Theoretical Study of Amino Acid-Based Ionic Liquids Interacting with

Article pubs.acs.org/JPCC

Theoretical Study of Amino Acid-Based Ionic Liquids Interacting with Carbon Nanosystems Cesar Herrera,† Rafael Alcalde,† Gregorio García,† Mert Atilhan,‡ and Santiago Aparicio*,† †

Department of Chemistry, University of Burgos, 09001 Burgos, Spain Department of Chemical Engineering, Qatar University, P.O. Box 2713, Doha, Qatar

‡

S Supporting Information *

ABSTRACT: The properties of 1-ethyl-3-methylimidazolium glycinate ionic liquid regarding fullerenes, graphene, and single-walled carbon nanotubes are studied using classical molecular dynamics simulations. Endohedral fullerenes forming C60 to C540 containing a variable number of confined ions are studied, and the solvation of these systems by bulk liquid phases is also studied. The adsorption of the ionic liquid on top of graphene sheets and the confinement between two sheets are also analyzed as a function of intersheet separation. Likewise, confinement inside single-walled nanotubes as a function of nanotube diameter is analyzed together with ionic mobility in comparison with bulk phases. External solvation, densification, and layering around the nanotubes are also considered. The properties of these systems involving amino acid-based ionic liquids are compared with available studies involving classical imidazolium ionic liquids with other types of ions.

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INTRODUCTION Interest in the behavior of ionic liquids, ILs, regarding carbon nanosystems (fullerenes, graphene, and carbon nanotubes) has increased these last few years, leading to a number of relevant studies. The properties of these mixed materials together with the possibility of their design from the large number of ILs are among their main advantages. The characteristics of fullerenes, graphene, and nanotubes are very different, and thus the interaction of these nanomaterials with ILs has been studied for many technological purposes. The behavior of fullerene + IL was first studied by considering the problem of fullerenes’ low solubility in most of the traditional solvents, and thus the possible solubilization of fullerenes by ILs was explored. Liu et al.1 reported initial studies on C60 solubility in imidazolium-based ionic liquids containing anions such as [BF4]−, [PF6]− or [Tf2N]−, and although these results did not show high solubilities, they confirmed that fullerenes are not chemically altered upon dissolution in IL and the relationship between solubility and IL polarity. Further studies2 showed that although it was possible to obtain a certain solubilization of fullerenes in ionic liquids, especially through then introduction of long alkyl chains in the ions, the solubility values were 1 or 2 orders of magnitude lower than in molecular solvents such as methylcyclohexane and toluene. The suitability and mechanisms of fullerene solubilization by ILs were recently analyzed in detail using theoretical approaches. Maciel and Fileti3 analyzed the solvation structure of two ionic liquids with different polarities around C60, showing the weakened interionic interactions upon C60 © XXXX American Chemical Society

solubilization and the role of aliphatic domains in fullerene solvation. Chaban et al.4 showed that although almost null solubilit ies are obt ained for C 6 0 in [1-eth yl-3methylimidazolium][BF4] IL at close to ambient temperature, the solubility increases dramatically with increasing temperature, leading to values larger than 66 g L−1 for temperatures above 333 K. These authors also showed mutual polarization between the dissolved fullerene and the IL, which can be used to obtain well-dispersed solutions of fullerenes. Moreover, Fileti and Chaban5 also showed that ILs may be used to improve fullerene solubility in water, where the IL surrounds the fullerene and allows a proper dispersion in water-rich media. Most of the available studies on fullerene + IL considered traditional ILs such as those based on imidazolium cations and fluorinated anions, and the number of studied ILs was very limited. Considering these limitations, our research group reported systematic studies in which the solvation of fullerenes by a large collection of ILs was analyzed using density functional theory (DFT) and molecular dynamics (MD) approaches showing how the IL−fullerene interaction may be fine-tuned through the selection of suitable ions.6−8 Another remarkable application involving fullerenes and ILs is the possible encapsulation of these ions inside the fullerene cavity, leading to endohedral fullerenes with IL. Although the experimental development of IL-based endohedral fullerenes Received: October 20, 2015 Revised: November 11, 2015

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DOI: 10.1021/acs.jpcc.5b10269 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C has not been reported, a recent theoretical study reported by our group confirmed their suitability.9 The nanosystems formed by the interaction of graphene sheets and ILs have led to a large number of studies. From a theoretical viewpoint, the development of highly dense layers of IL when adsorbed on graphene and the particular orientation of ions at the interface have been proven,10−16 which has been confirmed experimentally.17−20 Additional studies have been reported on the behavior of an IL confined between graphene sheets,13,21−24 which showed how the properties of the confined IL can be fine-tuned through the separation of the graphene sheets, thus evolving from IL properties similar to those in bulk phases to almost crystal-like ones. Moreover, graphene + IL systems have been proposed in technological applications such as supercapacitors,25,26 graphene exfoliation,27 heat-transfer fluids,28 electricity generation,29 CO2 capture,30 and solar energy applications,31 and thus studies on the behavior of new types of ILs with respect to graphene deserve further attention. The study of the solvation of carbon nanotubes, NTs, by ILs, especially for single-walled ones (SWNTs), and the properties of ILs confined inside these NTs has led to a remarkable number of studies. This interest arises from the discovery of the possibility of dispersing SWNTs by ILs by Fukushima et al.,32,33 leading to the so-called bucky gels or ionanofluids,34,35 with applications in fields such as thermal storage 36 and lubrication.37,38 The solvation of CN by IL has been studied by theoretical13,16,39−42 and experimental approaches43−46 showing the densification and arrangement of ions around the NT, leading to drastically different behavior in comparison to that of bulk fluids. The behavior of ILs in confined geometries is remarkably different from that in bulk phases,47 and thus studies on the behavior of an IL confined inside an NT has shown the development of crystal-like phases with high melting points,48−50 with ionic arrangements for confined ILs leading to highly ordered structures being strongly dependent on temperature, NT diameter, or the type of involved ions.13,42,51,52 These changes in IL structuring upon NT confinement may lead to some favorable properties such as increasing mobility for very viscous fluids,53,54 although this is also dependent on the type of IL considered.42 The analysis of the literature regarding the available studies for IL−carbon nanosystems shows that most of the studies involved ILs belonging to the so-called second-generation ILs,55,56 with most of the involved imidazolium-based cations combined with halogenated anions, which have shown problems from environmental, toxicological, technical, and economic viewpoints.57−61 The need to develop and use ILs based on environmentally friendly and low-cost ions has been reported,62−66 but the use of these new ILs for applications involving carbon nanosystems has been very scarce.13,42,67 The development of ILs using ions based on natural sources is a very attractive option for both environmental and economic reasons, and thus ILs based on amino acid ions, AAILs, have suitable properties.68−71 Egorova et al.72 reported a certain cytotoxicity of some AAILs that was lower than for ILs involving classic anions such as BF4− and PF6− considered in most of the IL−carbon nanosystem studies. The literature contains several studies reporting physicochemical properties AAILs73−76 or theoretical characterization,77−85 but studies regarding their behavior with carbon nanosystems are absent. Therefore, 1-ethyl-3-methylimidazolium glycinate [EMIM][GLY] IL (Figure 1) was selected in this work as a model

