The solvation mechanism of molecular solutes in ionic liquids (ILs) can be quite different than in conventional molecular solvents not only because of the ionicity of ILs but also because of their nanoscale structural heterogeneity. This heterogeneity is characterized by nonpolar domains, formed by the aggregation of alkyl chains, that are embedded in or intertwined with the polar ionic network. Based on the chemical adage "like dissolves like", nonpolar solutes are therefore expected to be localized in the nonpolar domains. We find however that certain nonpolar aprotic solutes have a finite solubility in [C1C1im][NTf2] despite the alkyl groups on the imidazolium ring being too small to form nonpolar domains. OHD-RIKES measurements on CS2 in [C1C1im][NTf2] and [C4C1im][NTf2] indicate that the intermolecular dynamics of CS2 are higher in frequency in [C1C1im][NTf2] than in [C4C1im][NTf2]. To understand these results molecular dynamics simulations were performed on these two mixture systems. In these simulations, nonpolarizable and polarizable models were used for CS2. In the case of the nonpolarizable model, CS2 has a higher probability of being in the nonpolar domains near the butyl chains than near the imidazolium rings in [C4C1im][NTf2]. However in [C1C1im][NTf2], CS2 molecules can only interact with the charged imidazolium rings. Adding polarizability has a pronounced effect in [C1C1im][NTf2] causing CS2 molecules to be more localized on the C2 carbon, whereas there is much less of a change in [C4C1im][NTf2], because of the attraction of CS2 to the butyl chain. Simulated densities of states (DOSs) using the polarizable model are consistent with the OHD-RIKES measurements, with the DOS being higher in frequency for CS2 in [C1C1im][NTf2] than in [C4C1im][NTf2].

The Importance of Polarizability in the Solvation of Molecular Solutes in ionic liquids: OHD-RIKES measurements and molecular dynamics simulations on mixtures of CS2 and ionic liquids

255th National Meeting (2018)

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