Lamellar soap phase

During the drying process the lamellar, liquid crystalline phase of soap boilers neat soap is transferred into a mixture of solid soap phases with liquid crystalline and liquid phases. The system will not be in a state of equilibrium and the phase structure of the soap will change slowly on standing or more rapidly when the dried soap is subjected to the various physical work processes of soap finishing. 

Besides the lamellar liquid crystals just described, others are known to exist. We shall discuss only one here namely, the nematic liquid crystals illustrated by the middle soap phase of a typical soap-water system. An unoriented sample made up of many micro-liquid crystals of this sort will give a series of concentric... 

An acid soap of 2 1 complex ratio was discovered recently between triethanolamine stearate and stearic acid (13). In water the acid soap forms a lamellar liquid crystal phase at high temperatures, above 60°C, and transforms to a lamellar gel phase on cooling. The gel phase, however, is not stable at ambient temperature due to the occurrence of the hydrolysis reaction which converts the soap back to stearic acid which precipitates in the triethanolamine aqueous solution. A polymorphism of C, E, and possibly A forms of stearic acid crystals were found resulting from the hydrolysis reaction (8). 

Ekwall and Baltcheffsky [265] have discussed the formation of cholesterol mesomorphous phases in the presence of protein-surfactant complexes. In some cases when cholesterol is added to these solutions a mesomorphous phase forms, e.g. in serum albumin-sodium dodecyl sulphate systems, but this does not occur in serum albumin-sodium taurocholate solutions [266]. Cholesterol solubility in bile salt solutions is increased by the addition of lecithin [236]. The bile salt micelle is said to be swollen by the lecithin until the micellar structure breaks down and lamellar aggregates form in solution the solution is anisotropic. Bile salt-cholesterol-lecithin systems have been studied in detail by Small and coworkers [267-269]. The system sodium cholate-lecithin-water studied by these workers gives three paracrystalline phases I, II, and III shown in Fig. 4.37. Phase I is equivalent to a neat-soap phase, phase II is isotropic and is probably made up of dodecahedrally shaped lecithin micelles and bile salts. Phase III is of middle soap form. The isotropic micellar solution is represented by phase IV. The addition of cholesterol in increasing quantities reduces the extent of the isotropic... 

When coniferous wood is pulped in the strongly alkaline Kraft pulping process, also called the sulfate pulping process , the predominant chemical pulping process today, the resin and fatty acids are converted into sodium soaps, and are after cooking recovered as so-called sulfate soap from the surface of the pulping liquor, the black liquor. The sulfate soap is a lamellar crystalline phase where the resin and fatty acids form the lamellae, and neutral components, such as sterols, are included between the lamellae [7],... 

The phase condition for concentrations in the range close to the cmc are found in Fig. 4A. For the lowest soap concentrations, a liquid isotropic alcohol solution separated, when the solubility limit of the alcohol was exceeded. This was changed at concentrations approximately one half the cmc, when a lamellar liquid crystalline phase appeared Instead. After the relatively narrow three-phase region had been transversed, this liquid crystalline phase was the only phase in equilibrium with the aqueous solution. Solubilization of the long chain alcohol Increased at the cmc, as expected. 

Near the surfactant region the crystalline or lamellar phase is found. This is the region one finds in hand soaps. The ordinary hand soap is mainly the salt of fatty acid (coconut oil fatty acids or mixtures [85%] plus water [15%] and some salts. X-ray analyses have shown that the crystalline structure consists of a layer of soap separated by a water layer (with salts). The hand soap is produced by extruding under high pressure. This process aligns the lamellar crystalline structure lengthwise. If the degree of expansion versus temperature is measured, the expansion will be found... 

These structures are extensively described in the current literature (Fanum, 2008 Friberg, 1976 Birdi, 2002 Holmberg, 2004 Somasundaran, 2006). Even within the same phases, their self-assembled structures are tunable by the concentration for example, in lamellar phases, the layer distances increase with the solvent volume. Lamellar structures are found in systems such as the common hand soap, which consists of ca. 0% soap + 20% water. The layers of soap molecules are separated by a region of water (including, salts etc.) as a kind of sandwich. The x-ray diffraction analysis shows this structure very clearly. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is more difficult to analyze their structures and properties than those of thermotropic liquid crystals. Similar phases and characteristics can be observed in immiscible diblock copolymers. 

What is most amazing of all in this picture is the degree of microscopic order present in a solution that appears quite unexceptional to the imaided eye. Usually, we associate T>eauty and aesthetic appeal with symmetry and regular shapes, just as in the examples of the ordered lamellar phase and lamellar focal conics. However, sometimes also asymmetric shapes have that special quaUty about them that conveys what we call beauty. Figure 4 shows a water-rich foam composed of dish soap with coconut oil. It consists of tightly-packed bubbles of very different sizes that create an asymmetric pattern of astounding beauty [3]. 

In this situation, the equilibrium thickness at any given height h is determined by the balance between the hydrostatic pressure in the liquid (hpg) and the repulsive pressure in the film, that is n = hpg. Cyril Isenberg gives many beautiful pictures of soap films of different geometries in his book The Science of Soap Films and Soap Bubbles (1992). Sir Isaac Newton published his observations of the colours of soap bubbles in Opticks (1730). This experimental set-up has been used to measure the interaction force between surfactant surfaces, as a function of separation distance or film thickness. These forces are important in stabilizing surfactant lamellar phases and in cell-cell interactions, as well as in colloidal interactions generally. 

