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

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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.