Aqueous boundary lubrication: Molecular mechanisms, design strategy, and terra incognita

The molecular mechanisms for aqueous boundary lubrication are very different from those in the classic boundary lubrication, originating from the fluidity of the hydration shells surrounding the surfactant and lipid headgroups. We discuss the important molecular and structural criteria for effective aqueous boundary lubricants, and highlight the strategy for reinforcing the interfacial structure for aqueous boundary lubrication via synergistic interactions between amphiphilic polymers and lipids/surfactants. It is proposed that the energetic considerations of different molecular elastic deformations in the stalk model of cell membrane fusion can be applied to guide our design of molecular architectures for surfactants and lipids to implement structural integrity in aqueous boundary lubrication. We discuss a controversy associated with the quiescent bilayer structure in the context of boundary lubricant interfacial structures. We also highlight other effective aqueous boundary lubrication systems, including hydrated ions and biomimetic hierarchical constructs inspired by the enigmatic and extremely efficient biological lubrication. Finally, we suggest that the Stribeck curve might be re-considered in light of recent advances in aqueous boundary lubrication, although the exact scope of this new aqueous boundary lubrication regime remains terra incognita.