Adsorption of a Corticoid on Colloidal Hematite Particles of Different Geometries

An electrokinetic study of the adsorption of a glucocorticoid drug, betamethasone disodium phosphate (BMP), on hematite (alpha-Fe2O3) particles is presented in this work. We analyze the electrophoretic mobility of both spherical and elongated hematite as a function of the concentration of BMP in the dispersion medium; it is found that, even for the lowest BMP concentration, the drug molecules adsorb onto the particles, changing their surface charge from positive to negative. When the drug concentration is increased between 10(-5) and 5 x 10(-3) M, the mobility of spherical hematite shows a monotonous increase from approximately -1.5 to approximately -3.5 &mgr;ms-1/Vcm-1; in the case of elongated particles, the mobility increases with BMP concentration up to 10(-4) M, larger concentrations of the corticoid having little effect on the electrokinetic properties of the particles. The effect of pH on the electrophoretic mobility of hematite is also investigated for two constant drug concentrations (0.05 and 0.5 mM); the isoelectric point of the particles is shifted to a pH below 3, another indication of adsorption of the negatively charged phosphate groups of BMP. These qualitative reasonings are confirmed by direct spectrophotometric determinations of the amount of BMP adsorbed on both hematite samples: spherical and ellipsoidal. The effect of pH on the adsorption density, Gamma, suggests that electrostatic interactions are not the only forces responsible for adsorption. In fact, although Gamma decreases with pH for both types of particles, it remains measurable up to pH 10, when strong electric repulsion is to be expected between hematite and phosphate groups. The effect of initial BMP concentration on Gamma is also studied. It is shown that a plateau region is reached in both samples, this indicating that their surfaces get saturated with BMP: the saturation begins when the coverage is of the order of approximately 1/8 (spherical particles) and approximately 1/40 (elongated particles) of a drug monolayer. Such difference might be due to the ellipsoidal hematite particles having a lower positive surface charge, since they are synthesized in a different dispersion medium.