Figure 1. Molecular structure of ions forming the ionic liquids studied in this work. Atom labeling used in this work is also reported.

AAIL to characterize the behavior of AAIL−carbon nanosystems (Figure 1). The properties of [EMIM][GLY] with fullerenes (C60, C180, C240, and C540), graphene, and singlewalled nanotubes were studied using classical molecular dynamics simulations. The main objective of the work is to carry out a nanoscopic characterization of the solvation of carbon nanostructures by AAIL, the adsorption of AAIL on carbon nanostructure surfaces, and AAIL confinement by carbon nanosystems. This study allows us to extend the available literature information on the interaction between classical IL and carbon nanosystems with AAIL, with the possibility of fine-tuning the properties of IL−carbon nanosystems through the selection of suitable ions.

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METHODS Force field parametrization for [EMIM][GLY] is reported in Table S1 (Supporting Information). Fullerenes (C60, C180, C240, and C540), graphene, and single-walled nanotubes were described according to the parametrizations reported in a previous work.13 The interaction of ionic liquids with fullerene was previously characterized for other types of ionic liquids, showing charge transfer due to the ionic liquid−fullerene interaction; therefore, polarization effects were inferred and force fields involving atomic charges considering the charge transfer were used for molecular dynamics simulations.4,6,7 Nevertheless, these force field parametrizations involving polarization effects do not lead to remarkable structural changes. Therefore, because the analysis of structuring is the main objective of the present work, nonpolarizable force fields were used for all simulations. Likewise, nonpolarizable force fields have been applied successfully in the literature for the simulation of ionic liquid−carbon nanosystems.8,12,50 Atomic charges for ionic liquids used in this study led to a difference in total ionic charges of ±1, and they were obtained from density functional theory (DFT) calculations of ion pairs in a previous work.78 These previous DFT calculations for ion pairs showed relevant charge transfer between the involved ions in the studied ionic liquids. Therefore, considering that the largest contribution to anion−cation interactions is of the Coulombic type, the use of these atomic charges leads to a suitable description of the properties of the studied ionic liquids. The force field parametrization for the studied ionic liquids was validated against experimental density data reported in the literature,73,75 with percentage deviations between experimental and calculated density data being in the ±1% range. Parameterization reported by Liu et al.80 led to 2% deviations in density, and thus the atomic charges considered in the force field parametrization reported in this work lead to an improvement in the prediction of physicochemical properties. Simulations for [EMIM][GLY] + fullerenes were carried out in two stages. In the first stage, the properties of [EMIM][GLY] encapsulated in C60, C180, C240, and C540 fullerenes B


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The Journal of Physical Chemistry C

Figure 2. Structure of endohedral fullerenes containing one [EMIM][GLY] pair in vacuum at 303 K.

in vacuum were studied through NVT simulations at 303 K. The number of ions encapsulated by each fullerene was varied from 1 to the maximum number of ions that may be fit inside the fullerene cavity, and the properties of each system were analyzed as a function of the number of encapsulated ions. In the second stage, the solvation of empty fullerenes (C60, C180, C240, and C540 with no IL inside the fullerenes) or filled fullerenes (encapsulating the maximum number of ions, inferred from the previous stage) by [EMIM][GLY] was studied. For these studies of fullerene solvation by [EMIM][GLY], each fullerene was solvated by 200 ion pairs, and NPT simulations at 303 K and 1 bar were carried out. All of the simulations involving fullerenes were carried out with an equilibration step, with the equilibration condition inferred through the constancy of total potential energy, followed by 10 ns production runs. Simulations for graphene + [EMIM][GLY] were carried out using two different approaches. In the first type of study, the adsorption of IL on a graphene surface was analyzed by placing it in contact with 200 ion pairs on top of a fixed graphene layer (with the z coordinate being perpendicular to the interface). In the second study, 200 ion pairs were confined between two fixed graphene sheets. The distance between both sheets was changed, and the properties of the confined IL were analyzed as a function of graphene sheet separation. All of these simulations involving graphene were carried out in the NVT ensemble at 303 K using an equilibration step followed by 10 ns production runs. The analysis of the properties of [EMIM][GLY] + SWNT was carried out by considering SWNT(15,15) and SWNT(10,10) nanotubes, both in the armchair configuration, hydrogen-terminated, 30 Å long, and with their long axes placed on the z axis. The study involving carbon nanotubes was carried out in two stages. In the first stage, the filling of empty nanotubes was analyzed, and for this purpose a cubic box of [EMIM][GLY] containing 500 ion pairs was previously equilibrated in the NPT ensemble at 303 K and 1 bar for 10 ns. In this equilibrated [EMIM][GLY] box, ions from a central cylindrical region, with a long axis of 35 Å and a short axis equal to that of the corresponding nanotube plus 5 Å, were removed. The corresponding empty carbon nanotubes were placed in this central region with the nanotube long axis orientated along the z axis. Therefore, once these systems composed of empty nanotubes surrounded by equilibrated [EMIM][GLY] were built, simulations in the NPT ensemble at 1 bar and temperatures in the 303 to 403 K range were carried out to analyze the filling rates as a function of temperature and nanotube diameter. In the second stage, the properties of filled nanotubes solvated by [EMIM][GLY] were analyzed through

