Mass transport of SDS and AOT solutions during a rapid surface expansion: Relaxation of surface tension

Surface tension (ST) relaxations are influential in industrial processes that are characterized by substantial surface deformations. Despite this, the dynamics of surfactant adsorption onto a dilating surface is still unclear. Recently, the ST relaxation of a non-ionic surfactant (TX-100) was reported (Lin et al. (2017) [17]), but little is known about the behavior of ionic surfactants. In this study, the relaxations of ST and surface dilational rates (λ) of ionic aerosol-OT (AOT) and sodium dodecyl sulfate (SDS) solutions were investigated by monitoring a continuously expanding bubble using a pendant bubble tensiometer. The data showed that during the rapid bubble expansion, ST of AOT solutions increased with time, while that of SDS changed negligibly. This implied that the mass transfer rate of AOT molecules from bulk to the bubble surface during expansion (at λ < 3 s⁻¹) is much slower than the depletion rate of AOT at the interface. Contrastingly, the mass transport of SDS molecules from bulk to the bubble surface is more rapid than the surfactant depletion rate at similar λ. A theoretical ST relaxation profile of an isolated bubble surface was also simulated, and it well described the experimental ST data of AOT solutions.