Disjoining pressure and film tension in comb-graft copolymer-stabilized oil films

Thin, supported decane films stabilized with comb-graft copolymers were studied as models of polymeric surfactant stabilized water-in-oil emulsions. The stabilizing polymeric surfactant ("polysoap") was composed of a poly(dimethylsiloxane) backbone with hydrophobic alkyl and hydrophilic ethylene/propylene oxide grafts with a total molecular weight of 62 000. Electrical compressive stresses were imposed on the films, and their thicknesses were determined from measurements of capacitance and optical interference. The theory for the interpretation of capacitance versus applied electric field in terms of disjoining pressure was developed. Sessile drop measurements of interfacial tension were used to infer a polymer coverage of 1.7 nm2/molecule. Dynamic light scattering measurements showed hydrodynamic diameters of 6 nm at 5.2 wt % of the polymer in decane. The measured film thicknesses ranged from 32 to 62 nm over a compressive force range of 0-1400 Pa. The films were remarkably thick and compressible compared to films formed from simple surfactant or lipid systems. The films displayed compressive moduli ranging from 1000 to 6400 Pa. 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.