NPT simulations at 303 K and 1 bar using 10 ns simulations, with particular attention paid to the behavior of ions confined inside the nanotubes and the behavior of ions in the nanotubes’ outer solvation shells. All MD results reported in this work were carried out using an MDynaMix v.5.2 molecular modeling package.86 Simulations were carried out using the Ewald method,87 with a cutoff radius of 15 Å, for the treatment of Coulombic interactions. Lorentz− Berthelot mixing rules were applied for Lennard-Jones crossterm contributions. Equations of motion were solved with the Tuckerman−Berne double-time-step algorithm88 for 1 and 0.1 fs long and short time steps, respectively. Temperature and pressure in the corresponding ensembles were controlled using the Nosé−Hoover method. Some authors have shown problems arising from the possible nonergodicity of the Nosé−Hoover thermostat,89 which may cast doubt on the suitability of this method for modeling ionic liquids as done in this work. Nevertheless, the Nosé−Hoover method has been applied successfully for a large number of simulations involving the study of bulk ionic liquids90,91 and ionic liquid interfaces as done in the present work.92−95

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RESULTS AND DISCUSSION Fullerenes. In the first stage of the study the properties of C60, C180, C240, and C540 fullerenes encapsulating a variable number of [EMIM][GLY] ions were studied in vacuum. In the case of C60, it was not possible to encapsulate one [EMIM][GLY] pair because of the fullerene cavity size (42.9 Å3 for C60 inner volume),96 which is remarkably lower than the calculated Connolly volume (189.7 Å3) for the ion pair. Therefore, for C60 only it was possible to develop an endohedral fullerene containing a single [GLY]− anion ([GLY]@C60), but this system is characterized by an interaction energy between C60 and [GLY]− of 737.1 kJ mol−1, which shows the poor stability of the studied system due to steric effects. This is confirmed by the fact that the [GLY]−− fullerene interaction energies for fullerenes encapsulating a single anion (no cation) are −129.6, −114.3, and −83.4 kJ mol−1 in C180, C240, and C540. Literature studies on endohedral fullerenes containing ionic liquids are almost absent in the literature, and only a previous work reported by our group on the properties of choline benzoate encapsulated in C540 is available.9 Hence, the effect of the fullerene size on the properties of the encapsulated ionic liquid was first analyzed for systems containing a single ion pair (Figure 2). These results show the trend of [EMIM]+ ions staying parallel to the fullerene walls. The strength of anion−cation interaction energies (E+‑ in Figure 2) increases (in absolute value) with increasing fullerene size, thus showing that different ionic C


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The Journal of Physical Chemistry C arrangements are obtained for the studied fullerenes. Moreover, the confinement also changes the molecular structures of the involved ions, e.g., the dihedral angles formed by C3−N1− N2−C4 in [EMIM]+ are 0.5, 13, and 17° upon encapsulation in C180, C240, and C540, respectively, whereas this angle is 3.3° for the free ion pair in the gas phase. Regarding the strength of ion−fullerene interactions, [EMIM]+−fullerene interaction energies are larger than [GLY]−−fullerene interaction energies for all of the studied fullerenes because these interactions are mainly of the Lennard-Jones type, which increase with the number of interacting atoms, and [EMIM]+ cations are closer to the fullerene walls than are [GLY]− anions. Nevertheless, the strength of ion−fullerene interactions decreases with increasing fullerene size both for anions and cations. Therefore, with increasing cavity size the strength of ion−fullerene interactions is weakened to improve anion−cation interactions, which are remarkably larger. Likewise, the results in Figure 3 show that

Figure 4. Total ion−ion intermolecular interaction energy, Eion−ion, obtained by summing Lennard-Jones and Coulombic contributions for anion−anion, cation−cation, and anion−cation for ions confined inside C540 at 303 K in vacuum as a function of the number of confined [EMIM][GLY] ion pairs, n.

trend, especially in the n = 1−5 region, which follow a completely different trend from that of n > 5. Regarding the strength of ion−fullerene interactions, the results in Figure 5

Figure 3. Number of fullerene carbon atoms, N, surrounding the center of mass of [EMIM]+ for endohedral fullerenes reported in Figure 2, as obtained from the integration of the corresponding radial distribution functions at 303 K in vacuum.

the closest distances between [EMIM]+ ions and the fullerene walls are 3.6, 3.8, and 4.1 Å for C180, C240, and C540, respectively, but the number of fullerene carbon atoms around the cation follows C540 < C240 < C180 for distances larger than the closest contact, and thus ion−fullerene interactions are weakened with increasing cavity size. It should be remarked that [GLY]−−fullerene interaction energies are lower for those systems containing one [EMIM][GLY] ion pair than when only the [GLY]− anion is encaged, thus showing that the prevailing role of anion−cation interactions over ion−fullerene ones moves anions farther away from the fullerene wall, leading to weaker anion−fullerene interactions. Once the properties of endohedral fullerenes containing a single ion pair were studied, the properties of systems encaging different numbers of ions were studied. The maximum numbers of [EMIM][GLY] ion pairs that can be fit inside the fullerene cavities are 2, 3, and 10 for C180, C240, and C540, respectively. Nevertheless, the properties of the endohedral fullerenes were analyzed in those systems containing a single ion pair to the maximum number of ion pairs inside each cavity. Results for the evolution of the total ion−ion interaction energy (anion− cation plus anion−anion plus cation−cation, Eion−ion) as a function of the number of encapsulated ion pairs, n, are reported in Figure 4. It should be remarked that for n = 1 only the anion−cation interaction is present. Eion−ion increases (in absolute value) with increasing n but follows a clearly nonlinear

Figure 5. Normalized intermolecular interaction energy, E/n, between ions and carbon atoms in fullerenes for ions confined inside C540 at 303 K in vacuum as a function of the number of confined [EMIM][GLY] ion pairs, n.

