Surface-enhanced crystal nucleation and polymorph selection in amorphous posaconazole

Molecules at a liquid/vapor interface have different organizations and mobilities from those in the bulk. These differences potentially influence the rate of crystal nucleation, but the effect remains imperfectly understood. We have measured the crystal nucleation rates at the surface and in the bulk of amorphous poscaconazole, a rod-like molecule known to have a preferred interfacial orientation. We find that surface nucleation is vastly enhanced over bulk nucleation, by ∼9 orders of magnitude, and selects a different polymorph (II) from bulk nucleation (I). This phenomenon mirrors the recently reported case of D-arabitol and stems from the similarity of anisotropic surface molecular packing to the structure of the surface-nucleating polymorph. In contrast to these two systems, the surface enhancement of nucleation is weaker (though still significant) in acetaminophen and in water and does not select a different polymorph. Together, the systems investigated to date all feature surface enhancement, not suppression, of crystal nucleation, and those showing a polymorphic change feature (1) structural reconstruction at the surface relative to the bulk and (2) existence of a different polymorph that can take advantage of the surface environment to nucleate. These results help predict the effect of a liquid/vapor interface on crystal nucleation and polymorph selection, especially in systems with a large surface/volume ratio, such as atmospheric water and amorphous particles.