Whisker Growth from Sodium Valproate-Synthetic Aluminum Silicate Systems

Abstract

The whisker growth on the surface of particles of the sodium valproate (NaVPA)-synthetic aluminum silicate (SAS) systems were investigated under various conditions of humidity and temperature. The chemical composition of whiskers was found to be the complex of NaVPA and valproic acid (VPA) with molar ratio 1 : 1. The whiskers grew in the range of the critical relative humidity (CRH) of NaVPA and a complex of NaVPA-VPA when the NaVPA-SAS systems contained 30-50% (w/w) of NaVPA. The whiskers grew at the bottom of them on the surface of particles. The whiskers grew until various length at gradually decreasing rate. The mechanism of the whisker growth was assumed that the VPA was formed from NaVPA by the surface acidity of SAS and it complexed with NaVPA. The complex moved to the roots of whiskers in a particle, then crystallized and grew as whiskers. The addition of sodium carbonate or monoethanolamine to the NaVPA-SAS systems inhibited the whisker growth. This may be due to the alkalinity of these compounds which poisoned the surface acidity of SAS. Hydrogenated castor oil also inhibited the growth of whiskers by another mechanism.