Crystallization of 4′-Hydroxyacetophenone from
Water: Control of Polymorphism via Phase Diagram Studies
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Abstract
The preparation of polymorphs and solvates and the characterization
of their stability domains have received considerable attention in
recent years, due to the importance of these studies for fundamental
research and for the production of new materials for task-specific
applications. In this work, the selective and reproducible crystallization
of different solid forms of 4′-hydroxyacetophenone (HAP) from
water was investigated, through the determination of a temperature–concentration
(<i>T</i>–<i>c</i><sub>HAP</sub>) phase
diagram. This determination was mainly based on gravimetric solubility
measurements, slurry tests, and metastable zone width (MZW) studies
with thermometric and turbidity detection. The experimental conditions
for the formation of five different HAP phases by cooling crystallization
could be established: the previously characterized anhydrous forms
I and II and the hydrate HAP·1.5H<sub>2</sub>O (H1), and two
new hydrates, one of stoichiometry HAP·3H<sub>2</sub>O (H2) and
another (H3) which proved too unstable for a stoichiometry determination.
The crystallization precedence of the various phases, their approximate
lifetimes, and transformation sequences could also be elucidated.
It was finally found that for a specific <i>T</i>–<i>c</i><sub>HAP</sub> domain the crystallization of HAP solid
phases was mediated by a colloidal dispersion. Preliminary dynamic
light scattering experiments indicated that this dispersion consisted
of particles with diameters in the range of 100–800 nm