Interaction Energies of Ionic Liquids with Metallic Nanoparticles

Article pubs.acs.org/JPCC

Interaction Energies of Ionic Liquids with Metallic Nanoparticles: Solvation and Stabilization Effects Ajda Podgoršek,† Alfonso S. Pensado,† Catherine C. Santini,‡ Margarida F. Costa Gomes,† and Agílio A. H. Pádua*,† †

Laboratoire Thermodynamique et Interactions Moléculaires, Institut de Chimie de Clermont-Ferrand, Université Blaise Pascal & CNRS, 24 avenue des Landais, 63177 Aubière, France ‡ Université de Lyon, Institut de Chimie de Lyon, UMR 5265 CNRS-Université de Lyon 1-ESCPE Lyon, C2P2, Equipe Chimie Organométallique de Surface, ESCPE 43 Boulevard du 11 Novembre 1918, F-69616 Villeurbanne, France S Supporting Information *

ABSTRACT: The interaction energies and solvation structure of ruthenium nanoparticles (RuNPs) in ionic liquids are studied here by titration calorimetry and by molecular simulation. The size of metallic nanoparticles synthesized in situ in ionic liquids can be controlled, and the resulting suspensions are stable without additional surface-active molecules. However, little is known about the energetics and mechanisms of solvation of nanoparticles in these complex, structured solvents. Ionic liquids were added into a suspension of RuNPs in [C1C4Im][NTf2], and the heat effect was recorded. The background heat of mixing of the two ionic liquids was measured separately. The interaction energy of [C1CnIm][NTf2] (n = 6, 8, 10) with RuNPs is larger than that of [C1C4Im][NTf2] indicating that longer alkyl side chains enhance the interactions with RuNPs. [C1C2Im][NTf2] also has stronger interactions with the nanoparticles, but this cation does not possess a significant nonpolar moiety. Ionic liquids with lesser propensity to form Hbonds such as [C1C1C4Im][NTf2] or [C1C4Pyrro][NTf2] interact less favorably with RuNPs. No significant effect of the anion structure was observed when changing the ionic liquid from [C1C4Im][NTf2] to [C1C4Im][PF6]. Structural information from molecular simulation shows that the charged head groups of both the cations and the anions are in contact with the nanoparticle, with only small charge separation at the interface. Alkyl side chains tend to point away from the nanoparticle but are still within interaction range. The overall picture results from a balance between electrostatic, van der Waals, and H-bond forces, which changes between different ionic liquids.

1. INTRODUCTION Suspensions of metallic nanoparticles in ionic liquids are interesting systems in several fundamental and applied contexts, notably in catalysis but not exclusively. Metal nanoparticles in solution combine the mass-transport properties of homogeneous catalysis systems with high surface areas while retaining the selectivity of heterogeneous catalysts.1,2 Ionic liquids are tunable, stable solvents that enable separations and are recyclable. Furthermore, ionic liquids are among the most remarkable solvents for generation and stabilization of metallic nanoparticles,3−5 leading to a good control of size and to stabilization without any tensioactive or ligand additives.6,7 This possibility offers access to a variety of novel functional materials with a wide range of potential applications, for example, in catalysis, lubrication, and electrochemical devices.2,8,9 The mechanisms of solvation and stabilization of metallic nanoparticles in ionic liquids are the subject of different hypotheses at present,4,5,10−13 and we have recently studied theoretically the structure of the interfacial layer of ionic liquid solvating a metallic nanoparticle.14 Nevertheless, a complete understanding of the physical chemistry phenomena involved is not yet available, due to the complexity of the systems. The molecular interactions between the nanoparticles and the medium, and the structure of the interfacial layer, influence the © 2013 American Chemical Society

stability of the suspensions and govern the size distributions of metallic nanoparticles generated in situ.5,7 Ionic liquids (ILs) display a high degree of self-organization in the liquid state, in particular those salts that contain nonpolar alkyl chains in the structure of the ions. The positively and negatively charged moieties have a strong cohesive energy and form a continuous network, segregating the nonpolar groups into domains with sizes of nanometer scale.15 These persistent structural heterogeneities have been identified by experiments16−19 and molecular simulation,13,20 and they influence the solvation of molecular compounds.21,22 It has been postulated that the marked liquid-phase structure can be used as a template for the generation of spontaneous, well-defined nanoscale structures with long-range order.3 In the present work we determine experimentally the energies of interaction between ionic liquids and metallic nanoparticles, with the aim of improving our understanding about the mechanisms of solvation and stabilization of nanoparticle suspensions in these solvents. Received: September 12, 2012 Revised: December 28, 2012 Published: January 22, 2013 3537

dx.doi.org/10.1021/jp309064u | J. Phys. Chem. C 2013, 117, 3537−3547

The Journal of Physical Chemistry C

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[NTf2] and RuNPs in other ionic liquids, using isothermal titration calorimetry. We can thus evaluate the effect of the cation and the anion on the stabilization of RuNPs. The ionic liquids chosen were 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C1CnIm][NTf2] (n = 2, 4, 6, 8, 10), to test different alkyl side chain lengths; 1-butyl-2,3dimethylimidazolium bis(trifluormethylsulfonyl)imide, [C1C1C4Im][NTf2], and N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [C1C4Pyrro][NTf2], to test different cation head groups; and 1-butyl-3-methylimidazolium hexafluorophosphate, [C1C4Im][PF6], to test a change in the anion. The stabilization of colloids is related to free energy, which includes both energetic and entropic contributions, the latter being important in certain stabilization mechanisms such as steric stabilization by surface polymers or ligands. In this work we are not directly analyzing differences in stability, and our NPs are not decorated by surface ligands or polymers. We performed a study of the organization of the surface layer of the ionic liquids using molecular simulation, a technique that provides a microscopic view of the solvation mechanisms and completes the energetic properties obtained by calorimetry. Using an interaction model between ionic liquids and RuNPs developed recently,12 the structure of the solvation shell of ionic liquids surrounding the nanoparticles as well as energetic quantities were calculated.

