL interactions in the liquid phase was found to be −0.7 kJ mol−1 and −1.1 kJ mol−1, respectively, which suggests that the toluene−IL interactions 9970


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[Tf2N−] ionic liquid. Let us note that about 30 ns are needed to saturate the IL phase. In a second stage, we estimated the capacity of toluene molecules present in the liquid phase to migrate in the interfacial region after a virtual degradation of the interfacial toluene performed by removing explicitly the interfacial toluene molecules. We report in Figure 7b the time evolution of the center-of-mass density of toluene. Figure 7b shows that the interfacial region is again enriched with toluene molecules.

are most favorable in liquid phase. Energy was calculated by considering the electrostatic and van der Waals interactions. This explains the absence of the formation of the toluene clusters into the liquid phase (a flat toluene density profile if observed in Figure 5.) Let us note that 98% of the so-calculated energy corresponds to the van der Waals interactions. From an interfacial standpoint, the toluene−toluene energy was found to be close to −1.4 kJ mol−1, which explains the toluene migration toward the interfacial boundary. Modeling of the Interfacial Filling in Toluene. To model the first stage of the process, i.e., the absorption of the VOC from the interface, we contacted a toluene vapor phase with the [octiq+][Tf2N−] ionic liquid. This simulation allowed us to well check the affinity between the toluene and the [octiq+][Tf2N−] ionic liquid. Thus, the initial configuration of molecular dynamics simulation consisted in an IL phase (without any toluene molecule inside) in contact with a toluene vapor phase. Algorithms, equilibration, and acquisition phases are similar to those previously discussed in the Computational Method section. We display in Figure 7a the time evolution of the center-of-mass density profile of toluene. As depicted in Figure 7a, the interfacial toluene density decreases over time progresses while the absorbed toluene concentration in the liquid phase increases. This result clearly highlights the high affinity of the toluene for the [octiq+]-

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CONCLUSIONS Molecular dynamics simulations of toluene absorbed in the [octiq+][Tf2N−] ionic liquid were carried out for various toluene concentrations by considering explicitly the liquid/ vapor interface. The density of the pure ionic liquid was fairly reproduced. By means of the calculation of the center-of-mass and atomic density profiles we highlighted that toluene molecules were preferentially located in the interfacial region. We showed that the interfacial location was probably due to the most favorable hydrophobic−hydrophobic interactions between toluene molecules. By modeling a virtual interfacial degradation of toluene molecules, a further migration of toluene molecules toward the interface was observed and the interfacial enrichment of toluene was recovered. Thus, by progressively desorbing the interface by, e.g., a biodegradation process, it would be possible to extract toluene from the liquid phase and then to regenerate the ionic liquid. To continue this study, we envisage to explore the transport of toluene across a water/IL interface and the explicit modeling of the interfacial degradation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: aziz.ghoufi@univ-rennes1.fr (A.G.). Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 7. (a) Time evolution of the center-of-mass density profiles of toluene along z direction for Nt = 100 at 400 K (cS = 20.6 μmol m−2). The arrows allow showing the decrease both decrease and increase in toluene amount. (b) Time evolution of the center-of-mass density profiles of toluene along z direction where the interfacial toluene was initially removed. The dotted line corresponds to the initial configuration, while the solid line corresponds to the final configuration. The horizontal arrows allow showing the migration of toluene amount in the interfacial regions. 9971


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