Stability and behavior of a comb-graft copolymer stabilizing a thin oil emulsion film

Amphiphilic polymers are able to adsorb at liquid/liquid interfaces and provide stability for emulsions. Studies on thin liquid films provide a way to measure forces between two approaching interfaces which are responsible for emulsion and foam stability. This paper begins with a brief review of some of the work which examine the effects of polymeric surfactants on stabilizing thin films. Then, a more detailed look at the effects of a graft-comb copolymer on stabilizing an oil-in-water film is given. The stabilizing polymeric surfactant ("polysoap") comprised a polydimethylsiloxane backbone with hydrophobic alkyl and hydrophilic ethylene/propylene oxide grafts. Electrical compressive stresses were imposed on the films, and their thicknesses were determined from measurements of capacitance and optical interference. Also, larger compressive stresses were applied in order to rupture the films and observe film breakdown behavior. The films were remarkably thick and compressible compared with films formed from simple surfactant or lipid systems. The film properties were relatively insensitive to the surfactant concentration and moderately sensitive to polymeric surfactant purity. The observed thicknesses are shown not to arise from interfacial electrostatic effects or van der Waals forces but from steric interactions. The observed thicknesses are consistent either with strongly stretched chains adsorbed at the interface or with multichain aggregate structures at the interface. The exact mechanism is still unclear.