Crystallisation of amorphous mannitol is retarded using boric acid

Abstract

An approach to inhibit the crystallisation of amorphous mannitol was investigated. Boric acid was selected as a model additive for a fundamental study of its ability to retard crystallisation and to facilitate characterisation of the properties of the amorphous solid. At concentrations above 5% (w/w) of boric acid, the DSC scans indicated that a totally amorphous solid could be prepared by cooling the melted pre-mixture under ambient conditions. An increase in the glass transition temperature ( T g) was observed with a corresponding increase in boric acid content, and their relationship was well fitted by the Gordon–Taylor equation. This result suggested that mannitol and boric acid mixed homogeneously. The crystallisation profiles of the resultant amorphous compositions were best described by the Avrami–Eroféev equation ( n=1/3), which indicated that random nucleation and three-dimensional crystal growth was the best-fitting mechanism of this crystallisation. The activation energy of crystallisation decreased with increasing boric acid content, indicating that the temperature dependency for crystallisation decreased with increasing boric acid content. Furthermore, the rate of crystallisation at 30 °C for mannitol alone was 7000 times higher than that of mannitol containing 7.5% (w/w) of boric acid.