Liquid lamellar

Chandrasekhar S 1998 Columnar, disootio, nematio and lamellar liquid orystals Their struotures and physioal properties Handbook of Liquid Crystais Voi 2B. Low Moiecuiar Weight Liquid Crystais / ed D Demus, J Goodby, G W Gray, H-W Spiess and V Vill (New York Wiley-VCH)... 

Fig. 14. A sample of a lamellar liquid crystal between crosses polarized in an optical microscope gives a pattern of "oily streaks" and Maltese crosses (a) while the Hquid crystal consisting of an array of cylinders shows the characteristic sectional pattern (b).

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... 

Lamella clarifier/thickener, 22 63, 67-69 Lamellar liquid crystals... 

Orienting systems of quasi-ternary mixtures composed of cetylpyridinium chloride/hexa-nol/NaCl [26] and cetylpyridinium bromide/hexanol/NaBr [27] have been reported to form lamellar liquid crystalline phases that allow a large temperature range over which dipolar couplings can be measured. The optimum condition for protein alignment consists of a 1 1.33 (w/w) ratio of cetylpyridinium bromide/hexanol. The residual 2H quad-rupolar splitting of the HOD resonance increases from 5 Hz to 20 Hz as the concentration of the mixture is varied from 30 to 65 mg/mL. These quasi-ternary mixtures are positively charged. 

FIG. 3. Geometry of hydrated molecules cylinders associate to a lamellar liquid crystal, cones to a hexagonal and an inverse hexagonal. Adapted from The Physical Chemistry ofMembranes (Silver, B., ed.), Allen Unwin, Inc. Solomon Press, Winchester, MA, 1985. 

The so ealled a phase of the fatty alcohols is a thermotropic type smectic B liquid crystal with a hexagonal arrangement of the moleeules within the double layers. It is initially formed from the melt during the manufaeturing proeess and normally transformed into a erystalline modifleation on eooling. However, the erystallization of the gel matrix ean be avoided if the a phase ean be kept stable as it eools to room temperature. This ean be achieved by eombining appropriate surfaetants, such as myristyl or lauryl alcohol and cholesterol, a mixture of whieh forms a lamellar liquid... 

Similar to Voltaren" Emulgel, oily droplets of an eutectic mixture of lidocaine and prilocaine are dispersed in a hydrogel to provide local anesthesia to the skin for injections and siugical treatment (Emla cream). A further possibility is the dermal administration of a liposome dispersion as a spray (Heparin PUR ratiopharm Spriih-gel "). After administration, water and isopropylic alcohol evaporate partially resulting in an increase of concentration and in a transition from the initial liposome dispersion into a lamellar liquid crystal [32]. The therapeutic effect appears to be influenced favorably by the presence of lecithins rather than by the degree of liposome dispersion. 

Mueller-Goymann, C.C., and Hamann, H.-J., Sustained release from reverse micellar solutions by phase transformation into lamellar liquid crystals, J. Contr. Rel., 23 165-174 (1993). 

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 short bulky aromatic compound does not pack well in a lamellar liquid crystalline structure, the mutual stabilizing action of the straight hydrocarbon chains is lost, and instability results. 

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. 

With these factors in mind, a new method to evaluate the conformation of an amphiphilic molecule at the site of interest was Introduced. The method is built on the fact that the determination of Interlayer spacings of a lamellar liquid crystal using low angle X-ray diffraction methods in combination with density measurements will provide sufficient information to calculate the cross-sectional areas occupied by each amphiphlle (19). 

Employing x-ray methods, Flaim and Friberg (2 ) studied the conformation of C21-DA as the acid or monosoap in a lamellar liquid crystalline matrix. At low water concentrations, the conformations are the same (extended). At somewhat higher water... 

As indicated above, miscibiUty gaps are small and intermediate lamellar liquid crystalline phases dissolve rapidly into the aqueous phase if the surfactant or surfactant mixture is rather hydrophihc with a high spontaneous curvature (low (v/la)), for instance at temperatures below Tq for pure nonionic surfactants. In this case dissolution, which converts lamellae of zero curvature to aggregates with significant curvature as surfactant concentration decreases, occurs spontaneously because it reduces system free energy. 

L micellar solution phase L lamellar liquid crystalline phase V viscous isotropic phase H2 reverse hexagonal phase... 

Region I. Relative to lamellar liquid crystalline phase Region IIl. Relative to hexagonal liquid crystalline phase Region IV. Relative to isotropic micellar solution... 

For mixtures of lecithin plus Na cholate it appears possible to infer the molecular arrangement in the dispersed micelles from the most likely structure of the liquid crystalline phase suggested by x-ray analysis. However, there are cases where dispersion is not possible because neither component is sufficiently hydrophilic to be dispersed even when alone in water. This is shown by the association of cholesterol and lecithin in the presence of water. The ternary diagram of Figure 4 is relative to these systems. Here only the lamellar liquid crystalline phase is obtained (region 1< in Figure 4). This phase is already given by lecithin alone, which can absorb up to 55% water. Cholesterol can be incorporated within this lamellar phase up to the proportion of one molecule of choles-... 

The detection of liquid crystal is based primarily on anisotropic optical properties. This means that a sample of this phase looks radiant when viewed against a light source placed between crossed polarizers. An isotropic solution is black under such conditions (Fig. 12). Optical microscopy may also detect the liquid crystal in an emulsion. The liquid crystal is conspicuous from its radiance in polarized light (Fig. 13). The structure of the liquid crystalline phase is also most easily identified by optical microscopy. Lamellar liquid crystals have a pattern of oil streaks and Maltese crosses (Fig. 14a), whereas ones with hexagonal arrays of cylinders give a different optical pattern (Fig. 14b). 

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. 

Figure 1 demonstrates the drastic influence on the stability region of a lamellar liquid crystalline phase when an aromatic hydrocarbon is substituted by an aliphatic one. The lamellar phase formed by water and emulsifier is stable between 20 and 60 wt % water. Addition of an aromatic hydrocarbon (p-xylene) to the liquid crystalline phase increased the maximum amount of water from 45 to 85% (w/w) (Figure 1 left). Inclusion of an aliphatic hydrocarbon (n-hexadecane) gave the opposite result the maximum water content in the liquid crystalline state was reduced (right). Some of the factors which govern the association behavior of these surfactants and cause effects such as the one above are treated below. 

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