No abstract

Centrifugation can be used to remove the continuous aqueous phase of an oil-in-water emulsion. The cream that remains after most of the water has been removed has the structure of a biliquid foam; it can be redispersed in water. Examination of this cream through electron microscopy shows polyhedral oil cells separated by thin films. The thickness of these films has been measured through small-angle neutron scattering. The results yield a disjoining pressure isotherm, where the film thickness is solely determined by the pressure applied to extract water during centrifugation. For hexadecane-in-water biliquid foams, stabilized with sodium dodecyl sulfate (SDS), this isotherm has two states, the common black film (CBF; water thickness beyond 25 Å) and the Newton black film (NBF; (water thickness of 13 Å). At low pressures (1−50 atm), the films are in the CBF state, where the measured disjoining pressure matches the entropic pressure of the counterions, calculated from the Poisson−Boltzmann equation. At high pressures (20−300 atm), ionic correlations in the counterion layer reduce the disjoining pressure and the films jump discontinuously to the NBF. The thickness of the NBF is stabilized by hydration forces, which resist the dehydration of counterions and headgroups. The surface density of SDS molecules in these films has also been measured. As water is extracted, the concentration of counterions increases, and they screen the headgroups more efficiently; as a result, the surface density of SDS in the monolayers rises. In the NBF state, the monolayers are tightly packed, with an orientational order that exceeds that of the lamellar phase. This tighter packing of surfactant molecules may explain the surprisingly high metastability of biliquid foams when the films are in the NBF state.