Solvation of C60 Fullerene and C60F48 Fluorinated Fullerene in

Article pubs.acs.org/JPCC

Solvation of C60 Fullerene and C60F48 Fluorinated Fullerene in Molecular and Ionic Liquids Joanna Szala-Bilnik, Margarida F. Costa Gomes, and Agílio A. H. Pádua* Institut de Chimie de Clermont-Ferrand, Université Blaise Pascal & CNRS, 63178 Aubière, France S Supporting Information *

ABSTRACT: The association and dispersion of C60 fullerene and C60F48 fluorinated fullerene in molecular and ionic solvents are studied using molecular dynamics simulations and dissolution experiments, with the aim of improving our understanding of nanocarbon materials interacting with liquid media. Fullerenes have uniform sizes and are devoid of edges and were chosen as models of wider classes of carbon nanomaterials. We parametrized a new atomistic force field for fluorinated nanocarbon materials and then used molecular dynamics to calculate structural quantities and potential of mean force. The aggregation in different solvents was assessed by UV/vis spectroscopy. The solvents considered include water, organic molecules, and ionic liquids, and we studied the effects of functional groups on the cation and of changing anions. Ionic liquids with a long alkyl chain on the cation are better solvents for C60F48 than for C60, revealing the stronger hydrophobic nature of C60F48. Surprisingly, an ionic liquid with a benzyl group has no particular affinity for C60. We observed nonadditive solvent effects related to the chemical structure of the ionic liquids, with the combination of a favorable anion (thiocyanate) with a long cation side chain (also favorable) not leading to enhanced affinity toward C60F48.

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INTRODUCTION Since their discovery, zero-, one-, and two-dimensional carbon nanomaterials1,2 have attracted much interest because of their molecular structure and their electronic and mechanical properties, which are extremely promising for applications in nanotechnology, materials science, and electronic, electrochemical, and optical devices.3−6 Additionally, physical and chemical modifications of carbon nanomaterials are routes to tailor and improve their properties; e.g. the fluorination of fullerenes allows controlling their electronic or tribological properties.7−11 Especial attention has been attached to carbon nanotubes (CNTs) and graphene (GP). The fluorination of CNTs considerably changes their properties. Fluorinated carbon nanotubes lead to improved performance of Li−O2 in comparison with nonfluorinated CNTs.12 After sonication in alcoholic solvents, fluorinated CNTs can form the stable suspensions, in contrast to nonfunctionalized CNTs.13 Fluorination changes the chemical nature of the carbon atoms from sp2 to sp3 geometry upon bonding with electronegative F atoms, thus perturbing the mechanical and electronic properties of the material. Besides, the C−F covalent bond is strong, leading to stable materials, analogous to perfluorinated hydrocarbons which are rather unreactive. Finally, highly fluorinated carbon materials possess a “hard shell” of weakly polarizable F atoms the interactions of which are weakly attractive in general, giving rise to both hydrophobic and lipophobic characteristics.14−16 This inertness and weak attractive forces are the source of the excellent lubricant properties of fluorinated nanocarbon materials.17 © XXXX American Chemical Society

The interactions of perfluorinated compounds are particular and associated with special kinds of nonideality,18 leading to the concept of fluorous phases.19,20 Highly fluorinated carbon nanomaterials share some of the properties of these molecular counterparts, and the study of their interactions is the main objective of this work. In particular, the exfoliation and the stabilization of fluorinated carbon nanomaterials in different solvents are still not well-understood. In the present work, we are interested in typical molecular solvents, such as water and organic liquids, and also in ionic liquids. Ionic liquids (ILs) are a class of purely ionic, saltlike materials that are composed solely of cations and anions. Their unique physicochemical properties (e.g., low melting temperature, nonflammability, low vapor pressure) have been leading to an increasing number of applications as solvents or reaction media.21−27 The combination of the properties of ILs with those of carbon nanomaterials or fluorinated carbon nanomaterials can be of interest, as ionic liquids form “buckygels” with CNTs28 and are suitable media for exfoliation of graphene.29−33 We wish to compare the interactions of nonfluorinated nanocarbon materials with those of fluorinated counterparts, in different solvents. Ionic liquids are structured phases, in first place because of the charge ordering in molten salts but also because ionic liquids with sufficiently long nonpolar side chains show a Received: May 21, 2016 Revised: August 6, 2016

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DOI: 10.1021/acs.jpcc.6b05140 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

were performed for the pristine and fluorinated nanocarbon materials in water, organic solvents, and ionic liquids. To investigate the influence of the molecular structure and functional groups of the ionic liquids, different cations and anions were considered, e.g., cations with various lengths of alkyl side chains and functional groups and anions of different families and sizes. In order to study the organization of the solvent around the fullerenes, radial distributions functions were produced. The free energy corresponding to the separation of two fullerenes in the different solvents was obtained through potential of mean force (PMF) calculations. Experiments were performed to assess the ability of solvents to hold dissolved C60. By relating the information from structural and energetic properties, we analyze the behavior of fluorinated and nonfunctionalized C60 in solution and try to formulate rules for the design of solvents and exfoliation media for carbon nanomaterials.

