Trapping of Sodium Dodecyl Sulfate at the Air-Water Interface of Oscillating Bubbles

We report that at very low initial bulk concentrations, a couple of hundred times below the critical micellar concentration (CMC), anionic surfactant sodium dodecyl sulfate (SDS) adsorbed at the air-water interface of a gas bubble cannot be removed, on the time scale of the experiment (hours), when the surrounding solution is gently replaced by pure water. Extremely sensitive interferometric measurements of the resonance frequency of the bubble-forced oscillations give precise access to the concentration of the surfactant monolayer. The bulk-interface dynamic exchange of SDS molecules is shown to be inhibited below a concentration which we believe refers to a kind of gas-liquid phase transition of the surface monolayer. Above this threshold we recover the expected concentration-dependent desorption. The experimental observations are interpreted within simple energetic considerations supported by molecular dynamics (MD) calculations.