Surfactant films in biliquid foams

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 wat er. Examination of this cream through electron microscopy shows polyhedral oil cells separated by thin films. The thickness of these films has been me asured through small-angle neutron scattering. The results yield a disjoini ng 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 is otherm has two states, the common black film (CBF; water thickness beyond 2 5 Angstrom) and the Newton black film (NBF; (water thickness of 13 Angstrom ). At low pressures (1-50 atm), the films are in the CBF state, where the m easured disjoining pressure matches the entropic pressure of the counterion s, calculated from the Poisson-Boltzmann equation. At high pressures (20-30 0 atm), ionic correlations in the counterion layer reduce the disjoining pr essure and the films jump discontinuously to the NBF. The thickness of the NBF is stabilized by hydration forces, which resist the dehydration of coun terions and headgroups. The surface density of SDS molecules in these films has also been measured. As water is extracted, the concentration of counte rions increases, and they screen the headgroups more efficiently; as a resu lt, the surface density of SDS in the monolayers rises. In the NBF state, t he monolayers are tightly packed, with an orientational order that exceeds that of the lamellar phase. This tighter packing of surfactant molecules ma y explain the surprisingly high metastability of biliquid foams when the fi lms are in the NBF state.