show that the [EMIM]+ cation leads to a stronger interaction with the C540 fullerene than does [GLY]−. This effect is also obtained for C180 and C240 for all of the studied n values. Moreover, ion−fullerene interactions decrease (in absolute value) on average up to n = 5, and then they remain almost constant. The evolution of interaction energies with increasing n is justified by the behavior of radial distribution functions reported in Figure 6. These results show that for up to n = 5 both anions and cations develop an internal solvation shell close to the fullerene wall, and thus all ions are close to the fullerene wall. For n ≥ 5, a second layer of ions is developed close to the fullerene center, placed in slightly outer regions close to the layer in the vicinity of the fullerene wall for [GLY]− (Figure 6a) and very well defined in the case of [EMIM]+ for which both layers are well defined (Figure 6b). Once the layer close to the fullerene wall is fully developed, n = 5, additional ions are placed in the region closer to the fullerene center of mass, and thus the strength of ion−fullerene interactions does not change remarkably (Figure 5) because the layer close to the walls suffers minor changes upon filling the inner fullerene D


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Figure 6. Radial distribution functions, g(r), between the center of mass of C540 fullerene and those for [EMIM]+ and [GLY]− for endohedral fullerenes filled with n ion pairs in vacuum at 303 K. Vertical dashed lines show the position of C540 fullerene carbon atoms.

regions. Moreover, for n > 5, the filling of internal regions leads to behavior closer to that of the liquid bulk phase, whereas for n < 5 a single layer close to the fullerene wall is formed, whose behavior is very different from that of the liquidlike phase. For n > 5, the ion−ion interaction energy (Figure 4) increases (in absolute value) with increasing n. In the previous paragraphs, the behavior of endohedral fullerenes filled with [EMIM][GLY] was analyzed with these systems placed in vacuum. In the following paragraph, the behavior of these fullerenes solvated by liquid [EMIM][GLY] will be analyzed. Results in Figure 7 shows radial distribution functions for the solvation of endohedral fullerenes containing the maximum number of ions they can fit inside their cavities. Regarding the behavior of confined ions, their properties do not change remarkably upon solvation, as can be inferred from the comparison of Figures 6 and 7d for C540. The structure of the fullerene external solvation shells is closely dependent on the fullerene size. As a rule, the results in Figure 7 show peaks corresponding to [EMIM]+ close to the external fullerene walls whose intensities are larger than those corresponding to [GLY]−, and thus the first external solvation of fullerenes is richer in cations than in anions, in parallel to the behavior for internal encaged ions. This effect is particularly relevant for C60 fullerene. The appearance of further peaks in radial distribution functions confirms the development of additional external solvation shells beyond the first one close to the fullerene walls. The orientation of ions in the external and internal solvation shells of the C540 endohedral fullerene may be inferred from the radial distribution functions reported in Figure 8. Regarding ions placed inside the C540 cavity, the first shell close to the fullerene walls is characterized by [EMIM]+ cations and [GLY]− anions lying parallel to the fullerene surface, for the second shell placed close to the fullerene center of mass [EMIM]+ is also almost parallel to the fullerene internal surface, and there is a region in which oxygen atoms in [GLY]− are closer to the fullerene center of mass. For the first external solvation shell, although both ions are slightly skewed, the results in Figure 8 confirm the trend of staying parallel to both the external and internal fullerene walls. These results are in

Figure 7. Radial distribution functions, g(r), between the center of mass of the corresponding fullerene and those for [EMIM]+ and [GLY]− for endohedral fullerenes filled with the maximum number of ion pairs that each fullerene cavity may fit (0, 2, 3, and 10 for C60, C180, C240, and C540, respectively) surrounded by [EMIM][GLY] at 303 K. Vertical dashed lines show the position of fullerene carbon atoms.

Figure 8. Site−site radial distribution functions, g(r), between the center of mass of the corresponding fullerene and relevant atoms in [EMIM]+ and [GLY]− for C540 endohedral fullerene filled with 10 [EMIM][GLY] ion pairs and surrounded by liquid [EMIM][GLY] at 303 K. Vertical dashed lines show the position of fullerene carbon atoms.

agreement with previous results reported by our group for choline benzoate ionic liquid, C540,9 and by Maciel and Fileti for 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4].3 Moreover, spatial distribution functions for ions placed E


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Figure 9. Spatial distribution functions for the ions in the first external solvation shells around the studied fullerenes at 303 K. Isodensity plots show 5 times the bulk number density. Color code: (blue) [EMIM]+ and (red) [GLY]−.

Figure 10. Number density profiles, ρ, for [EMIM][GLY] on top of one graphene sheet and with a vacuum layer above the IL layer at 303 K. Panel a shows the results for the center of mass of each ion, and the remaining panels show profiles for relevant atoms in the vicinity of the graphene surface (b, c) and the vacuum layer (d, e). z stands for the coordinate perpendicular to the graphene surface, and zgraphene stands for the position of the graphene sheet.

in the first solvation shells around the studied fullerenes (Figure 9) show that [EMIM]+ and [GLY]− ions occupy alternating regions around the fullerenes, and although the first shell is richer in cations (blue spots in Figure 9), the presence of anions at almost the same distances as the fullerene walls is also remarkable for all of the considered fullerenes. These ionic spatial distributions are different from those reported in the literature for [BMIM][BF4],3 which showed a very dense cationic region close to the fullerene walls and an outer anionic region characterized by well-defined high-density spots. Therefore, these results confirm that both the anion and cation develop pivotal roles in the structuring of solvation shells around fullerenes. Likewise, in the case of [BMIM][BF4], literature studies showed that the shortest cation alkyl chains are placed toward the fullerene (C60), with longer chains pointing outward, whereas for [EMIM][GLY] both methyl and ethyl chains seem to be placed parallel to the surface because the chains are shorter than in the case of [BMIM][BF4], and thus there is a much lower steric hindrance for ions in the first solvation shell and because C540 allows a stronger interaction with ions than in the case of C60. Nevertheless, in the case of [BMIM][BF4] solvating C60, the strength of the imidazolium cation with fullerene is larger than that for anion−fullerene,3

