Macro- and micromorphology of monoclinic paracetamol grown from pure aqueous solution

The morphology of monoclinic paracetamol crystals has been investigated bot h theoretically and experimentally. Calculations using the computer program HABIT 95 with both DREIDING II and MOMANY force fields predict prismatic f orms in which {100}. {001}, {110}, and {201} show approximately equivalent morphological importance. Whereas all of these faces are observed experimen tally, the real crystals showed a {110} dominance at low supersaturations w hich gave way to an increasing {001} dominance as the supersaturation incre ased. This variation was accompanied by a change from a columnar to a plate like habit. Surface examinations using phase contrast microscopy showed the habit changes to be due principally to changes in the growth mechanism of the {110} faces. A slow growth process involving two-dimensional nucleation at a few growth sources occurred at low supersaturations. This gave way to dislocation growth and finally at high supersaturations, to a fast growing mixed mechanism combining two-dimensional growth from the edges and vertex es of the {110} faces with the operation of dislocation sources at the face center. The increasing dominance of the two-dimensional growth contributio n at the highest supersaturations coupled with an increase in macrostep for mation resulted in the development of inclusions in the {110} sectors. This phenomenon will result in significant increases in the solvent impurity co ntent of crystals at the high supersaturations normally used in the product ion of this material. The results of this study show well the dominant part that the growth mechanism can play in the definition of the morphology of crystals and hence the care which must be taken in the interpretation of mo deling calculations.