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

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Physics

Background:

• Water-glycerol mixtures exhibit glass polymorphism, existing as low-density amorphous (LDA) and high-density amorphous (HDA) forms.
• Understanding the transitions between these amorphous states is crucial for materials science and condensed matter physics.

Purpose of the Study:

• To investigate the molecular dynamics of LDA-HDA transformations in water-glycerol mixtures under compression/decompression.
• To determine the influence of glycerol concentration and temperature on glass polymorphism and the LDA-HDA transition.
• To elucidate the molecular-level mechanisms driving the density changes during the LDA-HDA transformation.

Main Methods:

• Out-of-equilibrium molecular dynamics (MD) simulations were employed to model water-glycerol mixtures.
• Simulations covered a range of glycerol molar concentrations (0-13%) at constant temperature.
• Analysis included density changes, hysteresis, molecular coordination, and hydrogen bond networks.

Main Results:

• MD simulations qualitatively reproduced experimental density changes during LDA-HDA transitions.
• The LDA-HDA transformation became smoother and exhibited decreased hysteresis with increasing temperature and/or glycerol content.
• Glass polymorphism was found to be inaccessible in mixtures with approximately >5% glycerol, as LDA density increased significantly while HDA density remained constant.
• Glycerol OH groups contribute to density increase by approaching water molecules, filling interstitial spaces.

Conclusions:

• Glycerol content significantly impacts glass polymorphism in water-glycerol mixtures, limiting the LDA-HDA transition above ~5% glycerol.
• The LDA-HDA transformation involves molecular rearrangements around both water and glycerol molecules, affecting coordination numbers more than hydrogen bond counts.
• The choice of glycerol force field influences molecular conformations but not the fundamental mechanisms of the LDA-HDA transformation.