Liquid crystals solubilization

In n-octane/aqueous systems at 27°C, TRS 10-80 has been shown to form a surfactant-rich third phase, or a thin film of liquid crystals (see Figure 1), with a sharp interfacial tension minimum of about 5x10 mN/m at 15 g/L NaCI concentration f131. Similarly, in this study the bitumen/aqueous tension behavior of TRS 10-80 and Sun Tech IV appeared not to be related to monolayer coverage at the interface (as in the case of Enordet C16 18) but rather was indicative of a surfactant-rich third phase between oil and water. The higher values for minimum interfacial tension observed for a heavy oil compared to a pure n-alkane were probably due to natural surfactants in the crude oil which somewhat hindered the formation of the surfactant-rich phase. This hypothesis needs to be tested, but the effect is not unlike that of the addition of SDS (which does not form liquid crystals) in partially solubilizing the third phase formed by TRS 10-80 or Aerosol OT at the alkane/brine interface Til.121. 

A further possibility is the formation of liquid crystals on contact with body fluids at the site of application. The initially applied drug solution interacts with body fluids such as plasma, tears, or skin lipids and undergoes a phase transition into a mono-or multiphasic system of liquid crystals (Fig. 15). For example, oily solutions of reverse micellar solutions of phospholipids, which solubilize additional drug, trans-... 

Replacing part of the soap with the dicarboxylic acid at a sufficiently high pH value to ensure its complete ionization gave the results in Fig. 4B. At the site for the onset of the 3-phase area in Fig. 4A with the liquid crystal, the alcohol and the aqueous solution, solubilization of the alcohol now showed a sudden maximum. 

While aliphatic-containing polyamides are given the name nylons, those in which at least 85% of the amide groups are attached to an aromatic are called aramids. Aramids are stronger and tougher than nylons but they are also more difficult to solubilize and fabricate. Because the presence of the aromatic groups causes the aramids to be stiff, they often form liquid crystals (LCs) that are present in a nematic EC state in concentrated solution. 

Micellar properties are affected by changes in the environment, eg, temperature, solvents, electrolytes, and solubilized components. These changes indude complicated phase changes, viscosity effects, gd formation, and liquefication of liquid crystals. Of the simpler changes, high concentrations of water-soluble alcohols in aqueous solution often dissolve micelles and in nonaqueous solvents addition of water frequendy causes a sharp increase in micellar size. 

Liquid crystals are a fascinating topic of study in their own right, but we limit our discussion to a brief description of the ordering in some of the possible structures. In all cases the amphipathic molecules are oriented in such a way as to minimize the contact between water and the alkyl chains. Whether the polar head points outward or not depends on which component dominates the continuous phase the minor component is solubilized inside the micellar structures. 

The order and mobility of a labeled flexible alkyl spacer in the linear thermotropic polymeric nematic liquid crystal poly(2,2 -dimethyl-4,4 -dioxyazoxybenzenedodecanedioyl-dj0) (poly[oxy(3-methyl-1,4-phenylene)azoxy 2-methyl-1,4-phenylene)oxy(1,12-dioxo-1,12-dodecanediyl-d2oll) is explored with deuterium NMR. The quadrupol splittings of the spacer methylene segments in the nematic melt of the polymer are reported as a function of the temperature and are contrasted with observations on model compounds solubilized in a nematic solvent. 

Phase diagrams of water, hydrocarbon, and nonionic surfactants (polyoxyethylene alkyl ethers) are presented, and their general features are related to the PIT value or HLB temperature. The pronounced solubilization changes in the isotropic liquid phases which have been observed in the HLB temperature range were limited to the association of the surfactant into micelles. The solubility of water in a liquid surfactant and the regions of liquid crystals obtained from water-surfactant interaction varied only slightly in the HLB temperature range. 

These studies throw light on the initial aggregation phenomena, which results in micelles and may also (17) constitute the necessary basis for understanding the subsequent agglomeration to liquid crystals. Very little is known about the thermodynamic conditions for the latter associations. Instead we must rely on empirical data to illuminate the basic mechanisms which determine the association behavior of those substances in concentrated systems. Valuable information for understanding the drastic influence of weak intermolecular forces on the association structures (Figure 1) is obtained from the pronounced temperature dependence of the solubilization which was observed early by Shinoda (29). He and his collaborators (30, 31, 32 33) have since developed this subject. 

Winsor (17) describes how electrical conductivity varies during addition of an alcohol to an aqueous micellar solution containing some solubilized oil. Conductivity initially decreases as mixed (and probably larger) micelles containing both surfactant and alcohol are formed. When liquid crystal (presumably having a lamellar structure) starts to appear in equilibrium with the micellar solution, conductivity decreases even faster. As more alcohol is added, the aqueous solution disappears, only liquid crystal is present, and the conductivity reaches a minimum. Addition of still more alcohol results in the appearance of an oil-continuous micellar solution and an increase in conductivity. Eventually all liquid crystal disappears, the increase in conductivity ceases, and conductivity... 

