Surfactants versus surface functionalization to improve the stability of graphene nanofluids

Abstract

Graphene nanofluids have emerged as potential alternatives to improve the thermal efficiency of different engineering devices. However, their use in different applications that include polar liquids as base fluids is a problem due to the hydrophobic nature of graphene. In this study, we compared methods that use surfactants with those that employ surface oxidation (surfactant-free) to improve the dispersion of graphene in water and its stability over time. Cetyltrimethylammonium bromide and sodium dodecylbenzenesulfonate (SDBS), at their critical micelle concentration in water, were used as surfactants to prepare commercial graphene dispersions, and the electrochemical method was adopted to synthesize GO. Additionally, the effect of the concentration of electrolyte (sodium sulfate, Na2SO4) on oxygen functionalities in GO synthesis was evaluated and associated with nanofluid stability. The nanomaterials were characterized by thermogravimetric analysis, Fourier transform infrared spectroscopy, Raman spectroscopy, and scanning electron microscopy. Nanofluid stability was evaluated using the zeta potential and UV–VIS spectroscopy. The results show that the electrolyte has an important effect on the oxidation degree of the GO obtained: a high electrolyte concentration (0.5 M) produced the highest oxygen functionalities. Finally, the stability of the nanofluid produced with the electrolyte at 0.5 M was similar to that of the graphene nanofluid prepared with SDBS as surfactant.