Physicochemical Properties of Imidazolium-Based Ionic Nanofluids: Density, Heat Capacity, and Enthalpy of Formation

The heat capacity of ionic nanofluids (INF) of stacked-cup multiwalled carbon nanotubes (MWCNT) and [C(4)mim]BF4 and [C(4)mim]PF6 ionic liquids (IL) as well as their components was measured over the temperature range of 80-370 K. The specific heat capacity of INF was found be an additive quantity of specific heat capacities of the components. The temperatures of glass transition and fusion of IL in INF did not observably change compared to pure IL. The enthalpy of formation of ([C(4)inim]BF4+MWCNT) INF from its components was found to be negligible compared to the uncertainty of the measurements. All these facts confirm liophobic nature of the studied INF and provide the opportunity to predict thermodynamic properties of similar INF from the data on individual components. The apparent density of the studied MWCNT in INF is lower than those in their unstable dispersions with ionic and molecular liquids due to the high viscosity of INF not allowing penetration of ions into MWCNT and removal of gases from inner parts of MWCNT. The structural parameters of the studied MWCNT were estimated from the obtained density data.