The chemical potentials measured so far do not allow the formulation of thermodynamic criteria for the formation of lyotropic mesophases. Some qualitative remarks, however, can be made. Of particular interest are Ekwall s studies of the relations between the water binding of the mesophases, their ionization, x-ray parameters, and vapor pressures (4). For common soaps at room temperature mesophases can be observed only in the presence of amounts of water that hydrate the ionic and polar groups. Hydration is therefore characteristic of aqueous lyotropic mesophases as well as micellar systems (1, 2, 3). The binding of counterions to the micelles and to the mesoaggregates seems to be of a similar electrostatic nature. The addition of NaCl greatly affects the lamellar phase D and, to a lesser extent, phase E in these phases the counterions are more strongly bound than by micelles in the solution... 

The lamellar spacing of a monoglyceride gel phase as a function of water content is plotted in Figure 14. The gel phase of the neutral monoglyceride has a lipid bilayer thickness of 49.5 A, and it swells to a unit layer thickness of 64 A (20). If an ionic amphiphilic substance (e.g. a soap) is solubilized in the lipid bilayer, it is possible to obtain a gel phase with high water content. As with the gel phases with infinite swelling that were discussed above, there is, however, a minimum water layer thickness which in this monoglyceride gel is about 40 A. 

T extures of lyotropic mesophases have been the object of numerous observations by optical (1,2,3) and electronic (4, 5, 6,7) microscopy. Except for the pioneering work of Lehmann (1) and Friedel (2) who intended to identify the various kinds of defects which constitute the textures, the purpose of these observations was to recognize the different existing phases—lamellar, hexagonal (or in the soaps language neat phase, median phase, etc.)—in correlation with x-ray data. 

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. 

Figure 1. Schematic representation of two lyotropic mesophases. The lamellar phase (left) is a periodical stacking along one dimension of soap and water lamellae. In the hexagonal phase (right), the soap cylinders are organized in a two-dimensional array.

A mesomorphic (liquid-crystal) phase of soap micelles, oriented in a hexagonal array of cylinders. Middle soap contains a similar or lower proportion of soap (e.g., 50%) as opposed to water. Middle soap is in contrast to neat soap, which contains more soap than water and is also a mesomorphic phase, but has a lamellar structure rather than a hexagonal array of cylinders. Also termed clotted soap . See Neat Soap. 

For soap/alcohol combinations — g will depend not only on the soap counter ion but also on the alcohol/soap ratio. Furthermore, when a certain alcohol/soap ratio is exceeded (=2 for the potassium oleate system) S becomes Independent of the water content of the lamellar phase. This condition applies for Inverse structures and the water/pentanol/potassium oleate inverse micellar system will be examined for the structure determining ratio in Table I. 

A central issue in the field of surfactant self-assembly is the structure of the liquid crystalline mesophases denoted bicontinuous cubic, and "intermediate" phases (i.e. rhombohedral, monoclinic and tetragonal phases). Cubic phases were detected by Luzzati et al. and Fontell in the 1960 s, although they were believed to be rare in comparison with the classical lamellar, hexagonal and micellar mesophases. It is now clear that these phases are ubiquitous in surfactant and Upid systems. Further, a number of cubic phases can occur within the same system, as the temperature or concentration is varied. Luzzati s group also discovered a number of crystalline mesophases in soaps and lipids, of tetragonal and rhombohedral symmetries (the so-called "T" and "R" phases). More recently, Tiddy et al. have detected systematic replacement of cubic mesophases by "intermediate" T and R phases as the surfactant architecture is varied [22-24]. The most detailed mesophase study to date has revealed the presence of monoclinic. 

Experimental results (12) showed a transition to a lamellar liquid crystal for 14 added water molecules. Our calculations (to be reported at a later occasion) showed no discontinuity or any other indication of instability of the soap/acid water complex for the subsequent water molecules added in excess of 14. It appears reasonable to assume that the isotropic liquid/liquid crystal transition does not depend on the energy levels of the polar group interactions. The phase transition probably depends on the hydrophobic/hydrophilic volume ratio and estimations according to Israelachvili/Ninham (15) approach may offer a better potential for an understanding. 

Monoglycerides form an inverse hexagonal phase with glycerol, as in water [112], Mixtures of triethanolamine and oleic acid form a nonaqueous lamellar liquid crystal with a surfactant bilayer of soap and acid with intercalated ionized and unionized alkanolamine as solvent [113,114], Lamellar liquid crystals form analogously with dodecylbenzenesulfonic acid and triethanolamine [115]. 

Further removal of the dispersion medium results in a conversion of gel into a solid macroscopic phase, i.e. into the soap crystal. Based on the results of the X-ray diffraction analysis, soap crystals were shown to have a lamellar structure. The surfactant - water system can thus undergo transitions into various states, depending on the content of components from a homogeneous system (surfactant molecular solution) to lyophilic colloidal state and further to macroscopic heterogeneous system (soap crystals in water). Different states of the system can be described by a particular thermodynamic equilibrium, i.e. ... 

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