Ruthenium nanoparticles (NPs) were created by decomposition of (η4-1,5-cyclooctadiene)(η6-1,3,5-cyclooctatriene)ruthenium(0), [Ru(COD)(COT)], with hydrogen. This route is well-known in both organic23,24 and ionic solvents.25 The good size control of the RuNPs produced in 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide, [C1C4Im][NTf2], was attributed to the structured nature of the reaction medium and depended also on the conditions employed,6 which limit the diffusion of the precursor and nuclei, resulting in restricted growth of NPs. However, the most important result in the context of the present work is that the size control of NPs generated in 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids was achieved by altering the length of the alkyl chains, which dictates the size of the nonpolar domains.7 Experimental information on the nature of the interfacial layer of ionic liquids near metal nanoparticles is not categorical. Certain sources in the literature postulate the existence of an electric double layer (the Derjaugin−Landau−Verwey−Overbeek model), in which a first solvation shell of anions surrounds the electropositive NP surface, followed by less ordered, alternating layers of cations and anions.26 A small-angle X-ray scattering study of IrNPs in various ionic liquids supports ordering of the ions around the NP surface, corroborating the DLVO model.24 Other results, namely, from surface-enhanced Raman spectroscopy of a suspension of gold nanoparticles (AuNPs) in 1-triethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate, point to close interactions of the AuNPs with the imidazolium cations.27 In other studies, the enhanced stability of NPs in ILs was attributed to the presence of the surface hydrides10 or to N-heterocyclic carbenes, formed in situ from imidazolium cations, as was shown for IrNPs.28 Still other studies suggest that NPs are solvated in nonpolar regions formed by aggregation of hydrophobic alkyl side chains of the ions since there is a relationship between the length scale of the structural heterogeneities of the ionic liquids13 and the size of nanoparticles synthesized therein.7 Moreover, the close proximity of the metallic surface to the nonpolar alkyl group of the imidazolium cation was suggested by labeling experiments.11 A recent molecular simulation study on solvation of RuNPs in the ionic liquid [C1C4Im][NTf2] shows that both cation head groups and anions are present near the nanoparticle, with only small charge separation, while alkyl side chains are directed away from the surface. This study also indicates that stabilization does not arise from the electrostatic double layer but from a template effect due to the heterogeneous structure of the ionic liquid.12 Structural and stability effects of polar solutes (water, 1octylamine) on the RuNPs in [C1C4Im][NTf2] have been studied recently, and the binding of 1-octylamine, as an additional stabilizing ligand, on the RuNP surface was shown by isothermal titration calorimetry (ITC).29 ITC has been demonstrated as a powerful and very sensitive technique for measuring interactions between nanoparticles (mainly in aqueous suspensions) and various molecules, such as amino acids,30 proteins,31 and DNA bases,32 for applications in biochemistry. Research has also focused on the enthalpy of formation of metallic nanoparticles,33 as well as interactions between different NPs.34 However, the interactions between NPs and their medium, particularly ionic liquids, have not yet been studied experimentally to the best of our knowledge. We devised an original experimental protocol to obtain the difference in interaction energy between RuNPs in [C1C4Im]-

2. EXPERIMENTAL SECTION Materials. 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C1C4Im][NTf2], and 1-butyl2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, [C1C1C4Im][NTf2], were synthesized following a route reported in the literature.35 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (99.5%), [C1C2Im][NTf2], 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (99.5%), [C1C6Im][NTf2], 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (99.5%), [C1C8Im][NTf2], and 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (99.5%), [C1C10Im][NTf2], were acquired from Iolitec, Ionic Liquids Technologies. N-Butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (98.5%), [C1C4Pyrro][NTf2], and 1-butyl-3-methylimidazolium hexafluorophosphate (98.5%), [C1C4Im][PF6], were purchased from Aldrich. Ionic liquids were degassed and dried under vacuum at room temperature. The water content of the ionic liquids was determined by coulometric Karl Fischer titration (Mettler Toledo DL31) and was found to be less than 100 ppm in all samples used. In the following text, for simplification [C1C4Im][NTf2] is denoted as IL1 and any other ionic liquid ([C 1 C 2 Im][NTf 2 ], [C 1 C 6 Im][NTf 2 ], [C 1 C 8 Im][NTf 2 ], [C1C10Im][NTf2], [C1C1C4Im][NTf2], [C1C4Pyrro][NTf2], and [C1C4Im][PF6]) as IL2. Ruthenium nanoparticles were synthesized in [C1C4Im][NTf2] by decomposition of (η4-1,5-cyclooctadiene)(η6-1,3,5cyclooctatriene)ruthenium(0) under hydrogen atmosphere (4 bar), for 18 h at 25 °C (2.3 nm) and for 3 days at 0 °C (1.1 nm), as already reported.6 TEM images of suspensions of ruthenium nanoparticles with size distribution histograms are presented in the Supporting Information (Figure S1). The samples of 2.3 ± 0.6 nm and 1.1 ± 0.2 nm RuNPs with concentration of 0.13 mM were used, as reported in Figure 3 for each case. 3538



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xIL2 → 0. This allows a check on the consistency and accuracy of the results. All data of partial enthalpies of mixing of IL2 in IL1, IL2 in IL1, and IL2 in IL1(NP) are reported in the Supporting Information (Tables S1 and S2). The differential energies measured as described include not only direct terms due to the different intrinsic interactions of the ions with the nanoparticle, as would be obtained from isolated ion−NP pairs in vacuum, but also contributions arising from reorganizations of the interfacial layer of the ionic liquid in the presence of the nanoparticle. The latter are related to structural, or “entropic”, effects and are an integral part of the thermodynamic quantities obtained in the condensed phase. The reduction of the raw calorimetric data to derive partial molar enthalpies of mixing proceeded as described hereafter. The heat effects involved in injections of small quantities of IL2 into IL1 (and to a IL1 + IL2 mixture), QIL2, are directly related to the partial molar excess enthalpy of IL2 in the mixture, H̅ EIL2, according to eq 1. Analogously, the heat effects involved in injections of small quantities of IL1 into IL2 (and to a IL1 + IL2 mixture), QIL1, are related to partial molar excess enthalpies of IL1, H̅ EIL1.

Isotermal Titration Calorimetry. Calorimetric measurements were performed at 298.15 K and atmospheric pressure using an isothermal titration Nanocalorimeter (TA Instruments) equipped with 1 mL glass cells in a Thermal Activity Monitor TAM III thermostat (TA Instruments). The temperature of the thermostat is controlled precisely within 10−5 K. Detailed information about the calibration procedure is reported elsewhere.9 During the course of the titration experiments the sample in the measuring cell (pure IL, mixture of IL, or suspension of nanoparticles in IL) was stirred by a propeller at 120 rpm. In all the experiments the reference cell of the calorimeter was filled with pure ionic liquid, with approximately the same mass as in the measuring cell. Pure IL was injected in volumes of 4 μL (from a 250 μL gastight Hamilton syringe) during 60 s using a motor-driven pump (Thermometric 3810 Syringe Pump), allowing 15−20 min intervals between consecutive injections. In each experiment around 55 injections were performed. The cannula linking the syringe and the cell was immersed in the sample 5 min prior to the first injection. The measurements of each system were performed three times (IL1 + IL2) or twice (IL1(NP) + IL2) to obtain reproducible values in calculated mixing enthalpies, i.e., within ±3% and ±5% (max RAAD, %) for pure ionic liquids and NP suspensions, respectively. The area under each peak, corresponding to an addition of ionic liquid, is proportional to the heat effect Qi. Integration of all peaks from the recorded calorimetric plots was performed using the TAM Assistant software. The interaction energy between RuNPs and ionic liquid was determined from a combination of two titration experiments. In the first experiment, the sample cell of the calorimeter was carefully filled with around 0.52 mL (∼0.73 g) of the black colloidal suspension of RuNPs (1.1 or 2.3 nm, as noted in Figure 3) in [C 1 C 4 Im][NTf 2 ] (IL1) with the molar concentration of 0.13 mM. Handling the RuNPs in the presence of air was avoided as much as possible: when the titration unit containing the cell with IL1(NP) suspension was inserted to the calorimetric block, the atmosphere above the sample was flushed with nitrogen, and the system was hermetically closed. The reference cell of the calorimeter was filled with pure IL1. IL2 was then injected into the suspension of RuNPs in IL1, and the heat effect measured corresponds to the enthalpy of mixing of two ionic liquids IL1 and IL2 in the presence of NPs, ΔmixHIL1(NP)+IL2. This term comprises the gain in the interactions between NPs and the ionic liquid added (IL2), the loss in the interactions between NPs and IL1, and the gain in the interactions between ion pairs of the two different ionic liquids, IL2 and IL1. To subtract the information about the interactions between the two ionic liquids, a second (blank) experiment was performed following the analogous procedure described above, but without NPs. Around 0.7 mL of degassed IL1 was introduced into a 1 mL glass measuring cell, and degassed IL2 was loaded in a 250 μL gastight Hamilton syringe. IL2 was injected into IL1, and the heat effects, related to the mixing enthalpies of two ILs, ΔmixHIL1+IL2, were measured and subtracted from the enthalpy values obtained in the presence of NPs (ΔmixHIL1(NP)+IL2) to remove the contribution from the interactions between two different ionic liquids. Because of the importance of the values of mixing enthalpies of two ionic liquids, heat effects were determined also on the other part of the composition range by adding IL1 to IL2, thus covering both limiting composition ranges in mole fraction, xIL2 → 1 and

