Electric Field Regulation of Boundary Lubrication in Aqueous Solutions of Compounded Surfactants

The development of rapid-response electro-controlled friction systems is crucial for advancing modern precision equipment. However, conventional single-component aqueous lubricants generally suffer from limited tunability and delayed response under applied electric fields. In this study, we systematically investigate the electro-controlled friction behavior of mixed sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) aqueous solutions at ceramic interfaces. The results reveal that, at low concentrations (≤1.0 mM), the mixed system exhibits superior electro-frictional performance compared with pure SDS, particularly at an SDS:CTAB molar ratio of 7:1 and a total concentration of 1.0 mM, where the coefficient of friction can be reversibly tuned by up to 0.47 with a response time shortened by approximately 56-91%. At higher concentrations (≥2.0 mM), although the modulation amplitude shows no advantage over SDS alone, the response time remains reduced by 24-78%. Density functional theory (DFT) and molecular dynamics (MD) simulations indicate that electric-field-induced frontier orbital redistribution and the formation of ordered bilayers are the key mechanisms underlying this tunability. Specifically, enhanced mobility of SD^- anions at low concentrations facilitates rapid responses, whereas at higher concentrations or with excessive CTAB, molecular crowding and clustering reduce diffusivity and prolong the response time. This work not only elucidates the intrinsic mechanisms of electro-controlled friction in mixed surfactant systems but also provides feasible theoretical guidance for the design of high-performance, rapid-response electro-lubrication systems.