which confirm the strong tendency of imidazolium cations to interact with fullerene surfaces through π−π interactions. Graphene. In the second step of this study, the behavior of [EMIM][GLY] adsorbed on top of a graphene sheet was studied. In the arrangement used for this research, two interfaces were built: one ionic liquid−graphene and another one corresponding to ionic liquid−vacuum. The results in Figure 10 show number density profiles in the direction perpendicular to the graphene sheet surface, z. Figure 10a reports the number density profiles for the anion and cation centers of mass showing a strong densification in the vicinity of the graphene surface, with intense and narrow peaks at 3.55 Å for the graphene surface for both [EMIM]+ and [GLY]−, whose densities are 4 and 3 times larger than in the bulk ionic liquid phase for cations and anions, respectively. The presence of the graphene sheet leads to further perturbations in the fluid structure of the first adsorbed layer, as shown in Figure 10a by the presence of a second well-defined high-density peak at roughly 5.0 Å, slightly denser for [GLY]− than for [EMIM]+, and even a third high-density peak. Therefore, the results reported in Figure 10a show the presence of three layers above the graphene sheet in which densification is reported. This spatial heterogeneity in the vicinity of graphene (or graphite) F


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Figure 11. Charge density profiles, ρe, for [EMIM][GLY] on top of one graphene sheet and with a vacuum layer above the IL layer at 303 K. Panels b and c are extended plots of panel a in the vicinity of graphene and vacuum layers, respectively. z stands for the coordinate perpendicular to the graphene surface, and zgraphene stand for the position of the graphene sheet.

thus the adsorbed layer has solidlike structuring.98 Regarding the ion orientation at the interface, the results in Figure 10b show the parallel arrangement of imidazolium cations and the graphene surface, in agreement with previous literature results for all of the imidazolium-based ionic liquids for any involved anion.11,12,14,16,97,98 In the case of the [GLY]− anion, the results in Figure 10c show that the carboxylate and methylene groups are aligned parallel to the graphene surface but the amino group is placed closer to the graphene surface, in this way leading to the most effective interaction between the anion and the surface in the first high-density adsorbed layer. Charge-density profiles are reported in Figure 11a, showing oscillatory behavior in the bulk liquid phase, in agreement with the behavior of the number density reported in Figure 10a showing alternating anionic and cationic regions, which is very different from those regions in contact with the graphene sheet (Figure 11b). The charge distribution in the first adsorbed layer on graphene is characterized by a positively charged peak (at 2.7 Å in graphene) because of cation and anion hydrogen atoms in this layer, followed by a very intense and sharp negative peak (at 3.3 Å on the surface) rising from the presence of nitrogen atoms in anion amino groups (with large negative charge, Table S1, Supporting Information) in this region. The distribution of ions in the first adsorbed layer on the graphene surface is reported in Figure 12, and these results show the appearance of hot spots for [EMIM]+ cations whereas a more continuous distribution is obtained for [GLY]− anions. Nevertheless, the distribution of [EMIM]+ cations leads to low-density regions with a rectangular shape (yellow lines in Figure 12a) in which a

layers has been previously reported for other families of ionic liquids. Kislenko et al.11 reported high-density peaks in the vicinity of a graphene sheet for 1-butyl-3-methylimidazolium PF6, although in this case the anion is placed farther from the imidazolium cation than in the case of [EMIM][GLY] because the spherical shape of the PF6− anion interacts less effectively with the graphene sheet than does [GLY]−. The role developed by the anion with respect to the properties of the high-density layer in contact with the graphene sheet is inferred from literature results for ionic liquids containing imidazolium cations and chlorine,12 BF4−,14 PF6−,97,98 or SCN−,16 showing that those anions that allow an efficient graphene−imidazolium cation interaction (because of their size and/or shape) are also strongly adsorbed in the first high-density layer, whereas those whose presence in this layer would lead to a weakening of cation−graphene interactions are shifted toward the outer regions. Therefore, these results confirm the prevailing role of π−π interactions between the imidazolium ring in the cation and the graphene sheet, which control the structuring of the first high-density layer being able to disrupt the anion−cation interactions in spite of their strong Coulombic character. Nevertheless, it should be remarked that not only do imidazolium-based ionic liquids lead to a high-density layer in the vicinity of graphene sheets but also previous results reported by our group showed densifications for ionic liquids based on cholinium67 or alkylpiperazinium13 cations similar to those reported in this work for [EMIM][GLY]. The adsorption of [EMIM][GLY] on graphene is characterized by stronger [EMIM]+−graphene interactions (−1272.0 kJ mol−1 at 303 K) compared to [GLY]−−graphene interactions (−905.1 kJ mol−1 at 303 K), which is justified by the lower anionic density in the first adsorbed layer (Figure 10a) and the Lennard-Jones nature of ion−graphene interactions, which is directly dependent on the number of interacting sites (smaller for [GLY]− than for [EMIM]+), being very efficient for the imidazolium ring in the cation. Nevertheless, the [EMIM]+−graphene interaction energy is only 1.4 times stronger than the [EMIM]+−graphene interaction energy, which is in contrast to the results reported by Paek et al.98 showing that in the case of the [BMIM][PF6] ionic liquid, the cation−graphene interaction energy is 2.2 times larger than the anion−graphene interaction. Therefore, [GLY]− led to a more efficient interaction with the graphene sheet that did other anions such as [PF6]−. The strengths of ion−graphene interactions for [EMIM][GLY] are remarkably larger than the thermal energy at 303 K (2.52 kJ mol−1), and

Figure 12. Number density, ρ, and contour plots for [EMIM][GLY] on top of one graphene sheet at 303 K. Panels show the results for the center of mass of each ion. Dashed yellow lines show an example of the low-density cation region and the high-density anion region. G


