Patterned glycine crystals nucleated on functionalized metallic square islands. This approach can be used to fabricate particles with micron dimensions and screen solid forms under different conditions. The size of the glycine crystals is controlled by the dimensions of the islands. High energy metastable beta-glycine crystallizes on small metallic islands, whereas for large islands, the polymorphic outcome becomes biased toward the alpha-form.
Hollow needles can be produced by immersing certain hydrate crystals in alcohol solution (methanol or ethanol). The fact that these anhydrate needles are hollow was proven by confocal fluorescence microscopy. For one material (sodium 2-keto-l-gulonate anhydrate) it is shown here that the internal diameter can be controlled by changing the water content in the initial methanol solution. Interestingly, the internal diameter of sodium 2-keto-l-gulonate anhydrate needles can be decreased from about 4 μm to approximately 500 nm (ca. 88%) by increasing the water content in the alcohol phase.
Crystal growth is a process that only takes place under non-equilibrium conditions and a necessary prerequisite is that the crystal is exposed to a phase that is supersaturated in the material the crystal is composed of, be it a solution, a vapour or a supercooled melt. In industrial mass crystallization the growth rate for a population of crystals (in suspension growth processes [1]) rarely exceeds mean linear velocities of 10 -7 ms -1 . Here we present a mass crystallization process which is accompanied by rapid crystal growth several orders of magnitude faster and into a region of solution that is without inherent supersaturation. The material investigated is a solid hydrate that exhibits a solution mediated phase transition to its anhydrous form in the presence of methanol [2]. The phase transition is initiated simply by placing an amount of hydrate crystals into the solvent and is characterized by the rapid emergence of needle-shaped crystals. The needles emanate from the crystal faces of the hydrate crystals and grow into the solution, which is nominally free of the substance to be crystallized. The high growth rate of the crystals, which of the order of up to 10 -4 ms -1 is surprising. Although rapid needle growth has been observed before [3][4][5][6][7][8][9], to date a satisfactory explanation for needles growing under the abovementioned conditions is still outstanding. Based upon the topology of the crystals we propose a tentative mechanism for this phenomenon capable of explaining the unusually rapid growth and highlight those questions that need addressing in order to verify this mechanism. X-ray powder diffraction is used to characterize the crystal phase of the needles; confocal fluorescence microscopy reveals that the needles are hollow. The width of these needles is between 0.5 and 5 µm, their length appears to be limited only by the amount of hydrate available for their formation.
Freeze concentration is a method used to remove pure water from juice, wine, or beer by means of crystallization. The closed and fully filled liquid system completely preserves the quality of the fresh juice while operating at subzero Celsius temperatures. The aroma (including the volatile aroma that characterizes freshly squeezed juice), color, and flavor remain in the concentrated juice. Another advantage of this proven technology is the low operating temperature used that allows concentration of heat sensitive products such as black currant juice. The quality of freeze concentrated products cannot be achieved by any other technology. The solid concentration of the freeze concentrated black currant juice was increased to 39 Degrees Brix (Bx). It will be shown that the quality of this juice is equal to that of the fresh juice in all aspects. None of the aroma or fruitiness is lost.
The water activities at the transition point of ten inorganic hydrates were determined by different techniques. The techniques used were compared and evaluated aiming to identify which one is the most efficient. It could be shown that measuring the relative humidity is an easy, accurate, direct, and low-priced method and as such more efficient than other methods tested (measuring the density, hardness, and heat of fusion). IntroductionCrystals incorporating different amounts of solvent (water) in their lattice are solvates (hydrates) also frequently referred to as pseudopolymorphs. Depending on the thermodynamic conditions (temperature, pressure, relative humidity or solvent ("solvent mediated phase transformation" [1]), phase transformations can occur in crystals which are not in the thermodynamically stable phase. Solvates are mostly enantiotropic systems, meaning that the two stable phases can exist simultaneously under certain conditions, at the so-called transition point.In industrial processes and for pharmaceutical products it is extremely significant to know or to promote a favored modification as well as the conditions of phase transformations because various modifications of a product exhibit different color, crystal shape, density, solubility, dissolution rate, hardness and shelf life.Until now, there has been no model or tool for predicting different modifications. So, there is a need to develop a method/model of how to predict the transformation behavior at the transition point of such substances.The dehydration process of hydrates (phase transformation of a hydrate to an anhydrate or a reduced hydrate) is represented by:with Dn as the number of water molecules being released by dehydration (Dn = n complete dehydration).1)The equilibrium constant for dehydration is:K only denotes the water activity, a, because the hydrate and anhydrate are in the solid state, meaning that their activities are equal to one [2].Thus, water activity can be calculated as follows:Here, DG r 0 is the free standard reaction enthalpy, R is the ideal gas constant, and T is the temperature. DG r 0 can be calculated by Eq. (5), where DH r 0 is the standard reaction enthalpy and DS r 0 the standard reaction entropy.Consequently, it is possible to characterize the transition point in hydrate systems by water activity (activity of the bound solvent) (see Eq. (3)).Water activity is an important parameter that might be used to predict phase transformations and the stability of a product and to define optimal conditions for different modifications and their various physical properties. -1) List of symbols at the end of the paper.
Immersing a crystalline solvate in a suitable anti-solvent can induce phase transformation to solvent-free solid phase. In certain cases the solvent-mediated phase transition results in the generation of hollow, tubular structures. Both the tube dimensions of sodium-2-keto-L-gulonate anhydrate (skga) and the dehydration kinetics of sodium-2-keto-L-gulonate monohydrate (skgm) can be modified by the antisolvent employed. An explanation for the variable dehydration behaviour of skgm in the antisolvents is presented here. Furthermore, other crystalline hydrates were dehydrated in dry methanol. Providing an operational window can be found, any hydrate material could possibly find use in the production of tubes (micro-or nanotubes for different applications). The experimental conditions selected (dry methanol as antisolvent, dehydration temperature at 25 °C) for the dehydration did not lead to the anhydrate tube growth for all hydrates investigated. Based upon the results presented here a first hypothesis is presented to explain this effect.
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