The physicochemical properties of aqueous ternary ionic liquid (IL) and nanoparticle systems are affected by the various intermolecular interactions such as van der Waals, electrostatic and structural. Recently these systems have been considered both to increase the stability of nanofluids and to change the surface activity of the surface active ILs. A systematic experimental study was conducted on aqueous solutions containing a surface active ionic liquid, 1-octyl-3-methylimidazolium bromide ([C8mim][Br]), in pure water and in CuO-water nanofluids to a better understanding of the aggregation behavior occurring in these systems. The electrical conductivity (κ) data were obtained for [C8mim][Br] + water and [C8mim][Br] + CuO–water nanofluid (with 0.00066 wt%, 0.005 wt% and 0.01 wt% of CuO NPs) systems at 288.15, 293.15, 298.15 and 303.15 K. For these systems, surface tension (γ) measurements were taken at 298.15 K. The accurate analysis of the obtained data showed that the critical micelle concentration (cmc) for [C8mim][Br] in aqueous solution is increased with addition of CuO NPs to pure water, as well as with increasing concentration of CuO NPs in water-based fluid. The values of Gibbs energy, ΔG°mic, enthalpy, ΔH°mic, and entropy, ΔS°mic, of micellization were determined from the specific electrical conductivity data. The values of ΔG°mic were negative and magnitude of ΔG°mic (kJ⋅mol−1) followed the order: 288.15 K (− 21.95) < 293.15 K (− 22.86) < 298.15 K (− 23.62) < 303.15 K (− 24.16). At the lower temperatures, the micellization of [C8mim][Br] in aqueous solutions is entropically driven process (ΔH°mic > 0) and with a temperature raise, this process becomes enthalpically driven process (ΔH°mic < 0). The presence of CuO NPs in aqueous medium reduces the surface activity of [C8mim][Br], so that, the cmc value is shifted to the higher concentrations of [C8mim][Br]. Although the surface tension of the CuO-nanofluid is almost same as that of pure water, the interaction between [C8mim][Br] and CuO NPs surface leads to increasing of γ compared to the binary aqueous IL solutions. This phenomenon revealed a decrease in some surface tension parameters like maximum surface excess concentration, Гmax, adsorption efficiency, pC20, surface pressure at the saturated air/liquid interface, πcmc, and increase in the another parameters such as cmc, surface tension at the micellization, γcmc, and minimum area per IL molecule at the surface, Amin, respectively.