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The Journal of Physical Chemistry C high density for [GLY]− anions is obtained (Figure 12b), and thus alternating cationic and anionic regions on the first adsorbed surface are obtained, in agreement with the presence of both ions at this interface (Figure 10a). Results for the ionic liquid−vacuum interface (z > 32 Å in Figure 10a,d,e) show that [EMIM]+ is placed in outer regions close to the vacuum layer, whereas [GLY]− occupies inner layers (Figure 10a). The ion orientation at the vacuum interface is clearly different from that on the graphene surface, and the results in Figure 10c show [EMIM]+ being perpendicular to the interface, with the longer alkyl chain (ethyl) pointing toward vacuum. In the case of [GLY]− (Figure 10e), a perpendicular arrangement is also obtained, with carboxyl groups in the inner layers and amino groups pointing to the vacuum. The perpendicular arrangement of imidazolium cations at the vacuum interface is in agreement with results in the literature for ionic liquids containing different anions.99−101 The charge distribution at the interface (Figure 11c) shows that the region in closer contact with the vacuum layer is slightly positively charged, although this effect is very minor compared to that obtained in the vicinity of the graphene surface (Figure 11b). In the second stage of this research on the behavior of [EMIM][GLY] regarding graphene, the confinement of this ionic liquid between two parallel graphene sheets was studied. The effect of the distance between the graphene sheets, d, on the properties of the confined ionic liquid was considered using d in the 29.5 to 47.5 Å range. The strength of ion−ion intermolecular interaction energies should change upon confinement as a function of d, and the results reported in Figure 13a show that attractive anion−cation interactions increase (ion absolute value) with decreasing d but also increase the repulsive anion−anion and anion−cation interactions. Therefore, the balance between these opposite contributions leads to a net increase (in absolute value) of the total ion−ion interaction energy (Figure 13b), which follows an almost linear trend with d. Nevertheless, although repulsive and attractive terms for intermolecular interaction energies change remarkably with d (by roughly 20% on going from 29.5 to 47.5 Å) the total ion−ion interaction energy changes by only 5% in the same range. Therefore, upon confinement between graphene sheets, the decreasing separation of sheets leads to an increase in repulsive interionic interactions, but at the same time attractive and strong Coulombic interactions are also favored. Likewise, with decreasing d values, ion−graphene interactions are also improved for both ions, although always being larger (in absolute value) for [EMIM] + −graphene than for [GLY]graphene interactions (Figure 13c). Therefore, the total energy (considering both ion−ion and ion−graphene contributions) is remarkably more negative with increasing d. Alibalazadeh and Foroutan24 studied the behavior of [EMIM][BF4] confined between two graphene sheets as a function of d in the 10 to 28 Å range and reported a decrease (in absolute value) of total potential energy with increasing d, in agreement with the results reported in Figure 13c. Alibalazadeh and Foroutan24 reported a minimum in the evolution of potential energy with d at d = 16 Å, and the d range studied in the present work does not reach such small sheet separation. For very small intersheet distances, the ion−ion structuring is almost lost, thus leading to a confined system whose properties are dominated by ion−graphene interactions. Number density profiles for confined [EMIM][GLY] reported in Figure 14 show that decreasing d leads to an increase in the densification near both sheets for [EMIM]+ and

Figure 13. Intermolecular interaction energy, E, for [EMIM][GLY] confined between two graphene sheets as a function of the distance between sheets, d, at 303 K.

Figure 14. Number density profiles, ρ, for [EMIM][GLY] confined between two graphene sheets as a function of the distance between the sheets, d, at 303 K. z = 0 stands for the coordinate of the center plane between the two graphene sheets.

[GLY]−. Likewise, the ions structuring the region close to the graphene sheets are also highly dependent on the sheets’ separation. In the case of [EMIM]+ ions at short d, three welldefined and narrow peaks are obtained in the vicinity of the H


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the temperature, which may be justified by considering (i) the viscosity of [EMIM][GLY] in the bulk liquid phase (47.75 and 8.28 mPa s at 303.15 and 353.15 K, respectively)75 and (ii) the entropically disfavored mechanism of accessing, especially for narrow nanotubes.40 Moreover, the lower entering rates for [GLY]− in comparison to those for [EMIM]+ arising from the presence of hydrogen atoms and positively and negatively charged carbon atoms in the nanotube entrance regions, which retain (through these positive hydrogens) or push back (through the positive carbons) anions when trying to access the nanotube. This behavior has been previously reported for other ionic liquids such as [EMIM][BF4],40 choline lactate,67 N-methylpiperazinium lactate,13 and N-ethyl-N-(furan-2ylmethyl)ethanaminium dihydrogen phosphate.42 Results in Figure 16 show that for simulations at 403 K the maximum number of ions encaged in the nanotube cavity is reached in roughly 1 ns and the total filling of the cavities for lower temperatures is reached only for simulation times longer than 5 ns, with very low filling rates for temperatures close to ambient. In the case of SWNT(10,10), the maximum number of ions inside the cavity is 8, whereas in the case of SWNT(15,15) this number is 27 for [EMIM]+ and 23 for [GLY]−. Therefore, for the wider nanotube there are more cations than anions inside the nanotube on average, which was also previously reported for other imidazolium-based ionic liquids such as [EMIM][BF4],40 choline lactate,67 and N-methylpiperazinium lactate.13 The confinement of [EMIM][GLY] leads to a density reduction in comparison to the bulk liquid phase; the number density is 1.8 nm−3 for the anion and cation inside SWNT(10,10), 2.8 (for [EMIM]+), and 2.4 nm−3 (for [GLY]−) inside SWNT(15,15), remarkably lower than 3.7 nm−3 in the bulk liquid phase at 303 K. Shim and Kim40 reported number density data for [EMIM][BF4] confined inside SWNT(10,10) and SWNT(15,15) to be 2.0 nm−3 for the anion and cation inside SWNT(10,10) and 2.7 (for the cation) and 2.4 nm−3 (for the anion) inside SWNT(15,15). The comparison of results for [EMIM][GLY] and [EMIM][BF4] shows that only for the narrower nanotube does the presence of the [GLY−] anion leads to lower densities in comparison to that in the presence of [BF4]−, whereas for the wider one, densities are roughly independent of the type of involved anion. The arrangement of confined ions is reported in Figure 17. For SWNT(10,10), the size of the cavity (13.6 Å of diameter) leads to a single internal solvation shell (Figure 17a) in contrast to SWNT(15,15) for which a solvation layer in the vicinity of the nanotube internal wall is obtained together with a second internal shell around the nanotube center (Figure 17b). The development of several internal solvation shells has been previously reported for other imidazolium-based ionic liquids with anions such as [BF4]−,40 [PF6]−,103 [Cl]−,104 and [SCN−].16 The arrangement of the centers of mass along the nanotube longitudinal axis is reported in Figure 17c,d, leading to ordered structures following a zigzag arrangement in both nanotubes. Shim and Kim40 showed that for [EMIM][BF4] confined inside SWNT(15,15), anions and cations in the first solvation shell (close to the nanotube wall) developed pentagonal distributions in staggered arrangements. This particular arrangement is not obtained in the case of [EMIM][GLY] because of the nonspherical shape of [GLY]−. A more detailed picture of the ionic distribution inside the nanotubes may be obtained from number density profiles along the nanotube axis reported in Figure 18. For both nanotubes, oscillatory profiles are obtained