⎛ ∂(n + nIL2)Δmix HIL1 + IL2 ⎞ E = ⎜ IL1 H̅IL2 ⎟ ∂nIL2 ⎝ ⎠ p,T ,n

≈

IL1

Q IL2 ΔnIL2

⎛ ∂(n + nIL2)Δmix HIL1 + IL2 ⎞ E H̅IL1 = ⎜ IL1 ⎟ ∂nIL1 ⎝ ⎠ p,T ,n

and

IL2

≈

Q IL2 ΔnIL1

(1)

where nIL1 and nIL2 denote the amounts of IL1 and IL2, respectively; ΔmixHIL1+IL2 is the enthalpy of mixing of the two ionic liquids. ΔnIL1 and ΔnIL2 are the quantities of IL1 and IL2 per injection calculated from the injected volumes and the densities reported in the literature ([C1C2Im][NTf2],36,37 [C 1 C 4 Im][NTf 2 ], 36,37 [C 1 C 6 Im][NTf 2 ], 37,38 [C 1 C 8 Im][NTf2],37,38 [C1C10Im][NTf2],37,38 [C1C1C4Im][NTf2],39 [C1C4Pyrro][NTf2],40 and [C1C4Im][PF6]41). If the enthalpy of mixing, ΔmixHIL1+IL2, is represented with a Redlich−Kister equation, eq 2, where xIL1 and xIL2 are mole fractions of IL1 and IL2 in the mixture, respectively, then partial excess enthalpies H̅ EIL1 and H̅ EIL2 are obtained by the appropriate derivatives with respect to composition, yielding eqs 3 and 4. n

Δmix HIL + IL2 = (1 − x IL2)x IL2 ∑ Ai (1 − 2x IL2)i i=0 n

= (1 − x IL1)x IL1 ∑ Ai (2x IL1 − 1)i i=0

⎛ ∂Δ H ⎞ E = Δmix HIL1 + IL2 + (1 − x IL2)⎜ mix IL1 + IL2 ⎟ H̅IL2 ∂x IL2 ⎝ ⎠ p,T ,x

(2)

IL1

n −1 + i

= (x IL2 − 1) (x IL2 ∑ −2iAi (1 − 2x IL2) 2

i=0 n

+

∑ Ai(1 − 2xIL2)i ) i=0

3539

(3)


The Journal of Physical Chemistry C ⎛ ∂Δ H ⎞ E = Δmix HIL1 + IL2 + (1 − x IL1)⎜ mix IL1 + IL2 ⎟ H̅IL1 ∂x IL1 ⎝ ⎠ p,T ,x

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the enthalpy of mixing of ionic liquids IL1 and IL2 in the presence of NPs, ΔmixHIL1(NPs)+IL2, is calculated using the integrated form, eq 8. The parameters Ai,NPs are listed in Table S4 (Supporting Information). The system in the presence of NPs was simply treated as a binary system as the concentration of NPs was small, xNPs < 4 × 10−5.

IL2

n

= (x IL1 − 1)2 (x IL1 ∑ −2iAi (1 − 2x IL1)−1 + i i=0 n

+

∑ Ai(1 − 2xIL1)i ) i=0

n E = (x IL2 − 1)2 (x IL2 ∑ −2iAi ,NPs(1 − 2x IL2)−1 + i H̅IL2,NPs

(4)

i=0 n

When IL2 was added to IL1, the experimentally obtained partial excess enthalpies of H̅ EIL2 were directly fitted to eq 3. In the same way, adding IL1 to IL2 gave partial excess enthalpies of H̅ EIL1 which were fitted to eq 4. Thus, two series of Redlich− Kister parameters, Ai, for two concentration ranges measured (0.0 < xIL2 < 0.3 and 0.7 < xIL2 < 1.0) were obtained (Table S3, Supporting Information). For each series of parameters the corresponding values of ΔmixHIL1+IL2 were calculated in the measured composition range. Next, all the obtained data of ΔmixHIL1+IL2 were refitted to eq 2 to obtain sets of Ai parameters valid for the entire composition range (Table S3, Supporting Information). The functions of the calculated enthalpies of mixing are presented in Figure 1.

+

∑ Ai(1 − 2xIL2)i )

(7)

i=0

n

Δmix HIL1(NPs)IL2 = (1 − x IL2)x IL2∑ Ai ,NPs(1 − 2x IL2)i i=0

(8)

The difference in the interaction energy between RuNPs with IL2 and RuNPs with IL1 was calculated as the difference of the enthalpy of mixing two ionic liquids in the presence and absence of ruthenium nanoparticles, shown in eq 9. ΔΔHIL2(NPs) − IL1(NPs) = Δmix HIL1(NPs) +IL2 − Δmix HIL1 + IL2 n

= (1 − x IL2)x IL2(∑ (Ai ,NPs − Ai )(1 − 2x IL2)i ) i=0

Molecular Dynamics Simulations. Three ionic liquid systems were simulated in the presence of RuNP: [C1C4Im][NTf2], [C1C10Im][NTf2], and an equimolar mixture of [C1C4Im][NTf2] + [C1C10Im][NTf2]. In the three cases, 848 ion pairs solvating one nanoparticle composed of 323 ruthenium atoms (around 2 nm in diameter) were present in the simulation boxes, with cubic periodic boundary conditions. To assess the effect of system size the simulations containing [C1C10Im][NTf2] and the RuNP were also performed with 670 ion pairs. Details of the simulation methods and conditions and of the interaction potential model were reported in a previous publication,12 so here only the main features of the calculations are reported. The ionic liquid was represented by an all-atom classical force field developed specifically for the series of cations and anions used here.42 The ruthenium atoms interacted through a Finnis−Sinclair potential43 with specific parameters for this metal. The interactions of the atoms of the ions with the metal atoms were described by a specific potential model obtained from quantum calculations using the M06L density functional,44 which is adapted to describe nonbonded interactions. Explicit polarization of the metal atoms by the electrostatic charges in the ions was taken into account using a Drude oscillator model.45 This set of interaction potentials provides a complete description of the atomistic interactions in the systems containing the nanoparticle solvated by the ionic liquids and allows the production of molecular dynamics trajectories from which several thousand configurations are stored for further analysis. The results given below are averages from equilibrated molecular dynamics trajectories of 1 ns with a time step of 2 fs (after proper equilibration as described in ref 12) at constant NpT with T = 423 K and p = 1 bar. The higher temperature is justified to improve sampling because of the high viscosity and slow dynamics of the ionic liquids. Uncertainties are calculated by a block average procedure. Long-range corrections for the electrostatic energy were calculated using the Ewald summation method with a relative error below 10−4 in the electrostatic energy.

