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Glass polymorphism in glycerol–water mixtures: I. A computer simulation study

We perform out-of-equilibrium molecular dynamics (MD) simulations of water–glycerol mixtures in the glass state. Specifically, we study the transformations between low-density (LDA) and high-density amorphous (HDA) forms of these mixtures induced by compression/decompression at constant temperature....

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Autores principales: Jahn, David A., Wong, Jessina, Bachler, Johannes, Loerting, Thomas, Giovambattista, Nicolas
Formato: Online Artículo Texto
Lenguaje:English
Publicado: Royal Society of Chemistry 2016
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4847106/
https://www.ncbi.nlm.nih.gov/pubmed/27063705
http://dx.doi.org/10.1039/c6cp00075d
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author Jahn, David A.
Wong, Jessina
Bachler, Johannes
Loerting, Thomas
Giovambattista, Nicolas
author_facet Jahn, David A.
Wong, Jessina
Bachler, Johannes
Loerting, Thomas
Giovambattista, Nicolas
author_sort Jahn, David A.
collection PubMed
description We perform out-of-equilibrium molecular dynamics (MD) simulations of water–glycerol mixtures in the glass state. Specifically, we study the transformations between low-density (LDA) and high-density amorphous (HDA) forms of these mixtures induced by compression/decompression at constant temperature. Our MD simulations reproduce qualitatively the density changes observed in experiments. Specifically, the LDA–HDA transformation becomes (i) smoother and (ii) the hysteresis in a compression/decompression cycle decreases as T and/or glycerol content increase. This is surprising given the fast compression/decompression rates (relative to experiments) accessible in MD simulations. We study mixtures with glycerol molar concentration χ (g) = 0–13% and find that, for the present mixture models and rates, the LDA–HDA transformation is detectable up to χ (g) ≈ 5%. As the concentration increases, the density of the starting glass (i.e., LDA at approximately χ (g) ≤ 5%) rapidly increases while, instead, the density of HDA remains practically constant. Accordingly, the LDA state and hence glass polymorphism become inaccessible for glassy mixtures with approximately χ (g) > 5%. We present an analysis of the molecular-level changes underlying the LDA–HDA transformation. As observed in pure glassy water, during the LDA-to-HDA transformation, water molecules within the mixture approach each other, moving from the second to the first hydration shell and filling the first interstitial shell of water molecules. Interestingly, similar changes also occur around glycerol OH groups. It follows that glycerol OH groups contribute to the density increase during the LDA–HDA transformation. An analysis of the hydrogen bond (HB)-network of the mixtures shows that the LDA–HDA transformation is accompanied by minor changes in the number of HBs of water and glycerol. Instead, large changes in glycerol and water coordination numbers occur. We also perform a detailed analysis of the effects that the glycerol force field (FF) has on our results. By comparing MD simulations using two different glycerol models, we find that glycerol conformations indeed depend on the FF employed. Yet, the thermodynamic and microscopic mechanisms accompanying the LDA–HDA transformation and hence, our main results, do not. This work is accompanied by an experimental report where we study the glass polymorphism in glycerol–water mixtures prepared by isobaric cooling at 1 bar.
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spelling pubmed-48471062016-05-03 Glass polymorphism in glycerol–water mixtures: I. A computer simulation study Jahn, David A. Wong, Jessina Bachler, Johannes Loerting, Thomas Giovambattista, Nicolas Phys Chem Chem Phys Chemistry We perform out-of-equilibrium molecular dynamics (MD) simulations of water–glycerol mixtures in the glass state. Specifically, we study the transformations between low-density (LDA) and high-density amorphous (HDA) forms of these mixtures induced by compression/decompression at constant temperature. Our MD simulations reproduce qualitatively the density changes observed in experiments. Specifically, the LDA–HDA transformation becomes (i) smoother and (ii) the hysteresis in a compression/decompression cycle decreases as T and/or glycerol content increase. This is surprising given the fast compression/decompression rates (relative to experiments) accessible in MD simulations. We study mixtures with glycerol molar concentration χ (g) = 0–13% and find that, for the present mixture models and rates, the LDA–HDA transformation is detectable up to χ (g) ≈ 5%. As the concentration increases, the density of the starting glass (i.e., LDA at approximately χ (g) ≤ 5%) rapidly increases while, instead, the density of HDA remains practically constant. Accordingly, the LDA state and hence glass polymorphism become inaccessible for glassy mixtures with approximately χ (g) > 5%. We present an analysis of the molecular-level changes underlying the LDA–HDA transformation. As observed in pure glassy water, during the LDA-to-HDA transformation, water molecules within the mixture approach each other, moving from the second to the first hydration shell and filling the first interstitial shell of water molecules. Interestingly, similar changes also occur around glycerol OH groups. It follows that glycerol OH groups contribute to the density increase during the LDA–HDA transformation. An analysis of the hydrogen bond (HB)-network of the mixtures shows that the LDA–HDA transformation is accompanied by minor changes in the number of HBs of water and glycerol. Instead, large changes in glycerol and water coordination numbers occur. We also perform a detailed analysis of the effects that the glycerol force field (FF) has on our results. By comparing MD simulations using two different glycerol models, we find that glycerol conformations indeed depend on the FF employed. Yet, the thermodynamic and microscopic mechanisms accompanying the LDA–HDA transformation and hence, our main results, do not. This work is accompanied by an experimental report where we study the glass polymorphism in glycerol–water mixtures prepared by isobaric cooling at 1 bar. Royal Society of Chemistry 2016-04-28 2016-04-11 /pmc/articles/PMC4847106/ /pubmed/27063705 http://dx.doi.org/10.1039/c6cp00075d Text en This journal is © The Royal Society of Chemistry 2016 http://creativecommons.org/licenses/by-nc/3.0/ This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial 3.0 Unported License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
spellingShingle Chemistry
Jahn, David A.
Wong, Jessina
Bachler, Johannes
Loerting, Thomas
Giovambattista, Nicolas
Glass polymorphism in glycerol–water mixtures: I. A computer simulation study
title Glass polymorphism in glycerol–water mixtures: I. A computer simulation study
title_full Glass polymorphism in glycerol–water mixtures: I. A computer simulation study
title_fullStr Glass polymorphism in glycerol–water mixtures: I. A computer simulation study
title_full_unstemmed Glass polymorphism in glycerol–water mixtures: I. A computer simulation study
title_short Glass polymorphism in glycerol–water mixtures: I. A computer simulation study
title_sort glass polymorphism in glycerol–water mixtures: i. a computer simulation study
topic Chemistry
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4847106/
https://www.ncbi.nlm.nih.gov/pubmed/27063705
http://dx.doi.org/10.1039/c6cp00075d
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