ON THE MORPHOLOGY OF NORMAL ALKANE CRYSTALS WITH MONOCLINIC STRUCTURES - THEORY AND OBSERVATIONS

The morphology of crystals of monoclinic even n-alkanes over a chain-length range 10 less-than-or-equal-to m less-than-or-equal-to 40 is investigated. The overall van der Waals interactions between the paraffin molecules are calculated from the structure parameters using the Buckingham potential. Subsequently, a crystal graph is obtained, from which 12 connected nets are derived. To describe the morphology, the Hartman-Perdok theory, the theory of the roughening transition and the Donnay-Harker theory are applied to the system. The results are compared with each other. The influence of the chain lengths of paraffin molecules on the Ising temperatures and on the morphology is analysed. Experimentally, it can be seen that the predicted growth morphology of the crystals is in good agreement with the observed one. In addition, the roughening temperature of the {110} faces, for crystals of n-C32H66 and n-C28H58 grown from n-hexane solutions, are determined. They are about 306 (3) and 297 (5) K, respectively. Morphological transitions around the roughening temperature are also observed. It is found that an unusual morphological phenomenon occurs on long-chain paraffin crystals at low supersaturations. This is interpreted as the influence of solvent adsorption at the roughened faces.