The solubilization of an aqueous sodium chloride solution by potassium oleate in the pentanol isotropic solution was determined. The presence of sodium chloride increased the minimum concentration for solubilization, reduced the maximum solubilization at high pentanohpotassium oleate ratios, and altered this ratio to lower values for maximal solubilization of the electrolyte solution. The increased minimum amount of electrolyte solution for solubilization arose from the fact that no micelles were present at the lowest fractions of water in the pentanol solution. The increased potassium oleate. pentanol ratio for maximal solubilization of the electrolyte was related to the destabilization of the lamellar liquid crystal with which the inverse micellar pentanol solution of high water content was in equilibrium. 

For the 1 M NaCl system the solubility region was further reduced. Fig. 13, and the water solubilization maximum found at even higher surfactant/cosurfactant ratio. The series with the lower ratios of surfactant to cosurfactant showed an uptake of the aqueous solution somewhat similar to the series in the system with 0.5 M NaCl. The series with the surfactant/(cosurfactant + surfactant) ratio equal to 0.4 gave an initial liquid crystal formation lasting for 2-3 days folllowed by a middle phase lasting a longer time. The liquid crystalline and the middle phase layer were both more pronounced for the sample with initial salt concentration equal in the water and in the microemulsion. Fig. 14A, than for the sample with all the salt in the water. Fig. 14B. 

A liquid crystal is a general term used to describe a variety of anisotropic structures formed by amphiphilic molecules, typically but not exclusively at high concentrations. Hexagonal, lamellar, and cubic phases are all examples of liquid crystalline phases. These phases have been examined as drug delivery systems because of their stability, broad solubilization potential, ability to delay the release of encapsulated drug, and, in the case of lamellar phases, their ability to form closed, spherical bilayer structures known as vesicles, which can entrap both hydrophobic and hydrophilic drug. This section will review SANS studies performed on all liquid crystalline phases, except vesicles, which will be considered separately. Vesicles will be considered separately because, with a few exceptions, generally mixed systems, vesicles (unlike the other liquid crystalline phases mentioned) do not form spontaneously upon dispersal of the surfactant in water and because there have been many more SANS studies performed on these systems. 

Mueller-Goymann CC, Usselmann B. Solubilization of cholesterol in liquid crystals of aqueous systems of polyoxyethylene cetyl ethers. Acta Pharm Jugosl 1988 38(4) 327—329. 

Structural Considerations of Lamellar Liquid Crystals Containing Large Quantities of Solubilized Hydrocarbon Alkanes... 

The order parameters of aliphatic hydrocarbons solubilized in a lamellar liquid crystal were determined from NMR data. The variation of order parameters along the hydrocarbon chain with varying amounts of hydrocarbon solubilized supports a model with the main part of the hydrocarbon forming a layer between the amphiphilic layers with only a small amount of it penetrating between the amphiphilic molecules. 

The stability of a lamellar liquid crystal with such a large amount of oil is an intriguing problem. The main structural entity to be clarified before a serious attempt at an examination of the problem may be made is the degree of order of the hydrocarbon chains. This factor in turn depends on the location of the solubilized hydrocarbon chains are they penetrating the amphiphilic layer or are they forming a liquid layer between the layers of amphiphilic molecules. 

Surfactant aggregates (microemulsions, micelles, monolayers, vesicles, and liquid crystals) are recently the subject of extensive basic and applied research, because of their inherently interesting chemistry, as well as their diverse technical applications in such fields as petroleum, agriculture, pharmaceuticals, and detergents. Some of the important systems which these aggregates may model are enzyme catalysis, membrane transport, and drug delivery. More practical uses for them are enhanced tertiary oil recovery, emulsion polymerization, and solubilization and detoxification of pesticides and other toxic organic chemicals. 

It should be mentioned that crystal structure data are often obtained from co-solvent systems (Desai and Klibanov, 1995). For example, crystals of ubiqui-tin, papain and a heptapeptide were grown from 30% polyethylene glycol (PEG) 4000, 62% MeOH and a DMSO/isopropanol mixture, respectively (Kamphius et al., 1984 Karle et al., 1993 Love et al., 1997). The orthogonal and tetratogonal crystal forms of cyclosporine were prepared from 25% PEG 300 and acetone, respectively (Petcher et al., 1976 Kessler et al., 1985 Loosli et al., 1985 Verheyden et al., 1994a). Furthermore, cyclosporine and leuprolide form thermotropic liquid crystals when dried from EtOH and lyotropic liquid crystals when solubilized in PG, respectively (Tan et al., 1998 Lechuga-Ballesteros et al., 2003 Stevenson et al., 2003). 

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. ... 

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