Figure 1. Partial excess enthalpies of ionic liquid IL2 in the mixture IL1 + IL2 determined by fitting experimental data into eq 3. Parameters Ai of fitted functions are reported in Table 1. IL1: [C1C4Im][NTf2]. IL2: red, [C1C2Im][NTf2]; blue, [C1C6Im][NTf2]; dark green, [C1C8Im][NTf2]; purple, [C1C10Im][NTf2]; pink, [C1C1C4Im][NTf2]; bright green, [C1C4Pyrro][NTf2]; and orange, [C1C4Im][PF6]. (Symbols are plotted just to help distinguish curves.)

The partial molar excess enthalpies for IL2 and IL1 at infinite dilution were calculated as follows in eqs 5 and 6, respectively, from the third series of Ai parameters (Table S3, Supporting Information). n E, ∞ E = lim H̅IL2 = H̅IL2 x IL2 → 0

∑ Ai i=0

(5)

n E, ∞ E = lim H̅IL1 = H̅IL1 x IL1→ 0

∑ (−1)i Ai i=0

(9)

(6)

Analogously, in the presence of ruthenium nanoparticles the experimental data of the partial excess enthalpies of IL2, H̅ EIL2,NPs, were fitted with eq 7, yielding parameters Ai,NPs, and 3540



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The total system energy was decomposed into “intermolecular” energies between pairs of entities, such as cation− anion, cation−NP, etc. These pair interaction energies were obtained by subtracting the energies of parts of the system ruthenium atoms, cations, or anionsfrom the total energy of the system’s configurations. For example, to obtain the interaction energy of cations with the nanoparticle, first the energy of the configurations was recalculated with only Ru atoms present (in the same positions), ENP, while zeroing the interaction terms of all other particles in the system. Second, the energy with only the cations, Ecat, was recalculated likewise. Third, the energy of cations and Ru atoms both present, Ecat+NP, was recalculated. The “intermolecular” interaction energy between the cations and the nanoparticle is the difference ΔEcat−NP = Ecat+NP − (ENP + Ecat).

Information). Enthalpies of mixing were calculated from these and fitted using a Redlich−Kister series (eq 2) to obtain sets of parameters valid for the entire composition range (cf. Supporting Information for calculation procedure). Figure 1 shows the partial molar excess enthalpies of added ionic liquid IL2 as a function of its mole fraction. Figure 2 contains plots of enthalpy of mixing. When one ionic liquid IL2 is added into another with a common anion, for instance, [C1CnIm][NTf2] (n = 2, 6, 8, 10), [C1C1C4Im][NTf2], or [C1C4Pyrro][NTf2] into their respective mixtures with [C1C4Im][NTf2], the partial excess enthalpies at infinite dilution directly reflect the interactions between the cations. E,∞ The values of H̅ E,∞ IL1 and H̅ IL2 reported in Table 1 lead to are positive indicating several conclusions. All values of H̅ E,∞ i that interactions in the presence of unlike cations are less favorable than in pure ionic liquids. Comparison across the homologous series C1CnIm+ shows that the unfavorable interactions increase as the difference in the alkyl chain length increases. The difference between the two partial enthalpies at E,∞ infinite dilution, H̅ E,∞ IL1 and H̅ IL2 , increases with increasing number of carbons in the alkyl chain and becomes important for n ≥ 8. This difference is small for [C1C4Im][NTf2] + [C1C2Im][NTf2] and [C1C4Im][NTf2] + [C1C6Im][NTf2] systems, meaning that there is no important difference in energetic effects when either [C1C2Im][NTf2] or [C1C6Im][NTf2] is dissolved in [C1C4Im][NTf2] or vice versa. It is enthalpically favorable to dissolve ionic liquids with shorter alkyl chains in ionic liquids with longer alkyl chains in the imidazolium cation, i.e., [C1C4Im][NTf2] in [C1C8Im][NTf2] or in [C1C10Im][NTf2], than the opposite. The H̅ E,∞ IL1 values are larger than H̅ E,∞ when IL2 has a longer alkyl side chain due to IL2 the importance of van der Waals interactions between these cations. Other effects can be discerned, namely, that both the presence of an additional methyl group on the C2 carbon of the imidazolium cations and changing the cation from imidazolium to pyrrolidinium lead to much smaller values for the excess partial molar enthalpies, due to weaker cation−anion interactions through reduced hydrogen bonding. Romani et al.46 reported that the mixing of two ionic liquids with a common cation, such as [C1C4Im][PF6] + [C1C4Im][BF4], is exothermal, leading to the conclusion that interactions between unlike anions are more favorable than between like ones. Contrary to those results, we have observed that, likewise to the systems containing ionic liquids with a common anion, mixing two different ionic liquids sharing a common cation, such as [C1C4Im][PF6] + [C1C4Im][NTf2], is endothermic. The large positive values of H̅ E,∞ indicate much less favorable i

3. RESULTS AND DISCUSSION Partial excess enthalpies of ionic liquid IL2 ([C1C2Im][NTf2], [C 1 C 6 Im][NTf 2 ], [C 1 C 8 Im][NTf 2 ], [C 1 C 10 Im][NTf 2 ],

Figure 2. Enthalpies of mixing of ionic liquids IL1 + IL2 as a function of mole fraction of IL2, xIL2. Parameters Ai of fitted functions are reported in Table 1. IL1: [C1C4Im][NTf2]. IL2: red, [C1C2Im][NTf2]; blue, [C1C6Im][NTf2]; dark green, [C1C8Im][NTf2]; purple, [C 1 C 10 Im][NTf 2 ]; pink, [C 1 C 1 C 4 Im][NTf 2 ]; bright green, [C1C4Pyrro][NTf2]; and orange, [C1C4Im][PF6]. (Symbols are plotted just to help distinguish curves.)

[C1C1C4Im][NTf2], [C1C4Pyrro][NTf2], and [C1C4Im][PF6]) in mixtures with IL1 ([C1C4Im][NTf2]) were determined experimentally at 298.15 K at both ends of the composition range (0 < xIL2 < 0.3 and 0.7 < xIL2 < 1, Table S1, Supporting

Table 1. Parameters Ai in Equation 3 Obtained by Fitting Experimental Data of Partial Excess Enthalpies When Mixing IL1 and IL2 at the Concentration Range of 0 < xIL2 < 0.3 and 0.7 < xIL2 < 1a IL2

A0 (J mol−1)

A1 (J mol−1)

A2 (J mol−1)

[C1C2Im][NTf2] [C1C6Im][NTf2] [C1C8Im][NTf2] [C1C10Im][NTf2] [C1C1C4Im][NTf2] [C1C4Pyrro][NTf2] [C1C4Im][PF6]

232 206 726 1434 52 17 1656

−33 12 184 447 −6 17 −211

−3 −6 46 70 14 1 88

−1 H̅ E,∞ IL1 (J mol )

262 189 588 1057 71 1 1955

± ± ± ± ± ± ±

17 6 12 28 11 2 275

−1 H̅ E,∞ IL2 (J mol )

