Phase behavior and heat capacities of the 1-benzyl-3-methylimidazolium ionic liquids

• Heat capacities and phase behavior of benzyl ionic liquids are reported.
• Impact in the aromaticity cation–anion hindrance and charge delocalization.
• Heat capacities of [Bzmim][X] are similar to [C5C1im][X] analogs.
• Two different crystalline forms for [Bzmim][BF4] are reported.
• Increase of the Tg and Tm arising from the π–π interactions.

The thermal behavior, solid and liquid heat capacities of the 1-benzyl-3-methylimidazolium, [Bzmim]+, based ionic liquid series with Cl−, PF6−, BF4−, CHF2CF2SO3− and NTf2− as anions were used to evaluate the effect of the insertion of an aromatic character (benzyl group) and the expected increase of the π–π interactions as well as the impact in the cation–anion hindrance and charge delocalization in the ionic liquid properties. It was found that the molar liquid heat capacities of [Bzmim][X] are in the same order as the 1-pentyl-3-methylimidazolium, [C5C1im][X] analogs. A good correlation between the molar (solid and liquid) heat capacity and the number of atoms of the anion was found. Two different crystalline forms for [Bzmim][BF4] differing by 10 K in the melting temperature and about 10% in the heat capacity were found. The crystallization behavior, melting temperatures, and enthalpies and entropies of fusion show that the insertion of an aromatic character (benzyl group) in the imidazolium cation leads to a significant and systematic change of the thermophysical properties of the ionic liquids associated with an increase of the glass transition and melting temperatures arising from the additional π–π interactions.