Glass polymorphism in glycerol–water mixtures: II. Experimental studies

Abstract

We report a detailed experimental study of (i) pressure-induced transformations in glycerol–water mixtures at T = 77 K and P = 0–1.8 GPa, and (ii) heating-induced transformations of glycerol–water mixtures recovered at 1 atm and T = 77 K. Our samples are prepared by cooling the solutions at ambient pressure at various cooling rates (100 K s⁻¹–10 K h⁻¹) and for the whole range of glycerol mole fractions, χ_g. Depending on concentration and cooling rates, cooling leads to samples containing amorphous ice (χ_g ≥ 0.20), ice (χ_g ≤ 0.32), and/or “distorted ice” (0 < χ_g ≤ 0.38). Upon compression, we find that (a) fully vitrified samples at χ_g ≥ 0.20 do not show glass polymorphism, in agreement with previous works; (b) samples containing ice show pressure-induced amorphization (PIA) leading to the formation of high-density amorphous ice (HDA). PIA of ice domains within the glycerol–water mixtures is shown to be possible only up to χ_g ≈ 0.32 (T = 77 K). This is rather surprising since it has been known that at χ_g < 0.38, cooling leads to phase-separated samples with ice and maximally freeze-concentrated solution of χ_g ≈ 0.38. Accordingly, in the range 0.32 < χ_g < 0.38, we suggest that the water domains freeze into an interfacial ice, i.e., a highly-distorted form of layered ice, which is unable to transform to HDA upon compression. Upon heating samples recovered at 1 atm, we observe a rich phase behavior. Differential scanning calorimetry indicates that only at χ_g ≤ 0.15, the water domains within the sample exhibit polyamorphism, i.e., the HDA-to-LDA (low-density amorphous ice) transformation. At 0.15 < χ_g ≤ 0.38, samples contain ice, interfacial ice, and/or HDA domains. All samples (χ_g ≤ 0.38) show: the crystallization of amorphous ice domains, followed by the glass transition of the vitrified glycerol–water domains and, finally, the melting of ice at high temperatures. Our work exemplifies the complex “phase” behavior of glassy binary mixtures due to phase-separation (ice formation) and polyamorphism, and the relevance of sample preparation, concentration as well as cooling rates. The presence of the distorted ice (called “interphase” by us) also explains the debated “drift anomaly” upon melting. These results are compatible with the high-pressure study by Suzuki and Mishima indicating disappearance of polyamorphism at P ≈ 0.03–0.05 GPa at χ_g ≈ 0.12–0.15 [J. Chem. Phys., 2014, 141, 094505].