196 212 956 1951 60 35 1533

± ± ± ± ± ± ±

7 8 44 78 6 1 27

a

Partial excess enthalpies at infinite dilution of ionic liquids IL2 and IL1 in the mixture IL1 + IL2. IL1: [C1C4Im][NTf2]. IL2: [C1C2Im][NTf2], [C1C6Im][NTf2], [C1C8Im][NTf2], [C1C10Im][NTf2], [C1C1C4Im][NTf2], [C1C4Pyrro][NTf2], and [C1C4Im][PF6]. 3541



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Figure 3. Enthalpies of mixing of ionic liquids IL1 + IL2 obtained by fitting experimental data. Parameters Ai of fitted functions are reported in Table S3 and Table S4 (Supporting Information). Open symbols represent IL1 + IL2 systems, whereas full symbols represent IL1(NPs) + IL2. IL1: [C1C4Im][NTf2]. IL2: red, [C1C2Im][NTf2]; blue, [C1C6Im][NTf2]; dark green, [C1C8Im][NTf2]; purple, [C1C10Im][NTf2]; pink, [C1C1C4Im][NTf2]; bright green, [C1C4Pyrro][NTf2]; and orange, [C1C4Im][PF6]. For (a) and (d) the mean size of RuNPs was 2.3 ± 0.6 nm and for (b), (c), (e), (f), and (g) 1.1 ± 0.2 nm. Dotted lines represent error bars. (Symbols are plotted just to help distinguish curves.)

interactions between the PF6− and NTf2− than was observed with any two different cations in this study. We can reach the same conclusions looking at the calculated enthalpies of mixing of ionic liquids IL1 + IL2 as a function of mole fraction of IL2, presented in Figure 2. Uncertainties in enthalpies of mixing are within ±3%, and details are given together with the coefficients of the Redlich−Kister polynomials in the Supporting Information. Mixing enthalpies are positive and hence reflect the net unfavorable interactions in the mixing process. It can be observed that the values are

smaller (the maximum for the less favorable system, i.e., [C1C4Im][PF6] + [C1C4Im][NTf2], is around 400 J mol−1) in comparison with systems containing an ionic liquid and an associative compound, such as water or alcohol, where the maximum values of ΔmixH are around 1.5−2.5 kJ mol−1. Not surprisingly, mixing two components with the same ionic nature leads to a smaller difference in the interactions in the mixture from the respective pure substances, in comparison to mixing an ionic and a molecular compound with strongly associated molecules. 3542



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otherwise three parameters were sufficient. The best-fit coefficients Ai are given in Table S4 (Supporting Information). Figure 3 represents the enthalpies of mixing of IL1 and IL2 as a function of xIL2, with open symbols presenting ionic liquid systems and full symbols the suspensions of NPs. When 1-alkyl-3-methylimidazolium ionic liquids, [C1CnIm][NTf2] (n = 2, 4, 6, 8, 10), were added to the suspension of RuNPs in [C1C4Im][NTf2], values of enthalpy of mixing are below the values in the absence of NPs. On the contrary, adding ionic liquids containing the 1-butyl-2,3-dimethylimidazolium or N-butyl-N-methylpyrrolidinium cations results in higher values of enthalpies of mixing when NPs were present. No significant difference in the enthalpies of mixing is observed for [C1C4Im][PF6] added to [C1C4Im][NTf2] with or without NPs. The difference between the two sets of data corresponds to the difference in the interaction energy between RuNPs and IL2 and RuNPs and IL1. As can be seen from Figure 4 the values of ΔΔHIL2(NP)‑IL1(NP) are negative for 1-alkyl-3methylimidazolium bis(trifluoromethylsulfonyl)imides indicating that RuNPs interact favorably with the ionic liquids with longer alkyl chains in the imidazolium cations than with [C1C4Im][NTf2]. The strength of these interactions increases with increasing length of the alkyl chains, except for [C1C2Im][NTf2]. This short-chain ionic liquid is an outlier in the homologous series and can be considered as not possessing a nonpolar group. Indeed, the [C1C2Im][NTf2] ionic liquid does not show nonpolar domain aggregation19 and does not behave as longer alkyl chain counterparts in terms of size control of nanoparticles synthesized in situ.7 Comparison of [C1CnIm][NTf2] and [C1C1C4Im][NTf2] shows that introduction of an additional methyl group at the C2 carbon of the imidazolium ring strongly affects the interactions with NPs, which become less favorable. The same behavior was perceived when N-butyl-N-methylpyrrolidinium ionic liquid was added to suspensions in 1-butyl-3methylimidazolium ionic liquid. Finally, no significant effect of the anion structure was seen when changing the ionic liquid from [C1C4Im][NTf2] to [C1C4Im][PF6]. To complement the enthalpy data from the calorimetric experiments, molecular simulations were performed as described above. Molecular simulations of the present systems do not allow the calculation of differences in enthalpy of mixing within a few tens of J mol−1 because of statistical errors. However, the results of energy decomposition that yield the interaction energies of the ions with the nanoparticle are pertinent and are summarized in Table 2. It is seen that the differences observed from the several ionic liquids are still small compared to the statistical uncertainties of the simulations. Also, a measurable effect of system size is noticeable, with attractive energies increasing when using a larger number of ion pairs, although at the limit of the mutual uncertainties. We can presume that the results for the larger systems are closer to convergence. The results from energy decomposition do not

Figure 4. Difference in the interaction energy between RuNPs in IL2 and RuNPs in IL1, ΔΔHIL2(NPs)−IL1(NPs), obtained from eq 9. IL1: [C1C4Im][NTf2]. IL2: red, [C1C2Im][NTf2]; blue, [C1C6Im][NTf2]; dark green, [C1C8Im][NTf2]; purple, [C1C10Im][NTf2]; pink, [C1C1C4Im][NTf2]; bright green, [C1C4Pyrro][NTf2]; and orange, [C1C4Im][PF6]. Dotted lines represent error bars.

For [C1CnIm][NTf2] + [C1C4Im][NTf2] the values increase with increasing the difference in the alkyl chain length of the imidazolium cation, as could be expected from the partial molar enthalpies at infinite dilution. Again, mixing [C1C4Im][NTf2] with the other ionic liquids containing NTf2− but with weaker cation−anion interactions, such as [C1C1C4Im][NTf2] and [C1C4Pyrro][NTf2], results in very small values of mixing enthalpies, indicating a slight difference between the interactions in the pure systems and in the mixtures. For a change of anion in the system [C1C4Im][PF6] + [C1C4Im][NTf2], large positive values of enthalpies of mixing show a strongly endothermic mixing. The partial molar excess enthalpies of ionic liquid IL2 were determined in the mixture with IL1 in the presence of ruthenium nanoparticles. Injections of pure IL2 into suspension of RuNPs in [C1C4Im][NTf2] were performed at 298.15 K to cover the concentration range of the added ionic liquid IL2 up to 0.4 in fraction molar. It should be noted that two different samples of RuNPs were used. [C 1 C 2 Im][NTf 2 ] and [C1C10Im][NTf2] were injected into a suspension of RuNPs in [C1C4Im][NTf2] with a mean size of 2.3 ± 0.6 nm, whereas in all other cases samples with 1.1 ± 0.2 nm NPs were used. In fact, when the effect of the size of the NPs on the interaction energy with ionic liquids was studied no significant influence was observed, as the difference in heat effects measured for the two samples of NPs was within the experimental error (±5%). In the presence of NPs, the measured values of partial excess molar enthalpies were fitted into eq 2, as was done for the mixtures of ionic liquids. To better fit the experimental data to Redlich−Kister equations, five coefficients were used only in the case of [C1C4Pyrro][NTf2] and [C1C1C4Im][NTf2];

