Ionic liquids at the surface of graphite: Wettability and structure.

The aim of this work is to provide a better understanding of the interface between graphite and different molecular and ionic liquids. Experimental measurements of the liquid surface tension and of the graphite-liquid contact angle for sixteen ionic liquids and three molecular liquids are reported. These experimental values allowed the calculation of the solid/liquid interfacial energy that varies, for the ionic liquids studied, between 14.5 mN m-1 for 1-ethyl-3-methylimidazolium dicyanamide and 37.8 mN m-1 for 3-dodecyl-1-(naphthalen-1-yl)-1H-imidazol-3-ium tetrafluoroborate. Imidazolium-based ionic liquids with large alkyl side-chains or functionalized with benzyl groups seem to interact more favourably with freshly peeled graphite surfaces. Even if the interfacial energy seems a good descriptor to assess the affinity of a liquid for a carbon-based solid material, we conclude that both the surface tension of the liquid and the contact angle between the liquid and the solid can be significant. Molecular dynamics simulations were used to investigate the ordering of the ions near the graphite surface. We conclude that the presence of large alkyl side-chains in the cations increases the ordering of ions at the graphite surface. Benzyl functional groups in the cations lead to a large affinity towards the graphite surface.

Exfoliation of graphene and fluorographene in molecular and ionic liquids.

We use molecular dynamics simulations to study the exfoliation of graphene and fluorographene in molecular and ionic liquids, by performing computer experiments in which one layer of the 2D nanomaterial is peeled from a stack, in vacuum and in the presence of solvent. The liquid media and the nanomaterials are represented by fully flexible, atomistic force fields. From these simulations we calculate the potential of mean force, or reversible work, required to exfoliate the materials. Calculations in water and organic liquids showed that small amides (NMP, DMF) are among the best solvents for exfoliation, in agreement with the experiment. We tested ionic liquids with different cation and anion structures, allowing us to learn about their solvent qualities for the exfoliation of the nanomaterials. First, a long alkyl side chain on the cation is favourable for exfoliation of both graphene and fluorographene. The presence of aromatic groups on the cation is also favourable for graphene. No beneficial effect was found between fluorine-containing anions and fluorographene. We also analysed the ordering of ions in the interfacial layers with the materials. Near graphene, nonpolar groups are found along with charged groups, whereas near fluorographene almost exclusively non-charged groups are found, with ionic moieties segregated to the second layer. Therefore, fluorographene appears to be the more hydrophobic surface, as expected.

Polycyclic aromatic hydrocarbons as model solutes for carbon nanomaterials in ionic liquids.

The aim of this work is to understand the details of the interactions of ionic liquids with carbon nanomaterials (graphene and nanotubes) using polyaromatic compounds as model solutes. We have combined the measurements of thermodynamic quantities of solvation with molecular dynamics simulations to provide a microscopic view. The solubility of five polycyclic aromatic hydrocarbons (naphthalene, anthracene, phenanthrene, pyrene and coronene) was determined in seven ionic liquids ([C4C1im][C(CN)3], [C4C1pyrr][Ntf2], [C10C1im][Ntf2], [C2C1im][C(CN)3], [C2C1im][Ntf2], [C3C1pyrr][N(CN)2] and [C4C1im][N(CN)2]) at 298 K. The enthalpies of the dissolution of naphthalene, anthracene and pyrene were measured in four of the ionic liquids. Free energies were estimated from those measurements in order to analyse the entropic or enthalpic contributions to the dissolution process. Molecular dynamics simulations provided solvation free energies that were compared to experimental and structural information. Spatial distributions of solvent ions around the solutes when combined with IR measurements elucidate the structure of solvation environments. Interactions between the imidazolium rings of cations and the π system of the solutes have been identified. However, ionic liquids with pyrrolidinium cations appeared as better solvents due to favourable enthalpic contributions compared to imidazolium cations. Long alkyl side chains on cations lead to higher solubility and lower enthalpy of dissolution by creating a "softer" solvation environment. Considering the effect of anions, small and planar anions lead to higher solubilities and lower enthalpies of dissolution of polyaromatic hydrocarbons. These findings provide the design principles based on molecular interactions and the structure of solvation environments to choose or formulate ionic liquids in view of their affinity for carbon nanomaterials.

Corrigendum: Dispersion and Stabilization of Exfoliated Graphene in Ionic Liquids.

[This corrects the article DOI: 10.3389/fchem.2019.00223.].

Dispersion and Stabilization of Exfoliated Graphene in Ionic Liquids.

The liquid-phase exfoliation of graphite is one of the most promising methods to increase production and commercial availability of graphene. Because ionic liquids can be easily obtained with chosen molecular structures and tuneable physicochemical properties, they can be use as media to optimize the exfoliation of graphite. The understanding of the interactions involved between graphite and various chemical functions in the solvent ions will be helpful to find liquids capable of dissociating and stabilizing important quantities of large graphene layers. After a step of sonication, as a mechanical precursor, samples of suspended exfoliated graphene in different ionic liquids have been characterized experimentally in terms of flake size, number of layers, total concentration and purity of the exfoliated material. Nine different ionic liquids based on imidazolium, pyrrolidinium and ammonium cations and on bis(trifluoromethylsulfonyl)imide, triflate, dicyanamide, tricyanomethanide, and methyl sulfate anions have been tested. UV-vis, Raman and X-ray photoelectron in addition to high resolution transmission electron and atomic force microscopy have been selected to characterize suspended exfoliated graphene in ionic liquids. The number of layers in the flakes exfoliated, the size and concentration depend of the structure of the ionic liquid selected. In order to obtain large flake sizes, ionic liquids with bis(trifluoromethylsulfonyl)imide anions and a cation with an alkyl chain of medium length should be selected. Smaller cation and anion favors the exfoliation of graphene. The exfoliation caused the formation of C-H bonds and the oxidation of the graphitic surface.