Soaps lamellar structure

Well-ordered lamellar structures. The lamellae are arranged in parallel, giving rise to long-range order. Examples are soaps, phosphoUpids and clays. 

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. 

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. 

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. ... 

G. Friedel also understood that a lamellar structure would lead to thin films a bit like those which can be made with soap bubbles. Such experiments were... 

Amphiphiles, the representatives of which are soap, surfactant and lipid, have a hydrophilic polar head and lipophilic nonpolar tails. They always remain on the interface between water and oil and form monolayers of surfactants in a water/oil/amphiphile ternary system. This monolayers or interfacial film reduce the surface tension between water and oil domains. In a three-component system the surfactant film exists in various topologically different structures such as micelles, vesicles, bicontinuous microemulsions, hexagonal arrays of cylinders or lamellar structures depending upon the pressure, temperature and the concentration of the components [1,2]. Microemulsions are thermodynamically stable, isotropic and transparent mixtures of ternary amphiphilic systems. When almost equal volume fractions of water and oil are mixed with a dilute concentration of surfactants, they take... 

These structures are commonly referred to as hexagonal Hquid crystals. As the surfactant concentration is further increased, the tubules expand in a second direction to form large, stacked lamellar sheets of surfactants, commonly referred to as lamellar Hquid crystals. These Hquid crystals are very important in soap making. 

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... 

Although McBain suggested over 80 years ago that soap molecules form micellar structures of lamellar and spherical shape (McBain 1913), most of the subsequent work focused on spherical micelles. The earliest concrete model for spherical micelles is attributed to Hartley (1936), whose picture of a liquidlike hydrocarbon core surrounded by a hydrophilic surface layer formed by the head groups, has been essentially verified by modern techniques, and the Hartley model still dominates our thinking. We present an overview of the structure of the micelle first and then go on to examine the details a little bit more closely. 

Discussion. We can now propose a coarse description of the paraffinic medium in a lamellar lyotropic mesophase (potassium laurate-water). Fast translational diffusion, with D 10"6 at 90 °C, occurs while the chain conformation changes. The characteristic times of the chain deformations are distributed up to 3.10"6 sec at 90 °C. Presence of the soap-water interface and of neighboring molecules limits the number of conformations accessible to the chains. These findings confirm the concept of the paraffinic medium as an anisotropic liquid. One must also compare the frequencies of the slowest deformation mode (106 Hz) and of the local diffusive jump (109 Hz). When one molecule wants to slip by the side of another, the way has to be free. If the swinging motions of the molecules, or their slowest deformation modes, were uncorrelated, the molecules would have to wait about 10"6 sec between two diffusive jumps. The rapid diffusion could then be understood if the slow motions were collective motions in the lamellae. In this respect, the slow motions could depend on the macroscopic structure (lamellar or cylindrical, for example)... 

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. 

The special restriction caused by tying low molecular mass liquid crystalline substances to a polymer chain was also illustrated with amphiphilic liquid crystals. A hexagonally close-packed structure of rod-like micelle cylinders of sodium 10-undecenoate with about 50% water lost during polyma-ization at 60 °C its structure and became isotropic. On cooling, a lamellar liquid crystalline structure, more suitable to accommodate the macromolecular backbone was found. Bas l on the discussions of Sect. 5.3.4 it is likely that with longer side-chain amphiphiles condis crystals could be grown in analogy to the soaps described in Sect. 5.2.3... 

The lyotropic behavior of these carbohydrate derivatives has been studied very carefully [96]. It follows the classical sequence of mesophases established for the ionic detergents such as soaps. In contact preparations of l-0-octyl-/3-D-glucopyranoside (10 e) with water at room temperature three types of lyomesophases are observed (from high to low amphiphile concentration) lamellar, cubic and columnar hexagonal, their principal structures are depicted in Fig. 12. 

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. 

Whereas soaps may form any of the layer or micelle structures in Figure 10.31, the favoured structure of phospholipids in aqueous media is the bilayer (lamellar micelle). Phospholipid bilayers have a great tendency to close in on themselves to form bilayer spherical micelles which are known as liposomes or phospholipid vesicles (Figure 10.32a). These may lie in the size range 500-10,000 A diameter. 

---