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

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 (c_TTAO ≤ 5 wt%), fluorescence microscopy observations revealed the formation of vesicles (the L_αv phase). Further increasing c_TTAO induced a transition from the L_αv 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 c_TTAO = 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.