We describe a novel apparatus for the investigation of the forces between two liquid surfaces. In the configuration described here, an oil drop is formed at the tip of a thin flexible micropipet. The force exerted on the oil drop as it is pressed up to an oil-water interface is determined from the deflection of the pipet shaft. The disjoining pressure in the thin, oil-water-oil emulsion film formed by contact of the drop with the oil-water interface is determined by the hydrostatic pressure applied to the oil contained in the micropipet. The radius and the thickness of the film is derived from the optical interference pattern observed using a microscope. A simple theory is presented for the variation in force and film radius as the drop is moved up into the interface. Experimental results are given for dodecane-water-dodecane emulsion films stabilized by the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT). Data for the force and film radius as a function of the pipet position relative to the oil-water monolayer show good agreement with theoretical predictions. The variation of disjoining pressure with film thickness for the emulsion films is in accord with electrostatic theory.

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-β-d-glucopyranoside (C10-β-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 C10-β-Glu film shows coalescence at this salt concentration. For film thicknesses greater than ∼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 ∼20 nm.