Hydrophobic substances, effect

Ethoxylated castor ods or ethoxylated castorwaxes are used as solubilizers of hydrophobic substances in cosmetics. Examples are Cremophor EL (ethyoxylated castor od) and Cremophor RH (40/60 ethoxylated hydrogenated castor od). Other ethoxylated triglycerides are not as effective as castor od. Ethoxylated castor od is also a good solubilizer for vitamin A palmitate (121). 

Dendrimers can be designed to have a hydrophobic interior and a hydrophilic periphery. This gives them properties that are similar to those of conventional surfactants, and they can solubilize hydrophobic substances such as pyridine in aqueous solution by including them as guest molecules. They are therefore effectively mimolecular micelles. 

A number of innovations made in the 1920s and 1930s may be noted. Several attempts were made to reduce the dissolution of these cements in oral fluids and their adverse effect on the pulp by inclusion of oils and greases (Simon, 1929, 1932 Eberly, 1934). None have been considered beneficial (Palfenbarger, Schoonover Souder, 1938), a not surprising result because the inclusion of hydrophobic substances is bound to interfere in the setting of an aqueous cement. 

The rates of multiphase reactions are often controlled by mass tran.sfer across the interface. An enlargement of the interfacial surface area can then speed up reactions and also affect selectivity. Formation of micelles (these are aggregates of surfactants, typically 400-800 nm in size, which can solubilize large quantities of hydrophobic substance) can lead to an enormous increase of the interfacial area, even at low concentrations. A qualitatively similar effect can be reached if microemulsions or hydrotropes are created. Microemulsions are colloidal dispersions that consist of monodisperse droplets of water-in-oil or oil-in-water, which are thermodynamically stable. Typically, droplets are 10 to 100 pm in diameter. Hydrotropes are substances like toluene/xylene/cumene sulphonic acids or their Na/K salts, glycol.s, urea, etc. These. substances are highly soluble in water and enormously increase the solubility of sparingly. soluble solutes. 

What characterizes surfactants is their ability to adsorb onto surfaces and to modify the surface properties. At the gas/liquid interface this leads to a reduction in surface tension. Fig. 4.1 shows the dependence of surface tension on the concentration for different surfactant types [39]. It is obvious from this figure that the nonionic surfactants have a lower surface tension for the same alkyl chain length and concentration than the ionic surfactants. The second effect which can be seen from Fig. 4.1 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the washing liquor has at least to be right above the cmc. 

A variety of factors affect the horizontal and vertical migration of PAHs, including contaminant volume and viscosity, temperature, land contour, plant cover, and soil composition (Morgan Watkinson, 1989)- Vertical movement occurs as a multiphase flow that will be controlled by soil chemistry and structure, pore size, and water content. For example, non-reactive small molecules (i.e., not PAHs) penetrate very rapidly through dry soils and migration is faster in clays than in loams due to the increased porosity of the clays. Once intercalated, however, sorbed PAHs are essentially immobilized. Mobility of oily hydrophobic substances can potentially be enhanced by the biosurfactant-production capability of bacteria (Zajic et al., 1974) but clear demonstrations of this effect are rare. This is discussed below in more detail (see Section 5 5). 

Another important event contributing to the progress in this field was the development of reaction microcalorimetry, which has permitted direct measurement of heat effects involved with the transfer of hydrophobic substances from a nonpolar environment to water. These processes have been thought to mimic the unfolding of compact protein, structures. Prior to the development of direct calorimetric techniques, all information on the interaction of a hydrophobic substance with water was obtained from equilibrium studies. However, the results were limited in accuracy, particularly those properties that are obtained by consecutive temperature differentiation of the solubility, for example, the change in heat capacity. 

S. Koga, S. Sasaki, and H. Maeda, Effect of hydrophobic substances on the volume-phase transition of N-isopropylacrylamide gels, J. Phys. Chem. B 105(19), 4105-4110 (2001). 

The use of organic solvents is an alternative to enhance availability of hydrophobic substances, either substrates or products of the reaction, as well as to reduce side reactions. The first studies of peroxidase catalysis in water-miscible and water-immiscible organic solvents were initiated by Dordick and Klibanov using commercial HRP, one of the most often investigated enzymes [60, 61]. Several studies have been focused on the effect of solvents in peroxidase structure and function... 

Consequently, the method suggested in the current paper allows one to estimate not only the thickness of the layer of water that is affected by a solute molecule, but also the mutual affinity of solute molecules at infinite dilution. The compounds selected, hydrocarbons and alcohols, allowed one to investigate the hydrophobic effect for pure hydrophobic molecules (hydrocarbons) and for less hydrophobic substances such as the alcohols. 

The presence of organic solvents in the sample suppresses the analyte absorption onto the fibre. Thus, for benzene is decreased by 20% in the presence of 3% methanol [214]. Other well-sorbing hydrophobic substances such as dissolved organic carbon in natural water can have a similar effect. 

The polar carboxylate head dissolves in water, while the long, non-polar hydrocarbon-chain tail is hydrophobic (Box 4.14) but mixes well with greasy (lipophilic. Box 4.14) substances, effectively floating the grease into solution in a sheath of COO- groups. 

If purified poliovirus is not substantially concentrated at a trichloro-trifluoroethane (C2Cl3F3)-water interface, since fluorocarbons are among the most hydrophobic substances known, it is rather improbable that hydrophobic interactions could be involved in poliovirus adsorption to any material. This is particularly true with our oxide surfaces, which in being wet with water, are demonstrated to be hydrophilic in character. This C2CI3F3 extraction procedure is commonly used in enterovirus purification, and is highly effective in removing hydrophobic materials (for example, lipids) from partially purified preparations. 

Because of the high solubilization power of both hydrophilic and hydrophobic substances and the ultralow interfacial tension as their inherent property, microemulsions are an excellent medium for textile detergency. The structure of microemulsions and its effect on detergency are presented in Chapter 29 by Ddrfier. A complete discussion of aU such applications would render this volume prohibitively long, so only eclectic applicahons are included. 

That the concept of hydrophobic bonding does not square with the facts, is pointed out by Hildebrand (1979) who maintained that the very concept of a hvdrophobic effect is unreal. No hydrophobic substance has yet been discovered every substance is hydrophilic, although hydrocarbons are very little so. In support of the hydrophilicity of hydrocarbons, Hildebrand instanced that the energy required to evaporate a mole of butane from its aqueous solution (at 1 atm and 25°C) is 0.65 kcal greater than from its own pure liquid. He also pointed out that if you pour some octane on ice, you can see that the ice is instantly wet by it (Hildebrand, 1979). 

The toxicity of organic solvents or hydrophobic substances for microorganisms depends mainly on their effects on biological membranes - similar to membrane effects of several anesthetics. This concerns especially effects on cytoplasmatic membranes. The following main changes of membrane structures and functions have been observed ... 

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