Simple coacervation of hydroxypropyl methylcellulose phthalate (HPMCP) I. Temperature and pH dependency of coacervate formation

Abstract

The simple coacervation of hydroxypropyl methylcellulose phthalate (HPMCP) on the addition of 20% (w/w) sodium sulphate solution was investigated as a function of the temperature and pH value of the aqueous polymer solution. Phase diagrams, quantitative investigations of the isolated polymer-rich phases and charge density measurements served to characterize phase separation. The existence of coacervate and precipitate phases of HPMCP was attributed to the chemical structure of the polymer. Analogous to other polymers such as gelatin or cellulose acetate phthalate, HPMCP formed polymer-rich coacervate and precipitate phases from aqueous solutions following the addition of electrolyte solutions. Increased temperature and total electrolyte content had a synergistic effect on phase separation. With increasing temperature the polymer content in the polymer-rich phase rose up to 23.6% (w/w) at 60°C corresponding to a polymer yield of 63.7%. This was accompanied by the gelation of the coacervate phase which was attributed to the temperature-dependent hydration of the methyl and hydroxypropyl substituents of the polymer. Moreover, a minimum pH value of the HPMCP solution was required for coacervate formation, otherwise the polymer was salted out as a precipitate. Charge density measurements showed that this was accompanied by the almost entire dissociation of the HPMCP carboxyl groups. They were hence responsible for the pH-dependent polymer hydration and essential to coacervate formation. The adjustment of an appropriate pH value is therefore a prerequisite for the formation of coacervate phases in the process of microencapsulation with HPMCP by simple coacervation. Owing to the temperature dependency of coacervate formation, the temperature course required for the microencapsulation process was found to run opposite to that described for cellulose acetate phthalate.