Lifetime Assessment of Thin Liquid Film Stability in Binary Surfactant Mixtures

Binary surfactant formulations can modify thin-film stability through non-additive changes in adsorption, interfacial mobility, and surface rheology. This study extends an electrical liquid-bridge method previously developed for single surfactants to binary systems containing sodium dodecyl sulfate (SDS), benzalkonium chloride (BAC), Ethylan 1005, sodium oleate, and polypropylene glycol 600 (PPG 600). Sixteen formulations were examined at fixed drainage rates of 10 and 20 µL h−1 using normalized apparent-conductance measurements synchronized with optical recording and TechDig contour reconstruction. Fifteen formulations formed measurable films at both rates, whereas 250 ppm SDS + 500 ppm sodium oleate (nominal mass–concentration ratio 1:2) failed by neck rupture before film formation. Binary mixtures either prolonged film lifetime relative to matched single-component references or enabled film formation where the reference was non-film-forming. Optical measurements independently supported the electrical ranking: longer-lived films generally reached smaller terminal neck diameters. The longest-lived formulation, 3000 ppm SDS + 500 ppm sodium oleate, also reached the smallest terminal neck. Pump-based mass balance and terminal bubble deformation further supported greater liquid removal and axial stretching before rupture. The combined electrical–optical approach provides a reproducible comparative framework for composition-dependent thin-film stabilization.