Effect of Fe2+, Zn2+, and Ca2+ on the Solvent-Mediated Phase Transformation of Glycine

Abstract

This study investigated the effects of iron, zinc, and calcium ions (Fe²⁺, Zn²⁺, and Ca²⁺, respectively) on the solvent-mediated polymorphic transformation of glycine and subsequent crystal properties. The transformation from β- into α-glycine was monitored in real time using ultrasonic velocity measurements and revealed that these metal ions delayed the transformation in a concentration-dependent manner, with Fe²⁺ showing the strongest delay. x-ray diffraction and microscopy analyses confirmed the complete transformation into α-glycine, with significant morphology changes induced by the additives. Scanning electron microscopy and image-based analyses showed that Fe²⁺ led to compact, agglomerated crystals with irregular shapes, while Zn²⁺ and Ca²⁺ caused moderate thickening of the rod-like α-glycine crystals. Zeta potential measurements demonstrated metal ion adsorption on crystal surfaces, reducing surface charge and promoting agglomeration, particularly in the Fe²⁺ system. Filtration tests revealed a sharp increase in specific cake resistance with Fe²⁺, while Zn²⁺ and Ca²⁺ improved filtration performance. Thermogravimetric analysis and in-situ Fourier transform infrared spectroscopy analyses confirmed a consistent two-step thermal degradation pattern across all samples, with minor variations in decomposition temperatures and evolved gas profiles.