Instant crystallization temperature

Instant crystallization temperature (onset) for spray-dried and freeze-dried lactose as a function of water content, determined using DSC during heating at 5°C/min. 

Discussion/Colncidence of Crystallization Temperatures. Let us first consider the PVDF/PA-6 blend. In view of the nonaltered T of PVDF, we suppose that the PVDF crystallization induces the PA-6 crystallization rather than vice versa. Hence, the just created crystals of the PVDF matrix act as nucleating heterogeneity for the PA-6. The A y-value between PVDF crystals and PA-6 melt, obviously, is smaller than that of all other heterogeneities which are present in PA-6 to a sufficient extent except, possibly, the species "A". Its associated specific undercooling, moreover, must be so small that the PVDF crystals can induce the crystallization of the PA-6 from the instant of their own creation. 

When the blend is now further cooled, two possible ways of primary nucleation are possible. In a first case, the matrix phase is nucleated by heterogeneous species present in this phase and instantly, newly created crystals appear. Hence, the crystallization temperature of the matrix will be situated at its bulk T. A second possibility for coincident crystallization occurs in the case one finds again a single crystallization peak for the matrix phase, which however takes place above its bulk T. Some novel mutual nucleating mechanism was suggested in such blends a molten component (minor phase) acts as nucleating substrate for the matrix, which instantaneously crystallizes [Erensch and Jungnickel, 1989]. 

In order to guide design of solid dispersions, a simple supersaturation test/dissolution study for rank ordering of polymers and drug loading was reported without actually making solid dispersions (Konno et al. 2008). Dissolution of amorphous IMC at different temperatures (25 °C, 37 °C) in simulated intestinal fluid without pancreatin showed that IMC underwent solution-mediated transformation with higher concentration and lower temperature. The inclusion of polymer inhibited crystallization (Konno and Taylor 2008 Alonzo et al. 2009). For the amorphous felodipine, extensive crystallization was observed in 10-15 min at 25 °C, whereas instant crystallization was observed at 37 °C. A supersaturated solution was not formed when polymer was not included (Konno et al. 2008 Alonzo et al. 2009). 

There is a linear relationship between the crystallization temperatures and the inverse lamella thicknesses, which is quite in accordance with Gibbs-Thomson equation. There is also a linear relationship between the melting temperatures and the inverse lamella thicknesses. Crossover of these two linear curves is considered to be the triple point of mesophase transition. Recently, the crossover was reproduced in the molecular simulations of lattice polymers, and the interpretation was updated to an uplimit of instant thickening at the lateral growth front of lamellar crystals (Jiang et al. 2016). 

MnF4, known since 1928, has now been obtained in single crystals,740 which have a trigonal unit cell, but the structure has not yet been reported. The ultramarine blue solid is very hygroscopic, loses F2 slowly at normal temperatures and hydrolyses instantly in contact with water. 

Picking a Solvent. To pick a solvent for crystallization, put a few crystals of the impure solute in a small test tube or centrifuge tube and add a very small drop of the solvent. Allow it to flow down the side of the tube and onto the crystals. If the crystals dissolve instantly at room temperature, that solvent cannot be used for crystallization because too much of the solute will remain in solution at low temperatures. If the crystals do not dissolve at room temperature, warm the tube on the hot sand bath and observe the crystals. If they do not go into solution, add a drop more solvent. If the crystals go into solution at the boiling point of the solvent and then crystallize when the tube is cooled, you have found a good crystallization solvent. If not, remove the solvent by evaporation and try another solvent. In this trial-and-error process it is easiest to try low-boiling solvents first, because they can be removed most easily. Occasionally no single satisfactory solvent can be found, so mixed solvents, or solvent pairs, are used. 

Temperature changes—for example, warming a gas from 25°C to 50°C. As any sample is warmed, the molecules undergo more (random) motion hence entropy increases ( sys > 0) as temperature increases. Likewise, as we raise the temperature of a solid, the particles vibrate more vigorously about their positions in the crystal, so that at any instant there is a larger average displacement from their mean positions this results in an increase in entropy. 

If one melts a particular salt like sodium acetate and allows the molten matter to cool, it amazingly remains molten, even at room temperature, and does not crystallize. If one adds a little sodium acetate crystal into this molten mass, one instantly gets white salt crystals the heat of crystallization is immediately released, and the temperature is about 50°C (see E5.11). This reaction is used to produce so called pocket warmers . Following the crystallization, they are heated again to form the molten salt after cooling them to room temperature, they will deliver heat again. Precipitation reactions are also suitable for demonstrating the crystallization of certain types of ions (see E5.12). 

Observation The molten matter crystallizes instantly when the seeding crystal is added. The temperature of the crystalline mass rises during this procedure to more than 50°C. The test can be repeated through continued melting and cooling down. 

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