heterogeneous structure composed of ionic and nonpolar domains.34 Molecular solutes can interact with these different domains and be solvated therein. But nanometer-sized objects are too large and perturb much more the organization of solvent ions. This effect is still not sufficiently studied and may be critical for the design of the best solvent for exfoliation and stabilization. The present study is mainly computational, using atomistic interaction models and molecular simulation methods to provide a detailed, microscopic view of the interactions and ordering in liquid phases containing simple nanocarbon materials. We chose to focus the present work on relatively simple, well-characterized carbon nanomaterials devoid of particle-size or edge effects: fullerene, C60, and highly fluorinated fullerene, C60 F48. These are considered as representatives of wider classes of carbon nanomaterials for the purpose of studying weak, intermolecular forces with different solvents. The behavior of nanoparticles in solution depends on a delicate balance among particle−particle interactions, solvent− solvent interactions, and structure and also on the particle− solvent interactions. This topic has been the object of a recent review and is an active field today.35 If the description of these three terms is not coherent, then simulation may lead to unrealistic results. At the present state of the art, atomistic force fields for molecular liquids are very well established,36 and for ionic liquids, force fields are also established and tested;37 therefore, the main deficiencies are identified and can be mitigated. Interactions of fullerene are also described at an equivalent level.38 However, fluorinated fullerenes have not been studied sufficiently and we felt the need to develop interaction parameters for fluorinated fullerenes, which we present in the first part of this paper. When studying weak interactions involved in solvation, the molecular force field has to be parametrized to represent details of electrostatic and van der Waals forces. Therefore, the most suitable models for the present work are classical force fields of the OPLS-AA family36 or similar ones that pay special attention to the thermodynamic and solvation properties in the liquid state. Other force fields have been proposed for carbon nanomaterials, including fluorinated compounds, namely, ReaxFF.39,40 However, these reactive force fields are mainly aimed at applications where higher energies are involved (in covalent bond breaking and creation) or to describe mechanical properties of the materials, such as the thermal rippling behavior and the mechanical response of fluorographene under uniaxial stress.40 Although the OPLS-AA force field has been parametrized for perfluoroalkanes,41 it cannot be assumed that those parameters are transferable to molecules containing a mixture of fluorinated (sp3) and aromatic (sp2) carbon atoms. Specially in the regions where fluorinated and aromatic domains meet, certain terms in the force field will likely have to be specific, in particular, the electrostatic partial charges. We follow here a systematic approach, based on quantum chemical calculations, so that the parameters we develop are applicable a large diversity of fluorinated carbon nanomaterials. In the second part we have studied the stabilization of fluorinated carbon nanomaterials in different solvents. To do that, molecular dynamics (MD) simulation was used with the atomistic force fields, as these are the best tools at present to model the weak dispersive, polar, or hydrogen-bonded interactions that dominate the solvent−nanomaterial interface in the time scale considered for our systems. MD simulations

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FORCE FIELD PARAMETERIZATION In order to investigate the exfoliation and the stabilization of fluorinated carbon nanomaterials in solutions, it is necessary to establish appropriate interaction potential models representing the forces between the materials and between materials and fluids. The potential energy function (force field) consists of Coulomb and Lennard-Jones terms for the nonbonded interactions, combined with harmonic bond stretching and angle bending terms and cosine series describing torsional energetics. The functional form is identical to the widely used OPLS-AA force field for organic compounds,36 including perfluorinated41 and partially fluorinated42 hydrocarbons, providing compatibility with a large variety of molecular and ionic compounds.33 The functional form of the force field is given by eq 1. nonbonded

∑

U=

ij bonds

+

∑ ij

krij 2

torsions

+

⎧ q q e2 ⎡⎛ ⎞12 ⎛ ⎞6 ⎤⎫ σij σij ⎪ ⎪ i j ⎨ + 4ϵij⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥⎬ r ⎪ rij ⎝ rij ⎠ ⎦⎪ ⎣⎝ ij ⎠ ⎩ ⎭ angles

(rij − req )2 + ij

ijk

⎧ Vnijkl

k θijk 2

(θijk − θeq )2 ijk

⎫ [1 + ( − 1)n + 1 cos(nφijkl)]⎬ 2 ⎭ n=1 ⎩ 4

⎪

⎪

⎪

⎪

∑ ∑⎨ ijkl

∑

(1)

Geometric combining rules are used for the Lennard-Jones parameters between unlike sites: σij = (σiiσjj)1/2, εij = (εiiεjj)1/2. The present interaction model for fully and partially fluorinated carbon nanomaterials was derived on the basis of two model structures of fluorinated aromatic compounds, presented in Figure 1. These are smaller than extended nanomaterials, such as nanotubes or graphene flakes, to shorten the calculation time, but contain the relevant atom types. The two structures were chosen to model fluorinated fullerenes, nanotubes, and graphene, by considering fluorination only on one side (Figure 1, structure A) and also on both sites (Figure 1, structure B) of a graphenic plane. Chemical methods allow fluorination of both sides of graphenic planes43 or of just one side44 when fullerenes or nanotubes are considered, for example. The first structure (A) has six fluorine atoms on the same side of a 54-carbon aromatic plane, and the second molecule (B) is a coronene molecule functionalized with six fluorine B


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torsions) in the present model are taken from the OPLSAA47 and AMBER48 parameter databases and are given in Tables S1 and S2 of the Supporting Information. Figure 2 presents the names given to all atoms in structures A and B. The charges and the Lennard-Jones parameters are presented in Tables 1 and 2. Table 1. Charge Distribution for Structures A and B Obtained from ESP-Lumped Fit (ChelpG) to M06-2X/ccpVTZ(-f) Electron Densities and Those of OPLS-AA for Aromatic and Fluorinated Hydrocarbons, Which Are Adopted in the Present Force Field q/e ESP-lumped

