In this work, the preparation and characterization of some new nanofluids based on deep eutectic solvents (DESs) consisting of a hydrogen bond acceptor, ethylene glycol (EG), and a hydrogen bond donor, choline chloride (ChCl), as well as water, are presented. The nanofluids were designed by the dispersion of spherical MgO nanoparticles in four different DESs, ChCl:EG (molar ratio of 1:2), 1ChCl:5EG, 1ChCl:2EG:2Water, and 1ChCl:5EG:2Water. The stability of nanofluids was carried out by measuring size distribution for five days, which discovered the best results obtained with nanofluids of dispersed MgO in DES 1ChCl:5EG. Thermophysical properties (thermal conductivity and density) were measured and the influence of nanoparticles’ mass fraction, temperature, and water content all were examined. The acquired outcomes revealed that the trend of density was reducing by increment in temperature since for pure base fluids and DES-based nanofluids 1.3% decrement were recorded, averagely (the decline in density was sharper in the case of DES 1ChCl:5EG and its based nanofluids). The thermal conductivity was almost constant during the range of 283.15–333.15 K. It confirmed that the thermal conductivities of prepared nanofluids based on DESs with water were higher in comparison to the ones based on DESs without water and nanoparticles concentration could promote thermal conductivity. The greatest enhancement was gained at 10 wt% of MgO suspended in DES 1ChCl:2EG. The isobaric thermal expansivity was also determined at different temperatures. Eventually, the general conclusions were drawn and concerning the results, the MgO/DES 1ChCl:2EG 10 wt% nanofluids was introduced as the most efficient.
