Solubilization inverse

Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

For carotenoids, the type of matrix varies from relatively simple matrices in which the free carotenoid is dissolved in oil or encapsulated in supplements to more complex matrices in which the carotenoid is within plant foods. It is clear that the efficiency of the process by which the compound becomes more accessible in the gastrointestinal tract is inversely related to the degree of complexity of the food matrix. Carotenoid bioavailability is indeed far greater in oil or from supplements than from foods and usually the pure carotenoid solubilized in oil or in water-soluble beadlets is employed as a reference to calculate the relative bioavailability of the carotenoid from other foods. ... 

Carboxylate anions derived from somewhat stronger acids, such as p-nilrobcnzoic acid and chloroacetic acid, seem to be particularly useful in this Mitsunobu inversion reaction.53 Inversion can also be carried out on sulfonate esters using cesium carboxy-lates and DMAP as a catalyst in toluene.54 The effect of the DMAP seems to involve complexation and solubilization of the cesium salts. 

Zandonella G, Haalck L, Spener F, Faber K, Paltauf F, Hermetter A (1995) Inversion of lipase stereospecificity for fluorogenic alkyldiacyl glycerols. Effect of substrate solubilization. Eur J Biochem 231 50-55... 

Dispersants Keep contaminants of the lubricant in suspension and avoid their aggregation. Dispersants are amphiphilic molecules. Their long hydrocarbon tail helps to solubilize polar molecules in the base oil. The polar head group interacts with contaminants and facilitates the formation of (inverse) micelles around them. 

The solubility and release of naproxen from Pluronic PF-127 micelles were studied as a function of temperature and pH by Suh and Jun (1996). The solubility of the drug at pH 2 was signiLcantly increased as a linear function of PF-127 concentrations for three temperatures. Naproxen was highly entrapped by the micelles as indicated by large partition coefLcient. The micellar solubilization was a spontaneous (AG 0) and exothermic (AH< 0) process that resulted in a less ordered state (AS > 0). In the presence of PF-127, the release of naproxen was sustained at pH 2 and inversely proportional to the surfactant concentration. In contrast, at pH 7, PF-127 had little effect on the membrane transport of naproxen. The release of naproxen from the PF-127 gel into isopropanol myristate was also found to be dependent on the medium pH with the highest release observed at pH 6.3. 

The cholesterol-lowering properties of dietary plant sterols have been known for decades (Best et al., 1954 Peterson, 1951 Poliak, 1953), due specifically to reductions in cholesterol absorption. Inverse correlations between plant sterol intake and cholesterol absorption have been reported in animals (Carr et al., 2002 Ntanios and Jones, 1999) and humans (Ellegard et al., 2000). The exact mechanism by which plant sterols inhibit cholesterol absorption is unclear, and several mechanisms of action have been proposed, including (1) competition with cholesterol for solubilization in micelles within the intestinal lumen, (2) cocrystallization with cholesterol to form insoluble crystals, (3) interaction with digestive enzymes, and (4) regulation of intestinal transporters of cholesterol. 

It is widely recognized that the system IFT reaches a minimum in the middle phase microemulsion region. At the same time, the solubilization parameter (a), defined as mass of oil solubilized per unit mass of surfactant, is maximized in middle phase microemulsion systems (see Figure 1). This inverse relationship between the solubilization parameter (c)and IFT (y)has been defined by the Chun-Huh equation (Huh 1979, Sunwoo et. al. 1992, Abe et. al. 1987) ... 

An aggregate of surfactant molecules or ions in solution. Such aggregates form spontaneously at sufficiently high surfactant concentration, above the critical micelle concentration. The micelles typically contain tens to hundreds of molecules and are of colloidal dimensions. If more than one kind of surfactant forms the micelles, they are referred to as mixed micelles . If a micelle becomes larger than usual as a result of either the incorporation of solubilized molecules or the formation of a mixed micelle, then the term swollen micelle is applied. See also Inverse Micelle. 

Figure 2.8 Reprinted from Tribol. Int., Vol. 24, M.F. Fox, Z. Pawlak and D.J. Picken, Inverse micelles and solubilization in hydrocarbon formulations, pp. 341-349. Copyright 1991, with permission from Elsevier.

Shinoda and Kuineda [8] highlighted the effect of temperature on the phase behavior of systems formulated with two surfactants and introduced the concept of the phase inversion temperature (PIT) or the so-called HLB temperature. They described the recommended formulation conditions to produce MEs with surfactant concentration of about 5-10% w/w being (a) the optimum HLB or PIT of a surfactant (b) the optimum mixing ratio of surfactants, that is, the HLB or PIT of the mixture and (c) the optimum temperature for a given nonionic surfactant. They concluded that (a) the closer the HLBs of the two surfactants, the larger the cosolubilization of the two immiscible phases (b) the larger the size of the solubilizer, the more efficient the solubilisation process and (c) mixtures of ionic and nonionic surfactants are more resistant to temperature changes than nonionic surfactants alone. 

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. 

These inverse micelles will solubilize electrolytes in their aqueous core but the presence of the electrolytes also will influence the stability of the inverse micelle. A change in the stability of the inverse micelle will be reflected in modifications of the solubility region of the inverse micellar solution. This chapter will relate the changes in solubility areas from addition of electrolytes to the water to the structure of inverse micelles and other association complexes in the pentanol solution. 

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