DISJOINING PRESSURE MEASUREMENTS FOR FOAM FILMS STABILIZED BY A NONIONIC SUGAR-BASED SURFACTANT

Disjoining pressure isotherms for foam films made from a nonionic surf actant, octyl beta-glucoside, are measured at different surfactant con centrations, ionic strengths, and solution pH values. Below the cmc an electrostatic double-layer repulsion is present and dominates the lon g-range interaction. The decay length of the forces agrees with the ex pected Debye length and the measured long-range interactions are consi stent with solutions to the nonlinear Poisson-Boltzmann equation using constant charge conditions. The deduced surface charge densities incr ease with pH and ionic strength but decrease with increasing surfactan t concentration. At, or just above, the cmc, surfactant covers the int erface and suppresses the charge sufficiently to induce a transition f rom a common black film to a Newton black film. Ultimately, the film s tability is determined by both surface forces and elasticity. Combinin g both, via an overall film tension, leads to a general expression for the film elasticity.