Experimental Study of the Interactions of Fullerene with Ionic Liquids

Chapter 12

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Experimental Study of the Interactions of Fullerene with Ionic Liquids M. F. Costa Gomes,* L. Pison, and A. A. H. Padua Institut de Chimie de Clermont-Ferrand, CNRS and Université Clermont-Auvergne, F-63000 Clermont-Ferrand, France *E-mail: [email protected]

The interactions between fullerenes and room temperature ionic liquids can be investigated experimentally using titration calorimetry. In the present work, we have added the ionic liquid 1-decyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide, [C10C1Im][NTf2], into a solution of C60 in 1,2-dichlorobenzene and the heat effect was recorded. The background heat of mixing of the two liquids was measured separately. It was observed that the enthalpy of mixing of the ionic liquid with the organic solvent in presence of C60 is more negative by approximately 4 J·mol−1 than in the absence of fullerene.

Introduction Fullerenes are carbon materials that are not chemically inert and thus can be transformed, most often when present in liquid media, to find applications in materials chemistry, electrochemistry or catalysis (1). Therefore, the quantitative characterization and the understanding of the nature of fullerene solutions are important, the principal thermodynamic property to be studied being solubility. Fullerenes are scarcely soluble in any solvent, the highest reported solubilities at 298 K being 0.07 and 0.069 mol·L-1 in 1-chloronaphthalene or 1-phenyl-naphthalene, respectively (2). Other good solvents for C60 include 1,2-dichlorobenzene and o-xylene, with reported solubilities of 0.032−0.037 (2, 3) and 0.011−0.013 mol·L-1 (4, 5), respectively. Toluene or benzene only dissolve © 2017 American Chemical Society Shiflett and Scurto; Ionic Liquids: Current State and Future Directions ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


around 10−3 mol·L-1 of C60 at 298 K (1). A general observation is that solubility is enhanced in highly polarizable solvents with relatively low polarity and low cohesive energy density (6). Ionic liquids have been considered as possible solvents for carbon materials (7) namely carbon nanotubes (8, 9) or C60 (6, 10) and also as effective media for the exfoliation of graphite (11). These purely ionic, salt-like materials that are composed solely of cations and anions form structured phases because of charge ordering and also because ionic liquids with sufficiently long nonpolar side chains show a heterogeneous structure composed of ionic and nonpolar domains (12). It has been shown that different molecular solutes can interact with these domains and thus be solvated in different molecular environments (13). There have been few studies reported in the literature concerning the solvation of fullerenes in ionic liquids as the ionic nature of these solvents, like the polarity of organic solvents, is believed to cause a low solubility of C60 (14). Ionic liquids have, nevertheless, shown promising properties as solvents for the synthesis of functionalized fullerenes (15) and show remarkable properties for different applications in materials science and electrochemistry (16, 17). Following a previous computational study on the solvation of fullerene and fluorinated fullerene in molecular and ionic liquids (10), in this work we present for the first time an experimental study on the energy required to replace a good solvent of C60 – 1,2-dichlorobenzene, DCB – by an ionic liquid – 1-decyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide [C10C1Im][NTf2]. We have used an experimental protocol previously tested for the study of metallic nanoparticules in ionic liquids (18) that allows to obtain an estimation of the difference in interaction energy between C60 in 1,2-dichlorobenzene and C60 in [C10C1Im][NTf2]. Our aim is to contribute with experimental data to improve the understanding about the mechanisms of solvation and stabilization of this kind of carbon nanomaterial in ionic liquids.

Experimental Materials The ionic liquid 1- decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C10C1Im][NTf2] was purchased from Iolitec with a purity better than 98%. The liquid was used without further purification but was dried under primary vacuum at room temperature during 24 h. 1,2-dichlorobenzene (DCB) from Fluka, grade purum ≥98% from GC, was used in the measurements. Fullerene C60 was purchased at Sigma-Aldrich with a 99.9% purity and was used as received from the manufacturer.

