Solubility of lipids

Available literature solubility data of pure lipids belonging to major (fatty acids, mono-, di- and triacylglycerols, and fatty acid esters) and minor lipid classes (pigments, sterols, vitamins, and hydrocarbons) in SCCO2 were compiled (26, 27). These references (26, 27) contain exhaustive bibliography on lipid + SCCO2 binary systems. Literature data were correlated using Chrastil s equation, which is an empirical model used quite commonly to correlate the solubility of lipid components (30). This model is based on the formation of a solute-solvent complex on association of the solute and solvent molecules and establishes a linear relationship between In(solubility) and In(density) as follows ... 

If the classification of lipides were to be examined, some points would have to be made clear. In the first place, a definition of lipides should be made that excludes esters of mineral acids such as orthophosphoric acid. The phosphoric esters of alcohols and phenols are alkalino-stable—excluding those possessing a free carbonyl group moreover, they are not very soluble in organic solvents. The definition of lipides should therefore take into account the hydrolysis of ester linkage by heating with alkalies and the solubility of lipides in some organic solvents. 

The solubility of lipids in nonpolar organic solvents results from their significant hydrocarbon component. The hydrocarbon portion of the compound is responsible for its oiliness or fattiness. The word lipid comes from the Greek lipos, which means fat. ... 

There is one additional difficulty in the determination of water solubility of lipids, which often is neglected. The surface activity of lipid molecules... 

Walnut, 52,97 Washing, 191,195 Washingtonia robusta, 79 Water-borne paints, 245 Water solubility of lipids, 353 Water swelling of monogalactosyl-diglycerides, 334 Water phases in soaps, 328 Waxes, 41,42,51-3,133,135,138,142-47, 287, 283, 436, 557... 

Reversed-phase packing materials (Cig and Cg) are utilized especially for the separation of the different retinyl esters and for combined vitamin A and carotenoid mixtures such as those in food extracts and clinical samples. Most semiaqueous mobile phases, consisting of mixtures of methanol or acetonitrile and water, result in chromatographic runs of more than 1 h to elute retinyl stearate. Gradient elution shortens this elution time but results in a loss of separation of two predominant esters, retinyl palmitate and retinyl stearate. More recently, isocratic non-aqueous reversed-phase (NARP) conditions with an acetonitrile-dichloromethane mixture resulted in a successful separation of the different esters within 15 min. The big advantage of NARP conditions over semiaqueous systems is the increased solubility of lipids. To compensate for this increased affinity of the compounds of low polarity for the eluent, highly retentive stationary phases are recommended. 

In addition to normal- and reversed-phase chromatography, silver ion HPLC, RP ion-pair HPLC, chiral separation, and supercritical fluid chromatography (SFC) have been used for analysis of lipids. In silver-ion HPLC, the counterion of an ion-exchange column such as sulfonate is exchanged with silver ions. Only the degree of unsaturation in the lipid molecule determines retention. In RP ion-pair HPLC, different ion-pairing agents, such as alkylamonium phosphates, are added to the ODS column for molecular species separation. In SFC, which is used in combination with flame ionization detection (FID), solubility of lipids in carbon dioxide allows the separation to be performed. This mode of analysis is not widely used. 

Figure 50 shows the potential fo packed-column SFC for an efficient and fast control of the chiral purity of pharmaceuticals. Because of the excellent solubility of lipids in supercritical carbon dioxide, SFC in combination with the universal FID detector is a very helpful tool in lipid analysis. 

Solvent(s) used in the infused lipid solution not only affect the solubility of lipids but also have a large impact on ionization efficiency in addition to the separation in... 

Intercalating species By analogy with the general mutual solubility of lipids and conventional alkyl chain amphiphiles or with the co-miscibility of smectic phases of the same type, it might be expected that the co-miscibility of chromonic species would be widespread. Complete miscibility has been found in only a few cases, but the intercalation of sol-... 

Since the solubility of lipids in water is very low, the number of lipid molecules in a membrane is essentially constant over typical experimental time scales. Also, the osmotic pressure generated by a small number of ions or macromolecules in solution, which cannot penetrate the lipid bUayer, keeps the internal volume essentially constant. The shape of fluid vesicles [176] is therefore determined by the competition of the curvature elasticity of the membrane, and the constraints of constant volume V and constant surface area S. In the simplest case of vanishing spontaneous curvature, the curvature elasticity is given by (98). In this case, the vesicle shape in the absence of thermal fluctuations depends on a single dimensionless parameter, the reduced volume V = V/Vo, where Vb = (47t/3)1 o nd Ro = (5/4 r) are the volume and radius of a sphere of the same surface area S, respectively. The calculated vesicle shapes are shown in Fig. 23. There are three phases. For reduced volumes not too far from the sphere, elongated prolate shapes are stable. In a small range of reduced volumes of V e [0.592,0.651], oblate discocyte shapes have the lowest curvature energy. Finally, at very low reduced volumes, cup-like stomatocyte shapes are found. 

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