graphene surfaces (with maxima separating roughly 2.4 Å), but with increasing d these three peaks change to wider bands (Figure 14a). Therefore, in the case of cations the different adsorbed layers increase their width with increasing d. In the case of [GLY]−, the widening of the adsorbed layers it also inferred, although for large d values the separation between the consecutive adsorbed layers is not as well defined (Figure 14b). The comparison of number density profiles reported by Alibalazadeh and Foroutan24 for [EMIM][BF4] and those in this work for [EMIM][GLY] shows that in the case of similar d values (28 Å for [EMIM][BF4] and 29.5 Å for [EMIM][GLY]) the presence of [GLY]− leads to more structured adsorbed layers (three peaks) in contrast to those for [BF4]− that lead to a single number density peak in the vicinity of the graphene sheets. This may be justified by considering the globular shape of the [BF4]− anion in contrast to that of [GLY]−, which leads to a more disruptive effect on the structuring of the [EMIM]+ cations at the adsorbed layers. Nevertheless, the development of several well-defined adsorbed layers for ionic liquids confined between graphene sheets containing spherical anions has also been reported in the literature. Wang et al.102 reported the development of up to three narrow peaks for number density profiles of 1-alkyl-3-methylimidazolium PF6 ionic liquid confined between graphite layers separated by 86.79 Å. Therefore, the development of a single adsorbed layer for [EMIM][BF4] reported by Alibalazadeh and Foroutan24 at 28 Å should be a consequence of the short separation between graphene sheets, which led to a maximization of the disruptive effect of the spherical anion. The structuring in the vicinity of graphene sheets upon confinement for [EMIM][GLY] is confirmed by the charge density profiles reported in Figure 15 showing the presence of a

Figure 15. Charge density profiles, ρ, for [EMIM][GLY] confined between two graphene sheets as a function of the distance between the sheets, d, at 303 K. z = 0 stands for the coordinate of the center plane between the two graphene sheets.

positively charged region followed by a negative one close to the graphene surfaces, followed by two positive−negative regions on going to the bulk liquid phase. The maxima of these peaks decrease with increasing d, especially for those corresponding to the first adsorbed layer. Nanotubes. In the third stage of this study, the behavior of [EMIM][GLY] with respect to SWNT(15,15) and SWNT(10,10) nanotubes was studied. The filling rates of empty nanotubes surrounded by [EMIM][GLY] in the 303 to 403 K range were analyzed for both nanotubes. The dynamics of ions penetrating the inner nanotube region is largely dependent on I


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Figure 16. Number of confined ions, Nc, inside carbon nanotubes as a function of the simulation time, t, for different temperatures and 1 bar.

Figure 17. Snapshots of ions solvating and confined in SWNT(10,10) and SWNT(15,15) at 403 K and 1 bar. Color code: (blue) [EMIM]+ and (red) [GLY]−. (b, d) Only the centers of mass of each ion are plotted to improve visibility.

of the entrance regions are inferred. Regarding the axial arrangements, radial distribution functions reported in Figure 19a confirm the development of a single internal shell for both the anion and cation inside SWNT(10,10). In the case of [GLY]− inside SWNT(10,10), the center of mass of this cation is placed 4.3 Å from the nanotube walls, whereas for [EMIM]+ the internal solvation shell is characterized by two regions 4.4 and 5.6 Å from the nanotube wall. For SWNT(15,15), the two internal solvation shells, 4.3 and 8.1 Å from the nanotube walls, are well defined, whereas in the case of the [EMIM]+ cation a wide band is obtained showing the overlap of ions between both shells, in agreement with literature results for other imidazolium-based ionic liquids.103 Charge density profiles along the nanotube longitudinal axis with confined ions are reported in Figure 20, showing oscillating behavior inside the nanotube, in agreement with the ionic distribution reported in Figure 17, and charge accumulation in the vicinity of the nanotubes’ entrance as explained in previous paragraphs. One of the most remarkable issues involving ionic liquids upon confinement is the development of highly ordered structures leading to high-melting-point, crystal-like arrangements.48 This structuring upon confinement, confirmed by the layering behavior reported in previous paragraphs for [EMIM][GLY], is strongly dependent on temperature, which leads to

Figure 18. Number density, ρ, profiles for ions along the nanotube long axis, z, for SWNT(10,10) and SWNT(15,15) at 403 K and 1 bar. The dashed vertical lines show the position of the SWNT terminal hydrogen atoms.

inside the cavities, with alternating regions of high density for cations and anions. Likewise, high-density peaks in the vicinity J


