Hydrocarbon chains, long-range

A wide variety of long-chain fatty acids increase transdermal delivery the most popular is oleic acid. It is relevant that many penetration enhancers contain saturated or unsaturated hydrocarbon chains and some structure-activity relationships have been drawn from the extensive studies of Aungst et al. [22,23] who employed a range of fatty acids and alcohols, sulfoxides, surfactants, and amides as enhancers for naloxone. From these experiments, it appears that saturated alkyl chain lengths of around Cio to C12 attached to a polar head... 

In the solid state the translational motion of the molecules is slow and the molecules are arranged with long-range orientational and positional order. However, for compounds with long hydrocarbon chains the molecules may rotate in their lattice sites at the same time as they maintain full positional order, forming so-called plastic crystals (Evans and Wennerstrom, 1994, p. 412). The stability of these plastic crystalline phases (cy-forms) increases with chain length and with the presence of impurities (e.g., broad chain-length distributions) (Larsson, 1994, p. 27). 

In this section, a theoretical model-163 is proposed on the basis of which one can predict both the large sizes of the islands as well as the non-horizontal nature of the LE/LC phase transition. The present calculations involve not only the long-range dipole—dipole interactions, but also the van der Waals interactions among the hydrocarbon chains as well as the conformational free energy of the chains. In this manner, some of the molecular details are taken into account, and one can explain the dependence of the isotherms on the hydrocarbon chain length and the nature of the headgroup. 

The increase in (salt) for these onium salts can be understood in terms of mutual structure-forming or long-range hydrophobic interaction. The hydrocarbon side chains occupy voids in the solvent-lattice , which is stabilized by co-solvent but which collapses when x2 > xf. It is interesting therefore to note that the partial molar volume of R4N + ions decreases as x2 increases in ethyl alcohol + water mixtures, reaching a minimum near x2 0-1 which becomes more clearly defined with increase in size of the R-groups (Lee and Hyne, 1970). A similar trend is observed in acetone + water mixtures (Macdonald and Hyne, 1970). By way of contrast, the partial molar volumes, F (ion), for I-, Br , and Cl- ions decrease steadily to minima near x2 = 0-3 in ethyl alcohol + water mixtures where V2 (R4N+) is a maximum (Lee, 1972 1973). 

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