Effect of temperature on the rheological behavior of a new aqueous liquid crystal bio-lubricant

Abstract

The rheological behavior of bis[(2-hydroxyethyl)ammonium] palmitate (DPA) fatty acid-derived ionic liquid crystal (ILC) and its dispersions in water has been determined. Neat DPA shows non-Newtonian behavior, with a temperature-thickening effect in the mesomorphic state. The dispersion of 1 wt% DPA in water yields a Newtonian fluid, with viscosity values close to that of water. In contrast, increasing DPA concentration to 2 wt% leads to non-Newtonian behavior, with a shear thinning effect at high shear rates. Rheo-microscopy, dynamic light scattering, Zeta potential and conductivity measurements show that the new Water+2%DPA dispersion presents sharp transitions in viscosity, transmitted light, turbidity, particles size, particle charge and conductivity in the critical temperature interval from 37 to 42 °C, at which neat DPA exhibits a transition from a solid phase to a liquid crystalline one. The tribological performance of Water+2%DPA has been studied at 37 °C, showing good friction reduction and excellent antiwear ability, without tribocorrosion. This lubricating behavior has been related to low viscosity, high conductivity, particle size reduction and bilayer organization with carboxylate groups oriented to the outer surface, in contact with the sliding materials. The results described here open the feasibility of developing new biocompatible aqueous lubricants with controlled temperature response.