Competition between vitrification and crystallization of methanol at high pressure

Abstract

Competition between vitrification and crystallization of methanol at high pressure Brugmans, M.J.P.; Vos, W.L. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We have studied methanol at high pressure up to 33 GPa at room temperature with x-ray diffraction, optical ͑polarization͒ microscopy, Raman spectroscopy, and detection of hydrostaticity. A competition between crystallization and vitrification is observed when methanol is superpressed beyond the freezing pressure of 3.5 GPa: between 5.0 and 10.5 GPa crystals can nucleate, but if this region is surpassed quickly enough ͑within a few seconds͒, methanol remains amorphous. For the first time the nucleation rate and the crystal growth velocity have been studied as a function of pressure. These kinetic properties can be described by classical nucleation theory in agreement with, respectively, Turnbull-Fisher and Wilson-Frenkel type behavior using one and the same activated hard-sphere diffusion coefficient. The experimental nucleation rate and the crystal growth velocity are both effectively reduced to zero above 10.5 GPa, because the diffusion is suppressed. At these pressures methanol is compressed into a glass.

Numerical calculation of the rate of crystal nucleation in a Lennard-Jones system at moderate undercooling

“…Most experiments also indicate that the kinetic prefactor is significantly larger than predicted by classical nucleation theory. 1 However, it is interesting to note that in recent experiments by Brugmans et al 54 the opposite was found; a kinetic prefactor that is many orders of magnitude smaller than the estimate of classical nucleation theory.…”

Structure of crystalline methanol at high pressure

“…After the nucleation of many crystallites near 7 GPa, 5 the temperature was cycled at a pressure just above the melting curve to reduce the number of crystal grains, similar to Ref. 13.…”

“…The nucleation rate of crystals rises to a maximum near 7 GPa before vanishing at 10.5 GPa. 5,6 If the liquid is compressed sufficiently rapidly beyond 10.5 GPa, crystallization never happens and the liquid becomes a pressure induced glass. 5,6 Because the crystal structure at high pressures is unknown, there is no microscopic explanation why glass formation may occur.…”

“…5,6 If the liquid is compressed sufficiently rapidly beyond 10.5 GPa, crystallization never happens and the liquid becomes a pressure induced glass. 5,6 Because the crystal structure at high pressures is unknown, there is no microscopic explanation why glass formation may occur. For comparison, at ambient pressure methanol cannot be vitrified by rapid cooling of bulk liquid, but only by vapor deposition on a cold substrate ͑see, e.g., Ref.…”

“…3 A systematic study of hydrogen bonding is rewarding, because competition between various types of interactions causes phase transitions, including glass formation, and affects crystal nucleation. [4][5][6][7] Moreover, it has been proposed that with increasing pressure hydrogen bonding diminishes relative to the repulsive forces. 8 To develop a systematic understanding of hydrogen bonded molecular systems at high pressure, it is vital that a homologous series of chemical compounds is studied.…”