Crystallisability

Crystallisability is the propensity of a substance to form a crystalline phase under given conditions. It describes whether a substance will crystallize from a melt, solution, or during precipitation, and how readily crystals form and grow. It is governed by thermodynamics, which determine the relative stability of crystalline versus amorphous forms, and by kinetics, which control the rate of nucleation and crystal growth.

In solids such as small molecules and polymers, crystallisability depends on molecular structure and symmetry, intermolecular forces, and, for polymers, chain mobility. High regularity and strong directional interactions promote crystallization, while irregular backbones, bulky substituents, or solvent interactions that disrupt orderly packing hinder it. Process conditions such as temperature, cooling or evaporation rate, solvent quality, supersaturation, and impurities or additives also influence crystallisability.

Assessment typically uses differential scanning calorimetry to detect crystallization exotherms and estimate crystallinity; X-ray diffraction to quantify crystalline fraction and identify polymorphs; and polarized light microscopy to observe crystal shapes and growth. Kinetic models, such as Avrami or Ozawa analyses, help describe nucleation and growth rates. Crystallisability is related to, but not identical with, crystallinity, which is the actual fraction of material in crystalline form.

In pharmaceuticals, crystallisability affects solubility, stability, and bioavailability; in polymers, it governs mechanical properties and barrier performance. Practitioners seek to enhance or suppress crystallisation through solvent choice, thermal history, seeding with specific polymorphs, nucleating agents, or co-crystallisation, and sometimes via additive or polymeric blends. Related topics include polymorphism and crystallization kinetics.