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Crystals 2017, 7(1), 11; doi:10.3390/cryst7010011

From Rate Measurements to Mechanistic Data for Condensed Matter Reactions: A Case Study Using the Crystallization of [Zn(OH2)6][ZnCl4]

Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA
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Academic Editors: Helmut Cölfen and Semën Gorfman
Received: 31 August 2016 / Revised: 29 October 2016 / Accepted: 23 December 2016 / Published: 31 December 2016
(This article belongs to the Special Issue Development of Time-Resolved X-Ray Crystallography)
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Abstract

The kinetics of crystallization of the R = 3 hydrate of zinc chloride, [Zn(OH2)6][ZnCl4], is measured by time-resolved synchrotron x-ray diffraction, time-resolved neutron diffraction, and by differential scanning calorimetry. It is shown that analysis of the rate data using the classic Kolmogorov, Johnson, Mehl, Avrami (KJMA) kinetic model affords radically different rate constants for equivalent reaction conditions. Reintroducing the amount of sample measured by each method into the kinetic model, using our recently developed modified-KJMA model (M-KJMA), it is shown that each of these diverse rate measurement techniques can give the intrinsic, material specific rate constant, the velocity of the phase boundary, vpb. These data are then compared to the velocity of the crystallization front directly measured optically. The time-resolved diffraction methods uniquely monitor the loss of the liquid reactant and formation of the crystalline product demonstrating that the crystallization of this hydrate phase proceeds through no intermediate phases. The temperature dependent vpb data are then well fit to transition zone theory to extract activation parameters. These demonstrate that the rate-limiting component to this crystallization reaction is the ordering of the waters (or protons) of hydration into restricted positions of the crystalline lattice resulting in large negative entropy of activation. View Full-Text
Keywords: time-resolved diffraction; crystallization; phase transition; kinetics; mechanism; crystalline hydrate time-resolved diffraction; crystallization; phase transition; kinetics; mechanism; crystalline hydrate
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Hillis, B.G.; Losey, B.P.; Weng, J.; Ghaleb, N.; Hou, F.; Martin, J.D. From Rate Measurements to Mechanistic Data for Condensed Matter Reactions: A Case Study Using the Crystallization of [Zn(OH2)6][ZnCl4]. Crystals 2017, 7, 11.

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