HOMOGENEOUS NUCLEATION IN A MONODISPERSE OIL-IN-WATER EMULSION

A monodisperse emulsion of oil droplets in water (hydrocarbon radius a pproximately 8 nm) stabilized by the nonionic surfactant pentaoxyethyl ene glycol dodecyl ether (C(12)E(5)) is prepared from a stable microem ulsion containing the oil droplets by lowering the temperature across a phase boundary. At equilibrium a droplet emulsion phase exists in co njunction with excess oil. The first stage in the equilibration proces s is followed by measuring the turbidity. It is observed that for smal l temperature quenches the system is metastable, showing no measurable changes over at least 1 h. There is a sharp boundary between metastab le and unstable regions, and this, like the phase boundary, is virtual ly independent of droplet concentration in the range 5-20%. The observ ation is interpreted qualitatively as an Ostwald ripening-like process , where a few oil droplets grow, allowing the majority to shrink in si ze. The presence of a nucleation threshold can be understood in terms of the curvature free energy of the surfactant film.