Table 2. Interaction Energies between Ionic Liquids and a Ruthenium Nanoparticlea ΔEcat−NP/kJ mol−1 ΔEan−NP/kJ mol−1 ΔEIL−NP/kJ mol−1 a

[C1C4Im][NTf2]

[C1C4Im][NTf2] + [C1C10Im][NTf2]

[C1C10Im][NTf2]

[C1C10Im][NTf2] 670 ion pairs

−5330 ± 73 −3853 ± 51 −9182 ± 91

−5354 ± 78 −3865 ± 52 −9210 ± 69

−5380 ± 105 −3828 ± 79 −9209 ± 77

−5276 ± 73 −3749 ± 88 −9025 ± 104

Energies are given per mole of nanoparticle; 848 ion pairs present except where otherwise indicated. 3543



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Figure 5. Radial distribution functions of the ionic liquids surrounding a ruthenium nanoparticle. Left: distribution of atoms of [C1C10Im][NTf2] from the center of the NP. C2 indicates the carbon atom in the imidazolium ring. Right: an equimolar mixture of [C1C4Im][NTf2] + [C1C10Im][NTf2].

Figure 6. Radial distribution functions of the anions surrounding a ruthenium nanoparticle. Left: distribution of atoms from the anions in [C1C10Im][NTf2] from the center of the NP. Right: distribution of atoms from the anions in an equimolar mixture of [C1C4Im][NTf2] + [C1C10Im][NTf2].

Figure 7. Electrostatic charge density profiles from the center of the NP. Left: [C1C10Im][NTf2]. Right: equimolar mixture of [C1C4Im][NTf2] + [C1C10Im][NTf2].

structured first shell of anions in contact with the NP surface, as seen in Figure 6. The same effect is noticeable in the positively charged head groups of the cations (Figure 5, right-hand plot). These structural features indicate that the greater attractive forces between RuNPs and [C1C10Im][NTf2] are due to stronger electrostatic interactions with the ions, as a result of a more structured solvation layer. As in our previous work on [C1C4Im][NTf2], a small charge separation is observed around the nanoparticle, with charge-density fluctuations screened at about 5 Å from the nanoparticle surface. In Figure 7 it is seen that in [C1C10Im][NTf2] the charge separation is more pronounced than in the mixture or in [C1C4Im][NTf2], again as a result of a more structured solvation layer.

contradict the experimental data obtained by titration calorimetry: although lying within the error bars, the interactions with the ionic liquids containing the longer alkyl chains correspond to equivalent or slightly more attractive values. It is interesting to compare these energetic quantities with the study on the solvation structure of the [C1C4Im][NTf2] ionic liquid surrounding the Ru nanoparticle reported previously.12 From the structural point of view, it was noticed that the positively charged imidazolium head groups of the cations and the anions were both close to the nanoparticle surface, whereas the alkyl side chains pointed preferentially away from the nanoparticle, as shown in Figure 5. When comparing the solvation structure between [C1C4Im][NTf2],12 [C1C10Im][NTf2], and their mixture, it is seen that the longer alkyl chains are farther away from the nanoparticle, allowing a more 3544



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been identified in studies of cohesive energy47 and is characteristic of room-temperature ionic liquids.

4. CONCLUSIONS In this work, we studied the interactions between ruthenium nanoparticles (RuNPs) and ionic liquids changing several characteristics of the ions. Isothermal titration calorimetry enabled experimental access to the difference in the interaction energy between RuNPs in [C1C4Im][NTf2] and RuNPs in other ionic liquids and therefore to the effect of the structure of the cation and the anion on stabilization of RuNPs. In parallel to the experimental studies, molecular simulations provided access to energetic quantities and also to the distribution of the ions and of electrostatic charges surrounding the nanoparticle. First, we studied the interactions of ionic liquids [C1CnIm][NTf2] having alkyl chains of different lengths and suspensions of RuNPs in [C1CnIm][NTf2]. It was found that the difference in the interaction energy between RuNPs in [C1CnIm][NTf2] and RuNPs in [C1C4Im][NTf2] is negative over the studied composition range, which indicates that RuNPs interact favorably with the ionic liquids that have longer alkyl chain of imidazolium cation than with [C1C4Im][NTf2]. The strength of these interactions increases with increasing the length of the alkyl chain, showing that van der Waals interactions are important in these systems. One exception is [C1C2Im][NTf2], a short chain ionic liquid that has stronger attractive interactions with the RuNPs, but this ionic liquid is particular within the series studied since it does not contain a significant nonpolar moiety. Second, ionic liquids with the same anion but either with an additional methyl group on the C2 carbon of the cation, [C1C1C4Im][NTf2], or composed of the N-butyl-Nmethylpyrrolidinium cation interact less favorably with RuNPs than [C1C4Im][NTf2], showing that reducing the role of hydrogen bonds leads to less favorable interactions with RuNPs. The values of difference in the interaction energy between RuNPs in [C1C1C4Im][NTf2] and RuNPs in [C 1 C 4 Im][NTf 2 ] are much more positive than for [C1C4Pyrro][NTf2]. Finally, no significant effect of the anion structure when changing the ionic liquid from [C1C4Im][NTf2] to [C1C4Im][PF6] on the interaction energy with ruthenium nanoparticles was observed. Energetic quantities obtained from simulation on systems [C1C10Im][NTf2] + RuNP and [C1C4Im][NTf2] + RuNP agree with the experimental values, although the differences obtained are close to the limit of resolution of the simulations. The most interesting results from the simulation concern the structural information about how the ions surround the nanoparticle. It is seen that both the charged head groups of the cations and the anions are in close contact with the nanoparticle, with small charge separation effects. The alkyl side chains tend to point away from the nanoparticle but are still within interaction range (4 Å). This concurs with the experimental observation that changing both the nature of the cation, for example, the ability to establish H-bonds, and the length of the alkyl side chains influences the interactions with the metal clusters. Surprisingly, no significant difference is seen between anions NTf2− and PF6−, a result we cannot explain except by a coincidence: although NTf2− is larger and flexible and its electrostatic charge is more distributed, the four O atoms do provide significant attractive forces with a metal cluster.12 The overall picture is a complex one, with both H-bond, electrostatic, and van der Waals terms contributing to the interactions of the ionic liquids with the metallic nanoparticle. Such a balance between different terms in the interactions has

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ASSOCIATED CONTENT

S Supporting Information *

TEM pictures of RuNPs in [C1C4Im][NTf2] after titration with [C1C10Im][NTf2]. Values of partial molar excess enthalpies of IL2 in IL1 + IL2 and in IL1(NPs) + IL2 suspension measured experimentally by isothermal titration calorimetry at 298.15 K and atmospheric pressure. Calculation procedure used in the treatment of experimental data. Tables with parameters Ai obtained by fitting experimental data of partial excess enthalpies of IL2 in the mixture of IL1 + IL2 or IL1(NPs) + IL2 to derivative of the Redlich−Kister equation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +33473407166. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.P. is financed by the Contrat d’Objectifs Partagés, CNRSUBP, Région Auvergne, France. A.P. thanks Dr. J. Jacquemin (Queens University, Belfast) for discussions concerning the fitting of the experimental data.

