Rate of solvent evaporation and solubility parameter effects on microencapsulation

A nonsolvent/solvent evaporation method of microencapsulation using ethylcellulose and ion-exchange resin-drug complex was used to determine the effect of the rate of solvent evaporation and the effect of the solubility parameters of the microencapsulation system on the morphological characteristics and release rates of the microcapsules. The rate of solvent evaporation affected the shape, surface morphology, porosity and the purity of the ethylcellulose coat. At a very slow solvent evaporation rate, tails were formed and the coats were porous. Shorter evaporation times eliminated tails, led to smooth but wrinkled coats and plasticization with light liquid paraffin. Coat porosity was minimized with intermediate evaporation rates. The microcapsules formed by the very fast solvent evaporation rate had rapid release rates, microcapsules formed by slow and very slow evaporation rates had intermediate release rates and coats formed at fast and intermediate rates had variable and slow release rates. Microcapsules prepared using solvent mixtures which had a solubility parameter in the same area of a two-dimensional solubility parameter map usually exhibited similar release rates and morphological characteristics. Microencapsulations using solvent systems which had solubility parameters well within the polyisobutylene soluble reaction of a two-dimensional solubility parameter map produced mononucleated microcapsules which exhibited similar morphological characteristics. Whereas multinucleated microcapsules were produced when solvent systems were used which had solubility parameters close to the region of insolubility for polyisobutylene. Further examination of solubility parameter maps of ethylcellulose as affected by polyisobutylene demonstrated that polyisobutylene altered the phase changes of ethylcellulose. The formation of two liquid phases induced by polyisobutylene can be used to explain how mononucleated microcapsules are produced using the adjunct polymer polyisobutylene. The use of a solubility parameter map for the coating polymer ethylcellulose, as affected by the adjunct polymer, polyisobutylene, can be used to choose solvent systems which provide suitable phase changes and consequently similar microcapsule coats.