No abstract

In this review, the delicate interplay between stereochemical control, monitoring at the subnanometer level, and an understanding of crystal nucleation is probed. Control of crystal nucleation may be achieved employing tailor-made auxiliaries, which are either nucleation inhibitors or promoters. The process may be monitored at an interface via grazing incidence X-ray diffraction (GIXD). By these means, we can glean experimental knowledge of crystal nucleation in various molecular systems. A hypothesis was invoked that supersaturated solutions containing molecular clusters adopt various arrangements and shapes, some of which resemble the crystals into which they develop. This hypothesis was taken advantage of for the design of tailored inhibitors in achieving kinetic resolution of enantiomers and induced precipitation of particular crystal polymorphs. The control and behavior of polymorphic crystallization may be understood at the molecular level through the interplay between inhibitor, solvent, solute, crystal and layer lattice energies, as well as surface layer structures. With respect to promotion of crystal nucleation, it may be achieved by Langmuir monolayers at the air−aqueous solution interface, acting as a templating agent. Determination of the monolayer crystal structure by GIXD yields the extent and nature of the complementary fit between nucleator and nucleant. Finally, GIXD was applied to monitor by a snapshot technique the layer-by-layer crystalline assembly of cholesterol molecules at the air−water interface, which involved changes in molecular packing as the film grew in thickness.

Nucleation, growth, and dissolution of crystals have been studied by stereochemical approach involving molecular recognition at interfaces. A methodology is described for using ;;tailor-made'' additives designed to interact stereospecifically with crystal surfaces during growth and dissolution. This procedure was instrumental in controlling crystal morphology and in revising the concept of the structure and symmetry of solid solutions. Consequently, it was applied to the transformation of centrosymmetric single crystals into solid solutions with polar arrangement displaying second-harmonic generation and to the performance of asymmetric synthesis of guest molecules inside centrosymmetric host crystals. The method has led to a discovery of a new ;;relay'' mechanism explaining the effect of solvent on crystal growth. Finally, it allowed for the design of auxiliary molecules that act as promoters or inhibitors of crystal nucleation that can be used to resolve enantiomers and crystallize desired polymorphs.

No abstract

No abstract

Although ice melts and water freezes under equilibrium conditions at 0 degrees C, water can be supercooled under homogeneous conditions in a clean environment down to -40 degrees C without freezing. The influence of the electric field on the freezing temperature of supercooled water (electrofreezing) is of topical importance in the living and inanimate worlds. We report that positively charged surfaces of pyroelectric LiTaO3 crystals and SrTiO3 thin films promote ice nucleation, whereas the same surfaces when negatively charged reduce the freezing temperature. Accordingly, droplets of water cooled down on a negatively charged LiTaO3 surface and remaining liquid at -11 degrees C freeze immediately when this surface is heated to -8 degrees C, as a result of the replacement of the negative surface charge by a positive one. Furthermore, powder x-ray diffraction studies demonstrated that the freezing on the positively charged surface starts at the solid/water interface, whereas on a negatively charged surface, ice nucleation starts at the air/water interface.

Monolayers of aliphatic long-chain alcohols induced nucleation of ice at temperatures approaching 0 degrees C, in contrast with water-soluble alcohols, which are effective antifreeze agents. The corresponding fatty acids, or alcohols with bulky hydrophobic groups, induce freezing at temperatures as much as 12 degrees C lower. The freezing point induced by the amphiphilic alcohols was sensitive not only to surface area per molecule but, for the aliphatic series (C(n)H(2n + 1)OH), to chain length and parity. The freezing point for chains with n odd reached an asymptotic temperature of 0 degrees C for an upper value of n = 31; for n even the freezing point reached a plateau of -8 degrees C for n in the upper range of 22 to 30. The higher freezing point induced by the aliphatic alcohols is due to formation of ordered clusters in the uncompressed state as detected by grazing incidence synchrotron x-ray diffraction measurements. The diffraction data indicate a close lattice match with the ab layer of hexagonal ice.

Dedicated to Professor J. Michael McBride on the occasion of his 65th birthdayCrystal polymorphism, which embodies the ability of molecules to form diverse packing arrangements displaying different physical and chemical characteristics, is of paramount importance in fields such as pharmacology, solid-state chemistry, and material science.[1] However, the conditions to induce the precipitation of various (metastable) polymorphs is invariably achieved by "mix and try" methods, which are kinetically driven. Various factors should be considered in trying to understand these complex processes, for example, the formation of structured clusters in solution prior to crystallization, the structure of growing surfaces that delineate emerging nuclei, the interaction between these surfaces and the solvent, as well as solvent-solute and solutesolute interactions. Herein, we attempt to unravel some of these factors to rationalize the preferred crystallization of the b form of glycine (gly) in water-alcohol solutions as opposed to the more stable a or g polymorphs.The thermodynamic stability of the three polymorphs of glycine at room temperature is in the order g > a > b.[2-5] The a form [6] (space group P2 1 /n), grown from supersaturated aqueous solutions (33.3 g/100 mL water) at 25 8C, has a bipyramidal habit and is composed of centrosymmetric bilayers formed by strong NH···O hydrogen-bonding interactions between cyclic hydrogen-bonded zwitterionic molecular pairs. These bilayers are related along the b axis by glide symmetry through weak CH···O interactions (Figure 1 a). Previous studies indicated that a-gly crystallizes primarily in aqueous solutions through hydrogen-bonded cyclic dimer growth units: Diffusion-coefficient [7] measurements of supersaturated aqueous solutions of glycine point to the formation of clusters with an average of 1.8 molecular growth units. Furthermore, atomic force microscopy (AFM) and phase interferometry microscopy measurements established that steps approximately 1 nm in size were formed, which correspond to the thickness of a glycine bilayer. [8,9] Grazingincidence X-ray diffraction studies on growing a-gly {010}