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LIST OF ABBREVIATIONS AND SYMBOLS IL1: [C1C4Im][NTf2], 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide IL2: [C1CnIm][NTf2], 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides (n = 2, 6, 8, 10) [C1C1C4Im][NTf2], 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [C1C4Pyrro][NTf2], N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C1C4Im][PF6], 1-butyl-3-methylimidazolium hexafluorophosphate RuNPs, NPs, ruthenium nanoparticles ΔmixHIL1+IL2, molar enthalpy of mixing of IL1 and IL2 ΔmixHIL1(NPs)+IL2, molar enthalpy of mixing of suspension of RuNPs in IL1 and IL2 H̅ EIL1, partial molar excess enthalpy of IL1 H̅ EIL2, partial molar excess enthalpy of IL2 H̅ E,∞ IL1 , partial molar excess enthalpy of IL1 at infinite dilution H̅ E,∞ IL2 , partial molar excess enthalpy of IL2 at infinite dilution REFERENCES

(1) Roucoux, A.; Schulz, J.; Patin, H. Reduced Transition Metal Colloids: A Novel Family of Reusable Catalysts? Chem. Rev. 2002, 102 (10), 3757−3778. (2) Astruc, D.; Lu, F.; Aranzaes, J. R. Nanoparticles As Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis. Angew. Chem., Int. Ed. 2005, 44 (48), 7852−7872. (3) Antonietti, M.; Kuang, D.; Smarsly, B.; Zhou, Y. Ionic Liquids for the Convenient Synthesis of Functional Nanoparticles and Other Inorganic Nanostructures. Angew. Chem., Int. Ed. 2004, 43, 4988− 4992. (4) Dupont, J.; Scholten, J. D. On the Structural and Surface Properties of Transition-Metal Nanoparticles in Ionic Liquids. Chem. Soc. Rev. 2010, 39 (5), 1780−804.

3545



Article

particles: Synthesis, Organization, And Dynamics. J. Am. Chem. Soc. 2001, 123 (31), 7584−7593. (24) Pelzer, K.; Vidoni, O.; Philippot, K.; Chaudret, B.; Colliere, V. Organometallic Synthesis of Size-Controlled Polycrystalline Ruthenium Nanoparticles in the Presence of Alcohols. Adv. Funct. Mater. 2003, 13 (2), 118−126. (25) Silveira, E. U., AP; Rossi, L. M.; et al. The Partial Hydrogenation of Benzene to Cyclohexene by Nanoscale Ruthenium Catalysts in Imidazolium Ionic Liquids. Chem.Eur. J. 2004, 10 (15), 3734−3740. (26) Fonseca, G. S.; Machado, G.; Teixeira, S. R.; Fecher, G. H.; Morais, J.; Alves, M. C. M.; Dupont, J. Synthesis and Characterization of Catalytic Iridium Nanoparticles in Imidazolium Ionic Liquids. J. Colloid Interface Sci. 2006, 301 (1), 193−204. (27) Schrekker, H. S.; Gelesky, M. A.; Stracke, M. P.; Schrekker, C. M. L.; Machado, G.; Teixeira, S. R.; Rubim, J. C.; Dupont, J. Disclosure of the Imidazolium Cation Coordination and Stabilization Mode in Ionic Liquid Stabilized Gold(0) Nanoparticles. J. Colloid Interface Sci. 2007, 316 (1), 189−195. (28) Scholten, J. D.; Ebeling, G.; Dupont, J. On the Involvement of NHC Carbenes in Catalytic Reactions by Iridium Complexes, Nanoparticle and Bulk Metal Dispersed in Imidazolium Ionic Liquids. Dalton Trans. 2007, 47, 5554−5560. (29) Salas, G.; Podgorsek, A.; Campbell, P. S.; Santini, C. C.; Padua, A. A. H.; Costa Gomes, M. F.; Philippot, K.; Chaudret, B.; Turmine, M. Ruthenium Nanoparticles in Ionic Liquids: Structural and Stability Effects of Polar Solutes. Phys. Chem. Chem. Phys. 2011, 13, 13527− 13536. (30) Joshi, H.; Shirude, P. S.; Bansal, V.; Ganesh, K. N.; Sastry, M. Isothermal Titration Calorimetry Studies on the Binding of Amino Acids to Gold Nanoparticles. J. Phys. Chem. B 2004, 108 (31), 11535− 11540. (31) De, M.; You, C. C.; Srivastava, S.; Rotello, V. M. Biomimetic Interactions of Proteins with Functionalized Nanoparticles: A Thermodynamic Study. J. Am. Chem. Soc. 2007, 129 (35), 10747− 10753. (32) Gourishankar, A.; Shukla, S.; Ganesh, K. N.; Sastry, M. Isothermal Titration Calorimetry Studies on the Binding of DNA Bases and PNA Base Monomers to Gold Nanoparticles. J. Am. Chem. Soc. 2004, 126 (41), 13186−13187. (33) Fini, P.; Curri, M. L.; Castagnolo, M.; Ciampi, F.; Agostiano, A. Calorimetric Study of US Nanoparticle Formation in w/o Microemulsions. Mater. Sci. Eng., C 2003, 23 (6−8), 1077−1081. (34) Toshima, N.; Kanemaru, M.; Shiraishi, Y.; Koga, Y. Spontaneous Formation of Core/Shell Bimetallic Nanoparticles: A Calorimetric Study. J. Phys. Chem. B 2005, 109 (34), 16326−16331. (35) Magna, L.; Chauvin, Y.; Niccolai, G. P.; Basset, J. M. The importance of Imidazolium Substituents in the Use of ImidazoliumBased Room-Temperature Ionic Liquids As Solvents for PalladiumCatalyzed Telomerization of Butadiene with Methanol. Organometallics 2003, 22 (22), 4418−4425. (36) Jacquemin, J.; Husson, P.; Padua, A. A. H.; Majer, V. Density and Viscosity of Several Pure and Water-Saturated Ionic Liquids. Green Chem. 2006, 8, 172−180. (37) Costa Gomes, M. F.; Pison, L.; Pensado, A. S.; Padua, A. A. H. Using Ethane and Butane As Probes to the Molecular Structure of 1Alkyl-3-methylimidazolium Bis[(trifluoromethyl)sulfonyl]imide Ionic Liquids. Faraday Discuss. 2012, 154, 41−53. (38) Tariq, M.; Carvalho, P. J.; Coutinho, J. A. P.; Marrucho, I. M.; Canongia Lopes, J. N.; Rebelo, L. P. N. Viscosity of (C2−C14) 1-Alkyl3-methylimidazolium Bis(trifluoromethylsulfonyl)amide Ionic Liquids in an Extended Temperature Range. Fluid Phase Equilib. 2011, 301, 22−32. (39) Campbell, P. S.; Podgorsek, A.; Gutel, T.; Santini, C. C.; Padua, A. A. H.; Costa Gomes, M. F.; Bayard, F.; Fenet, B.; Chauvin, Y. How Do Physical−Chemical Parameters Influence the Catalytic Hydrogenation of 1,3-Cyclohexadiene in Ionic Liquids? J. Phys. Chem. B 2010, 114, 8156−8165.

