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Issue 16, 2016
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Glass polymorphism in glycerol–water mixtures: I. A computer simulation study

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Abstract

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 χg = 0–13% and find that, for the present mixture models and rates, the LDA–HDA transformation is detectable up to χg ≈ 5%. As the concentration increases, the density of the starting glass (i.e., LDA at approximately χg ≤ 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 χg > 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.

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

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Publication details

The article was received on 05 Jan 2016, accepted on 11 Mar 2016 and first published on 17 Mar 2016


Article type: Paper
DOI: 10.1039/C6CP00075D
Phys. Chem. Chem. Phys., 2016,18, 11042-11057
  • Open access: Creative Commons BY-NC license
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    Glass polymorphism in glycerol–water mixtures: I. A computer simulation study

    D. A. Jahn, J. Wong, J. Bachler, T. Loerting and N. Giovambattista, Phys. Chem. Chem. Phys., 2016, 18, 11042
    DOI: 10.1039/C6CP00075D

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