Permeation of oestrone, oestradiol, oestriol and dexamethasone across cellulose acetate membrane.

The permeation across cellulose acetate of three oestrogens, differing only in the number of hydroxyl groups attached to the nucleus, and a 'standard' steroid, dexamethasone, was investigated using the lag-time method for calculating diffusion parameters, between 10 and 40 degrees. Diffusion coefficients for the similarly-sized oestrogens were relatively insensitive to marked changes in polarity, but increased permeation was correlated with increased partition coefficients, decreased polarity and fewer hydroxyl groups on the nucleus. Permeation increased with temperature and energies of activation were calculated from Arrhenius-type plots. Ep values ranged from 4-84 k cal mol-1 (20 kJ mol-1) for the least polar steroid (oestrone) to 6-91 k cal mol-1 (29 kJ mol-1) for the most polar steroid (oestriol). The results implied that steroid diffusion occurred through aqueous membrane channels, but that it was impeded to various extents by both obstruction and polar interaction effects.