Self-emulsification methods

In some systems, when the limit of stability of the phase with zero curvature is exceeded, a direct single phase (direct microemulsion, [22,59], or direct cubic liquid crystal [25,60]) instead of a two-phase region, is formed. Then, further addition of water leads the system to the region with two-phase equilibria (W + O), where kinetically (but not thermodynamically) stable dispersions (i.e., nano-emulsions) can form. This emulsification method could be classified as a self-emulsification method since once the system is in one-phase region with direct-type structure, no inversion takes place. 

In some cases under the conditions similar to those corresponding to the formation of lyophilic colloidal systems, a spontaneous formation of emulsions, the so-called self-emulsification, may take place. This is possible e.g. when two substances, each of which is soluble in one of the contacting phases, react at the interface to form a highly surface active compound. The adsorption of the formed substance under such highly non-equilibrium conditions may lead to a sharp decrease in the surface tension and spontaneous dispersion (see, Chapter III, 3), as was shown by A.A. Zhukhovitsky [42,43], After the surface active substance has formed, its adsorption decreases as the system reaches equilibrium conditions. The surface tension may then again rise above the critical value, acr. Similar process of emulsification, which is an effective method for preparation of stable emulsions, may take place if a surfactant soluble in both dispersion medium and dispersed liquid is present. If solution of such a surfactant in the dispersion medium is intensively mixed with pure dispersion medium, the transfer of surfactant across the low surface tension interface occurs (Fig. VIII-10). This causes turbulization of interface... 

Emulsions are formed by mixing two liquids, a process which creates discrete droplets in a continuous phase. During emulsification,by mechanical agitation for example, both liquids tend to form droplets resulting in a complex mixture of 0/W and W/0 emulsions. Which of the components forms the continuous phase depends on the emulsifier used since one of the types of droplet is unstable and coalesces. Therefore, there is a need to identify the continuous phase in emulsion systems not only in the final emulsion system, but also at short times after emulsion formation or even dining the emulsification process. The NMR self-diffusion method may easily distinguish the continuous and... 

J. You, ED. Cui, X. Han, Y.S. Wang, L. Yang, Y.W. Yu, O.P. Li, Study of the preparation of sustained release microspheres containing zedoary turmeric oil by the emulsion solvent diffusion method and evaluation of the self emulsification and bioavailabihty of the oil. Colloids SurfB Biointerfaces, 48 35-41,2006. 

As mentioned in introduction, nano-emulsions being nonequilibrium systems require an energy input for their formation, which can be supplied mechanically (high-energy emulsification), or from the chemical energy of the components (condensation or low-energy emulsification methods). In this section, the different condensation methods, which are classified as self-emulsification and phase inversion methods, will be discussed. 

In self-emulsification or direct emulsification methods, a solution of the oil and surfactant in an appropriate solvent (that is also soluble in the continuous aqueous phase) is simply added to water to form oil-in-water (0/W) emulsions in one step, under agitation. Similarly, a solution of water and surfactant in an appropriate solvent (also soluble in the oil phase) is added to the oil to form water-in-oil (W/0) emulsions [19]. It is called direct or self-emulsification because the emulsion is just obtained by a dilution process without any phase inversion. This method uses the chemical energy of dissolution in the continuous phase of the solvent present in the initial system (which is going to constitute the dispersed phase). When the intended continuous phase and dispersed phase are mixed. 

The so-called self-emulsification is referred to those methods in which the nano-emulsion is just obtained by a dilution process without any inversion of phases. It is also called direct emulsification because the initial emulsion type is that of the intended final emulsion [19]. It should be taken into account that the term self-emulsification is frequently used in the Ulerature to describe emulsification mechanisms, in which not only dilution processes but also processes implying changes in the spontaneous curvature of the surfactant film are involved. Therefore, this terminology is often misleading. 

There are a large number of methods (Table 2) to prepare nanoparticulate systems. These depend to a large extent on the material (polymer, protein, metal, ceramic) that will form the basis of the carrier. One can, in essence, consider three approaches to their production (0 by comminution (in the case of solids, milling, and in the case of liquids, high pressure emulsification) (ii) molecular self-assembly, such as that occurs with polymeric surfactants to form polymeric micelles or with dendrons to form dendrimeric aggregates and (iii) precipitation from a good solvent as shown in Figure 6. 

Forster, T.,Schambil, F. and Tesmann, H. (1990) Emulsification by the phase inversion temperature method The role of self-bodying agents and the influence of oil polarity. Int. J. Cosmet. Sci., 12,217. 

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