Influence of pH adjustment on microcapsules obtained from complex coacervation of gelatin and acacia

The coacervation behaviour of commercial grade gelatin and acacia mixtures was studied with five different acids to adjust the coacervation pH, i.e. HCl, HNO3, H2SO4, acetic acid, and citric acid. The electrical equivalence pH value (EEP) of the polymer mixture was determined by means of a streaming current detector (SCD). With all acids--except H2SO4--maximum coacervate yield was observed at the EEP. Using H2SO4 the EEP was found at a lower pH value than compared with the point of maximum coacervate yield. The quantity of coacervate at the EEP was significantly reduced in the presence of H2SO4 whereas with all other acids, almost no differences were found. The dependence of the coacervate volume on the added amount of acid did not change in parallel to the dry coacervate yield and there was no coincidence of the maximum coacervate volume and the EEP. The barrier properties of the capsule shells of corresponding microcapsules using indomethacin as a model drug were examined by dissolution studies. Indomethacin microcapsules showed the slowest release rate when the coacervation pH was adjusted to the EEP and not to the pH of maximum coacervate yield. As expected from the coacervation behaviour, dissolution profiles of the microcapsules were quite similar even when different acids were used for pH adjustment.