Physicochemical characterisation of graphene-ammonium lactate ionic liquid nanofluid

Abstract

A new series of nanofluids based on graphene dispersed in 2-hydroxyethylammonium lactate (ML) ionic liquid was developed. Concentrations of 0.1, 0.5 and 1 wt% of graphene were studied and these dispersions were stable after 2 months. Raman spectra showed a strong interaction between ML and graphene. The effect of the concentration of graphene and temperature on the viscoelastic behaviour and conductivity of the nanofluids was studied. An unexpected decrease in the viscosity was found with a low concentration of graphene due to the suppression of hydrogen bonding of the ionic liquid. Shear thinning effects appeared with higher concentrations of graphene and Ostwald and Herschel-Bulkley equations were used to describe the steady-state viscosity results. Creep-recovery tests were also performed, and the data were fitted to a complex Burgers model for the nanofluid with 1 wt% of graphene, with a 47 % of elastic response. The complexity of the model was related to the presence of different molecular arrangements in the nanofluid. An enhancement of the conductivity was observed with increasing values of the graphene concentration. The effect of temperature on viscosity and electrical conductivity was successfully modelled by using both Vogel-Fulcher-Tammann and Power Law equations. Electrochemical characterisation at room temperature was also carried out, finding an irreversible oxidation at 1 V only for the highest concentration (1 wt%). The concentration of percolation was estimated in the range of 0.5 to 1 wt% of graphene.