Soap solutions phase diagram

Fig. 1. The fatty acid soap-water phase diagram of McBain (58) modified (1) to show the molecular arrangement in relation to aqueous concentration (abscissa) and temperature (ordinate). Ideal solution, i.e., true molecular solution, is to the left of the vertical dashed line, indicating the critical micellar concentration (CMC), which varies little with temperature. At concentrations above the CMC, provided that the temperature is above the critical micellar temperature (CMT), a micellar phase is present. At high concentrations, the soap exists in a liquid crystalline arrangement, provided that the solution is above the transition temperature of the system, i.e., the temperature at which a crystalline phase becomes liquid crystalline. The Krafft point is best defined (D. M. Small, personal communication) as the triple point, i.e., the concentration and temperature at which the three phases (true solution, micelles, and solid crystals) coexist, but in the past the Krafft point has been equated with the CMT. The diagram emphasizes the requirement for micelle formation (a) a concentration above the CMC, (b) temperature above the CMT, and (c) a concentration below that at which the transition from micelles to liquid crystals occurs. Modified from Hofmann and Small (1).

Fig. 10. Phase equilibria of the bile acid (as sodium salt)-fatty acid soap-water phase diagram at constant water concentration in relation to temperature. Mixtures with varying molar ratios of bile acid/sodium soap (total concentration 40 mM) were incubated, and the temperature at which the system became clear was plotted solutions were buffered top i 12. The curves indicate the critical micellar temperature of the system and have also been termed mixed Krafft points (46). The CMT of the bile acids is extremely low.

The fifth main type occurs in systems in which the soap component is not an association colloid of the paraffin chain type but a salt of a bile acid, with its condensed four-ring skeleton with two or three hydroxyl groups and with one carboxyl group at the end of a branched hydrocarbon chain. Figure 29 shows the phase diagram for the sodium cholate-decanol-water system (9). There is no mesomorphous phase but one extensive continuous area with homogeneous isotropic solutions. The cholate and decanol are mutually soluble in the presence of water, as in the case of the soap and the alcohol in the soap-alcohol systems, but here we have the remarkable phenomenon that water and decanol, which are practically insoluble in one another, become mutually soluble in all proportions in the presence of a certain quantity of a bile acid salt. 

In the phase diagrams of soap solutions to which electrolyte has been added (see Fig. 21) there occurs a region in which the solution separates into two layers under the influence of... 

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). 

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). 

Although lyotropic liquid crystals are characterized by the fact that concentration is the determining factor in their phase transitions, temperature also plays an important role. This can be seen on the phase diagram of a soap-water system, where the vertical axis is the concentration of amphiphilic molecules and the vertical axis is the temperature. The concentration at which micelles form in solution, called the critical micelle concentration, is shown as a dotted line. The line at low temperatures is called Krafft temperature T. It separates the crystal in water part from the liquid crystalline structures. Above the solutions have milky appearances, since the micelles scatter light (the larger the micelle, the milkier the solution). Below the solution becomes clear, as only crystals are suspended in the solvent. 

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. 

Figure 7 depicts a simplified block flow diagram (BFD) for a typical biodiesel production process using base catalysis. In the first step, methanol and catalyst (NaOH) are mixed with the aim to create the active methoxide ions (Figure 4, step 1(b)). Then, the oil and the methanol-catalyst solution are transferred to the main reactor where the transesterification reaction occurs. Once the reaction has finished, two distinct phases are formed with the less dense (top) phase containing the ester products and unreacted oil as well as some residual methanol, glycerol, and catalyst. The denser (bottom) layer is mainly composed of glycerin and methanol, but ester residues as well as most of the catalyst, water, and soap can also be found in this layer. 

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