Water-insoluble substances such as essential oils can be stabilized in an aqueous environment in the inner core of a micelle. Polymeric micelles have been recognised as a very promising carrier for water-insoluble drugs, due to their pro- longed stability times in vivo [84]. Therefore they can be suitable formulations for applications of essential oils as an- tibiotics. The effectiveness of micelle formulations of essen- tial oil active compounds for bacteria growth inhibition has indeed been proved [85]. Although micelles spontaneously form when the concentration of a surfactant or an am-
phiphilic block copolymer is higher than the critical micelle concentration, a more elaborate process is required if a high encapsulation efficiency of the oil in the micelle is to be achieved. Nowadays, the most popular micelle production method is to dissolve the active compound and the polymer in an organic solvent, which is then replaced with water by dialysis. This method is adequate for laboratory scale, but has important disadvantages for large scale implementation related to the small productivity of dialysis equipments, and the long processing times required (>1 day). An alternative is the formation of an organic solvent-in-water emulsion fol- lowed by the evaporation of the organic solvent, but this approach has his own disadvantages due to the presence of residual organic solvent in the final product and the necessity of exposing the product to high temperatures during pro- longed times [86].
A possible alternative is the use of compressed gases or supercritical fluids to remove the organic solvent from the emulsion in order to produce essential oil-loaded micelles, according to the Supercritical Extraction from Emulsions (SFEE) process [87]. A schematic diagram of this process is presented in Fig. (3). It consists in putting the emulsion into contact with a compressed gas. Possible contacting devices include packed columns and sprays. Gas extracts the organic solvent and an aqueous suspension is obtained.
In a recent study performed with the objective of precipi- tating -carotene from the organic phase of a dichloro- methane-in-water emulsion [88], it was demonstrated that with this technique it is possible to eliminate the organic solvent from the emulsion down to concentrations below 1 ppm. With a study of the evolution of drop sizes with time, it was shown that final particle size matched the particle size of empty micelles in the initial emulsion. Therefore, it could be concluded that the produced -carotene particles were en- capsulated in surfactant micelles. The feasibility of carrying out the process at moderate pressures (5 MPa) was also demonstrated. Due to this, this process in principle is also feasible for obtaining micelles loaded with active com-