Figure 1. Structures used to parametrize the force field for fluorinated carbon materials. (A) Fluorine atoms on the same side on a hydrogenterminated aromatic plane with 54 carbon atoms. (B) Fluorine atoms on both sides on a coronene structure.

atom type

A

B

OPLS-AA

CF CCF CG CA HA F

+0.26 −0.07 +0.00 −0.115 +0.115 −0.19

+0.36 −0.07

+0.12 0.0 0.0 −0.115 +0.115 −0.12

−0.115 +0.115 −0.29

Table 2. Nonbonded Parameters (Lennard-Jones and partial charges) for Fluorinated Nanocarbon Compoundsa

atoms, three on each site. A larger molecule was necessary when the six fluorine atoms are on the same side; otherwise, the optimized geometry obtained is distorted. The geometries of these structures were optimized at the HF/6-31G(d) level using Gaussian 09.45 The potential energy of interaction between two molecules of structures A or B was calculated at several distances using the M06-2X density functional,46 which is parametrized for energetics of noncovalent complexes. The cc-pVTZ(-f) basis set was used for the energy calculations with effects of basis set superposition corrected via the counterpoise method. The present force field for fluorinated nanocarbons is aimed at studying energetics and structure in condensed phases, and these properties are relatively insensitive to details of bond and angle vibrations. Therefore, the bonded terms representing the different bonds, angles, and torsions (including improper

atom type CF CCF CG CA HA F a

σ/Å 3.50 3.47 3.47 3.55 2.42 3.4

(47) (36) (36) (36) (36)

E/kJ mol−1 0.27614 0.27640 0.27640 0.29288 0.12552 0.08368

(47) (36) (36) (36) (36)

The parameters for the F atoms were derived in the present work.

Atomic partial charges for the atoms in structures A and B were derived using the ChelpG electrostatic surface potential (ESP) method on electron density calculated at the M06-2X/ cc-pVTZ(-f) level. The charges obtained directly from the ESP fit have different values for the same atom types between

Figure 2. Naming convention for the atom types in the force field representing partially fluorinated nanocarbon structures (structure A on the left and B on the right). CCF, CG, and CA denote aromatic carbon atoms (all three correspond to one single atom type CB for the purpose of bonded interactions), CF denotes fluorinated sp3 carbon atoms, and F are fluorine atoms (presented inside the ring not to obscure the view) and HA are H atoms saturating the edges. C


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charge set. An alternative good fit was obtained using OPLS-AA charges41 and εF = 3.4 Å and σF = 0.083 68 kJ mol−1. Therefore, the size of the F atoms had to be increased and the well depth of the van der Waals interactions decreased from those of perfluoroalkanes. Further essays using charge distributions and Lennard-Jones parameters are illustrated in the Supporting Information (Figures S1 and S2). The equivalent calculations for structure B are presented in Figure 3 (bottom) and were performed without any parameter adjustment, allowing for a test of the transferability of the force field. From the results obtained with structures A and B, we chose to adopt OPLS-AA charges and the Lennard-Jones parameters given in Table 2. Another force field for fluorinated hydrocarbon is ReaxFF.39,40 In the Supporting Information the inset within Figure S1 shows the interaction energy calculated with ReaxFF with cutoff radii of 10 and 15 Å, showing that this force field does not have enough sensitivity to the small energy scale of van der Waals interactions.

structure A and B, but in the interest of transferability of the force field parameters to a variety of fluorinated nanocarbons, we opted to smooth such differences to arrive at a unified set of partial charges. The values of partial charges are presented in Table S3 (Supporting Information). The discussion about their adjustment and the electronegativity equalization method QEq49 is also placed in the Supporting Information. The final choice of the set of charges to use will be discussed below. As initial approximations for the Lenard-Jones parameters for the fluorine atom, we took the values for perfluoroalkanes from the literature,41 which are σF = 2.95 Å and εF = 0.2218 kJ mol−1, respectively. The partial charge of fluorine atoms in perfluoroalkanes is qF = −0.12e,41 so in our structures we considered qCF = +0.12e and qCCF = 0.0e. We tested this set of charges, which is essentially the OPLS-AA specification for perfluoroalkanes, and also tested a set of charges that more closely corresponds to the ESP calculation, denoted in Table 1 as ESP-lumped. In Figure 3 are plotted the interaction energies as a function of the distance between two A structures (top) and two B structures (bottom) for various parameter sets. The interaction energy obtained using the Lenard-Jones parameters for perfluoroalkanes from the literature shows significant differences compared to the electronic structure calculations (Figure 3 top), and a good fit was achieved for σF = 3.4 Å and εF = 0.1212 kJ mol−1 when using the ESP-lumped

