Complex systems based on nanomaterials and common solvents have been shown to have thermophysical properties that can revolutionize current utilization of heat transfer fluids and heat storage cycles. This has been made possible by the existence of thermal conductivity enhancements derived from the presence of additional mechanisms of heat transfer in comparison with the base solvent. Ionic liquids have been shown to have thermophysical properties that justify the replacement of several of the chemical processes now under exploitation, and some of the solvents used, because they can in certain conditions, be considered as green solvents. Dissolving (or mixing as a thermally stable suspension) nanoparticles in ionic liquids, forms “bucky gels”, or IoNanoFluids, which we have recently shown to have thermal conductivity enhancements ranging from (5 to 35) %. This paper reports data on the thermal conductivity of the ionic liquids 1-hexyl-3-methylimidazolium tetrafluoroborate (CAS Number, 244193-50-8), [C6mim][BF4], 1-butyl-3-methylimidazolium hexafluorophosphate (CAS Number, 174501-64-5), [C4mim][PF6], 1-hexyl-3-methylimidazolium hexafluorophosphate (CAS Number, 304680-35-1), [C6mim][PF6], 1-butyl-3-methylimidazolium trifluoromethanesulfonate (CAS Number, 174899-66-2), [C4mim][CF3SO3], and 1-butyl-1-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl}imide (CAS Number, 223437-11-4), [C4mpyrr][(CF3SO2)2N], and IoNanofluids with multiwalled carbon nanotubes (MWCNTs) as a function of temperature and discuss the molecular theories of heat transfer and storage in these types of systems. Moderate thermal conductivity enhancements, between (2 and 9) %, were found for the systems studied, showing a week dependence on temperature. It also reports heat capacity values for [C4mim][BF4] and [C4mim][PF6]. Values of the heat capacity of an IoNanofluid, C4mim][PF6] with (1 and 1.5) % Baytubes, are reported for the first time, showing also an enhancement (8 %), a fact that deserves further investigation in a near future. The behavior of these nanofluids, along with that of ionic liquids of the type studied, suggests that nanocluster formation and preferred paths for heat transfer and storage are present and are likely to be the cause of the phenomena found. However, existing theories cannot yet explain the results obtained.

Ionic liquids have proved to be excellent heat transfer fluids and alternatives to common HTFs used in industry for heat exchangers and other heat transfer equipment. However, its industrial utilization depends on the cost/kg of its production, to be competitive for industrial applications with biphenyl and diphenyl oxide, alkylated aromatics, and dimethyl polysiloxane oils, which degrade above 200 °C and possess some environmental problems. The efficiency as a heat transfer fluid depends on the fundamental thermophysical properties necessary to model convective heat transfer (density, heat capacity, thermal conductivity, and viscosity) and calculate the heat transfer coefficients in given heat exchangers geometries. 1-Ethyl-3methylimidazolium methanesulfonate [C 2 mim][CH 3 SO 3 ] (CAS no. 145022-45-3) (ECOENG 110) is actually produced by BASF, under the trade name of Basionics ST 35, with an assay ≥97% with ≤0.5% water and ≤2% chloride (Cl − ). Density, speed of sound, heat capacity, viscosity, electrical, and thermal conductivity of the industrial product, after drying under vacuum, were obtained for the temperature range 283.15−363.15 K, at atmospheric pressure. Values of the thermal properties studied showed an interesting phase behavior below the melting point (303 K), which suggests the existence of second order−disorder transitions (λ-type) before reaching the freezing point. A study was carried out to investigate the toxicity of [C 2 mim][CH 3 SO 3 ] in saltwater crustaceous. The lethal concentration was determined with Artemia, and the LC 50 was found to be higher than with an organic solvent, such as ethylene glycol.

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Measurements of the thermal conductivity of ionic liquids are extremely important for current chemical plant design of new environmentally safe processes. Existing data are very scarce and inaccurate. IoNanofluids have emerged as a possible alternative to current engineering fluids for heat transfer applications, namely in small volume heat exchangers. In the present paper we report new data on the thermal conductivity of of 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonylimide ([C 4 mim][(CF 3 SO 2 ) 2 N]) and 1-ethyl-3methylimidazolium ethylsulfate ([C 2 mim][EtSO 4 ]) at temperatures between (293 and 343) K and IoNanofluids with multiwalled carbon nanotubes based on them, to understand the effect of adding nanomaterials to a ionic liquid matrix and its effect on the mentioned thermal property. The application of existing models to predict the behavior of the IoNanofluids, namely, the enhancement in the thermal conductivity, showed that it is fundamental to understand better the mechanism of heat transfer in these systems, namely, the role played by the interface ionic liquid (cation and anion)−nanoparticle, whatever molecular shape they have. A test of the effect of water on the purity of [C 4 mim][(CF 3 SO 2 ) 2 N] was also performed, and results showed that at any given temperature the thermal conductivity decreases with the amount of water added and that the effect increases with increasing temperature.

Ionic liquids have been suggested as new engineering fluids, namely in the area of heat transfer, as alternatives to current biphenyl and diphenyl oxide, alkylated aromatics and dimethyl polysiloxane oils, which degrade above 200 °C and pose some environmental problems. Recently, we have proposed 1-ethyl-3-methylimidazolium methanesulfonate, [C2mim][CH3SO3], as a new heat transfer fluid, because of its thermophysical and toxicological properties. However, there are some interesting points raised in this work, namely the possibility of the existence of liquid metastability below the melting point (303 K) or second order-disorder transitions (λ-type) before reaching the calorimetric freezing point. This paper analyses in more detail this zone of the phase diagram of the pure fluid, by reporting accurate thermal-conductivity measurements between 278 and 355 K with an estimated uncertainty of 2% at a 95% confidence level. A new value of the melting temperature is also reported, Tmelt = 307.8 ± 1 K. Results obtained support liquid metastability behaviour in the solid-phase region and permit the use of this ionic liquid at a heat transfer fluid at temperatures below its melting point. Thermal conductivity models based on Bridgman theory and estimation formulas were also used in this work, failing to predict the experimental data within its uncertainty.