Solvent lipophilicity

Salts and Derivatives. Generally the vitamers are high melting crystalline soHds that are very soluble in water and insoluble in most other solvents. Properties of the common forms are Hsted in Table 1. The only commercially important form of vitamin B is pytidoxine hydrochloride (7). This odorless crystalline soHd is composed of colorless platelets melting at 204—206°C (with decomposition). In bulk, it appears white and has a density of - 0.4 kg/L. It is very soluble in water (ca 0.22 kg/L at 20°C), soluble in propylene glycol, slightly soluble in acetone and alcohol (ca 0.014 kg/L), and insoluble in most lipophilic solvents. A 10% water solution shows a pH of 3.2. Both the hydrochloride and corresponding free base sublime without decomposition (16). 

The dipping solutions described in Part II of this book are usually less concentrated than the corresponding spray solutions. The solvents employed are specially chosen for their suitability to the special requirements of dipping solutions. Water, which on the one hand, can sit on the surface of RP plates and not penetrate them and, on the other hand, can cause disintegration of water-incompatible layers is usually replaced by alcohol or other lipophilic solvents. 

Separation by adsorption chromatography takes place preferentially as a result of hydrogen bonding or dipole-dipole interactions. Hence, separation of mixtures of substances on silica gel layers by lipophilic solvents primarily takes place according to polarity differences. Further separation within a polarity group can then be achieved either two-dimensionally or off-line by partition chromatography on anotho TLC plate (Fig. 4). 

Rapid dermal absorption of trichloroethylene is evident from a study in which peak blood and exhaled air concentrations occurred within 5 minutes after a human subject immersed one hand in a solution of unspecified trichloroethylene concentration for 30 minutes (Sato and Nakajima 1978). Studies on dermal absorption of trichloroethylene in humans, as well as animals, are complicated by the fact that exposure in these studies is usually by direct contact of the skin with the undiluted chemical. Trichloroethylene is a lipophilic solvent that defats the skin and disrupts the stratum comeum, thereby enhancing its own absorption. Thus, the rate of absorption probably increases in a nonlinear fashion with greater epidermal disruption. Although the extent of absorption through the skin may be relatively modest with normal industrial use (Sato and Nakajima 1978 Stewart and Dodd 1964), there is insufficient information to evaluate the effects of chronic, low-level exposure in hiunans, especially when multiple routes may be involved. 

It is believed that solubilization is initiated when a biomolecule with charged surface groups approaches the bulk aqueous-lipophilic solvent interface, where the interactions cause the bulk interface s surfactant layer to bend, such that the protruding biomolecule becomes surrounded by the surfactant layer [105]. Ultimately, a filled w/o-ME forms, and partitions to the bulk organic phase [105]. 

Although these reservoirs may be highly contaminated with PCDD/PCDFs, the chemical and physical properties of these compounds imply that dioxins and furans will stay adsorbed to organic carbon in soils or other particles. On the other hand, mobilization can occur in the presence of lipophilic solvents (leaching into deeper layers of soils and/or groundwater) or in cases of erosion or run-off from topsoil (translocation into the neighbourhood). Experience has shown that transport of PCDD/PCDFs due to soil erosion and run-off does not play a major role in environmental contamination and human exposure (Fiedler 1995, 1999). 

A solid to semi-solid substance containing volatile essential oils and a fatty-waxy material obtained by the extraction of plant tissue with lipophilic solvents like hexane, e.g. Orris concrete. 

Therefore, the partition coefficient (log P) enables to foresee with figures the behavior of a given substance (the solute) in a hydrophilic or lipophilic solvent environment. 

