Soap-water system binary

Binary Soap-Water System Mixtures of soap in water exhibit a rich variety of phase structures (4, 5). Phase diagrams chart the phase structures, or simply phases, as a function of temperature (on the y-axis) and concentration (on the x-axis). Figure 2.1 shows a typical soap-water binary phase diagram, in this case for sodium pahnitate-water. Sodium palmitate is a fully saturated, 16-carbon chain-length soap. At lower temperatures, soap crystals coexist with a dilute isotropic soap solution. Upon heating, the solubility of soap increases in water. As the temperature is increased the soap becomes soluble enough to form micelles this point is named the Krafft point. The temperature boundary at different soap concentrations above which micelles or hquid crystalline phases form is named the Krafft boundary (5). 

Phase diagram for a typical soap-water system. The nearly vertical dashed line shows the minimum concentration for micelle formation. The line separating the crystal in water part from the liquid crystaUine parts is called Kraft temperature (Tr). Vertical bars and symbols at the top of the figure indicate hypothetical sequence of phases in binary amphiphile-solvent systems. Here a, b, c and d indicate intermediate phases (for example the bicontinuous cubic phase), L2 denotes the inverse micelle solution, H2 is the inverse hexagonal phase, L is the lamellar phase, H, is the normal hexagonal phase and Lj is the normal micelle phase. Note, this idealized sequence has never been observed entirely and the phase boundaries are rarely vertical. 

These deviations were accounted by Strey et al.,8 who carried out experiments with the binary water—C12E5 system, by noting that the amplitude of the thermal undulations increased with the repeat distance d. If one considers the total area of the interface a constants which depends only on the number of surfactant molecules, the projection So of the total area on a plane perpendicular to d will decrease with increasing d. Hence, the apparent area per surfactant molecule, which is defined as the ratio between So and the total number of surfactant molecules, decreases with increasing d, while the ideal dilution law implies that the apparent area per surfactant molecule is a constant. The excess area, defined as AS = S — So, was related to the bending modulus of the interface,8 and the experimental results for the deviations from the ideal dilution law were used to determine. Kc-3,11 However, it should be noted that there are binary systems for which the deviations from the ideal dilution law occur in the opposite directions. For instance, in the binary systems of fatty acid alkali soaps/water, the apparent area per headgroup increases with water dilution, because of the incorporation of water in the interface.1... 

There are two types of lipid-water phase diagrams. The first type, discussed above, is obtained from polar lipids, which are insoluble in water (i.e. the solubility is quite small, monolaurin for example has a solubility of about 10 m). Fig. 8.12 illustrates the principles of phase equilibria in this type of lipid-water system. The second type of binary system is obtained when the lipid is soluble as micelles in water. Examples of such lipids are fatty acid salts and lysolecithin. An aqueous soap system is illustrated in Fig. 8.13. When the lipid concentration in the micellar solution is increased, the spherical micelles are transformed into rod-shaped micelles. At still higher lipid concentrations the lipid cylinders are hexagonally arranged and the liquid-crystalline phase Hi is formed. The lamellar liquid-crystalline phase is usually formed in the region between Hi and the anhydrous lipid. Excellent reviews of the association behaviour of amphiphiles of this type have been published (Wennerstrom and Lindman, 1979 Lindman and Wennerstrom, 1980). 

Figure 1 shows the results obtained by Francois and Skoulios (27) on the conductivity of various liquid crystalline phases in the binary systems water-sodium lauryl sulfate and water-potassium laurate at 50 °C. As might be expected, the water-continuous normal hexagonal phase has the highest conductivity among the liquid crystals while the lamellar phase with its bimolecular leaflets of surfactant has the lowest conductivity. Francois (28) has presented data on the conductivity of the hexagonal phases of other soaps. She has also discussed the mechanism of ion transport in the hexagonal phase and its similarity to ion transport in aqueous solutions of rodlike polyelectrolytes. 

Role of Adsorbed Surfactant Layer. Foams, irrespective of the nature of liquid and gas involved, require a third component for stabilization of thin films (lamellae) of the liquid. In the familiar case of aqueous soap films, this third component is the soap, a surface-active chemical that adsorbs at the gas—liquid interface and lowers the surface tension of water. The two effects, adsorption at the liquid surface and the depression of surface tension, are intimately linked and occur concomitantly. The adsorption is defined as the excess moles of solute per unit area of the liquid surface. In a binary system, this surface excess can be directly related to the lowering of surface tension by Gibbs adsorption equation ... 

Further milestones in the field of lyotropic liquid crystals are investigations of binary systems of soaps and detergents with water [21 ] and work on the lyomesomorphism of polypeptides and nucleic acids [22], In 1933 the first review on lyotropic liquid crystals appeared [23]. The author of this review investigated the amphotropic properties of various compounds. He suggested that in amphotropic materials raising the temperature or adding a solvent (water) lead to the same partial break up of the crystal lattice, however, a certain order is maintained. Furthermore, colloidal solutions showing partial supramolecular structures are also described here. But, due to different parameters of order these latter systems differ from lyotropic liquid crystals. 

Diffusivities of binary, ternary and multi-component liquid crystalline mixtures, e.g. of soap (potassium laurate (PL), water [25, 58], and lipid (dipalmitoylphosphatidylcho-line (DPPC) [25, 59] systems in lamellar, hexagonal, cubic, nematic and micellar mesophases [25,60,61] have been studied extensively by pulsed-field-gradient NMR [25] and optical techniques [62], partly because of their intimate relation to the structure and dynamical performance of biological membranes [18]. The main distinction from thermotropic phases is that for layered structures a noticeable diffusion occurs only within the layers (i.e. lateral, frequently written as Dl, but in our notation DjJ, whereas it is negligibly small and difficult to detect across the layers [60-62] (transverse migration, for bilayers denoted by flip-flop ) so the mobility is essentially two dimensional, and the anisotropy ratio is so great that it is seldom specified explicit-... 

---