Apparent molar volumes of ILs [EmimNTf2], [BmimNTf2], and [BmmimNTf2] in DEC and PEGMME solvents

•Densities of six dilute solutions of three ILs with two solvents were measured.•Compounds are [Emim][NTf2], [Bmim][NTf2], [Bmmim][NTf2], DEC and PEGMME.•Interactions of ion-ion and ion–solvent were discussed.•Influence of methyl and methylene introduction on the properties were discussed. Different groups have important influence on the interaction between molecules. In order to understand the effect of group introduction on the molecular interaction in solution, the densities of six diluted solutions including three ionic liquids (ILs) and two organic solvents were measured. The groups include methylene and methyl. The three ILs are 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Emim][NTf2]), 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Bmim][NTf2]), and 1-butyl-2,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Bmmim][NTf2]). The organic solvents are diethyl carbonate (DEC) and propylene glycol monomethyl ether (PEGMME). The measured densities were used to calculate apparent molar volumes, Vϕ/cm3·mol-1, from 293.15 to 328.15 K with ± 0.02 K. Then, the fitting of infinite dilution apparent molar volumes, Vϕ0/cm3·mol-1, were obtained by using the calculated values of apparent molar volumes. The effects of different structures of ILs cations and organic solvents on the apparent molar volume, infinite dilution apparent molar volume, limiting apparent molar expansibility and thermal expansibility coefficient were discussed.