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strength of all of the involved intermolecular interactions are reported in Table 1. These results show that all ion−ion interactions are weakened upon confinement, both attractive and repulsive contributions, and that this effect increases with decreasing nanotube diameter. Therefore, the structure of the ionic liquid is largely disrupted upon confinement (Figure 17), weakening Coulombic interactions inside the cavity in comparison with those in the bulk liquid phase. Although the confined ionic liquid also develop interactions with carbon atoms in the nanotube, which are weaker for [GLY]− than for [EMIM]+ (Table 1), the mobility of ions is increased when confined as the self-diffusion coefficients reported in Table 1 show. Therefore, one may obtain a confined ionic liquid with properties closer to those of the bulk liquid when confined inside nanotubes with large cavities whereas in narrow tubes low-viscosity fluids could be obtained. Chaban and Prezhdo54 reported that [EMIM][Cl] has self-diffusion constants 5 times larger when confined inside SWNT(10,10) and roughly 3 times larger inside SWNT(16,16). In the case of [EMIM][GLY], the increase in mobility is not so large because the average selfdiffusion coefficients are 1.2 and 2.0 times larger inside SWNT(15,15) and SWNT(10,10), respectively, which arises from geometric factors because of the larger size of the [GLY]− anion and its nonspherical shape in comparison to that of [Cl]−. Another remarkable feature from the results in Table 1 arises from the higher mobility of [GLY]− compared to that for [EMIM]+ inside the nanotube, whereas in the bulk ionic liquid the opposite behavior is inferred, which is justified by considering that although all ion−ion interactions are weakened inside the cavity, the ion−nanotube interactions are weaker for [GLY]− than for [EMIM]+, thus leading to larger anionic mobility. The solvation of the nanotubes is also characterized by the development of layering and densification out of the nanotube close to the walls (Figure 17a,b), as previously reported for other imidazolium-based ionic liquids.41 The properties of these external solvation shells are reported in Figure 21, which shows the development of two shells for [GLY]− at 4.4 and 8.4 Å and a wide region for [EMIM]+. The structuring of the external solvation shells around the nanotubes (Figure 21) is very similar to that inside SWNT(15,15) reported in Figure 19b. Moreover, cations and anions share the same regions around the nanotube. Regarding the orientation of ions in the external and internal solvation shells, results in Figure 22 shows that for those layers in the vicinity of the nanotube walls, imidazolium rings in the cation lie parallel to the surface, in this way maximizing π−π interactions and leading to large [EMIM]+−nanotube interaction energies (Table 1). On the contrary, confined imidazolium cations in the vicinity of the center of the nanotube develop perpendicular arrangements.

Figure 19. Radial distribution function, g(r), of ions confined inside the SWNT along the short axis of the SWNT as a function of distance from the SWNT wall at 403 K and 1 bar.

Figure 20. Charge density profiles along the nanotubes longitudinal axis at 403 K and 1 bar.

changes in properties such as hydrogen bonding upon heating.50 Nevertheless, it has also been reported that ionic mobility is remarkably enhances upon confinement in nanotube cavities.54,104 In the case of [EMIM][GLY], the average number of hydrogen bonds between the anion and cation (H2−O1 sites, Figure 1) is 1.20 (per ion pair), whereas it is 1.0 upon confinement in both SWNT(10,10) and SWNT(15,15). Therefore, a weakening of intermolecular forces is produced when [EMIM][GLY] is confined, which should improve ionic mobility. Nevertheless, when ions are confined the role of ion− nanotube new interactions should be considered, and thus the

Table 1. Self-Diffusion Coefficients, D, and Intermolecular Interaction Energies, E, for Ions Confined Inside SWNT at 403 K and 1 bar E/kJ mol−1

1012 D/m2 s1 [EMIM] bulk SWNT(15,15) SWNT(10,10)

+

7.6 ± 0.7 8.2 ± 0.9 12.3 ± 1.4

[GLY]

−

6.5 ± 0.7 9.1 ± 1.0 16.1 ± 1.9

+

+

−

[EMIM] /[EMIM]

[GLY] /[GLY]

1152.1 503.7 258.8

1128.8 499.9 225.9 K

−

[EMIM]+ /[GLY]−

[EMIM]+ /SWNT

[GLY]− /SWNT

−2837.7 −1467.5 −1055.1

−1459.9 −276.9

−276.9 −85.9


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developed, confined ions exhibit a larger amount of mobility than in bulk phases. At the same time, the external solvation of the nanotubes is also characterized by densification and layering.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10269. Force field parametrization for [EMIM][GLY] (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

Figure 21. Total charge density, ρe, along the SWNT long axis for SWNT with confined ions at 403 K and 1 bar.

ACKNOWLEDGMENTS This work was made possible by the Ministerio de Economiá y Competitividad (Spain, project CTQ2013-40476-R) and Junta de Castilla y León (Spain, project BU324U14). We also acknowledge The Foundation of Supercomputing Center of Castile and León (FCSCL, Spain) and the Computing and Advanced Technologies Foundation of Extremadura (CénitS, LUSITANIA Supercomputer, Spain) for providing supercomputing facilities. G.G. acknowledges funding from Junta de Castilla y León, cofunded by the European Social Fund, for a postdoctoral contract. The statements made herein are solely the responsibility of the authors.

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CONCLUSIONS The properties of the [EMIM][GLY] ionic liquid with respect to fullerenes, graphene, and carbon nanotubes are studied in this work using molecular dynamics simulations. The encaging of ions inside fullerenes in the C60 to C540 range was studied as a function of the number of confined ions, showing the building of internal solvation shells. For these endohedral fullerenes containing ionic liquids, the strength of ion−fullerene interactions follows an inverse trend with increasing fullerene size. The solvation of endohedral fullerenes by ionic liquids is characterized by layering and densification around the fullerene walls because of the large ion−fullerene interactions, especially through the imidazolium ring but also for the amino acid-based anion. A large amount of densification is also obtained when [EMIM][GLY] is adsorbed on graphene sheets, with cations being strongly adsorbed at the surface and anions occupying regions with low cation density. The presence of the graphene sheet leads to perturbations in the ionic liquid structuring not only for those ions in close contact with the surface but also for up to three adsorbed layers above the sheet, as inferred. Regarding the confinement of the studied ionic liquid between two graphene sheets, the effect of sheet separation was studied, showing that a decreasing intersheet distance leads to larger ion−graphene interactions and at the same time improving ion−ion intermolecular forces, thus improving the total system energy. The behavior of [EMIM][GLY] confined inside carbon nanotubes is largely dependent on the tube diameter, but a high-density layer in the vicinity of the nanotube walls is always obtained, followed by a second internal shell when the cavity size allows it. The confinement weakens ion−ion interactions, and although new strong ion−nanotube interactions are

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