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MOLECULAR DYNAMICS SIMULATION Information about the dispersion and association processes of fullerenes and fluorinated fullerenes in various solvents can be obtained by calculation of the potential of mean force (PMF), a free energy quantity which can be viewed as the interaction between two molecules mediated by the solvent.50−52 To calculate the PMF, we carried out molecular dynamics simulations with periodic cubic simulation boxes containing two C60 or C60F48 molecules and defined numbers of solvent molecules (water, organic solvents, and ionic liquids). Molecular dynamics simulation enables one to obtain information about the structure of solvation layers. The simulation boxes were created using the Packmol utility.53 Trajectories were generated at constant temperature and pressure, maintained by the Nosé−Hoover thermostat and barostat. The pressure was maintained at 1 bar and the temperature of the simulations was 300 K for water and organic solvents and 423 K for ionic liquids. Water molecules were described using the SPC/E model.54 Force field parameters for fullerenes, DMF, DMSO, and ionic liquids were taken from refs 33, 36, 48, 55−59. Force field parameters for fluorinated fullerenes were those developed here as described above. The SHAKE algorithm was used to constrain bonds involving hydrogen atoms of the solvent molecules and also to constrain the angle in water molecules. Lennard-Jones parameters between unlike atoms were obtained from geometric mixing rules. A cutoff distance of 10 Å was employed for LJ interactions and real-space electrostatics. Long-range electrostatic interactions were calculated using the particle−particle particle−mesh solver60 for a tolerance of 10−4 in the Coulomb energies. It is known that fixed-charge force fields for ionic liquids lead to slow dynamics when compared to experiment, and the simulated systems are too viscous. Thus, to increase the mobility of ionic liquids, higher temperature (423 K) and scaled ionic charges (±0.8e) were employed. Scaling down the charges of the ions can be interpreted as accounting for polarization or charge transfer, and the legitimacy of this approach is documented in the literature.61−63 The equilibration phase required 1 ns (106 steps, time step 1 fs) for simulations with water and organic solvents and 5 ns with ionic liquids (2.5 × 106 steps, time step 2 fs). Simulations were performed using the LAMMPS code.64

Figure 3. Interaction energy between two molecules of structures A (top) and B (bottom) obtained using various sets of Lenard-Jones parameters for fluorine atoms and different values of partial charges for F, CF, and CCF atom types. The black line corresponds to the M062X/cc-pVTZ(-f) energies, the red line to the partial charges and Lennar-Jones parameters taken from the OPLS-AA force field, the green line to the partial charges from the OPLS-AA force field and new Lennard-Jones parameters (Table 2), and the blue line to the ESPlumped partial charges and other Lennard-Jones parameters. D


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The Journal of Physical Chemistry C To calculate the PMF, we employed the umbrella sampling technique with the weighted histogram analysis method (WHAM).65,66 An example of a histogram obtained by application of the WHAM method is shown in the Supporting Information (Figure S3). We started from the equilibrated state with the shortest distance between the centers of mass of nanomaterials. All histograms were obtained every 0.25 Å for water and organic solvents and every 0.3 Å for ionic liquids, spanning ranges of separation from 9.0 Å for fullerenes and 11.0 Å for fluorinated fullerenes up to 20 Å. At each separation an acquisition run was performed for 0.2 ns. Biasing force constants were applied to maintain the centers of mass of the fullerenes near specified distances, and the values of the biasing potentials varied according to the separation: force constants were 251.2 kJ mol−1 Å−1 at distances below 12.0 Å, 209.3 kJ mol−1 Å−1 below 14.5 Å, 167.5 kJ mol−1 Å−1 below 16.0 Å, 125.6 kJ mol−1 Å−1 below 19.0 Å, and 83.7 kJ mol−1 Å−1 for larger distances. Ionic liquids have more complex molecular structures, including possible variations in the structure of cations and anions. In order to study these effects, we investigated four ionic liquids having the same anion, bis(trifluoromethanesulfonyl)amide (Ntf2−), and differing in the lengths of the alkyl side chains on the 1-alkyl-3-methylimidazolium cations, from ethyl (C2) to decyl (C10). Changing the length of the alkyl side chains affects the balance between Coulomb and van der Waals interactions in the ionic liquids and also determines the ordering of the liquid phases, since longer alkyl chains tend to segregate from the charged head-groups, forming spatial domains. It is important for us to understand the consequences of these structural effects to the solvation of fullerenes. With a benzyl-functionalized imidazolium and bis(trifluoromethanesulfonyl)amide anion, [BnC1im][Ntf2], we want to test the hypothesis if increasing the aromatic character of the cation leads to a higher affinity for C60. The effect of different anions was tested by studying ionic liquids composed of the C2C1im+ cation with two other anions: dicyanamide (DCA−) and thiocyanide (SCN−). Ntf2− is a lowcoordinating anion, qualified as a “hydrophobic” anion because it leads to ionic liquids with lesser miscibility with water. The DCA− and SCN− anions are smaller and have higher charge densities, leading to interactions of a different nature. The fluorinated character of Ntf2− is expected to play a role in the solvation of fluorinated fullerene. Ntf2−, SCN−, and DCA− all lead to low-viscosity ionic liquids where solvation phenomena can be observed in shorter time scales. Another ionic liquid, based on the small thiocyanide anion but with a long octyl chain on the cation, [C8C1im][SCN], was also considered. The composition of simulation boxes is presented in Table S4 in the Supporting Information.

