Thermodynamic behaviour and polymorphism of 1-butyl-3-methylimidazolium hexafluorophosphate composites with multi-walled carbon nanotubes

Highlights: • Densities of ionic nanofluids are used to estimate structural parameters of multiwalled carbon nanotubes. • No deviation from additivity is found in the heat capacities of (MWCNT + [C₄mim]PF₆) ionic nanofluids. • Nanoconfined [C₄mim]PF₆ gradually transforms to the liquid-like phase on heating. -- Abstract: Thermal behaviour of five (1-butyl-3-methylimidazolium hexafluorophosphate + multi-walled carbon nanotubes) materials was studied by adiabatic calorimetry over the temperature range (78–370) K. The samples differed by the nanophase content (0.11–0.92 mass fraction), preparative procedure, and appearance ranging from fluids to powders at room temperature. The specific heat capacity of the fluids was found to be an additive quantity of the heat capacities of the components for all phases, and the temperatures of phase transitions did not change relative to the bulk values for the ionic liquid. Evacuation of the contacting components was found to be a necessary step to reproducibly obtain the high-density fluids. However, no notable effect of the evacuation on thermal behaviour of the studied systems was detected. For the powder-like sample with the highest nanophase content, a sigmoidal heat capacity curve was observed. Based on this it was concluded that the internal diameter of the studied nanotubes was small enough to reveal a gradual transition from the crystal-like structures to the liquid-like ones instead of isothermal melting of the ionic liquid.