Morphology-Controlled Ultralow Aqueous Friction in Cationic-Anionic Mixed Surfactant Systems

Aqueous boundary lubrication depends critically on stable, hydrated interfacial layers, yet the role of molecular self-assembly in the tribological performance remains insufficiently understood. Here, we investigate a catanionic surfactant system, C12C3C12(SO3)2/CTAB, in which controlled variation of the gemini surfactant C12C3C12(SO3)2 molar fraction (Xg = 0.1, 0.2, 0.3) at a fixed total concentration of 2 mM drives structural transitions from spherical micelles to wormlike micelles and vesicles. Surface force balance and atomic force microscopy measurements reveal that all morphologies provide ultralow friction coefficients (10-3-10-4) via hydration lubrication but exhibit markedly different load-bearing capacities and mechanical stability. Spherical micelles provide the highest resilience and stability under pressures up to ∼70 atm; wormlike micelles display intermediate stability with partial structural damage under confinement; and vesicles yield the lowest friction (μ ≈ 10-4) but collapse at moderate pressures (15-30 atm). These findings demonstrate that aggregate morphology, controlled by stoichiometric charge neutralization, governs the balance between friction reduction and mechanical robustness, and highlight mixed surfactant self-assembly as an efficient strategy for low-concentration water-based lubricants.