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The Dual Nature of Crystallization
August 29, 2015   
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Scientists from the Polish Academy of Sciences’ Institute of Nuclear Physics in Cracow have discovered that, contrary to previous research, the physical process of crystallization is governed not solely by the laws of thermodynamics, but also by the diffusion phenomenon.

Organic substances present in pharmaceuticals should in general be in glassy form, the scientists say. But as a result of long-term storage these compounds may crystallize, and the drug loses its therapeutic properties. In order to better understand the fundamental processes that occur during such transformations, the Cracow scientists conducted a series of studies on the phenomenon.

“Crystallization is most often associated with the process of cooling. However, we know of another, quite counter-intuitive, phenomenon: some compounds under certain conditions begin to crystallize as temperatures rise,” explains Maria Massalska-Arod¼ from the Institute of Nuclear Physics. “This effect is known as cold crystallization, and we observed it nearly 20 years ago in liquid crystal substances that have a tendency to vitrify (transform into glass).”

Liquid crystals are liquids whose elongated molecules have a kind of ordering generally found in ordinary crystals. The simplest nematic liquid crystal can flow, but its molecules are oriented in one direction spatially, yet randomly distributed, in contrast to ordinary crystals.

The scientists used digital image processing techniques to assess changes in the arrangement of elongated molecules in one type of liquid crystal (4CFPB) when they were heated. They easily underwent vitrification in the nematic phase. The researchers reported their results in the Crystal Growth & Design journal.

"Selecting nematic liquid crystals as an object of the study was not accidental,” says Tomasz Rozwadowski, a Ph.D. student from the Institute of Nuclear Physics. “When we view them using a polarizing microscope, they show multicolored areas arranged in a characteristic texture. We wanted to see if the graphic evaluation of changes in the texture during the heating of the nematic glassy phase might reveal valuable information about the process of crystallization.”

The physicists from the Institute of Nuclear Physics performed a series of experiments looking at changes occurring in liquid crystal as the temperature rose.

The degree of crystallization obtained by numerical analysis of polarizing microscope images was compared with data collected at the same time with two conventional methods, calorimetery and dielectric spectroscopy. The results proved to be consistent, confirming the usefulness of the graphic evaluation of texture changes in the study of crystallization and allowing the scientists to get a closer insight into the process.

The data collected by the physicists showed the existence of two different mechanisms responsible for crystallization in the studied material. In experiments in which the temperature rose by more than 8 degrees Kelvin per minute, crystallization proceeded in accordance with classical thermodynamic predictions. However, when the sample was heated at a slower rate than 8 degrees Kelvin per minute, it was diffusion—or the intermingling of molecules and their movement from a region of high concentration to a region of low concentration—resulting from the mobility of the molecules that was chiefly responsible for the crystallization process. This had an influence on the temperature of crystallization: when the process was that of diffusion, the temperature was significantly lower because crystallization required less energy, the scientists concluded.

“If cold crystallization was a phenomenon governed solely by thermodynamics, it would be enough to keep the temperature just below 275 degrees [Kelvin] in order to safeguard the substance against uncontrolled transformation,” says Massalska-Arod¼. “Now we know that sometimes the process of diffusion of molecules begins to play a decisive role. Cold crystallization can therefore occur in a fairly wide range of temperatures, under conditions that were previously unexpected."

The Cracow Institute of Nuclear Physics has made some important contributions to international research on crystals. Polish scientist Jan Czochralski discovered a method for the production of monocrystals in 1916 that laid the foundations for the modern electronics industry. The Czochralski process is a method of crystal growth to obtain single crystals of semiconductors.

The Henryk Niewodniczański Institute of Nuclear Physics conducts basic and applied research in such areas as particle physics and astrophysics, hadron physics, high-, medium-, and low-energy nuclear physics and condensed matter physics (including materials engineering). The institute is involved with various applications of methods of nuclear physics in interdisciplinary research, covering medical physics, dosimetry, radiation and environmental biology, environmental protection and other related disciplines.
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