Bottom-Up Formation of Dodecane-in-Water Nanoemulsions from Hydrothermal Homogeneous Solutions**

Oil and water do not mix, but often they are made to mix in the form of emulsions by dispersing either component as fine droplets in the other. Emulsions are widely used in such industries as food, pharmaceutical, cosmetic, chemical, agricultural, print/ink, and petroleum. In recent years, considerable interest has emerged in emulsions containing droplets in the size range between 20 nm and 200 nm. Such emulsions, the so-called nanoemulsions, differ from macroemulsions in several aspects. Owing to the small droplet size, nanoemulsions are transparent or translucent, and creaming or sedimentation is significantly slower. The small droplet size also has potential benefits for developing a new realm of applications such as pharmaceutical and cosmetic formulations as well as reactors for synthesizing nanomaterials. Emulsions are usually prepared by top-down processes, in which external forces are applied to water/oil/surfactant mixtures to deform and disrupt large droplets into smaller ones. The deformation of the droplet, however, is opposed by the Laplace pressure that is inversely proportional to the droplet size. Thus, the disruption of the droplet becomes increasingly difficult as the size becomes smaller and the applied energy is rather dissipated as heat. Alternatively, the phase inversion temperature (PIT) method has been developed. PIT is the temperature at which the spontaneous curvature of a surfactant film at the oil/water interface is zero and the interfacial tension is extremely low. When emulsification is performed at PIT, nanosized droplets are easily formed owing to the near-zero interfacial energy. However, the PIT method can only be used with polyoxyethylene-type nonionic surfactants. Nanosized solid particles are usually prepared by bottomup processes. Unlike the top-down process, the bottom-up process starts with homogeneous solutions and solutes are allowed to assemble to form nanoparticles. It is plain to see that the bottom-up approach is also preferable for fabricating nanosized liquid droplets, but homogeneous solutions of oil and water need to be prepared. Water near its gas/liquid critical point (Tc= 374 8C, Pc= 22.1 MPa) exhibits properties that are remarkably different from those at ambient conditions, presumably because high thermal energy suppresses clustering of water molecules. For example, the dielectric constant of water, which is approximately 80 at ambient conditions, decreases to 2 at 400 8C and 25 MPa (Figure S1 in the Supporting Information), and this value is comparable to those of typical hydrocarbons. Accordingly, a wide variety of hydrocarbons become freely miscible with water. We found this opens up a whole new opportunity for emulsification, where oil droplets are formed in a bottom-up manner by phase separation of hydrothermal homogeneous solutions after a rapid and deep-temperature quench (i.e. from very high temperature to low temperature). The process that we call MAGIQ (monodisperse nanodroplet generation in quenched hydrothermal solution) was applied for emulsifying dodecane in water. The concept of MAGIQ is illustrated by in situ microscopic images in Figure 1, which were taken on an optical microscope with a high-temperature and high-pressure chamber. When coarse droplets of dodecane in water were heated in the chamber under a constant pressure of 25 MPa,