DISJOINING PRESSURE AND STRATIFICATION IN ASYMMETRIC THIN-LIQUID FILMS

We directly measure, for the first time, disjoining pressure isotherms for asymmetric oil/aqueous surfactant/gas (i.e., pseudoemulsion) film s using a modified version of the porous-plate technique first develop ed by Mysels in conjunction with thin-film interferometry. Dynamic fil m-thinning experiments are also performed on individual foam and pseud oemulsion films. At SDS surfactant concentrations above the critical m icelle concentration (CMC) (0.1 M SDS), the pesudoemulsion films exhib it the same step-wise layer thinning observed in foam films under simi lar conditions. Further, we conduct dynamic thinning experiments on so lid/liquid/gas systems and show that aqueous 0.2 M CTAB films sandwich ed between glass and air also display discrete thinning transitions. A ll of these stratification transitions arise from oscillations in the disjoining pressure isotherm, generated by amphiphilic structuring wit hin the film. For 0.1 M SDS dedecane/air pesudoemuslion films, the slo pe and peak height of the disjoining-pressure oscillations increase wi th each subsequent amphiphilic layer as film thickness decreases. Magn itudes of the structural forces are low (< 100 Pa) but the length scal e of the oscillations is large (approximately 10 nm) and rather far re aching (approximately 50 nm). Moreover, for 0.1 M SDS solutions, the c apillary pressures associated with film rupture are significantly lowe r for pseudoemulsion films (approximately 0.1 kPa) when compared to fo am films (approximately 15 kPa) at equivalent conditions. Taken togeth er, our dynamic thinning and equilibrium disjoining pressure measureme nts indicate that stratification in 0.1 M SDS films has little effect on both kinetic and thermodynamic films stability.