The morphology of monoclinic paracetamol crystals has been investigated both theoretically and experimentally. Calculations using the computer program HABIT 95 with both DREIDING II and MOMANY force fields predict prismatic forms in which {100}, {001}, {110}, and {201̄} show approximately equivalent morphological importance. Whereas all of these faces are observed experimentally, the real crystals showed a {110} dominance at low supersaturations which gave way to an increasing {001} dominance as the supersaturation increased. This variation was accompanied by a change from a columnar to a platelike 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 vertexes of the {110} faces with the operation of dislocation sources at the face center. The increasing dominance of the two-dimensional growth contribution at the highest supersaturations coupled with an increase in macrostep formation resulted in the development of inclusions in the {110} sectors. This phenomenon will result in significant increases in the solvent impurity content of crystals at the high supersaturations normally used in the production 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 modeling calculations.

When dissolved in aqueous solution, R-lactose, whether originally in the anhydrous or the monohydrated form, readily undergoes mutarotation to yield the β isomer. At equilibrium, which is reached in 6.5 h at 292 K and more rapidly with increasing temperatures, the latter is present to the extent of ca. 60% w/w. At saturation, R-lactose hydrate precipitates from this solution being considerably less soluble than the β form. The resulting crystals have a highly asymmetric, tomahawk shaped morphology characterized by large {01 h1} faces. It is confirmed that this shape arises from growth inhibition due to the incorporation of the β isomer in the {01 h1} sectors of the crystals coupled with a characteristic lack of growth in the [01 h0] direction. Growth under conditions that restrict the formation of the β isomer yields more symmetrically shaped, needlelike crystals. These show a gradual transition to the tomahawk shape as the concentration of the β isomer in solution and hence in the crystal is increased. An assessment is made of the degree of incorporation of the β isomer into the crystal, its distribution and its influence on the defect structure of the material.