ELECTROKINETICS AND STABILITY OF A CELLULOSE-ACETATE PHTHALATE LATEX

An experimental investigation is described on the surface electric cha racterization of a commercially available latex, Aquateric, composed o f cellulose acetate phthalate polymer particles, and used in enteric-c ontrolled drug release. Since the surface charge of dispersed systems is an essential parameter governing most of their behavior, it is of f undamental importance to characterize how that quantity changes in the different environments in which the colloids could be used. The exper imental method used in this work is electrophoresis; we report measure ments of electrophoretic mobility of the latex as a function of pH and ionic concentration in the dispersion medium. It is shown that the ze ta potential of the polymer particles is negative for the whole pH ran ge studied and increases with pH as the dissociation of surface acetic acid groups proceeds. A plateau value is found for pH > 5, correspond ing to complete dissociation of available ionizable sites. The values of the electrophoretic mobility (mu(e)) and the zeta potential (zeta) of Aquateric are also analyzed as a function of the concentration of 1 -1 (NaCl) and 2-1 (CaCl2) concentration. The anomalous surface conduct ance (associated to the mobility of counterions adsorbed in the inner part of the electric double layer of the particles) manifests in a max imum in the \mu(e)\-NaCl concentration plot for 10(-3)M concentration. No such behavior is observed in the presence of CaCl2 solutions, wher e 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 mainta ined constant, the hydrolysis of aluminum gives rise to a less efficie nt control of the charge of the particles and no positive mobilities a re observed. Electrophoretic mobility measurements as a function of pH at constant AlCl3 concentration show an abrupt change of mu(e) from n egative 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 potent ial) of Al(OH)(3). (C) 1997 John Wiley & Sons, Inc.