(5) Scholten, J. D.; Leal, B. C.; Dupont, J. Transition Metal Nanoparticle Catalysis in Ionic Liquids. ACS Catal. 2012, 2, 184−200. (6) Gutel, T.; Garcia-Anton, J.; Pelzer, K.; Philippot, K.; Santini, C. C.; Chauvin, Y.; Chaudret, B.; Besset, J.-M. Influence of the SelfOrganization of Ionic Liquids on the Size of Ruthenium Nanoparticles: Effect of the Temperature and Stirring. J. Mater. Chem. 2007, 17 (31), 3290−3292. (7) Gutel, T.; Santini, C. C.; Philippot, K.; Padua, A.; Pelzer, K.; Chaudret, B.; Chauvin, Y.; Basset, J. M. Organized 3D-Alkyl Imidazolium Ionic Liquids Could Be Used to Control the Size of in Situ Generated Ruthenium Nanoparticles? J. Mater. Chem. 2009, 19 (22), 3624−3631. (8) Boennemann, H.; Nagabhushana, K. S. In Metal Nanoclusters in Catalysis and Materials Science: The Issue of Size Control; Corain, B., Schmid, G., Toshima, N., Eds.; Elsevier B.V.: Amsterdam, 2008; pp 21−48. (9) Campbell, P. S.; Santini, C. C.; Bayard, F.; Chauvin, Y.; Colliere, V.; Podgorsek, A.; Costa Gomes, M. F.; Sa, J. Olefin Hydrogenation by Ruthenium Nanoparticles in Ionic Liquid Media: Does Size Matter? J. Catal. 2010, 275 (1), 99−107. (10) Redel, E.; Thomann, R.; Janiak, C. First Correlation of Nanoparticle Size-Dependent Formation with the Ionic Liquid Anion Molecular Volume. Inorg. Chem. 2008, 47 (1), 14−6. (11) Campbell, P. S.; Santini, C. C.; Bouchou, D.; Fenet, B.; Philippot, K.; Chaudret, B.; Padua, A.; Chauvin, Y. A Novel Stabilisation Model for Ruthenium Nanoparticles in Imidazolium Ionic Liquids: In Situ Spectroscopic and Labelling Evidence. Phys. Chem. Chem. Phys. 2010, 12 (16), 4217−23. (12) Khare, V.; Li, Z.; Mantion, A.; Ayi, A. A.; Sonkaria, S.; Voelkl, A.; Thunemann, A. F.; Taubert, A. Strong Anion Effects on Gold Nanoparticle Formation in Ionic Liquids. J. Mater. Chem. 2010, 20 (7), 1332−1339. (13) Vanecht, E.; Binnemans, K.; Patskovsky, S.; Meunier, M.; Seo, J. W.; Stappers, L.; Fransaer, J. Stability of Sputter-Deposited Gold Nanoparticles in Imidazolium Ionic Liquids. Phys. Chem. Chem. Phys. 2012, 14 (16), 5662−5671. (14) Pensado, A. S.; Padua, A. A. H. Solvation and Stabilization of Metallic Nanoparticles in Ionic Liquids. Angew. Chem., Int. Ed. 2011, 50, 8683−8687. (15) Canongia Lopes, J. N.; Padua, A. A. H. Nanostructural Organization in Ionic Liquids. J. Phys. Chem. B 2006, 110 (7), 3330− 3335. (16) Triolo, A.; Russina, O.; Fazio, B.; Appetecchi, G. B.; Carewska, M.; Passerini, S. Nanoscale Organization in Piperidinium-Based Room Temperature Ionic Liquids. J. Chem. Phys. 2009, 130 (16), 164521− 164527. (17) Hardacre, C.; Holbrey, J. D.; McMath, S. E. J.; Bowron, D. T.; Soper, A. K. Structure of Molten 1,3-Dimethylimidazolium Chloride Using Neutron Diffraction. J. Chem. Phys. 2003, 118 (1), 273−278. (18) Fuller, J.; Carlin, R. T.; Delong, H. C.; Haworth, D. Structure of 1-Ethyl-3-Methylimidazolium Hexafluorophosphate − Model for Room-Temperature Molten-Salts. Chem. Commun. 1994, 3, 299−300. (19) Mele, A.; Romano, G.; Giannone, M.; Ragg, E.; Fronza, G.; Raos, G.; Marcon, V. The Local Structure of Ionic Liquids: CationCation NOE Interactions and Internuclear Distances in Neat [BMIM][BF4] and [BDMIM]-[BF4]. Angew. Chem., Int. Ed. 2006, 45 (7), 1123−1126. (20) Wang, Y. T.; Izvekov, S.; Yan, T. Y.; Voth, G. A. Multiscale Coarse-Graining of Ionic Liquids. J. Phys. Chem. B 2006, 110 (8), 3564−3575. (21) Canongia Lopes, J. N.; Costa Gomes, M. F.; Padua, A. A. H. Nonpolar, Polar, And Associating Solutes in Ionic Liquids. J. Phys. Chem. B 2006, 110 (34), 16816−16818. (22) Padua, A. A. H.; Costa Gomes, M. F.; Canongia Lopes, J. N. Molecular Solutes in Ionic Liquids: A Structural, Perspective. Acc. Chem. Res. 2007, 40, 1087−1096. (23) Pan, C.; Pelzer, K.; Philippot, K.; Chaudret, B.; Dassenoy, F.; Lecante, P.; Casanove, M. J. Ligand-Stabilized Ruthenium Nano3546



Article

(40) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M. Pyrrolidinium Imides: A New Family of Molten Salts and Conductive Plastic Crystal Phases. J. Phys. Chem. B 1999, 103, 4164−4170. (41) Dzyuba, S. V.; Bartsch, R. A. Influence of Structural Variations in 1-Alkyl(aralkyl)-3-methylimidazolium Hexafluorophosphates and Bis(trifluoromethylsulfonyl)imides on Physical Properties of the Ionic Liquids. ChemPhysChem 2002, 3, 161−166. (42) Canongia Lopes, J. N.; Deschamps, J.; Padua, A. A. H. Modeling Ionic Liquids Using a Systemayic All-Atom Force Field. J. Phys. Chem. B 2004, 108, 2038−2047. (43) Li, J.; Kong, L.; Liu, B. Proposed Definition of Microchemical Inhomogeneity and Application To Characterize Some Selected Miscible/Immiscible Binary Metal Systems. J. Phys. Chem. B 2004, 108, 16071−16076. (44) Y. Zhao, Y.; Truhlar, D. G. A New Local Density Functional for Main-Group Thermochemistry, Transition Metal Bonding, Thermochemical Kinetics, And Noncovalent Interactions. J. Chem. Phys. 2006, 125, 194101−194102. (45) Iori, F.; Corni, S. Including Image Charge Effects in the Molecular Dynamics Simulations of Molecules on Metal Surfaces. J. Comput. Chem. 2008, 29, 1656−1666. (46) Navia, P.; Troncoso, J.; Romani, L. Excess Magnitudes for Ionic Liquid Binary Mixtures with a Common Ion. J. Chem. Eng. Data 2007, 52 (4), 1369−1374. (47) Santos, L. M. N. B. F. L.; Lopes, J. N. C. J.; Coutinho, J. A. P. J.; Esperança, J. M. S. S. J.; Gomes, L. R. L.; Marrucho, I. M. I.; Rebelo, L. P. N. L. Ionic liquids: First Direct Determination of Their Cohesive Energy. J. Am. Chem. Soc. 2007, 129 (2), 284−285.

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