Apparatus and Operation Calorimetric measurements were performed at 313 K and atmospheric pressure using an isothermal titration nanocalorimeter (TA Instruments) equipped 274 Shiflett and Scurto; Ionic Liquids: Current State and Future Directions ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


with 1 mL stainless steel cells housed in a Thermal Activity Monitor TAM III thermostat (TA Instruments). The temperature of the thermostat is controlled precisely within 10–5 K. An electrical calibration was done before each experiment, and the instrument was chemically calibrated several times by titration of a 0.01 M aqueous solution of 18-crown-6 ethers with an 0.2 M aqueous solution of BaCl2 as described in references (19, 22). The enthalpies of binding of Ba2+ ions to 18-crown-6 were found to be 2% higher than those reported in the literature. No correction attributable to these differences was introduced in the raw data and, based on these values, the authors estimate the overall uncertainty of the present calorimetric determinations as ±2%. For the determination of the enthalpy of mixing of [C10C1Im][NTf2] (IL) and DCB, ΔmixHIL+DCB, approximately 0.8 mL of degassed ionic liquid or degassed DCB were introduced into the 1 mL glass measuring and reference cells. The liquid in the measuring cell was stirred by a gold-platted propeller at 80 rpm and volumes of 9 μL of the second liquid (different from the one in the cells) were injected during 40 s using a motor driven pump (Thermometric 3810 Syringe Pump) equipped with a 100 μL gastight Hamilton Syringe. The intervals between consecutive injections were 40 min, which allowed for a good thermal stabilization of the solution and the return to a stable baseline. Each experiment consisted in 10 injections. Undesirable effects of diffusion of the liquid in the cell into the injection canula were avoided by immersing the canula only about 3 mm prior to the first injection. A peak with an area proportional to the resulting heat effect Qi translated to the thermal effect due to each injection of liquid. The integration of the peaks from the recorded calorimetric plots was performed using the TAM III Assistant software. For the measurement of the interaction energy between C60 and [C10C1Im][NTf2], the sample cell of the calorimeter was filled with approximately 0.8 mL of a solution of 0.0311 mol·L-1 of C60 in DCB and the reference cell was filled with the same amount of solution. The solution in the sample cell was stirred at 80 rpm and volumes of 9 μL of the ionic liquid were injected following the procedure described above. The heat effect measured corresponds to the enthalpy of mixing of the two liquids in the presence of C60, . This corresponds to the contributions due to the new interactions between C60 and the ionic liquid, the loss in the interactions between the C60 and DCB, and the gain of interactions between the ionic liquid and DCB. This last contribution is determined by “blank” experiment, without C60, where the enthalpy mixing of the two liquids, ΔmixHIL+DCB, is determined.

Data Reduction The heat effects involved in injections of small quantities of [C10C1Im][NTf2] in DCB (or of DCB in [C10C1Im][NTf2]) , QIL (or QDCB), are directly related to the (or of DCB in the IL, ), partial molar excess enthalpy of the IL in DCB, according to eq 1. 275 Shiflett and Scurto; Ionic Liquids: Current State and Future Directions ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


where nIL and nDCB denote the amounts of [C10C1Im][NTf2] and 1,2dichlorobenzene, respectively. ΔnIL and ΔnDCB are the quantities of [C10C1Im][NTf2] and DCB per injection calculated from the injected volumes and from the densitites reported in the literature (20, 21). If the enthalpy of mixing, ΔmixHIL+DCB, is represented by a Redlich-Kister equation, eq 2, where xIL and xDCB are mole fractions of [C10C1Im][NTf2] and DCB in the mixture, respectively, then and are obtained by appropriate derivatives with respect to composition, yielding eq 3 and 4 as described in references (18) and (22, 25).

The enthalpy of mixing of IL with DCB in the presence of C60, , is calculated the system in presence of C60 being simply treated as a binary system as the concentration was small, xC60 < 10–2.

