D.M. Kaminski

Thickness-dependent ordering of water layers at the NaCl(100) surface

We have determined the ordering properties of water adsorbed at room temperature on the rock salt (100) surface under four different conditions: ultrahigh vacuum, dry nitrogen atmosphere, 45% and 75% relative humidity. Details of the atomic structure are determined for both sides of the solid-liquid interface. The top most layer of NaCl shows a small relaxation that changes from an expansion to a contraction with increasing humidity. Under all measured conditions water monolayers with different ordering properties are present at the interface. Surprisingly, we find that the amount of ordering in the first layer is increasing with increasing thickness of the water film. At a solid-liquid interface, the ordering appears to be correlated with the solubility.

Stability of the polar {111} NaCl crystal face

We present a surface x-ray diffraction determination of the {111} NaCl-liquid interface structure. Using ultrathin water or formamide liquid layers we ascertained that the crystal surface is smooth at an atomic level and is not reconstructed. Our results reveal surprisingly small differences in surface structure between the two cases, which nevertheless lead to dramatic differences in crystal morphology. We determined that the rocksalt {111} surface is Na(+) terminated for both environmental conditions. A quarter to half a monolayer of laterally disordered Cl(-) ions is located on top of a fully ordered Na(+) crystal surface with occupancy 0.75-1.0. This means that the polar surface is stabilized through the formation of an electrochemical double layer.

Growth and morphology of thin fat crystals

The experimental and theoretical morphology of fat crystals under non-diffusion limited conditions is reviewed. In addition, recent work on the effect of diffusion limited growth is reported. Growth rate versus supersaturation curves of various flat-needle shaped triacylglycerol crystals grown from solution or melt are presented. In most cases these curves exhibit a transition from a non-linear to a linear regime. In the linear regime, the edges and corners of the flat-neeedle shaped crystals are often determined by diffusion. Due to the high anisotropy of fat crystals, the morphologically unstable patterns observed above the transition, appear as non-faceted or faceted dagger-like shapes, depending whether the top faces are kinetically roughened or not. For tripalmitate grown from trioleate, flat faces are still observed above the transition, despite completely linear kinetics. Only at even larger supersaturation these crystal faces become unstable, showing that flat faces can still be obtained under diffusion limited conditions.

Towards an atomic-scale understanding of crystal growth in solution

Our understanding of crystal growth continues to increase thanks to progress in theoretical models, computer simulations and experimental techniques. A discussion of the state-of-the-art in morphology prediction and of the determination of the solid-liquid interface structure using X-ray diffraction shows, however, that there is still a large gap between experiment and theory. We expect that computer modelling, in the form of both Molecular Dynamics simulations and first-principle calculations, will play a crucial role in filling this gap.