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

Thin, supported decane films stabilized with comb-graft copolymers were stu died as models of polymeric surfactant stabilized water-in-oil emulsions. T he stabilizing polymeric surfactant ("polysoap") was composed of a poly(dim ethylsiloxane) backbone with hydrophobic alkyl and hydrophilic ethylene/pro pylene oxide grafts with a total molecular weight of 62 000. Electrical com pressive stresses were imposed on the films, and their thicknesses were det ermined from measurements of capacitance and optical interference. The theo ry for the interpretation of capacitance versus applied electric field in t erms of disjoining pressure was developed. Sessile drop measurements of int erfacial tension were used to infer a polymer coverage of 1.7 nm(2)/molecul e. Dynamic light scattering measurements showed hydrodynamic diameters of 6 nm at 5.2 wt % of the polymer in decane. The measured film thicknesses ran ged 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 simpl e surfactant or lipid systems. The films displayed compressive moduli rangi ng from 1000 to 6400 Pa. The film properties were relatively insensitive to the surfactant concentration and moderately sensitive to polymeric surfact ant purity. The observed thicknesses are shown not to arise from interfacia l electrostatic effects or van der Waals forces but from steric interaction s. The observed thicknesses are consistent either with strongly stretched c hains adsorbed at the interface or with multichain aggregate structures at the interface.