Abstract
Crystal nucleation rates have been measured in the bulk and at the surface of acetaminophen melt. Of its six known polymorphs, Form III nucleates in the temperature range investigated (290–333 K), both in the bulk and at the surface. Nucleation is the fastest near 318 K (about 20 K above the bulk glass transition temperature Tg) at a rate of 3 × 106 s–1 m–3 in the bulk and 200 s–1 m–2 on the surface. On the per-molecule basis, surface nucleation outpaces bulk nucleation by 5 orders of magnitude, highlighting the importance of the liquid/vapor interface in crystal nucleation. In contrast to its strong effect on nucleation, the free surface has a weak effect on crystal growth. The observed nucleation kinetics is well described by the Classical Nucleation Theory (CNT) if the crystal growth rate is used to represent the kinetic barrier. Our finding is relevant for understanding the physical stability of amorphous materials for applications in drug delivery and electronics.
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