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Three ionic liquids were used for this study without any further purification: [C2C1im][Ntf2] (from Iolitec, >98% pure), [C10C1im][Ntf2] (from Iolitec, >98% pure), and [C2C1im][DCA] (from Iolitec, >98% pure). In order to compare the ionic liquids as solvents, solutions of different concentrations in C60 were characterized using UV−vis absorption measurements made with a JASCO V-650 spectrophotometer at room temperature. As illustrated in Figure 4, the UV−vis spectra of solutions of C60 in [C10C1im][Ntf2] present the absorption band around

Figure 4. Absorbance spectra for solutions of C60 in [C10C1im][Ntf2] at different concentrations: green, 0.01 mg/mL; dark blue, 0.02 mg/ mL; gray, 0.04 mg/mL; light blue, 0.25 mg/mL; red, 0.31 mg/mL; dark green, 0.40 mg/mL; orange, 0.45 mg/mL; black, C60 in CH2Cl2..

360 nm, being characteristic of C60.68 This band is herein subject to a bathochromic shift against the position reported for solvents like n-hexane (330 nm), as is usually found for C60 dissolved in aromatic or highly polar solvents.69 The spectra change significantly as a function of concentration. First, absorption around 360 nm increases related to the overall amount of C60. The increase and very broad absorption at wavelengths higher than 450 nm can be attributed to the formation of C60 aggregates, as described in previous solvation studies in molecular liquids.70,71 Several authors linked the formation of aggregates to the method used for the preparation of the solutions,69 with direct sonication or excessive stirring favoring formation of aggregates. Both of these procedures were avoided herein. In light of the data reported in Figure 4, the solubility of (molecular) C60 in [C10C1im][Ntf2] is equal to 0.030 ± 0.005 mg/mL (or 4.2 × 10−5 M). The visual aspect of the solutions prepared is compatible with this conclusion and exhibits a gradually more intense purple color as the total concentration of C60 increases (Figure S14, Supporting Information). The same study was made with [C2C1im][Ntf2] and [C2C1im][DCA] in order to investigate the influence of the size of the alkyl side chain of the cation and of the change of anion on the C60 solubility, respectively. Similar procedures were followed in these two cases, and the results obtained are illustrated in Figure 5. As with C60 in [C10C1im][Ntf2], the data in Figure 5 was used to estimate the solubility in both [C2C1im][Ntf2] and [C2C1im][DCA] as 0.010 ± 0.005 mg/mL (or 1.4 × 10−5 M).

DISSOLUTION EXPERIMENTS

In a typical dissolution experiment, ca. 2 mg of C60 (SigmaAldrich, 99.5% pure) was added to 10.0 mL of dichloromethane (Sigma-Aldrich, 99.9% pure). The mixture was sonicated in a low-power ultrasonic bath (Branson 2510) for approximately 5 min, followed by 12−18 h of stirring protected from light. Precise amounts of the resulting C60 solution in CH2Cl2 were then mixed with appropriate volumes of ionic liquid. After removal of the organic solvent by evaporation under primary vacuum, solutions of C60 in the ionic liquids with different concentrations were obtained.67 E


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Figure 5. (Left) Absorbance spectra of solutions of C60 in [C2C1im][DCA] at different concentrations: red, 0.002 mg/mL; light blue, 0.004 mg/mL; yellow, 0.02 mg/mL; green, 0.03 mg/mL; dark blue, 0.06 mg/mL; orange, 0.09 mg/mL; black, C60 in CH2Cl2. (Right) Absorbance spectra of solutions of C60 in [C2C1im][Ntf2] at different concentrations: green, 0.002 mg/mL; light blue, 0.004 mg/mL; yellow, 0.02 mg/mL; green, 0.03 mg/ mL; dark blue, 0.04 mg/mL; black, C60 in CH2Cl2.

decreasing of the height and the broadening of maxima are observed, which shows that the structure around C60F48 is less organized than around C60. The almost identical position (for anion a little bit closer to the center of the mass of nanomaterials) of the first maximum of g(r) CM−CR , g(r)CM−CW, and g(r)CM−CT, or g(r)CM−F and g(r)CM−O or g(r)CM−S means that cation and anion occupy the same regions in the vicinity of C60 or C60F48. The anion Ntf2− lies parallel to the surface of C60 as the height and position of maximum of g(r)CM−F and g(r)CM−O are identical. For the fluorinated fullerenes, the maximum of g(r)CM−F is higher than in g(r)CM−O, showing the higher affinity of fluorine atoms for fluorinated materials. The maximum of RDFs of anion and cation of nonfluorinated material have similar heights, demonstrating no preferences in solvation. The maximum of g(r)CM−CT of C60 is the highest, but the others are also quite high, which confirmed the parallel orientation of cation with a little closer position of alkyl chain on cation to the surface of fullerenes. An affinity between the cation alkyl chains and carbon atoms of graphene has been reported, with the side chain lying parallel to the surface.72 Fluorinated fullerenes in ionic liquids with small cations show broader peaks in the RDFs, because the surface of C60F48 is more complex than that of C60. A remarkable structural feature is that ionic liquids with a long alkyl chain on the cation show very intense first maxima of g(r)CM−CT with fluorinated fullerenes, indicating preferential solvation of the fluorinated compound by the nonpolar chains. The probability of finding Ntf2− in the close surroundings of fluorinated fullerenes is low (right panel of Figure 8) and for smaller anions it is even lower (right panel of Figure S8, Supporting Information). The aromatic character of the benzyl chain should show the better affinity for fullerenes than fluorinated fullerenes. However, what is a little bit surprising is that there are no significant differences in the radial distribution functions. The only observed differences, these being same as for the other ionic liquids, show the less structured solvation sphere around C60F48 than around C60. It is important to emphasize that the size of the benzyl group and butyl chain are similar, and in the contrast to the long alkyl chain, they do not support the separation of fluorinated fullerenes.

