Disjoining pressure isotherms for oil-water-oil emulsion films

We have used a novel liquid surface forces apparatus (LSFA) to determine the variation of disjoining pressure with film thickness for dodecane-water-dodecane emulsion films. The LSFA allows measurement of film thicknesses in the range 5-100 nm and disjoining pressure from 0 to 1500 Pa. Disjoining pressure isotherms are given for films stabilized by the nonionic surfactants n-dodecyl pentaoxyethylene glycol ether(C12E5) and n-decyl-beta-D-glucopyranoside (C-10-beta-Glu) and the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in the presence of added electrolyte. For C12E5 and AOT, the emulsion films are indefinitely stable even for the highest concentration of NaCl tested (136.7 mM), whereas the C-10-beta-Glu film shows coalescence at this salt concentration. For film thicknesses greater than similar to 20 nm with all three surfactants, the disjoining pressure isotherms are reasonably well described in terms of electrostatic and van der Waals' forces. For the nonionic surfactant emulsion films, the charge properties of the monolayers are qualitatively similar to those seen for foam films. For AOT emulsion films, the monolayer surface potentials estimated by fitting the isotherms are similar to the values of the zeta potential measured for AOT-stabilized emulsion droplets. For thin emulsion films (< 20 nm) certain systems showed isotherms that suggested the presence of an additional repulsive force with a range of similar to 20 nm.