Isobutane as a probe of the structure of 1alkyl3methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids

An experimental study of the solubility and of the thermodynamic properties of solvation, between temperatures (303 and 343) K and at pressures close to atmospheric, of 2methylpropane (isobutane) in several ionic liquids based on the bis(trifluoromethylsulfonyl)imide anion and on 1alkyl3methylimidazolium cations, [C_nC₁Im][NTf₂], with alkyl sidechains varying from two to ten carbon atoms is presented. The isobutane solubility increases with increasing size of the alkyl sidechain of the cation in the ionic liquid and decreases with increasing temperature (as typical of an exothermal dissolution process). The mole fraction solubility of isobutane varies from 0.904 · 10² in [C₂C₁Im][NTf₂] at T = 343 K to 1.002 · 10¹ in [C₁₀C₁Im][NTf₂] at T = 303 K. The values measured in this work are compared to the behaviour of nbutane in the same ionic liquids published in a previous study (Costa Gomes et al., 2012). Isobutane was found to be significantly less soluble than nbutane in all the ionic liquids. The differences found are interpreted in relation to the molecular structures obtained by molecular dynamics simulations for the solutions of nbutane and isobutane in the studied [C_nC₁Im][NTf₂] ionic liquids.