Boundary Lubrication with Adsorbed Anionic Surfactant Bilayers in Hard Water

The adsorption behavior of an anionic surfactant, hydroxy alkane sulfonate with an alkyl chain length of 18 (C18HAS), from its hard water solution onto a mica surface and resulting lubrication properties were investigated. Because of the double chain-like chemical structure and aggregation behavior, C18HAS formed vesicles in hard water, which adsorbed onto a negatively charged mica surface via cation (Ca²⁺) bridging and then transformed into a bilayer film. The number of bilayers formed on the surface was evaluated by force curve measurements using an atomic force microscope (AFM), and the results showed a time-dependent increase of the number of adsorbed bilayers. Friction and lubrication properties were evaluated for the confined film of the C18HAS hard water solution between mica surfaces using the surface forces apparatus (SFA). When the two surfaces were brought into contact under load and sheared against each other, the lubricating film consisted of two adsorbed C18HAS bilayers whose friction coefficient μ was of the order of 10^-3 or below. The detailed analysis of the friction features revealed that the slipping in the boundary film does not occur at the interface between the opposed headgroup region of the two adsorbed bilayers, which is the typical mechanism for the low friction of adsorbed phospholipid bilayers extensively studied in the literature. Instead, slipping occurs at the interface between opposed <i>liquid-like</i> alkyl chain tails within the adsorbed bilayers; the low friction coefficient comes from the existence of two slip planes in the boundary film.