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Partition coefficient, P

Figure 5.13 Relation of neurotoxicity to the hydrophobic character of various penicillins: the hydrophobic character is measured by the partition coefficient P and toxicity by a neurotoxicity index T.

Reproduced from T. R. Weihrauch et al., Arch. Pharmacol. (NS), 289, 55 (1975).

phase of an emulsion, to the micellar phase of a surfactant, or to a closure. Adsorption may also occur onto container closures and suspended solid particles.

Permeation of antimicrobial agents into rubber stoppers and other closures is another example of partitioning. Although rubber is an amorphous solid, partitioning between the aqueous phase and rubber depends, as in liquid systems, on the relative affinities of the solute for each phase.

Glyceryl trinitrate, a volatile drug with a chloroform/water partition coefficient of 109, diffuses from simple tablet bases into the walls of plastic bottles and into plastic liners used in packaging tablets. This partitioning can be prevented if there is included in the tablet formulation an agent (such as polyoxyethylene glycol) which complexes with the drug substance, thereby increasing its affinity for the 'tablet phase' - in other words, reducing its

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