Electrokinetics and stability of a cellulose acetate phthalate latex

Abstract

An experimental investigation is described on the surface electric characteriza- tion of a commercially available latex, Aquateric, composed of cellulose acetate phthalate polymer particles, and used in enteric-controlled drug release. Since the surface charge of dispersed systems is an essential parameter governing most of their behavior, it is of fundamental importance to characterize how that quantity changes in the different envi- ronments in which the colloids could be used. The experimental method used in this work is electrophoresis; we report measurements of electrophoretic mobility of the latex as a function of pH and ionic concentration in the dispersion medium. It is shown that the zeta potential of the polymer particles is negative for the whole pH range studied and increases with pH as the dissociation of surface acetic acid groups proceeds. A plateau value is found for pHu 5, corresponding to complete dissociation of available ionizable sites. The values of the electrophoretic mobility (me) and the zeta potential (z) of Aquateric are also analyzed as a function of the concentration of 1-1 (NaCl) and 2-1 (CaCl2) concentration. The anoma- lous surface conductance (associated to the mobility of counterions adsorbed in the inner part of the electric double layer of the particles) manifests in a maximum in the EmeE- NaCl concentration plot for 10 03 M concentration. No such behavior is observed in the presence of CaCl2 solutions, where only a decrease of the mobility with ionic strength is observed. The effect of AlCl3 concentration on the mobility is also considered; it is found that at pH 2 aluminum ions adsorb on the particles and render them positively charged. When the pH of the suspensions is not maintained constant, the hydrolysis of aluminum gives rise to a less efficient control of the charge of the particles and no positive mobilities are observed. Electrophoretic mobility measurements as a function of pH at constant AlCl3 concentration show an abrupt change of me from negative to positive, interpreted as due to surface precipitation of Al(OH)3. When the pH is further increased, a second charge reversal is found, corresponding to the isoelectric point (pH of zero zeta potential) of Al(OH)3. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2721-2726, 1997