Mechanistic Insightsinto Crystal Habit Regulationof Guanidinoacetic Acid by Cyanamide-Based Additives

Guanidinoacetic acid (GAA), a functional feed additive, has its industrial application hindered by poor powder flowability caused by an unfavorable crystal habit. This study investigates cyanamide-based additives, cyanamide (CA), Dicyandiamide (DICY), and melamine (MA), to regulate the crystal habit of GAA during aqueous crystallization. Increasing supersaturation accelerates crystal growth but does not significantly alter the crystal habit. In contrast, the additives induce a distinct transition from elongated hexagonal to near-equiaxed crystals, significantly improving flowability. Kinetic and surface characterization reveal that the additives selectively inhibit growth of the smooth (111) face, while exerting a weaker inhibitory effect on rough (011) face. This face selectivity originates from distinct growth mechanisms: on the (111) face, additive adsorption induces step pinning and preserves layer growth, whereas on the rough-growing (011) face, the additives weaken interfacial kinetic constraints and enhance solute diffusion near the crystal face, resulting in relatively weak growth inhibition despite stronger adsorption. The habit-modifying efficiency follows the order MA > DICY > CA, which is governed by molecular recognition and interfacial compatibility. These findings provide a mechanistic and practical strategy for industrial crystal habit engineering.