ADSORPTION OF BETAMETHASONE DISODIUM PHOSPHATE ON ETHYLCELLULOSE LATEX - AN ELECTROKINETIC STUDY

The use of polymer latexes as vehicles for drug transport and controll ed delivery in the body is a very active research field in applied col loid science. In this work, the polymer chosen is a commercially avail able ethylcellulose-based material, Aquacoat. We report electrokinetic (electrophoresis) data on this polymer in the presence of the soluble glucocorticoid betamethasone disodium phosphate (BMP). The electropho retic mobility of the particles becomes more negative as the concentra tion of BMP in the medium is increased between 10(-5) and 10(-3) M, th is demonstrating that the negative species produced upon dissolution o f BMP adsorb on the particles; this trend is reversed for higher conce ntrations due to the phenomenon of double layer thickness. When pH is increased at constant BMP concentration, the negative mobility increas es in absolute value, but it reaches lower values than attained in the absence of BMP. This is interpreted as another consequence of adsorpt ion, since the ionization constants of the phosphate and sulfate group s are different. Such adsorption is confirmed and measured by spectrop hotometric analysis of the supernatants after centrifugation of the su spensions; it is found that the adsorption density increases with the initial concentration of BMP in the medium, an almost vertical isother m being obtained for concentrations above 5 x 10(-3) M, possibly due t o bidimensional aggregation of the adsorbed molecules and multilayer a dsorption at higher concentrations. It is found that the adsorbed amou nt increases with the ionic strength of the medium as a consequence of screening of the particle and BMP charges, whereas these electrostati c contributions to adsorption manifest also in the pH dependence of ad sorption. IR analysis of the particles after equilibration with BMP so lutions of increasing concentration agrees well with adsorption data.