Zero-charged catanionic lamellar liquid crystals doped with fullerene C60 for potential applications in tribology.

Abstract

The formation of lamellar liquid crystals (LLCs) has been demonstrated in a few salt-free catanionic surfactant systems and their properties have been well documented. However, examples of their combination with other materials are relatively rare. Herein, a salt-free zero-charged catanionic surfactant with low chain melting temperature was prepared by mixing equimolar tetradecyltrimethylammonium oxide (TTAOH) and oleic acid (OA) in water, and its concentration-dependent aggregate transition was investigated. In the dilute region (cTTAO ≤ 5 wt%), fluorescence microscopy observations revealed the formation of vesicles (the Lαv phase). Further increasing cTTAO induced a transition from the Lαv phase to LLCs via a region where vesicles and lamellae coexist. With ordered hydrophobic domains, the LLCs can be used as hosts for the doping of fullerene C60 (refers to C60 hereafter) with the highest C60/TTAO weight ratio of 0.04. The doping of C60 effectively improves the viscoelasticity of the LLCs confirmed by rheological characterization while only slight modifications on their matrixes have been detected using small angle X-ray scattering measurements. The LLC/C60 hybrids with cTTAO = 80 wt% were then subjected to tribological measurements, and an obvious reduction in their friction coefficients and wear volumes was observed. The C60/TTAO weight ratio at which the best tribological performance appears was determined to be 0.01. Our results indicate that the combination of C60 and catanionic LLCs could lead to the appearance of a new generation of environmentally-benign lubricants.