Controlling crystallisation and the effect of interfaeial curvature

Abstract

Crystallisation induced at the air-water planar interface and oil-water interface in emulsions is explored. DL-Aspartic acid crystallisation under nylon 6 spread films is studied by neutron reflectivity and ER-FTIR, and shows that the resulting crystallisation occurs within, rather than underneath the spread film layer. It is shown that 50% of the nylon 6 surface layer is composed of DL-aspartic acid, despite only 1-5% being visible to the naked eye. This is attributed to sub-visible crystalline nuclei embedded in the nylon 6 film layer, whose growth to visible dimensions is impeded by the surrounding nylon 6 film. Calcium carbonate crystallisation induced by octadecanoic acid monolayers at moderate surface pressure is studied by GDCD and ER-FTIR and shows a tilting of the monolayer hydrocarbon chain towards the surface normal as crystallization progresses. Addition of polyacrylic acid to the subphase results in formation of an amorphous calcium carbonate film under the monolayer, with no corresponding tilt of the monolayer hydrocarbon chain observed. The crystallisation of ice induced by 1-heptacosanol at the oil-in-water (o/w) interface in emulsions is studied and shows that a high curvature interface is a less effective ice nucleator than the low curvature systems. A much better ice nucleating ability is even observed when the interfacial density of 1-heptacosanol is significantly less for the low curvature systems. This is attributed to the larger critical nucleus required for crystallisation induced at a high curvature interface and a poorer ability of the I-heptacosanol to pack in a way that favours ice nucleation.Glycine crystallisation in emulsions is shown for o/w and water-in oil (w/o) cases. Crystallisation in the o/w samples using Span surfactants typically show β-phase spherical crystal aggregates, whereas the o/w show β-phase crystals with elongated morphology. Systems prepared at the phase inversion boundary typically showed a poorer nucleating ability than analogous w/o and o/w emulsion systems. The use of water-soluble additives failed to alter the glycine polymorph, but use of octanoic acid did promote a glycine crystallisation and an unusual honeycomb structure due to the aggregation of octanoic acid attracted to the glycine crystals.