Glass transition of LiCl aqueous solutions confined in mesoporous silica

The thermal transitions of confined LiCl aqueous solutions were studied by differential scanning calorimetry for solutions with salt concentrations with eutectic (R = 7) and subeutectic (R > 7) compositions (R = moles of water/moles of LiCl). The confinement media consist of mesoporous silica with pore diameters between 2 nm and 58 nm, with a small negative surface charge density. The vitrification of confined LiCl aqueous solutions was observed in all samples, expanding the vitrification region up to R = 15, and probably beyond for cooling rates of ≈1000 K/min. Ice crystallization was observed in some samples, except for those confined in the narrower pores. The onset and endpoint glass transition temperatures for the confined eutectic samples increase by 2 K and 5 K, respectively, for the smallest pore diameters (2 nm), which is equivalent to the effect of applying a pressure of up to 100 MPa to the bulk sample. This behavior is opposite of that reported for aqueous subeutectic NaCl solutions confined in silica glasses of similar sizes. We speculate that this is due to the fact that the mechanism of double confinement of the NaCl solution, between the pore wall and the precipitated ice, is not operative for LiCl solutions. Instead, the Li+ ions might force the hydration water in to a high-density state.Fil: Longinotti, María Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Fuentes Landete, Violeta. Universidad de Innsbruck; AustriaFil: Loerting, Thomas. Universidad de Innsbruck; AustriaFil: Corti, Horacio Roberto. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin

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

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 s1–10 K h1) and for the whole range of glycerol mole fractions, wg. Depending on concentration and cooling rates, cooling leads to samples containing amorphous ice (wg Z 0.20), ice (wg r 0.32), and/or ‘‘distorted ice’’ (0 o wg r 0.38). Upon compression, we find that (a) fully vitrified samples at wg Z 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 wg E 0.32 (T = 77 K). This is rather surprising since it has been known that at wg o 0.38, cooling leads to phase-separated samples with ice and maximally freezeconcentrated solution of wg E 0.38. Accordingly, in the range 0.32 o wg o 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 wg r 0.15, the water domains within the sample exhibit polyamorphism, i.e., the HDA-to-LDA (low-density amorphous ice) transformation. At 0.15 o wg r 0.38, samples contain ice, interfacial ice, and/or HDA domains. All samples (wg r 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 E 0.03–0.05 GPa at wg E 0.12–0.15 [J. Chem. Phys., 2014, 141, 094505]

Nature of Water’s Second Glass Transition Elucidated by Doping and Isotope Substitution Experiments

Based on calorimetry and dielectric spectroscopy, the influence of dopants as well as H/D-isotope substitution on the dynamics and thermodynamics of expanded high-density amorphous ice (eHDA) is studied. We find that dopants do not significantly alter the phase behavior, the dielectric relaxation times, and the calorimetric glass transition of eHDA. These observations starkly contrast those made for crystalline ices such as ice I_{h}, ice V, ice VI, and ice XII, where suitable dopants enhance the dielectric dynamics by several orders of magnitude and can trigger hydrogen order-disorder transitions, then taking place below the orientational glass transition temperature of undoped samples. This conspicuous contrast to the behavior of crystalline ices strongly argues against point-defect dynamics in amorphous ices and against a previously suggested “crystallinelike” nature of the amorphous ices. Furthermore, H/D substitution also does not affect the calorimetric glass transition in eHDA much, whereas for crystalline ices, the heat capacity increase at the glass transition is roughly halved. In addition, the H/D-isotope shift of the glass transition onset is much larger for crystalline ices than it is for amorphous ices. This observation favors the notion of eHDA’s glass transition as a glass-to-liquid transition and is evidence against a mere molecular-reorientation unfreezing at water’s second glass transition. Comparing the isotope effect on activation energies for dielectric relaxation with ice V suggests that in amorphous ice water molecules move translationally above T_{g}. Thus, the present work strongly supports that above this glass transition, water does indeed exist in its contested high-density liquid state

Advances in the study of supercooled water

In this review, we report recent progress in the field of supercooled water. Due to its uniqueness, water presents numerous anomalies with respect to most simple liquids, showing polyamorphism both in the liquid and in the glassy state. We first describe the thermodynamic scenarios hypothesized for the supercooled region and in particular among them the liquid-liquid critical point scenario that has so far received more experimental evidence. We then review the most recent structural indicators, the two-state model picture of water, and the importance of cooperative effects related to the fact that water is a hydrogen-bonded network liquid. We show throughout the review that water's peculiar properties come into play also when water is in solution, confined, and close to biological molecules. Concerning dynamics, upon mild supercooling water behaves as a fragile glass former following the mode coupling theory, and it turns into a strong glass former upon further cooling. Connections between the slow dynamics and the thermodynamics are discussed. The translational relaxation times of density fluctuations show in fact the fragile-to-strong crossover connected to the thermodynamics arising from the existence of two liquids. When considering also rotations, additional crossovers come to play. Mobility-viscosity decoupling is also discussed in supercooled water and aqueous solutions. Finally, the polyamorphism of glassy water is considered through experimental and simulation results both in bulk and in salty aqueous solutions. Grains and grain boundaries are also discussed