Lamellar phases micelle self-assembly

Considerations of the packing parameter concept of Israelachvili et al. [1] suggest that double-chain surfactants, which form the basis of measurements described in this article, cannot readily form spherical micelles. With double-chain surfactants, a more likely aggregate structure is the formation of bilayer vesicles, which can be also thought of as a dispersed lamellar phase (La) as such the vesicular dispersed form is likely to be preferentially formed at low concentrations ( 1 mmol dm-3) of surfactant. Furthermore, it is necessary to consider the possibility, unlike in the case of micelles, that such vesicles, formed by self-assembly of surfactant monomers, will not be thermodynamically stable. The instability is then likely to be in the direction of growth to a thermodynamically-stable lamellar phase from the vesicles. This process will be driven, at least initially, by fusion of two vesicles. 

Evans et al. also showed that the 1 1 mixture of BAN and (3, y-distearoyl-phos-photidylcholine (DSPC) gives a smectic A texture in the temperature range of 57.3 to 100°C [21]. This is the first notice of lyotropic lamellar liquid crystals formed in the ionic medium. Additionally, Seddon et al. [28] and Neve et al. [29] have described the long-chained A-alkylpyridinium or l-methyl-3-alkylimidazolium ions to display smectic liquid-crystalline phases above their melting points, when Cl or tetrachloro-metal anions like CoCl " and CuCl " are used as the counter ions. Lin et al. have also noted the liquid crystal behavior of 1-alkylimidazolium salts and the effect on the stereoselectivity of Diels-Alder reactions [30]. However, liquid crystals are classified as ionic liquid crystals (ILCs), and they are distinguished from liquid crystals that are dispersed in ionic liquids. Although the formation of micelles and liquid crystal phases in ionic liquids have been thus reported, there has been no mention of the self-assembly of developed nano-assemblies that are stably dispersed in ionic liquids. In the next section the formation of bilayer membranes and vesicles in ionic liquids is discussed. 

Comparable to the binary systems (water-surfactant or oil-surfactant), self-assembled structures of different morphologies can be obtained ranging from (inverted) spherical and cylindrical micelles to lamellar phases and bicontin-uous structures. To map out these regions, a phase diagram is most useful. 

Surfactants are amphiphilic molecules which, when dispersed in a solvent, spontaneously self-assemble to form a wide variety of structures, including spherical and asymmetric micelles, hexagonal, lamellar, and a plethora of cubic phases. With the exception of the lamellar phase, each of these phase structures can exist in both normal and reverse orientations with the hydrophobic chains on the exterior of the aggregate, in contact with solvent or vice versa orientation. The range of structures a particular surfactant forms and the concentration range over which they form, depends upon the molecular architecture of the surfactant, its concentration, and the solvent in which it is dispersed. For example, some solvents such as ethanol do not support the formation of aggregates. As most pharmaceutical systems use water as their solvent, this entry will concentrate on aqueous-based systems, although other solvent systems, particularly other non-aqueous polar systems, will be mentioned where appropriate. 

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