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Aggregation Behavior in One-Phase (Winsor IV) Microemulsion Systems
Published in Promod Kumar, K. L. Mittal, Handbook of Microemulsion Science and Technology, 2018
Shmaryahu Ezrahi, Abraham Aserin, Nissim Garti
Between micellar solution and HI phase there is a micellar cubic phase with spherical or slightly anisotropic micelles. Between HI and Lα or between Lα and HII there are bicontinuous cubic phases as we have seen. Cubic phases based on the packing of inverse micelles between reverse micelle solution (L2) and HII are relatively rare, presumably owing to the low tendency of reverse micelles to organize in a regular manner [244], and have only quite recently been established [166,245]. Such a location of cubic phases might be anticipated for compounds having bulky hydrocarbon chains and small, weakly polar headgroups (ideally also with attractive lateral headgroup interactions such as hydrogen bonding). The Fd3m cubic phase has been discovered in fully hydrated phospholipids, mixtures of a strong polar lipid and a very weakly polar amphiphile [160,166,246]. In addition, it was shown that Fd3m cubic phases might be formed in purely binary glycolipid-water and nonionic surfactant-water systems [244]. It was suggested that the monolayer bending modulus in these cases was rather low, making the chain packing constraints less severe [244]. Also, a reverse (W/O) cubic phase was observed in a ternary system consisting of an amphiphilic diblock copolymer (EO17BO10, where EO represents ethylene oxide and BO represents butylene oxide), water, and p-xylene [247]. Because of the weak (excluded volume) interactions between the reverse micelles, high micelle densities (and consequently high amphiphile concentration) would be needed to make a crystalline packing possible. Upon an increase in the amphiphile concentration it is more likely that the reverse micelles (L2) elongate and then crystallize into a hexagonal array of crystals without going through a cubic ordering arrangement; this is usually the case with the typical surfactants. Apparently, in the case of macromolecular amphiphiles such as the block copolymer studied here, the micellar size and (steric) interactions between micelles are both large enough to facilitate the crystallization of the reverse micelles into a cubic lattice [247]. Along with Fd3m, a bicontinuous cubic phase Ia3d and a normal micellar cube of the space group Im3m were observed at the same temperature together with five more noncubic phases. In fact, this is the richest phase behavior reported to date in an amphiphile-containing system [248].
Geometric features in lyotropic liquid crystalline phase transitions observed in aqueous surfactant systems
Published in Journal of Dispersion Science and Technology, 2022
The solvent defines if the obtained structures, either micelles or liquid crystalline phases, are normal (Type I) or reverse (Type II), for aqueous and nonaqueous media, respectively. The influence of the surfactant molecular structure on its phase behavior is predicted by the well-known theory of critical packing parameter (CPP), where the increase in surfactant concentration leads to the formation of structures with lower curvatures in the expected order: micellar cubic (II)→hexagonal (HI)→bicontinuous cubic (QI)→lamellar (Lα)→reverse phases.[1–3] Intermediate phases between the hexagonal and lamellar structures may appear but, for the sake of simplicity, they will not be explored here. In a similar manner, the reverse phases will also be excluded of the present study. Such a phase sequence displayed above assumes that the starting micelles formed in the dilute regime are spherical.