In [C2C1im][DCA], the solvation of C60 seems to follow a similar mechanism as in [C10C1im][Ntf2], with concomitant increase in the intensity of both the 360 nm band and the broad feature at longer wavelengths. Even if the molecular solubility is similar in [C2C1im][Ntf2], here the intensity of the band at 330 nm does not increase at concentrations higher than 0.02 mg/ mL as it does in the other two ionic liquids. This experimental study confirms that, as previously reported in the literature,67 the solubility of fullerene in imidazoliumbased ionic liquids increases when the length of the alkyl side chain of the cation increases. The values reported herein are, nevertheless, smaller (but of the same order of magnitude) than those previously found for [C8C1im][Ntf2],67 even if the same procedure for preparing the solutions was followed. We consider that the solubility values reported here, which are lower, correspond more accurately to the solubility of molecular C60 at room temperature in the ionic liquids chosen.

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STRUCTURE OF THE SOLVATION LAYERS The solvation layers of ions around the fullerene molecules were investigated by calculation of radial distribution functions between the center of mass of fullerenes or fluorinated fullerenes and particular atoms of ionic liquids. The names of atoms within ionic liquids can be confusing, so our naming convention for the atom types is presented in Figure 6. The radial distribution functions for C60 and C60F48 in [C2C1im][Ntf2], [C10C1im][Ntf2], and [BnC1im][Ntf2] are presented in Figures 7−9. RDFs of fullerenes and fluorinated fullerenes in other ionic liquids are placed in the Supporting Information. In all ionic liquids, well-defined maxima of RDFs of C60 are seen, whereas for fluorinated fullerenes the

Figure 6. Naming convention for the atom types in the cation and anion. F


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Figure 7. Radial distribution function between the center of mass of fullerenes (left) or fluorinated fullerenes (right) and particular atoms of [C2C1im][Ntf2] (specified on the plots).

Figure 8. Radial distribution functions between the center of mass of fullerenes (left) or fluorinated fullerenes (right) and particular atoms of [C10C1im][Ntf2] (specified on the plots).

G


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Figure 9. Radial distribution functions of atoms of [BnC1im][Ntf2] (specified on the plots) with respect to the center of mass of fullerenes (left) and fluorinated fullerenes (right) at 423 K.

Figure 10. Example of PMF of C60 in ionic liquid. Points a−e on the chart correspond to snapshots a−e. For simplicity, only ions in the approximate plane of the two fullerenes are shown.

[BnC1im][Ntf2] with respect to the center of mass of fullerenes at 300 K are presented in the Supporting Information (Figure S9), which can be compared to Figure 9.

The choice of a higher temperature for simulation of nanomaterials in ionic liquids is due to their slow dynamics and difficulties in sampling configurations at lower temperatures. However, ionic liquids have a marked structure because of charge ordering, and the temperature differences in the present study do not cause major changes in the structure. For comparison, radial distribution functions of atoms of

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POTENTIAL OF MEAN FORCE Figure 10 presents an example of potential of mean force curve of fullerenes in ionic liquid. The first point a corresponds to the H


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single layer of hydrating water molecules between the two C60, whereas the minimum around 16 Å corresponds to two layers of intercalated water molecules. Similar shapes for PMF curves of fullerenes in water and DMSO were also reported in several simulation studies.73,75,76 Table 3 presents the differences

closest position of two C60 at contact, not separated by any solvent ions molecules. From the maximum at point b onward, solvent ions intercalate between fullerenes, and at the secondary minimum (point c) a single layer of solvent ions is intercalated. The points d and e correspond to the larger distances between the two C60 molecules and more solvent layers are found between them. Due to the high viscosity (slow dynamics) of the ionic liquids and also to their marked ordering, it is difficult to lower the level of noise in the PMF profiles. The main quantities we extract from the PMF profiles are the height of the first peak b with respect to the minimum at contact (zero in our scale). This explains why we chose to normalize all PMF curves at the principal energy minimum (at contact), which is this first barrier that must be overcome in order for the fullerenes to be solvated. The remaining oscillations of the PMF then correspond to a lesser energy scale. The final parts of the PMF profile are less-well-defined, and we will only distinguish large, very evident energy differences at the largest separation. The PMF curves of C60 in water, DMSO, and DMF are presented in Figure 11 (top), where it is seen that the shape of

Table 3. Difference between Energies at First Minimum (M1), Maximum (MAX), and Second Minimum (M2) of Fullerenes in Different Solvents (in kJ mol−1) water

