Surface modification of silica particles by a cationic surfactant: adsolubilization of steroids from aqueous solutions

The incorporation of three steroids, progesterone, testosterone and hydrocortisone onto fine silica particles surfaces as induced by the adsorption of a cationic surfactant, cetyltrimethylammonium bromide at the solid/water interface, has been studied below and above the surfactant critical micelle concentration (cmc). It is shown that the binding of the steroids is at a maximum at the equilibrium cmc and decreases above this concentration. This effect is due to the competing effects of drug adsolubilization in the adsorbed aggregates and the solubilization into the free micelles above the rmc. At higher surfactant concentrations complete desorption of the drugs from the silica/water interface is observed. Analytical expressions have been derived for the adsorption and the desorption processes. It is shown that at low drug concentration the partition coefficients of the steroids are equal in the surface adsorbed surfactant aggregates and in the micellar solutions. The analytical expressions enable the calculation of the surfactant concentration required for the complete desorption of the drugs from the silica surface. At higher drug concentrations, the same effects are observed, but the mathematical analysis is then complicated by drug adsorption onto silica in the absence of surfactant. (C) 1998 Elsevier Science B.V. All rights reserved.