High operational functionality nanofluids from choline chloride, ethylene glycol, and multi-walled carbon nanotubes – Economical substitutes for ionanofluids

Abstract

Deep eutectic solvents (DESs) are an emerging class of green, easy-to-synthesize, and inexpensive solvents with designable properties widely used in nanotechnology applications. The optimization of such working fluids for energy transfer purposes requires the simultaneous maximization of their thermal/electrical conductivities and thermal/shelf-life stabilities, as well as minimization of their viscosity and overall material/maintenance costs. In this work, there is provided a comprehensive analysis of physicochemical properties, including, first-ever measurements of electrical conductivity and refractive index, of new highly-concentrated DES-based nanofluids composed of choline chloride/ethylene glycol (ChCl/EG) with molar ratios of 1:15, 1:5, 1:3, and 0.25–1 wt% of originally 860-µm-long multi-walled carbon nanotubes (MWCNTs), cut upon sonication to 14.0–16.9 µm. The study deepens the understanding of the mechanisms occurring at the molecular level during heat and electricity transfer in DES-based nanofluids and provides new empirical transport-property correlations. The analyzed nanodispersions are highly viscous (150–840 mPa·s at 25 °C, 9.3 s–‍1) non-Newtonian fluids (flow behavior index 0.49 ≤ m ≤ 0.66), though their viscosity can be notably reduced by elevated temperature and shear rate (28.3–120 mPa·s at 40 °C, 186 s−1). Importantly, DES-based nanofluids exhibit significant improvement in thermal conductivity (λ = 0.307 W·m−1·K−1), AC electrical conductivity (σ = 42.6 mS·cm−1), and decomposition temperature (T50%=255 °C) for ChCl/EG 1:3 + 1 wt% MWCNTs at 25 °C, thus, they can potentially become an interesting alternative to ionanofluids with up to 83 times lower raw material costs and at least 30-month shelf-life stability.