DMSO DMF

ref

M1 − MAX

M1 − M2

MAX − M2

this work 75 76 73 this work 75 this work

−17.6 −18.0 −20.5 −20.9 −13.2 −14.0 −12.4

−11.1 −13.0 −16.3 −18.8 −4.6 −2.5 −3.2

6.6 7.5 4.2 2.1 8.6 11.5 9.0

between the free energies at the minimum at contact (M1), the first maximum (MAX), and second minimum (M2). Our results for water and organic solvents show good agreement with the simulations reported in the literature.73,75,76 The deepest first minimum is observed for water, which confirms that stronger aggregation is observed than for DMSO and DMF. Therefore, it is easier for a pair of C60 molecules to escape from the potential well in DMF and DMSO than in aqueous solution. The M1 − M2 difference for water is much higher than for organic solvents, which shows that water opposes a higher barrier to disaggregation. The low solubility of fullerenes in water and in organic solvents has been reported,77−80 so the tendency to aggregate is not surprising. The PMF curves of C60 in the six ionic liquids are presented in Figure 11, the center plot corresponding to the ionic liquids having the same anion but different side chains on the cations and the bottom plot corresponding to ILs sharing the same cation but with different anions. The first minimum corresponding to the contact separation is also placed around 10 Å in the ionic liquids. The ionic liquids with the same anion (Nft2−) and different alkyl chains on the cations have PMF curves with similar shape, including the position of the maximum and minimum. A significant difference in the height of the PMF is observed for [BnC1im][Ntf2]. The higher energy needed to separate C60 is a result probably of the bigger size of the cation with the benzyl group than with the alkyl chain. Table 4 presents the differences between the energies at the first minimum, first maximum, and second minimum, and also here a clear order between the features of the PMF curves with different cations does not appear as expect for [BnC1im][Ntf2].

Figure 11. PMF for two fullerenes in water, DMSO, and DMF (top); [CnC1im][Ntf2], n = 1, 4, 8, 10, and Bn (center); and [C2C1im][X], X = Ntf2, DCA, and SCN (bottom).

Table 4. Difference between Energies at First Minimum (M1), Maximum (MAX), and Second Minimum (M2) of Fullerenes in Different Solvents (in kJ mol−1)

the curve is quite different for water. In all cases, the PMF shows a deep minimum at a separation distance of around 10 Å, indicating the van der Waals contact distance for the fullerenes. Oscillatory features at larger separation, with a maximum at 12 Å, are observed for water and for the two organic solvents. The second minimum for water appears at 13.5 Å, whereas for organic solvents it is shifted to a higher value, at 14.5 Å. The oscillatory shape of the PMF for C60 in H2O was also discussed in the literature.73,74 The second minimum corresponds to a

[C2C1im][Ntf2] [C4C1im][Ntf2] [C8C1im][Ntf2] [C10C1im][Ntf2] [BnC1im][Ntf2] [C2C1im][DCA] [C2C1im][SCN] [C8C1im][SCN] I

M1 − MAX

M1 − M2

MAX − M2

−16.5 −13.2 −16.8 −13.2 −22.8 −14.8 −15.7 −14.8

−8.8 −8.5 −10.5 −10.4 −18.1 −4.6 −6.4 −7.4

7.7 4.7 6.3 2.8 4.7 10.2 9.3 7.4


Article

The Journal of Physical Chemistry C In a counterintuitive way, the present PMF calculations do not predict that longer alkyl side chains in the ionic liquids nor the benzyl group corresponds to a more favorable solvation of C60. On the other hand, the positions of maxima and minima are strongly related with the structure of the anions. The larger anion Ntf2− does not correspond to the PMF curve with the deepest first minimum or the lowest first maximum. Separation of two C60 molecules appears easier in the DCA− and SCN− ionic liquids, followed by the ionic liquid with Ntf2−, which shows the highest first maximum. In the case of [C8C1im][SCN] presented in Figure S10 (left) of the Supporting Information, a similar shape of PMFs for [C8C1im][SCN] and [C2C1im][SCN] is observed. The figure also shows that in the presence of a small anion, a longer alkyl chain on the cation does not help to dissolve nonfluorinated fullerenes. The PMF curves for C60F48 in the organic solvents are shown in Figure 12 (top) and the main features in terms of free energy

Table 5. Difference between Energies of First Minimum (M1), Maximum (MAX), and Second Minimum (M2) of Fluorinated Fullerenes in Different Solvents (in kJ mol−1) H2O DMF DMSO [C2C1im][Ntf2] [C4C1im][Ntf2] [C8C1im][Ntf2] [C10C1im][Ntf2] [BnC1im][Nft2] [C2C1im][DCA] [C2C1im][SCN] [C8C1im][SCN]

