Thermal analysis of some pharmaceutically relevant systems obtained by SOL-gel technique

Abstract

Thermal analysis has been performed of xerogels obtained by encapsulation of sodium thiosulfate (systems I and II), glycerol with I₂/KI (system III) and urea with I₂/KI (system IV) in silica gel. DTA, TG and DTG of the xerogels with thiosulfate differed markedly depending on the thiosulfate content. At the 22% thiosulfate content in the original silicic acid solution used for the preparation of the xerogel, a single exothermic peak appears in the DTA trace assignable to the combustion of elemental sulfur derived either from the reaction between silicic acid and the thiosulfate or from the thermal degradation of the latter. When the thiosulfate content in the original solution is raised up to 60%, the peak is split into two symmetrical components, one (at 553 K) attributable to combustion of the more finely dispersed sulfur and the other (at 703 K) to sulfur particles of coarser morphology. The former is likely to be formed by chemical interaction of the components of original solution and the latter is due to thermal degradation of residual thiosulfate.

The main features of a DTA, TG and DTG obtained from xerogel III are exothermic events recorded in the DTA trace suggesting that after the loss of iodine, thermal degradation of glycerol takes place with the onset at 523 K leaving a combustible solid. Under these conditions, glycerol alone distilled out with no evidence for degradation.

A derivatogram of xerogel IV is characterized by initial sublimation/evaporation of urea and iodine followed by partial condensation of urea to afford biuret and eventually cyanuric acid.