engleski

In-situ non-ambient x-ray diffraction analysis of oxitropium bromide and derivates.

Thermosalient materials are the ones that during heating/cooling undergo an energetic phase transition which is so sudden and abrupt that the crystals are ballistically projected to heights of several hundred times larger than their own dimensions. Apart from providing visually extremely attractive phenomenon, these materials have a tremendous technological potential as the future self-actuation device (nanoswitches, thermal sensors, artificial muscles, etc.)1. In situ X-ray analysis, often referred to as analysis at non- ambient conditions allow to study and investigate any macroscopic property of a material that is directly related to its structural property (e.g. crystallographic symmetry, crystallite size, vacancies, size and shape of nanoparticles or pores). Temperature, pressure, gas atmosphere and mechanical stress trigger phase transformation, chemical reactions, crystal proprieties and so on. Here we present a systematic experimental study of the thermosalient effect in Oxitropium bromide and Scopolamine methyl bromide with in-situ non- ambient x-ray diffraction measurements (in the range from 100K to 500K at low vacuum conditions), which enable us to elucidate the thermosalient phenomenon in this system. Oxitropium bromide and methylscopolamine bromide have very similar molecular structures, the only difference being that one ethyl group is replaced by methyl group in the case of methylscopolamine bromide. Both compounds have medical uses, oxitropium bromide is used as a bronchodilator, whereas methylscopolamine bromide used to prevent nausea and vomiting caused by motion sickness. They also both exhibit thermosalient effect – unexpected and abrupt jumping of the crystals during heating and cooling. Both systems are characterized by uniaxial negative thermal expansion, but there is an abrupt change of the unit cell parameters during the phase transition for the oxitropium bromide whereas the parameters are changing perfectly linearly in the whole temperature range of existence for methylscopolamine bromide. As in most of the thermosalient systems, immense negative thermal expansion seems to be the most likely candidate for the driving force behind this phenomenon. Therefore, the question remains – what is causing the jumping in methylscopolamine bromide – a system so close to oxitropium bromide but exhibiting totally different kind of thermosalient effect.

molecular crystals ; thermosalient materials ; thermal axpansion ; crystal polymorphic phase transition

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