EQUILIBRIUM MEASUREMENTS OF OSCILLATORY DISJOINING PRESSURES IN AQUEOUS FOAM FILMS

Disjoining pressure isotherms are measured for single, isolated foam films stabilized with sodium dodecyl sulfate above the critical micelle concentration (cmc). The measured isotherms exhibit an equilibrium oscillatory component that extends out to film thicknesses as large as 50 nm. The oscillations originate from surfactant structuring within the film and are responsible for the multiple black films extensively studied by Perrin in 1918. We find that as film thickness decreases, the slopes and peak heights Of the oscillations increase for each subsequent black film encountered. The discrete changes in film thickness observed at each black film transition range from 6 to 10 nm and are a function of the ionic strength and surfactant concentration. Pressures required to rupture the multiple black films are several orders of magnitude lower than the final film-rupture pressure. Finally, the oscillatory form of the disjoining pressure permits quantitative interpretation of the stepwise thinning behavior observed in films containing surfactant above the cmc.