Molecular Insights into the Crystallization of 4’-Hydroxyacetophenone from Water: Solute Aggregation, Liquid–Liquid Phase Separation, and Polymorph Selection

In this work, insights into the structural rearrangements occurring in aqueous solution, prior to the nucleation of different 4’-hydroxyacetophenone (HAP) forms from water, were obtained, through a combination of thermomicroscopy, microdifferential scanning calorimetry, density and speed of sound measurements, and molecular dynamics simulations. The results confirmed our previous observation that cooling crystallization of HAP is intermediated by liquid–liquid phase separation (LLPS) and, depending on the initially selected concentration range, selectively leads to the formation of different crystal forms. Analysis of the solution properties before the onset of LLPS revealed that, in the HAP mole fraction range x_HAP < 0.004 (Zone I), where hydrate H2 ultimately crystallizes, small, solvated clusters are initially present in solution, which remain approximately invariant in size, shape, and HAP/H₂O proportion as the temperature decreases. For the x_HAP > 0.005 range (Zone III), where anhydrous form I crystallizes, large HAP/water aggregates (that can even percolate the whole system as x_HAP approaches the 0.005 limit) are already initially present in solution. As cooling progresses, they become more compact, a process accompanied by a reduction in water content, which is more significant as the solution concentration increases. The 0.004 < x_HAP < 0.005 (Zone II) range corresponds to a transition region where, as x_HAP increases, the physical properties of the solution initially evolve from those typical of Zone I and, at a certain point, abruptly change and start converging to those typical of Zone III. In all zones, the colloidal particles formed upon LLPS (from which crystallization results) can also reduce their water content on cooling, but the extent of this process increases as x_HAP moves from Zones I and II, where hydrates are formed, to Zone III, where anhydrous form I is produced.