The barriers of the eye differ from that of skin in dimensions and mucosal organization there are epithelial cel-Inlar barriers made of lipid layers on the eye and dry snrfaces made of proteins in the stratihed snrfaces of the skin. Skin is protected by a superficial outer fatty hhn, the stratum comeum, and covered by a thick layer of cells, the basal cellnlar layer. Thanks to these thick layers, skin has a high resistance to initial diffusion of aqueous fluids and less resistance to lipophilic solvents and to acids. The initial chemical cracking of the lipid layer allows fluids to invade deeper layers of the skin. The basal cell layer and the hair follicles contain stem cells capable of epithelial regeneration. 

Free xanthophylls, both endogenous and present in the saponified samples, are more polar and extract less efficiently into lipophilic solvents. Frequently, the addition of a polar organic solvent (tetrahydrofuran, methylene chloride, diethyl ether) is required to thoroughly extract them from the sample matrix and aqueous phase. 

The new cyclosporine formulation (Sandimmun Neoral, Novartis Pharmaceuticals Corporation, East Hanover, NJ) is a self-microemulsifying drug delivery system, which consists of the drug in a lipophilic solvent (corn oil), hydrophilic cosolvent (propylene glycol) surfactant and an antioxidant [37]. Upon contact with GI fluids, Sandimmun Neoral readily forms a homogenous, monophasic microemulsion, which allows the absorption of the drug molecules. Unlike Sandimmun, the formation of this microemulsion is independent of bile salt activity, and indeed, studies have shown that the absorption of cyclosporine from the new formulation is much less dependent on bile flow [38] and is unaffected by food intake [39],... 

Although the solubility of (unsubstituted) linear oligophenyls, starting from p-terphenyl, becomes poorer and poorer also in lipophilic solvents, polybenzene... 

Canceill, J., Lacombe, L., Collet, A., New cryptophane forming unusually stable inclusion complexes with neutral guests in a lipophilic solvent. J. Am. Chem. Soc. 1986, 108, 4230-4232. 

The extraordinary ability of certain cryptophanes to bind neutral molecules of complementary size in lipophilic solvents is illustrated in Fig. 10, showing the... 

In addition to the ability of carbon dioxide to act as a lipophilic solvent, its dissolving power can be tailored to carry out fractionations and relative separations of components in a complex mixture, and this unique property is advantageously applied to the characterization of flavors. Several representative examples that point out this ability are presented. 

The use of tetrabenzo[a,c,g,i]fluorene (Tbf) derivatives as tagging reagents, such as 8.70 (224), was reported for acids each Tbf-tagged reagent is purified by specific adsorption on charcoal, typically stirring for 20 min in an ice bath, and subsequent washing with lipophilic solvents to recover pure intermediates, typically stirring with toluene aliquots at 40 °C until no more UV adsorbance is detected in the toluene aliquots. 

An alternative, more empirical but more accurate method for predicting miscibility has been given by Godfrey [103], using so-called miscibility numbers (M-numbers). These are serial numbers of 31 classes of organic solvents, ordered empirically by means of their lipophilicity i.e. their affinity for oil-like substances), using simple test-tube miscibility experiments. All pairs of solvents whose Af-numbers differ by 15 units or less are miscible in all proportions at 25 °C a difference of >17 corresponds to immis-cibility, and an M-number difference of 16 units indicates borderline behaviour (limited mutual miscibility) [103]. The central class of solvents with Af-number equal to 16 e.g. 2-n-butoxyethanol) comprises universal solvents, which are miscible with less lipophilic as well as with more lipophilic solvents. 

Examples of how the first condition can be met are given in Table II, which lists the slopes and intercept values of each equation of type (1) relating the least lipophilic solvent systems to octanol/water. The number of solute P values used to establish each equation is listed under... 

If hydrogen bonding could be accounted for separately, all of the slopes in the equations comparing the lipophilic solvent systems to octanol would be close to 1.0. For example, cyclohexane values correlate poorly with octanol values when all solutes are taken together. Even for phenols within the H-donor group, the correlation coefficient is only 0.76. However, if we add a log term which measures the hydrogen bonding capability of phenols (5), the correlation coefficient increases to 0.98 and the slope becomes 1.0. 

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