Spreading of Aqueous Dimethyldidodecylammonium Bromide Surfactant Droplets over Liquid Hydrocarbon Substrates

The dynamics of dimethyldidodecylammonim bromide (DDAB) aqueous surfactant solutions spreading over a deep layer of liquid hydrocarbons is studied by video-enhanced microscopy at 23 °C and at different relative humidities ranging from 35 to 100%. Aqueous DDAB droplets do not spread over simple liquid hydrocarbons until the DDAB concentration exceeds 0.005 wt % where the initial spreading coefficient is positive and the aqueous DDAB solutions consist of dispersed small vesicles and large liposome-like aggregates. The rates of spreading strongly depend on surfactant concentration, even though the initial spreading coefficient is essentially constant with concentration. Neither relative humidity nor drop volume significantly influences spreading rates, although larger drop volumes do spread to larger areas. Classical tension-gradient-driven spreading theory, developed for pure, nonvolatile, and immiscible liquid spreading on a second liquid predicts lens expansion rates that are an order of magnitude higher than those experimentally observed for DDAB solutions. With the aqueous DDAB surfactant solutions, spreading eventually ceases in the form of equilibrium lenses whose areas increase linearly with both surfactant concentration and drop volume. A surprising and important observation is that substrate viscosity has only a minor effect on the rate of surfactant solution spreading. Fascinatingly, DDAB solution droplets actually spread faster on mineral oil than on dodecane, which is 18 times less viscous, even though the initial spreading coefficients of these two substrates are essentially identical. We argue that the rate of surfactant arrival at the stretching air−water and oil/water interfaces determines the droplet spreading kinetics.