Abstract
The migration rates of ephedrine and salicylic acid from white soft paraffin-isopropyl myristate mixtures into water have been measured using a previously described procedure. The rates for ephedrine decreased progressively with increasing isopropyl myristate concentration, and this was attributed to the observed accompanying increase in solubility, which decreased the activity of the solute in the solution. Solubility and partition coefficient determinations established that the improved solubility resulted from complexation between ephedrine and isopropyl myristate. In contrast, migration rates of salicylic acid in white soft paraffin-isopropyl myristate mixtures increased with increasing isopropyl myristate concentration up to 50% isopropyl myristate, and then declined. Solubility and partition coefficient determinations indicated that salicylic acid dimerised and also complexed with isopropyl myristate in the lipid mixtures. The diffusion rate profile was attributed to a balance between declining solute-solute and increasing solute-solvent complexation with increasing isopropyl myristate concentration. This mechanism was supported by the behaviour of salicylic acid in Witepsol H15-isopropyl myristate mixtures.
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