Potential-dependent superlubricity of stainless steel and Au(1 1 1) using a water-in-surface-active ionic liquid mixture

Superlubricity (vanishing friction) is observed for both surfaces at OCP+1.0 V up to a surface-dependent critical normal force due to [AOT]^- bilayers adsorbing strongly to the positively charged surface thus allowing AFM tip to slide over solution-facing hydrated anion charged groups. High-resolution AFM imaging reveals ripple-like features within near-surface layers, with the smallest amplitudes at OCP+1 V, indicating the highest structural stability and resistance to thermal fluctuations due to highly ordered boundary [AOT]^- bilayers templating robust near-surface layers. Exceeding the critical normal force at OCP+1.0 V causes the AFM tip to penetrate the hydrated [AOT]^- layer and slide over alkyl chains, increasing friction. At OCP and OCP-1.0 V, higher friction correlates with more pronounced ripples, attributed to the rougher templating [BMIm]⁺ boundary layer. Kinetic experiments show that switching from OCP-1.0 V to OCP+1.0 V achieves superlubricity within 15 s, enabling real-time friction control.