COLLOIDAL STABILITY OF A PHARMACEUTICAL LATEX - EXPERIMENTAL DETERMINATIONS AND THEORETICAL PREDICTIONS

In this work, the colloidal stability of a pharmaceutical ethylcellulo se latex, Aquacoat, is analyzed by means of turbidity measurements. Th e effects of both electrolyte (KCI, CaCl2 and LaCl3) concentration and pH on the stability are considered, and it is found that at pH 4 (6), the critical coagulations concentrations (c.c.c.) are 70 (150) mM KCI , 12 (20) mM CaCl2, and 1 (1) mM LaCl3. The results are interpreted in terms of both classical and extended versions of the DLVO theory of t he stability of colloids; the Hamaker constant, A, characteristic of t he van der Waals interaction between the particles in suspension, is c alculated from measurements of the surface free energy of the polymer, based on contact angle measurements of selected liquids on polymer pe llets. Concerning the electrostatic contribution to the total interact ion energy between the polymer spheres, it is calculated from zeta pot ential data deduced from electrophoretic mobility of the latex suspens ions. The application of the so-called extended DLVO theory includes n ot only van der Waals and electrostatic interactions, but also acid-ba se or structural ones, estimated using a recent model of the surface f ree energy of solids. Using the well-known Fuchs equation, we have com puted the stability ratio W of the suspensions, using both the classic al and extended DLVO theories, for the different experimental conditio ns. The results show that, when the electrolyte is KCI, the classical model correctly predicts the c.c.c. values, although the slopes of the log W concentration trends are different from the experimental ones. The extended theory underestimates the c.c.c.'s and gives a better app roximation to the slopes. In the case of both CaCl2 and LaCl3, the bes t agreement is always found with the extended theory. It is suggested that consideration of ionic strength effects on the acid-base characte ristics of the solid could improve the extended DLVO results. (C) 1996 Academic Press,Inc.