Experimental and computational morphology of three polymorphs of the free base of Venlafaxine: a comparison of morphology prediction methods.

In this paper the experimental and the computational studies of the morphology of three polymorphs of the free base of Venlafaxine ((N,N-dimethyl)-2-(1-hydroxy cyclohex-1-yl)-2-(4-methoxyphenyl) ethylamine) are reported. The morphology of all polymorphs has been predicted using the Bravais-Friedel-Donnay-Harker method, the attachment energy method and kinetic Monte Carlo growth simulations and these predictions have been compared with experimental observations. The Monte Carlo simulations allow for a detailed simulation of the growth process, including driving force and growth mechanism, which leads to a semi-quantitative prediction of the growth morphologies of all three phases. For phase I two distinct growth habits are found experimentally under the same conditions. This is explained by the occurrence of a spiral growth mechanism in one of the two, which was observed using AFM and which is also supported by the Monte Carlo simulations. The habit of phase II could only be explained from simulations when a spiral growth mechanism is assumed; the shape of phase III could not be modeled accurately from the Monte Carlo simulations. Although the shape of the crystal is reproduced accurately, some of the indices of the faces predicted are not in agreement with the indices measured. The deviations are interpreted to be due to the presence of domains in the crystals as a result of the layered structure.