Phase Separation in Microcapsule Membranes Composed of Soybean Lecithin, Cholesterol, Stearic Acid and Polyvinylpyrrolidone.

Abstract

A phase diagram of a soybean lecithin (SL)-cholesterol (CH)-stearic acid (SA) system incorporating 42% polyvinylpyrrolidone was estimated by differential scanning calorimetry (DSC), X-ray diffiractometry (XRD) and polarizing microscopy in order to relate it to the dissolution and disintegration properties of microcapsules whose membranes consisted of these compounds. At the weight ratio of SL : CH=4 : 1, SL : SA=4 : 1 or SL : CH : SA=3 : 1 : 1, SL liquid crystalline phase (SL(LCP)) dissolved the maximum amount of CH and/or SA without any other crystalline phases detected on DSC and XRD. The existence of SL(LCP) was confirmed by observation under polarizing microscope : mixtures remained anisotropic over 20-90°C or SA-containing anisotropic phases became isotropic at 70-80°C without any peaks on DSC. With increase in CH and /or SA, these were separated from SL(LCP) dissolving CH and/or SA. The separated CH (CH(c)) was found to have the same crystalline structure as the parent CH from XRD. The separated SA, in contrast, showed two types of XRD patterns : one had the same pattern as the parent SA (SA(c)) and the other had a pattern with some peaks shifted to shorter spacings(SA(c')). The mixtures gave a phase diagram with six regions, each of which had the following phase(s) : 1)SL(LCP);2) SL(LCP) and CH(c); 3) SL(LCP), CH(c) and SA(c'); 4) CH(c), SA(c') and SA(c); 5) SA(c) and SA(c'); 6) SL(LCP) and SA(c'). It was not clear whether SL(LCP) was present in the fourth and fifth zones or not.