The Implication of the Connected Net Topology on the Morphology of β-Monoacid Triacylglycerol Crystals

Abstract

The crystal morphology of 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.

On the prediction of crystal morphology. III. Equilibrium and growth behaviour of crystal faces containing multiple connected nets

“…In the case of the presence of a DOF phase, it appears that faces can have a very high growth rate and still be macroscopically¯at. Experimental examples of such growth behaviour were found for the (011) face of naphthalene (Grimbergen, Reedijk et al, 1998), the (110) faces of orthorhombic n-paraf®ns (Grimbergen, van Hoof et al, 1998), the top faces of triacylglyceride (fat) crystals (Hollander et al, 1999) and the (110) faces of lysozyme . In this paper, it is shown that the BFDH and attachment-energy predictions are not generally applicable as the predictive value seems to be very poor even for many practical crystal graphs, when compared with the results of a complete connected-net analysis and MC simulation data.…”

Needle Crystal Morphology Explained

“…The 16.16.16 single crystals appear in the spherulites as long flat faceted needles with aspect ratios up to 500 and the 10.12.10 crystals as even narrower needles having aspect ratios up to 1000. In a recent paper the morphology of b-monoacid triacylglycerol (TAG) crystals was treated on the basis of a detailed connected net analysis [9]. In this paper the morphology of the crystals of n.n.n TAGS, with n even, was studied using a two dimensional (2D)-nucleation mechanism, taking into account the edge energies involved for the creation of such nuclei on the various crystal faces.…”

“…To find all connected nets giving rise to the possible flat (F-) faces the crystal graph representing all growth units (TAG molecules) and their mutual bonds is needed. For a discussion on the relevant bonds in case of TAG crystals we refer to Hollander et al [9]. In that paper it was shown that in order to predict the correct morphology for the b-TAGS merely a limited number of bonds between the growth units has to be considered.…”

“…The bond strengths are calculated using force fields. The method and force field applied to calculate the bond energies are similar to the ones used for the calculation of the bonds for the b-n.n.n structures [9].…”

“…For an explanation of the elongated morphology of the b-TAGS we refer to Hollander et al [9]. For the b¢-10.12.10-TAG the far more elongated morphology as compared to the b-TAGS was very recently explained by a zero edge energy for the {112} faces [12].…”