Coadsorption of the sodium salts of two steroid molecules at a silica/interface as induced by the adsorption of a cationic surfactant

Abstract

The incorporation of two ionic steroids, namely the sodium salts of hydrocortisone 21-hemisuccinate (HNa) and prednisolone 21-succinate (PNa), at a silica/water interface in the presence of adsorbed cetyltrimethylammonium bromide has been investigated first at a constant pH value. It is shown that this coadsorption effect is qualitatively similar to the adsolubilization effect which is described for neutral molecules. The adsorption of the cationic surfactant induces the coadsorption of the anionic drug molecules although the silica surface is negatively charged. At surfactant equilibrium concentration above the critical micelle concentration the drug molecules are distributed between the adsorbed aggregates and the free micelles. At larger surfactant concentration, the drugs may be completely depleted from the silica/water interface. Based upon Langmuir-type isotherms, the equilibrium constants of the drug molecules with the adsorbed aggregates and the free micelles are calculated. The constants are about three times larger for the former than for the latter aggregates. The signification of such results is discussed. The coadsorption of HNa at low surfactant surface coverage was also investigated in the pH interval between 3 and 9. HNa is strongly coadsorbed at lower pH onto the silica surface. The coadsorption goes through a maximum at a pH value which may be considered as equal to the apparent pK of the drug and decreases to zero at higher pH values. A pK value equal to 4.2 is proposed for HNa. This behaviour is interpreted as the result of the interplay of the drug dissociation and that of the surface silanol groups upon the change of pH.