Interferometric investigation of the gas-state monolayer of mono-rhamnolipid adsorbing at an oil/water interface

We present a study on biosurfactant uptake at the interface of a millimeter-sized drop of squalene in water, by a recently developed differential interferometric technique. The technique allows detecting capillary waves, with amplitudes of the order of 10−9 m, excited on the surface of the drop by an electric field in the order of 5 V/cm. By studying the resonant surface modes of the drop it is possible to assess the interfacial properties as a function of the surfactant concentration in the water bulk. The technique allows to follow the adsorption process at extremely low surfactant concentration, not accessible by other methods, investigating the gas state in the π-A diagrams of 2D amphiphilic monolayers.