Heating Profile of Long Alkyl Chain Ionic Liquid Doped Solvents Under Ultrasound Irradiation

Abstract

In this work, the heating profile of water, dimethyl sulfoxide (DMSO), acetonitrile (CH3CN) and ethanol (EtOH) and their solutions containing monocationic (CnMIMBr, n = 4, 6, 8, 10, 12, 14 and 16) and dicationic (di-CnMIMBr, n = 4, 6, 8 and 10) imidazolium-based ionic liquids (ILs) were evaluated under ultrasound irradiation. Similarly, ILs (CnMIMBr, n = 4, 8 and 14) with the BF 4 − anion and inorganic salts, such as NaBr and NaCH3CO2 were also evaluated. The results showed that water and DMSO have a linear heating profile; while CH3CN and EtOH have a logarithmic profile. In general, the IL-containing solutions heated up a little faster than the pure solvents, which indicates that the cationic moiety of the ILs did not cause notable changes in the heating profile of the solvents. The IL concentration and the consequent dissociation effects on this were not observed in any of the experimental conditions tested. The addition of NaBr and NaCH3CO2 changed the heating profile of EtOH, resulting in a linear profile. Additionally, our results confirmed that experimental parameters, such as irradiation amplitude and solution volume, significantly change the heating profile of solvents. Finally, under conventional heating and stirring (silent experiments), the heating profile of the pure solvents and the IL-containing solvents indicated that, under ultrasound, the cavitation process makes the heating faster and more efficient. Determination of the dissipated power reflected the solvent’s heating profile more than its heat capacity; therefore, the heating rate in ultrasound reveals the cavitation effects in the particular medium. These results are important because they show that the catalytic effects of ILs in chemical reactions or the solvent effects in micro extraction processes under ultrasound irradiation can be retained without raising the temperature and without decomposition of the ILs or products.