Abstract:
The crystal morphology of beta n.n.n TAGs, still the only triacylglycerol
series of which the crystallographic structures have been elucidated, is
predicted
using a revised version of the Hartman-Perdok theory. In an earlier
publication by Bennema et al.(1) the morphology was predicted on the basis
of
attachment energy. The platelike shape of the crystals was predicted
well, but the indices for the top faces failed. Second, the experimentally
observed
crystals are much more elongated. In the present approach, the morphology
is not only predicted using the attachment energy but also use is made
of
roughening: temperatures. For faces that grow below the roughening
temperature at low supersaturation, the rate-limiting step is the formation
of steps on
the surface. Therefore, under these growth conditions, the roughening
temperatures of the faces determine the morphology rather than the attachment
of
new growth units to the surfaces. The roughening temperature of a flat
face {hkl} can be estimated by the Ising temperatures, which are calculated
from
substitute nets constructed from all possible step energies found from
the connected nets. In some cases, depending on the topology of the connected
net,
exceptionally low Ising temperatures can be found. In these cases,
it is shown that the roughening temperature is not directly determined
by the slice
energy. If this concept is used for the prediction of the morphology
of TAG crystals, a good agreement with the experimentally observed faces
is obtained. Moreover, the dependence of the morphology for the various
TAGs on the chain length n is explained. Using this concept, however, the
presence of the
{31l} faces on the morphology cannot be explained. It is shown that
this face is probably an F-face that is stabilized by solvent molecules.