Inherent Low Interfacial Tension-Triggered Highly Stable, pH-Responsive and Temperature- and Salt-Tolerant Oil/Water Mixtures for Multi-Environment Adaptive Lubrication

Homogeneously mixing lubricating oil and water using surface-active additives will lead to liquid lubricants capable of combining high lubricative and cooling performances. They can further achieve effective separation and recycling using stimuli-responsive surface-active materials with tunable interfacial properties. Here, we show that oil/water mixtures with long-term stability, pH-controllable combination/dissociation, long-lasting friction resistance, and favorable adaptive lubrication to high normal load, temperature, and salinity environments can be created solely using deionized water and a commercially available phosphate ester-based lubricating oil Nycobase 5750 that can spontaneously yield very low interfacial tensions via tight but pH-sensitive hydrogen bonding at the contact interface, even when the external temperature and salt concentration in the aqueous phase reach as high as 80 °C and 5.4 M, respectively. The liquid mixtures can provide an enhanced lubricity and comparable extreme-pressure property to pure oil owing to the introduction of a thermally conductive water component. Our study may provide useful indications for creating very pure, economic, sustainable, and adaptive liquid lubricants.