Formation of 10-100 nm size-controlled emulsions through a sub-PIT cycle

We have re-examined the phase inversion temperature (PIT) emulsification process. This is a low-energy method that uses a physicochemical drive to produce very fine oil/water emulsions in the absence of high shear flows. We used the polyoxyethylene 8 cetyl ether (C(16)E(8))/hexadecane/water system, which has a PIT of 76.2 degrees C. We find that successful emulsification depends on two conditions. First, the mixture must be stirred at low speed throughout the whole process: this makes it possible to produce emulsions at surfactant concentrations that are too low to form an equilibrium microemulsion. Second, the stirred mixtures must be heated above a threshold called the clearing boundary (CB) and then quenched to lower temperatures. The clearing boundary is determined experimentally by a minimum in the turbidity of the stirred mixture, which results from solubilization of all the oil into swollen micelles. This matches the emulsification failure boundary, and it is expressed mathematically by the condition R*C(0) = 1, where R* is the radius that results from the oil/surfactant composition for monodisperse spheres and C(0) is the spontaneous spherical curvature of the surfactant. Thus, we show that such cycles do not need to cross the PIT. In fact, sub-PIT cycles and cross-PIT cycles give exactly the same result. These conditions lead to emulsions that have a narrow size distribution and a mean diameter controlled by the oil/surfactant ratio. The typical range of those diameters is 20-100 nm. Moreover, these emulsions have an excellent metastability, in contrast with emulsions made with shorter oil and surfactant molecules.