Lateral interactions of charges in thin liquid films and the Berezinskii-Kosterlitz-Thouless transition

At certain experimental conditions, foam films drawn from aqueous surfactant solutions can form highly stable Newtonian films. We show that the origin of the film stability can be attributed to specific patterning of surfactant molecules. Due to high dielectric contrast, the charges, together with their electrostatic images, interact as needlelike charged polymers. Below a critical thickness, such quasipolyelectrolytes undergo the Berezinskii-Kosterlitz-Thouless transition from a plasmalike state to a charge neutral one. In the latter, all the charges are bound into dipole pairs. Inherent only in thin films, the effect leads to the surfactant condensation into spots that cannot be observed on a single interface at the same surfactant concentration. We quantify a film resistance to rupture in terms of a pore line tension and show that the corresponding energy barrier needed for pore creation is much greater than the energy of thermal excitations.