Effects of Additives on the Morphology of Thiamine Nitrate: The Great Difference of Two Kinds of Similar Additives

The growth of thiamine nitrate, in supersaturated aqueous solutions in the absence and presence of sodium alkyl sulfates CH₃(CH₂)_nSO₄Na and sodium alkyl sulfonates CH₃(CH₂)_nSO₃Na was studied by a single seed crystal growth experiment. It was surprisingly found that the growth of the a-axis of thiamine nitrate is significantly inhibited by CH₃(CH₂)_nSO₄Na, thus reducing the aspect ratio of thiamine nitrate, while the aspect ratio of thiamine nitrate in the presence of CH₃(CH₂)_nSO₃Na remains almost constant. Furthermore, the mechanism of additives to modify the crystal morphology is proposed: both additives can inhibit the growth of thiamine nitrate by hindering the solute diffusion. However, their unusual behavior is due to the selective adsorption, which was caused by electrostatic and hydrogen bond interactions between solute and additive molecules. In particular, the anionic groups exposed at the end of the additives demonstrate an interesting case in which a small variation in the charge density and hydrogen bonding ability can lead to a marked difference in modifying the crystal growth behavior. The results obtained from this study should be helpful in the performance evaluation and selection of the morphology modifiers for thiamine nitrate crystals.