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Laminated double‐layered films comprising a drug‐containing and drug‐free layer were prepared using tripelennamine, barbitone, salicylic acid and caffeine dispersed in hydroxypropylcellulose (HPC) attached to ethyl cellulose (EC) films containing various proportions of polyethylene glycol (PEG) or HPC to enhance permeability. Drug release in vitro followed zero‐order kinetics, rate constants being dependent on the thickness of the drug‐free membrane, which was rate‐controlling. Thickness‐corrected zero order constants were independent of drug‐loading, which did, however, control the duration of release. Permeability coefficient measurements on the same rate‐controlling films used as single barrier membranes enabled the effective drug concentration (Co) at the interface between the laminated membranes to be estimated; Co was independent of drug loading and was of the order expected from the aqueous solubilities of the drugs. Release rates were enhanced by addition of hydrophilic polymer to the rate‐controlling membranes, either linearly with fraction of additive for PEG to 0·6 or HPC to 0·4, or logarithmically for HPC from 0·4 to 0·8. Enhancement coefficients, which were different for each system, reflected the different mechanisms of hydrophilic polymer action. PEG was leached out rapidly, pores being formed in the matrix. In contrast, HPC was largely retained, so that the enhancement was less. The logarithmic enhancement stemmed from formation of swollen hydrated channels, which, unlike the low HPC fractions or the PEG systems, allowed entry of buffer ions, so that only in these channel systems were the release rates altered by change of the external pH.

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Previous studies on the solubilisation of benzoic acid by aqueous solutions of the polyoxyethylene glycol monoalkyl ether (C, H, , & + ,.[OCH,*CH,];OH)Cetomacrogol (n = 16, m = 24) have been extended to the hydroxybenzoic acids in the same system, and to benzoic acid in glycol monoethers of the ranges 1 n = 12, m = 9, 16, and 20; n = 16, m = 20, 24, and 60. The results all fit the form of the Langmuir isotherm, indicating that solubilisation involves binding to definite sites in the system; this is interpreted in terms of a " core-and-capsule " model of the micelle, with the alkyl chains restricted to the core and the polyoxyethylene chains to the capsule. The results are consistent with the core-capsule interface's being the location of the solubilisate in the cases of benzoic and salicylic acids, and with the polyoxyethylene chains in the capsule being this location in the cases of the metaand para-hydroxy acids. s.w.3 THE potentiometric method of measurement of the uptake of acids and bases by surface-active agents in aqueous solution 1,2 was shown in Part I to be applicable to the binding of benzoic acid by Cetomacrogol, the monocetyl ether of a polyoxyethylene glycol containing 24 oxyethylene units; the results fitted the form of the Langrnuir is0therm.l The studies have now been extended to non-ionic surfactants of a range of poly-