Surfactant-water systems, phase sequence

Fig. 4 Idealized phase sequence in surfactant-water systems. (From Ref., reproduced by permission of the Royal Society of Chemistry.)...

Figure 1.2 Isothermal Gibbs triangles of the system water (A)-oil (B)-non-ionic surfactant (C) at different temperatures. Increasing the temperature leads to the phase sequence 2-3-2. A large miscibility gap can be found both at low and high temperatures. While at low temperatures a surfactant-rich water phase (a) coexists with an oil-excess phase (b), a coexistence of a surfactant-rich oil phase (b) with a water-excess phase (a) is found at high temperatures. At intermediate temperatures the phase behaviour is dominated by an extended three-phase triangle with its adjacent three two-phase regions. The test tubes illustrate the relative change in phase volumes.

Figure 17.2. A sequence of isothermal schematic phase diagrams of a ternary system L denotes the liquid microemulsion phase, W and O are essentially pure water and oil phases, respectively, while L -f O is the Winsor I, L + W is the Winsor II and W + L + O is the Winsor III equilibrium. For nonionic surfactant-water-oil systems, the sequence from (a) to (i) is obtained with increasing temperature...

Figure 8 The iV-dodecanoyl-iV-methylglucamine-water system, which illustrates the classical sequence of changes in the solubility boundary with temperature found in many nonionic and ionic surfactants. This compound is somewhat unusual in that the lamellar liquid-crystal phase extends to 100% and so exists as a thermotropic liquid crystal, which melts to an isotropic liquid at about 121 C. Physical studies in this temperature region are compromised by chemical instability. (From Ref. 91.)...

In two-component systems of association of colloid and water the sequence of phases, as the water content decreases, is micellar solution - hexagonally packed polar rods complex phases with rod-shaped aggregates lamellar mesophase D - crystalline surfactant. Some of these steps may be absent, depending, for example, on the temperature. 

From these phase diagrams, one important conclusion must be emphasized A comparison of the water-rich side and the oil-rich side of the phase diagram shows similar properties in their phase behavior and structures. In each side as the alcohol concentration is varied, the system exhibits the sequence micelle (or vesicle)-lamellar-sponge. This reveals that although the experimental situation seems opposite, the physics is the same and can be described with the flexible surface model [19]. Symmetry properties of phase behavior were found also with nonionic surfactants systems [104]. 

Vesicles are closed bilayers that can be observed in two forms. At low surfactant concentration, the vesicles are unilamellar and behave like a colloidal suspension of polydisperse particles. At more concentrated surfactant solutions, small multilayered vesicles are formed [134], Multilamellar vesicles (known also as spherulites) have also been observed in the lamellar phases of surfactant-brine (or even pure water-alcohol) systems [218]. The surfactant may be SDS [218,223] or DDAB (didodecyldimethylammonium bromide) [224]. In alcohol-containing systems the bilayer structural transformations are controlled by the alcohol/surfactant ratio [134].Thus, in many SDS-brine (or water)-alcohol systems, a vesicle (L4) phase is located between the micellar phase and the lamellar (L ) phase. At fixed surfactant concentration, the sequence of phases L4 -La-L3 (in water) is obtained by increasing the alcohol content, and the sequence L2 -La-L3 (in oil) is obtained by decreasing the alcohol content [ 134]. 

The aspect of the test tube sequence is depicted in Fig. 8, where the shaded area indicates the surfactant-rich phase. The composition of the system is typically 2% amphiphile and 49% each oil and water, so that the representative point is likely to be in the poly phasic region of the diagram. 

In order to illustrate the phase behaviour of microemulsions, it is most convenient to consider the systems with nonionic surfactants of the ethylene oxide type. These have been studied extensively by Shinoda and Kunieda and co-workers and by Kahlweit and Strey and co-workers (for more recent reviews, see, e.g. refs (9) and (10)). At low surfactant concentrations, there is a general sequence of phase equilibria, often referred to as Winsor equilibria (11). The equilibrium conditions for the microemulsion phase, L, changes from equilibrium with excess oil (Winsor I) to equilibrium with excess water (Winsor II), via a three-phase equilibrium with excess water and oil (Winsor III). For nonionics, this sequence occurs when increasing the temperature, while for quaternary or ternary systems, it can be observed with increasing salinity or cosurfactant-to-surfactant ratio. 

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