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

The incorporation of three steroids, progesterone, testosterone and hydroco rtisone onto fine silica particles surfaces as induced by the adsorption of a cationic surfactant, cetyltrimethylammonium bromide at the solid/water i nterface, has been studied below and above the surfactant critical micelle concentration (cmc). It is shown that the binding of the steroids is at a m aximum at the equilibrium cmc and decreases above this concentration. This effect is due to the competing effects of drug adsolubilization in the adso rbed 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 d erived for the adsorption and the desorption processes. It is shown that at low drug concentration the partition coefficients of the steroids are equa l in the surface adsorbed surfactant aggregates and in the micellar solutio ns. The analytical expressions enable the calculation of the surfactant con centration required for the complete desorption of the drugs from the silic a surface. At higher drug concentrations, the same effects are observed, bu t the mathematical analysis is then complicated by drug adsorption onto sil ica in the absence of surfactant. (C) 1998 Elsevier Science B.V. All rights reserved.