Naturally Occurring Micelle Forming Systems

In the previous section we discussed two-component lyophilic colloidal systems, namely the dispersions of micelle-forming surfactants. The third component, when introduced into the system, depending on its nature can either retard the formation of micelles or, oppositely (which occurs more... 

The solubility of clofazimine was enhanced in aqueous micellar systems, containing both naturally occuring surfactants e.g bile salts, and synthetic surfactants, e.g the non ionic Cremophor EL and Triton XI00, and the anionic sodium dodecyl sulphate. The incorporation of fatty acids to form mixed micelles brought about a further enhancement in drug solubility in the case of naturally occuring surfactants (approximately 300 fold with sodium cholate linoleic acid relative to buffer). In contrast, with synthetic surfactants this enhancement decreased (Fahelelbom et al., 1991 O Driscoll et al., 1991). 

A question that arises from data such as those in Fig. 2 concerns the mechanism by which the pressure-driven phase separation occurs. Two types of phase transitions are known from studies of liquid systems. At the solubilization phase boundary, reverse micelles expel excess water to form a second phase. Its location is determined by the natural curvature of the surfactant interface. The natural curvature is the preferred curvature of the interface when no interactions between droplets are present. At the haze point boundary, surfactant and water precipitate together to form a surfactant-rich second phase. This phase transition is driven by micelle-micelle interactions. It is analogous to the cloud point transition seen with increasing temperature in an aqueous micellar system. 

We have discussed the self-assembly of nonionic surfactants that occurs in RTILs. Overall, the self-assembly properties in RTILs are largely similar to the aqueous medium. Notable differences between the aqueous and nonaqueous systems are sometimes seen when nonionic surfactants form micelles or lyotropic liquid crystals at certain compositions and temperatures, and this mainly results from the different affinity of the nonionic surfactants with the liquids. In other words, it may be possible to expect the formation of micelles or lyotropic liquid crystals to a certain degree by considering the solvophobic or solvophilic nature of the nonionic surfactants in the RTILs. An interesting feature of RTILs is their self-assembly in bulk liquids and at interfaces. This feature also makes a significant impact on the self-assembly of nonionic surfactants in RTILs. Particularly, we have demonstrated the importance of this feature when nonionic surfactants adsorb at solid/RTIL interfaces. We believe that the self-assembled structures of amphiphilic molecules with RTILs are of great interest not only from academic but also from industrial standpoints. One of the potential applications based on such self-assembled structures should be high-performance ion-conductive electrolytes as a new device system with nanolevel order [50]. 

Surfactants are essential for the preparation of solid/liquid dispersions (suspensions). The latter are generally prepared using two main procedures (7) Bmlding up of particles from molecular units. (2) Dispersion of bulk performed powder in a liquid followed by dispersion and wet milling (comminution) to produce smaller particles. An example of the first system is the production of polymer latex dispersions by emulsion or dispersion polymerization. The monomer is emulsified in an aqueous solution containing a surfactant to produce an emulsion of the monomer. An initiator is added to initiate the polymerization process. In some cases, initiation occurs in the micelles that are swollen by the monomer. The number of particles produced and hence their size is determined by the number of micelles in solution. In dispersion polymerization, the monomer is mixed with a solvent in which the resulting polymer is insoluble. A surfactant (protective colloid) and initiator is added. The surfactant prevents flocculation of the polymer particles once formed. Again the size of the particles produced depends on the nature and concentration of the surfactant used. 

The main conclusion of the work was that oxidation of the aldehyde in betaine micelles did not depend simply on the saturation ratio R as defined earlier and which was shown to hold for aldehyde-cetomacrogol systems [44]. Detailed studies of betaine-benzaldehyde-water systems were carried out by Swarbrick and Carless [47] in order to elucidate the impact of the complex nature of these systems and the role of the different phases that could occur. Their results showed that, in general, it was wrong to assume that all material in excess of that taken up into micelles of the phase formed liquid dispersions in water. This situation obtained only with the Cg and Cio betaines in the others the presence of liquid crystalline phases as an alternative phase for solubilization was identified. Oxidation in the various phases of these systems was studied in some detail [45]. The maximum rate of oxidation of the aldehyde was related to the concentration of aldehyde present in any one phase, the conclusion being that the use of the saturation ratio which expresses concentrations in one phase in terms of another phase is an unnecessary consideration when attempting to establish explanations for the behaviour of t-aldehydes in these systems. 

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