Counterion effect on boundary lubrication of cationic surfactant at mica-water interface

While counterion effects on surfactant behavior in bulk solutions or at solid-liquid adsorption interfaces are well understood, their influence on interfacial friction and lubrication remains unclear. We hypothesize that counterions do not simply modify adsorption layer thickness through micellization but critically regulate the lateral uniformity and interfacial nanostructure of surfactant films, which in turn governs friction and lubrication performance at solid-water interfaces.

To test this hypothesis, cetyltrimethylammonium bromide (CTAB) was employed as a model cationic surfactant at mica-water interfaces, with sodium bromide (NaBr) as a counterion. Surface forces apparatus (SFA) was used to quantify adsorption layer thickness and adhesion, while friction tests evaluated boundary lubrication performance. Atomic force microscopy (AFM) and contact-angle measurements were conducted to characterize the lateral morphology and wettability of the adsorbed films under varying CTAB and NaBr concentrations.

With increasing CTAB concentration, the adsorbed structure evolves from sub-monolayer to monolayer and bilayer structures, accompanied by a reduction in friction. In contrast, although the addition of NaBr promotes micellization and increases the apparent thickness of the adsorbed layer, it unexpectedly increases friction. At high NaBr concentrations (100 mM), CTAB nearly loses its boundary lubrication capability. AFM observations reveal that the presence of NaBr induces laterally discontinuous structures on the mica surface, which account for the degraded lubrication performance. These findings reveal an important role of counterions in regulating friction through their influence on adsorption-layer organization, highlighting counterion concentration as a key and tunable parameter in aqueous lubrication systems.