Effects of Salt-Controlled Self-Assembly of Triblock Copolymers F68 on Interaction Forces between Oil Drops in Aqueous Solution

Nonionic triblock copolymers, surfactant Pluronic F68 (PEO76–PPO29–PEO76), are widely used in industrial processes, such as foaming, emulsification, and stabilization. The behaviors of triblock copolymers such as the salt-dependent self-assembly in bulk solution and the irreversible adsorption at the oil/water interface are mainly focused to explore their effects on the interaction forces between nano-spacing interfaces of oil droplets. In this study, the atomic force microscopy (AFM) technique was employed to measure the drop interaction forces with different F68 bulk concentrations. All selected bulk concentrations (≥100 μM) of copolymers can ensure the formation of a stable layer structure of stretched polymer chains (“brush”) at the oil/water interface, which behaved as a mechanical barrier at the interface. This study quantified the forces caused by the space hindrance of F68 copolymers both in the bulk phase and at the interface of oil/F68 aqueous solution during drop interaction. The effects of monovalent electrolyte (NaCl)-induced self-assembly behavior of triblock copolymers F68 in bulk solution on drop interaction forces were measured through the AFM technique.