The mesophase behaviour of surfactant- and lipid-water mixtures

7 The mesophase behaviour of surfactant- and lipid-water mixtures 

The free energy of surfactant interfaces is due to interactions between water and the surfactant head-groups, as well as interactions between the surfactant chains, both of which compete to set the curvatures of the interface. Consequently, all else being equal, homogeneous interfaces are preferred over other geometries for a monodisperse distribution of surfactant 

In most cases, the complex array of interactions at work within an actual surfactant-water mixture leads to variations of the surfactant parameter with surfactant dilution and temperature. In general then, the phase behaviour of a binary surfactant-water mixture follows a curved trajectory through the local/global domain plotted in Fig. 4.11. If these variations in molecular conformation are small, the phase progression with water dilution is expected to follow a nearly-vertical line in the plot if the molecular architecture is sensitive to these external parameters, die succession of phases with water dilution is more nearly horizontal. 

these mesostructures are predominantly lamellar, and identified as conventional (parabolic) lamellar phases, although they may in fact be hyperbolic. Indeed, unless v/al is exactly unity, a planar interface (lamellar mesophase) incurs a bending energy cost hyperbolic sponge monolayers or bilayers or mesh monolayer mesophases are favoured if v/al differs from unity. It is likely then that many lamellar phases in fact adopt a hyperbolic geometry. Careful neutron-scattering studies of a lamellar phase have revealed the presence of a large number of hyperbolic defects (pores within the bilayers) in one case [16]. (An example of this mis-identification of hyperbolic phases in block copolymers is discussed in section 4.10.) 

The effect of temperature on double-chain zwitterionic surfactants and uncharged lipids is to increase v/al further beyond unity, allowing a range of aggregation states. For example, the diacyl lipid didodecyl phosphatidylethanolamine mixed with about 20% water exhibits lamellar , cubic (V2) and reversed hexagonal (H2) phases upon heating [15]. This 

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