Lamellar liquid crystals, surfactant

Structured laundry liquids are currently available in Europe and were recently introduced in the United States [50,51]. These products typically contain high levels of surfactants and builder salts, as well as enzymes and other additives. In the presence of high ionic strength, the combination of certain anionic and nonionic surfactants form lamellar liquid crystals. Under the microscope (electron microscope, freeze fracturing) these appear as round droplets with an onion-like, multilayered structure. Formation of these droplets or sperulites permits the incorporation of high levels of surfactants and builders in a pourable liquid form. Stability of the dispersion is enhanced by the addition of polymers that absorb onto the droplet surface to reduce aggregation. 

The phase behavior of a-ester sulfonates has been studied in detail with methyl laurate and methyl palmitate [58]. In both cases, at higher temperatures, as the surfactant concentration increases, there is a transition from an isotropic solution to a hexagonal liquid crystalline phase and finally, at high surfactant concentrations, to a lamellar liquid crystal (Fig. 4). The crystal/liquid-crys-tal phase transition occurs at even higher temperatures as the chain length increases. On the other hand, chain length has practically no influence on the... 

A change in the perception of their mechanism of action came in the sixties when Lawrence (7) pointed out that short chain surfactants would delay the gelling to a liquid crystalline phase which takes place at high surfactant concentrations. Friberg and Rydhag (8) showed that hydrotropes, in addition, prevent the formation of lamellar liquid crystals in combinations of surfactants with hydrophobic amphlphiles, such as long chain carboxylic acids and alcohols. The importance of this finding for laundry action was evident. 

The conditions for surfactants to be useful to form liquid crystals exist when the cross-sectional areas of the polar group and the hydrocarbon chain are similar. This means that double-chain surfactants are eminently suited, and lecithin (qv) is a natural choice. Combinations of a monochain ionic surfactant with a long-chain carboxylic acid or alcohol yield lamellar liquid crystals at low concentrations, but suffer the disadvantage of the alcohol being too soluble in the oil phase. A combination of long-chain carboxylic acid plus an amine of equal chain length suffers less from this problem because of extensive ionization of both amphiphiles. 

Ci2E04 undergoes a change near 18 °C from a two phase W+L, system to a W+L, system. (6). The improvement in detergency performance of this W+L system over that of the micellar systems shown in Figure 1 is thus correlated with the appearance of lamellar liquid crystals of the surfactant in the bath. The temperature dependence of the performance of liquid crystals of C12E04 between 22 and 32 °C, on the other hand, is probably due to changes in the nature of tire... 

The importance of a surfactant - rich phase, particularly a lamellar one, to detergency performance was noted for liquid soils such as C16 and mineral oil (3.6). Videomicroscopy experiments indicated that middle phase microemulsion formation for C12E04 and Cjg was enhanced at 30 °C, while at 18 °C, oil - in - water, and at 40 °C, water - in - oil microemulsions were found to form at the oil - bath interface (3.6). A strong temperature dependence of liquid soil removal by lamellar liquid crystals, attributed to viscosity effects, has been noted for surfactant - soil systems where a middle - phase microemulsion was not formed (10). 

Galactomannans, particularly, become adsorbed and organized on an oil-drop surface as lamellar liquid crystals that perform as steric and mechanical barriers to coalescence (Reichman and Garti, 1991). The surfactancy of xanthan was found to be related to the amount adsorbed (Young and Torres, 1989). 

Few studies exist for ionic silicone surfactants. Several trisiloxane anionic, cationic and zwitterionic surfactants have been found to form micelles, vesicles and lamellar liquid crystals. As would be expected, salt shifts the aggregates toward smaller curvature structures [40]. 

In the second part of the paper, it will be shown that the partition of the alcohol between the phases can explain the deviations from the ideal dilution law for a water/ oil/surfactant/cosurfactant lyotropic lamellar liquid crystal. 

