The transfer of salicylic acid across a Cellophane membrane from micellar solutions of polysorbates 20 and 80

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Abstract
In the pH range 1 to 5 there is a linear relation between the rate constant for salicylic acid transfer across a cellophane membrane and the fraction of ionized drug present at each pH. The rate constant for ionized molecules is about 60% of that for unionized molecules. The presence of polysorbates 20 and 80 in the drug solution markedly decreases the apparent transfer rate constant of salicylic acid at low pH. When transfer rate constants obtained in the presence of surfactant are calculated in terms of the non-micellar concentration of salicylic acid they compare favourably with those obtained in the absence of surfactant. These results indicate that micellar drug does not participate in the transfer process.