Foams Stabilized by Surfactant Precipitates: Criteria for Ultrastability

Foams are ultrastable when all the aging processes arrest. We make such foams by precipitating sodium dodecyl sulfate with potassium chloride during the foaming process. The precipitate crystals adsorb onto the bubble surfaces to arrest coarsening and stop drainage by blocking in the interstices around the bubbles. However, if the concentration of SDS is too high, the foams are no longer ultrastable. The transition is sudden and corresponds to the point at which significant dodecyl sulfate remains in solution. The presence of the noncrystallized surfactant allows the foam to coarsen leading to the eventual disappearance of the foams, even if the crystals in the continuous phase can still block drainage. The transition occurs as the concentration of nonsolubilized KCl becomes higher than the concentration of SDS, giving us a linear stability boundary. The system offers an interesting alternative to other types of particles because the surfactant crystals break and reform as the temperature is cycled, which makes for reusable solutions and stimulable foams.