Figure 1. A sketch of a lyotropic lamellar liquid crystal, composed of water, oil, a surfactant (open circles), and cosurfactant (full circles). The distribution of the surfactant and cosurfactant between the water and oil phases and interface is discussed in section II.

When the lamellar liquid crystal is in equilibrium with an excess phase i (either water or oil) and one assumes that the concentrations of the surfactant and cosurfactant are the same in the excess phase and in the lamellae of the same kind, the equality of the chemical potentials leads (because of eq 3d) to p = p which when used in eqs3a or 3b provides the condition for equilibrium with an excess phase i, 9f[di,df)lddi = 0. After the lamellar phase reaches the equilibrium with an excess phase, <5 can no longer increase, since this will always increase the total free energy of the system. Consequently, the lamellar phase might exist only in the regions where... 

ELI. Uncharged Lyotropic Lamellar Liquid Crystals. A common procedure to evaluate the thicknesses of the water and oil layers was to assume that all the molecules of surfactant and cosurfactant are adsorbed on the interface, where they occupy constant areas. While very simple, this procedure is not always accurate, because the cosurfactant is also present in the water and particularly in the oil phase.28 Larche et al. explained the deviation from the ideal dilution law by taking into account the distribution of the cosurfactant (pentanol) between the interface and the oil (decane) phase.23... 

II.2. Charged Lyotropic Lamellar Liquid Crystals. In the case of a charged system, two changes in the system of eqs 17 must be made.80 81 First y should be replaced in eq 17a, for the surfactant, by y + fy=mole fraction of the surfactant in water, Xsw, should be replaced in the same equation by the mole fraction of the surfactant in water, in the vicinity of the interface CX SW). Assuming that the surfactant in the aqueous phase is totally dissociated, the concentration of surfactant near the interface (Xsw) can be related to the average surfactant concentration in the water phase, Xsw, by using the equilibrium and mass balance relations... 

Micellar aggregates are considered in chapter 3 and a critical concentration is defined on the basis of a change in the shape of the size distribution of aggregates. This is followed by the examination, via a second order perturbation theory, of the phase behavior of a sterically stabilized non-aqueous colloidal dispersion containing free polymer molecules. This chapter is also concerned with the thermodynamic stability of microemulsions, which is treated via a new thermodynamic formalism. In addition, a molecular thermodynamics approach is suggested, which can predict the structural and compositional characteristics of microemulsions. Thermodynamic approaches similar to that used for microemulsions are applied to the phase transition in monolayers of insoluble surfactants and to lamellar liquid crystals. 

Micellar solutions are isotropic microstructured fluids which form under certain conditions. At other conditions, liquid crystals periodic in at least one dimension can form. The lamellar liquid crystal phase consists of periodically stacked bilayers (a pair of opposed monolayers). The sheetlike surfactant structures can curl into long rods (closing on either the head or tail side) with parallel axes arrayed in a periodic hexagonal or rectangular spacing to form a hexagonal or a rectangular liquid crystal. Spherical micelles or inverted micelles whose centers are periodically distributed on a lattice of cubic symmetry form a cubic liquid crystal. 

In solutions of water and surfactant, the surfactant monolayers can join, tail side against tail side, to form bilayers, which form lamellar liquid crystals whose bilayers are planar and are arrayed periodically in the direction normal to the bilayer surface. The bilayer thickens upon addition of oil, and the distance between bilayers can be changed by adding salts or other solutes. In the oil-free case, the hydrocarbon tails can be fluidlike (La) lamellar liquid crystal or can be solidlike (Lp) lamellar liquid crystal. There also occurs another phase, Pp, called the modulated or rippled phase, in which the bilayer thickness varies chaotically in place of the lamellae. Assuming lamellar liquid crystalline symmetry, Goldstein and Leibler [19] have constructed a Hamiltonian in which (1) the intrabilayer energy is calculated... 

The middle diagram of Figure 11 illustrates phase behavior that may be more desirable for some applications. In this case the presence of a lamellar liquid crystal region in the phase diagram, although it will not be revealed by experiments confined to low surfactant concentrations, may be exploitable for the formation of very stable dispersions (82). 

