Fourier transform infrared spectroscopic studies of human serum albumin microcapsules prepared by interfacial cross-linking with terephthaloylchloride: influence of polycondensation pH on spectra and relation with microcapsule morphology and size.

Fourier transform infrared (FT-IR) spectroscopic studies were performed on microcapsules prepared through interfacial cross-linking of human serum albumin (HSA) with terephthaloylchloride at various pH values (5.9 to 11). Correlations were established with morphology and size of microcapsules. Increasing polycondensation pH resulted notably in a progressive increase of peaks at 1795 and 1724 cm-1, assigned to anhydride and ester; respectively, in a decrease of the carboxylate-assigned 1394 cm-1 peak, and in alterations of the 1340-1080-cm-1 region. These spectral changes were most pronounced from pH 9 and were shown to correspond to smaller-sized microcapsules (mean size decreased from 30-40 microns to less than 15 microns) with rough surfaces. Further soaking of highly cross-linked microcapsules in a pH 7.5 buffer resulted in the disappearance of the 1795 cm-1 peak, with a concurrent increase of the 1394 cm-1 peak and a decrease of the 1724 cm-1 peak. These changes, attributed to complete breaking of anhydride and partial hydrolysis of esters, were accompanied by an unwrinkling of the microcapsule membrane, then made smooth, and a significant increase in size. Treating microcapsules with hydroxylamine under alkaline conditions allowed complete reversal of the spectral alterations assigned to anhydride and ester formation. A comparable (slightly higher) increase in size was observed with microcapsules which exhibited smooth surfaces and a low density.