M1 − MAX

M1 − M2

MAX − M2

−16.2 −10.5 −10.1 −26.8 −19.7 −13.8 −12.9 −16.7 −24.7 −13.8 −13.8

−12.2 −8.2 −5.9 −22.4 −12.2 −11.7 −10.8 −14.6 −17.0 −10.6 −11.8

4.0 2.3 4.2 4.4 7.5 2.1 2.1 2.1 7.7 3.2 2.0

studied, namely, short-chain imidazolium liquids with bistriflamide, [C2C1im][Ntf2] and [C4C1im][Ntf2], and the liquid with the dicyanamide anion, [C2C1im][DCA]. Ionic liquids with longer alkyl chains on the cation ([C8C1im][Ntf2] and [C10C1im][Ntf2]) lead to free energy differences between the aggregated and dissolved situations that are similar to the values obtained in the molecular solvents. This result corresponds to a favorable dispersion of the fluorinated fullerene C60F48 in the ionic liquids where the respective weight of van der Waals interactions is more important. It is interesting to observe that the ionic liquid based on the thiocyanide anion, [C2C1im][SCN], also leads to relatively low free energy differences between the aggregated and dissolved situation, again similar to the values obtained in the molecular solvents. This shows that the size of the anion is not the only factor at play, since the other small anion, DCA−, does not appear as a good solvent for C60F48. Since we found that both a longer alkyl chain on the cation and the SCN anion lead, independently, to lower barriers to disaggregation of C60F48, we wished to investigate if the effect is cumulative. It is curious to see (Supporting Information, Figure S10, right) that designing an ionic liquid combining a longchain imidazolium cation with a favorable anion (SCN−) does not lead to significantly easier dispersion of the fluorinated fullerenes. The ability of an ionic liquid to disaggregate fluorinated fullerenes appears to be a complex property that depends on various structural features of the ions in a nonadditive way. Direct comparisons of PMF curves of pristine and fluorinated fullerenes in molecular and ionic liquid are presented in the Supporting Information (Figures S11−S13). The comparison of the PMF curves of C60 and C60F48 in water shows that the overall free energy cost of disaggregation is similar, at 12 kJ mol−1, although there is a small difference in the height of the first maximum. This overall energy cost becomes lower in DMSO and in DMF, around 8 or 10 kJ mol−1. There is a slight difference between C60 and C60F48, with a lower energy cost for C60, but the PMF curves for the fluorinated carbon still show oscillations at the highest distance we studied. In the ionic liquids, the striking feature is that in [C2C1im][Ntf2] and [C2C1im][DCA] there is a very large difference between the PMF profiles of C60 and C60F48. The energy cost for disaggregation of C60 is significantly lower in these two ionic liquids: it is above 20 kJ mol−1 for C60F48 and around 10 kJ mol−1 for C60. In the ionic liquids with longer alkyl side chains there are only minor differences in the free energy scale

Figure 12. PMF between two fluorinated fullerenes in water, DMSO, and DMF (top); [CnC1im][Ntf2] n = 1, 4, 8, 10, and Bn (center); and [C2C1im][X], X = Ntf2, DCA, and SCN (bottom).

differences are listed in Table 5. The results are significantly different from those obtained with C60. The contact distance is now around 12.5 Å, related to the larger diameter of the fluorinated fullerene. In water and in the organic solvents (DMSO and DMF), the PMF curves of C60F48 tend toward similar overall energy differences (between the pair of fullerenes at contact and at large separations), but the first maxima are very different, with water giving rise to a higher first peak, indicating a more difficult dissociation of the fluorinated fullerene. Features of the PMF curves listed in Table 5 quantify these observations. Some of the ionic liquids (Figure 12 center and bottom) show much deeper potential wells than the molecular solvents J


Article

The Journal of Physical Chemistry C between the pristine and fluorinated fullerenes, all being around 10−12 kJ mol−1. In [C2C1im][SCN] this free energy cost is quite low, below 10 kJ mol−1. A little bit higher energy is observed for [C8C1im][SCN], but still ionic liquid with small anions or cations with long alkyl chain are preferred. The inclusion of an aromatic group in the structure of the cation does not result in an increased affinity for C60. Actually, it is the fluorinated fullerene that has a lower overall energy difference between the contact pair and the solvated situations, which is a result of the less-ordered structure of solvation sphere around C60F48.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Prof. Marc Dubois for providing the C60 samples used for the experimental measurements. This work was supported by grant CLINT (ANR-12-IS10-003) of the Agence Nationale de la Recherche.

■

CONCLUSION The results of molecular simulations reported herein allowed us to discern trends in the structural and energetic properties of solutions of C60 and C60F48, both in molecular and ionic solvents. As expected, water appears to be a poor solvent for both nanocarbon compounds when compared to dimethyl sulfoxide or dimethylformamide, two polar organic solvents. Some of the ionic liquids we studied appear to be as good as organic solvents for the stabilization/solvation of fullerenes and fluorinated fullerenes. Concerning the relations between the structure of the ionic liquids and the solvation properties, we observed that the length of the alkyl nonpolar side chain of the ionic liquid has little effect on the structure of the solvation layer around fullerene and also little effect on the shape of the PMF profile. However, the length of the side chain affects the solubility, as shown experimentally for C60, whereas the effect on solubility of changing anion appears to be less, for the small sample of two ionic liquids we studied experimentally. The length of the side chain has a significant effect on the organization of ions and the PMF with fluorinated fullerene, resulting in a lower barrier to separation. This is interpreted as a consequence of the higher hydrophobicity of fluorinated carbon compounds increasing the affinity for the side chains when compared to the aromatic fullerene. Somewhat surprisingly, the ionic liquid with a benzyl side group does not show a particular affinity for C60, showing that the interactions in those systems are not trivial and simple rules such as “like dissolves like” do not necessarily apply. A very interesting result in our opinion is related to the nonadditivity of interactions: the SCN− anion proved effective for the solvation of C60F48 when compared with other anions. This was studied using a short-chain cation, ethylimidazolium. In parallel, longer side chains in the cations proved effective as well, in ionic liquids with the large Ntf2− anion. However, the combination of the SCN− anion with a long side chain on the cation did not prove better, showing that the solvation phenomena are not due to a simple additivity of the effects of individual functional groups, and more complex structural features matter.

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fullerene solutions with various concentrations in ionic liquids (PDF)

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REFERENCES

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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.6b05140. Force field parameters including details on the calculation of partial charges, composition of the simulation boxes, additional plots of radial distribution functions and of potential of mean force, and an image of K


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Solvation of C60 Fullerene and C60F48 Fluorinated Fullerene in

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