A position to the right of the microemulsion area means the presence of a lamellar liquid crystal as has been repeatedly demonstrated by Ekwall (19). The temporary appearance of liquid crystals when W/0 microemulsions are brought into contact with water have, with this result, been given a satisfactory explanation. The faster transport of the surfactant into the aqueous layers gives rise to temporarily higher surfactant concentrations and the stability limits for the water rich W/0 microemulsions phase are exceeded towards the liquid crystalline phase in water. 

The next system studied was novel in that the lower phase was a homogeneous lamellar liquid crystal containing a synthetic sulfonate surfactant in equilibrium with an excess oil phase. No previous observations of liquid crystalline flow through porous media have been reported. The initial viscosity of the liquid crystalIjiie phase was a relatively low 10 cp at a shear rate of 4.5 s -. The interfacial tension between liquid crystal and oleic phase was 0.018 dyne/cm (14). 

Therefore, methods of stabilizing dispersions against water injection (e.g., use of other surfactants, lamellar liquid crystals, use of very dilute solutions of water-soluble polymers with surfactants) might aid commercialization by reducing surfactant costs. 

Although the so-called a-phase of the fatty alcohols—a thermotropic type smectic B liquid crystal with hexagonal arrangement of molecules within the double layers—is initially formed from the melt during the manufacturing process, it normally transforms into a crystalline modification as it cools. However, the crystallization of the gel matrix can be avoided if the ot-phase can be kept stable as it cools to room temperature. This can be achieved by combining appropriate surfactants such as myristyl or lauryl alcohol and cholesterol, a mixture of which forms a lamellar liquid crystal at room temperature. Due to depression of the melting point, the phase transition temperature of crystalline to liquid crystalline as well as liquid crystalline to isotropic decreases. Therefore, a liquid crystalline microstructure is obtained at room temperature. 

The behavior of lyotropic crystals is usually non-Newtonian. Lamellar liquid crystals show thixotropic or negative thixotropic behavior,but shear thinning/thixotropy is more common. The shear thinning effect is ascribed to orientation of the liquid crystalline domains and some structural breakdown. Viscoelasticity has also been observed. If the concentration of the surfactant molecules is, however, low and close to the CMC, Newtonian flow will be observed. ... 

When surface active agents are considered, a further complication may be encountered. Because of their surface active nature, the surfactants not only emich at the surfaces, but also form extended structures themselves. At low concentrations, the surfactants remain as dissolved monomers or asssociate to oligomers. However, when the critical micellization concentration (cmc) is surpassed, a cooperative association is activated to micelles (1 to 10 nm) consisting typically of some 50 to 100 monomers. At stiU higher concentrations, or in the presence of cosurfactants (alcohols, amines, fatty acids, etc.), liquid crystalline phases may separate. These phases have an infinite order on the x-ray scale, but may remain as powders on the NMR (nuclear magnetic resonance) scale. When the lamellar liquid crystalline phase is in equilibrium with the liquid micellar phase the conditions are optimal for emulsions to form. The interface of the emulsion droplets (1 to 100 pm) are stabilized by the lamellar liquid crystal. Both the micelles and the emulsions may be of the oil in water (o/w) or water in oil (w/o) type. Obviously, substances that otherwise are insoluble in the dispersion medium may be solubilized in the micelles or emulsified in the emulsions. For a more thorough analysis, the reader is directed to pertinent references in the literature. ... 

FIG. 2.3 Solubilization of octanoic acid into a surfactant micellar solution (Li) is limited because the addition of the acid leads to the formation of a lamellar liquid crystal (La). 

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]. 

Ionic surfactants with two long (six or more carbons) alkyl chains have high VH values relative to lc, and probably do not form spherical micelles. They have values of n that increase with surfactant concentration, the increase becoming more pronounced with increase in the length of the chains. Some of these micellar solutions are in equilibrium with lamellar liquid crystal structures (Lianos, 1983). 

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