Isothermal thickening of single crystals of C294H590 in dilute solution

Crystals of the long, uniform alkane, C294H590 can be grown from dilute sol utions in a range of different integer folded forms. At suitable temperatur es, folded crystals will thicken from one folded form to another. The morph ology of the crystals as they grow depends both on the growth temperature a nd on the number of folds in the crystal. In this paper we report, for the first time, observations on the morphologies of crystals that have been gro wn, and allowed to thicken to varying extents, in DSC pans. From the DSC we have precise knowledge of the overall degree of crystallinity and extent o f thickening that has occurred over the whole sample. On thickening from twice-folded to once-folded crystals, the initial crysta l shape is lost and a fine, dendritic structure results, in which the thick ened regions exist in a surrounding matrix of unthickened material; the thi ckened regions have a characteristic width of 20-30 nm. It is suggested tha t this characteristic width is caused by the interplay of solid state diffu sion and the lattice strain caused by the reduction in top surface area. At lower temperatures, when thickening occurs from three times folded to twic e-folded forms, the initial lozenge shape of the three-times folded crystal s is maintained after thickening, forming "picture-frame" crystals, but wit h a fine web of thickened material within the frame. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.