The different in the interaction energy between C60 in DCB and C60 in [C10C1Im][NTf2] can be considered as the difference in the enthalpy of mixing of the two liquids in presence and in absence of fullerene using eq 7

276 Shiflett and Scurto; Ionic Liquids: Current State and Future Directions ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Results and Discussion The enthalpy of mixing of [C10C1Im][NTf2] with 1,2-dichlorobenzene was calculated from the experimental calorimetric data through the partial molar excess enthalpies of the two liquids in the mixture using eq 2. The experimental points were fitted to a Redlich-Kister function with three coefficients. The enthalpies of mixing as a function of composition, at 313 K, are represented in Figure 1. Because the experimental points were determined only at extreme mole fraction compositions, the Redlich-Kister fit at intermediary compositions, although still valid (25), is less significant. The mixing enthalpy is negative with a minimum of −164 J·mol-1 deviated to ionic liquid mole fractions lower than equimolar (at xIL = 0.5 ΔmixHIL+DCB = −84.2 J·mol-1), reflecting the energetically favourable interactions in the mixing process. These values are significantly less negative than the ones found for mixtures of 1-alkyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide ionic liquids with polar compounds and are closer to the reported values of the enthalpy of mixing with apolar liquids like toluene (e.g. ΔmixH ≈ −0.2 kJ·mol-1 for toluene + [C2C1Im][NTf2] at 298 K (23) or ΔmixH ≈ −0.6 kJ·mol-1 for toluene + [C4C1Im][NTf2] at 363 K (24)).

Figure 1. Enthalpy of mixing of [C10C1Im][NTf2] + 1,2-dichlorobenzene as a function of the mole fraction of [C10C1Im][NTf2], xIL, at 313 K. The Redlich-Kister fit at intermediary compositions is less significant as the experimental determinations are only at extreme compositions.

In the presence of C60, the measured values of the partial molar excess enthalpies were fitted to a Redlich-Kister polynomial as was done for the mixtures of [C10C1Im][NTf2] with 1,2-dichlorobenzene. The values are depicted in Figure 2 and it is observed that, when the ionic liquid [C10C1Im][NTf2] is added to the solution of C60 in 1,2-dichlorobenzene, the heat of mixing is more negative than when [C10C1Im][NTf2] is added to the solution pure 1,2-dichlorobenzene. 277 Shiflett and Scurto; Ionic Liquids: Current State and Future Directions ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 2. Enthalpies of mixing of [C10C1Im][NTf2] + 1,2-dichlorobenzene (empty symbols and dashed lines) and of [C10C1Im][NTf2] + C60 in 1,2-dichlorobenzene (full symbols and full lines). The differences encountered are already apparent in the heats directly measured by the calorimeter as seen in Figure 3.

Figure 3. Experimental heat effects of the additions of [C10C1Im][NTf2] to 1,2-dichlorobenzene (open symbols) and of [C10C1Im][NTf2] to C60 in 1,2dichlorobenzene (full symbols). The difference between the two curves represented in Figure 2 can be understood as the difference in the interaction energy of C60 with 1,2-dichlorobenzene and of C60 with [C10C1Im][NTf2], ΔΔHDCB(C60)−IL(C60). As can be seen in Figure 4, the values of ΔΔHDCB(C60)−IL(C60) are negative, indicating an exothermal heat effect due to interactions of the ionic liquid with C60, even if the solubility of the fullerene is much lower in [C10C1Im][NTf2] (10) than in 1,2-dichlorobenzene (2, 3). 278 Shiflett and Scurto; Ionic Liquids: Current State and Future Directions ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 4. Difference in the enthalpy of mixing of [C10C1Im][NTf2] + 1,2-dichlorobenzene and [C10C1Im][NTf2] + C60 in 1,2- dichlorobenzene obtained from eq 7.

Conclusion We present an experimental study on the thermodynamics of mixing of an imidazolium based ionic liquid with a solution of C60 in 1,2-dichlorobenzene. The authors show that isothermal titration calorimetry techniques are sufficiently precise to enable experimental access to the difference in the interaction energy between carbon-based materials (herein C60) and different solvents, in particular ionic liquids. It has been shown in this work that the addition of an imidazolium based ionic liquid to a solution of fullerene in an organic solvent (1,2-dichlorobenzene in the present case) is more exothermic than the addition of the ionic liquid to the pure organic solvent. The difference, low but significant (approximately −4 J·mol-1 of ionic liquid for xIL = 0.03 in a solution of concentration 0.0311 mol L-1 of C60 in DCB), can be attributed to the interaction between the ionic liquid and the fullerene.

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Experimental Study of the Interactions of Fullerene with Ionic Liquids

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