Double glass transitions in aqueous lithium chloride solutions vitrified at high pressures: evidence for a liquid-liquid immiscibility

Cooperative and Local Molecular Motion of High-Density Water in Glycerol Aqueous Solutions

The glass-to-liquid transition of water, particularly in high-density water (HDW), has long been a controversial topic due to challenges posed by inevitable crystallization. In this study, we addressed this issue by creating homogeneous high-density glass from a dilute glycerol aqueous solution under high pressure. Using dielectric spectroscopy, we explored the glass transition of HDW in glycerol solutions across the full concentration range under high pressures. HDW was found to exhibit two distinct relaxation modes: one linked to cooperative motion and the other to noncooperative local motion. The fragility index classification of HDW, derived from the cooperative motion of water, suggests that HDW behaves as a "fragile" liquid, contradicting previous suggestions. Extrapolation to pure HDW indicates that the dielectric relaxation observed in pure HDW originates from noncooperative local water motion.

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.

Glass polymorphism and liquid-liquid phase transition in aqueous solutions: experiments and computer simulations

One of the most intriguing anomalies of water is its ability to exist as distinct amorphous ice forms (glass polymorphism or polyamorphism). This resonates well with the possible first-order liquid-liquid phase transition (LLPT) in the supercooled state, where ice is the stable phase. In this Perspective, we review experiments and computer simulations that search for LLPT and polyamorphism in aqueous solutions containing salts and alcohols. Most studies on ionic solutes are devoted to NaCl and LiCl; studies on alcohols have mainly focused on glycerol. Less attention has been paid to protein solutions and hydrophobic solutes, even though they reveal promising avenues. While all solutions show polyamorphism and an LLPT only in dilute, sub-eutectic mixtures, there are differences regarding the nature of the transition. Isocompositional transitions for varying mole fractions are observed in alcohol but not in ionic solutions. This is because water can surround alcohol molecules either in a low- or high-density configuration whereas for ionic solutes, the water ion hydration shell is forced into high-density structures. Consequently, the polyamorphic transition and the LLPT are prevented near the ions, but take place in patches of water within the solutions. We highlight discrepancies and different interpretations within the experimental community as well as the key challenges that need consideration when comparing experiments and simulations. We point out where reinterpretation of past studies helps to draw a unified, consistent picture. In addition to the literature review, we provide original experimental results. A list of eleven open questions that need further consideration is identified.

Calorimetric study of water's two glass transitions in the presence of LiCl

A DSC study of dilute glassy LiCl aqueous solutions in the water-dominated regime provides direct evidence of a glass-to-liquid transition in expanded high density amorphous (eHDA)-type solutions. Similarly, low density amorphous ice (LDA) exhibits a glass transition prior to crystallization to ice Ic. Both glass transition temperatures are independent of the salt concentration, whereas the magnitude of the heat capacity increase differs. By contrast to pure water, the glass transition endpoint for LDA can be accessed in LiCl aqueous solutions above 0.01 mole fraction. Furthermore, we also reveal the endpoint for HDA's glass transition, solving the question on the width of both glass transitions. This suggests that both equilibrated HDL and LDL can be accessed in dilute LiCl solutions, supporting the liquid-liquid transition scenario to understand water's anomalies.

Glass-to-cryogenic-liquid transitions in aqueous solutions suggested by crack healing

Observation of theorized glass-to-liquid transitions between low-density amorphous (LDA) and high-density amorphous (HDA) water states had been stymied by rapid crystallization below the homogeneous water nucleation temperature (∼235 K at 0.1 MPa). We report optical and X-ray observations suggestive of glass-to-liquid transitions in these states. Crack healing, indicative of liquid, occurs when LDA ice transforms to cubic ice at 160 K, and when HDA ice transforms to the LDA state at temperatures as low as 120 K. X-ray diffraction study of the HDA to LDA transition clearly shows the characteristics of a first-order transition. Study of the glass-to-liquid transitions in nanoconfined aqueous solutions shows them to be independent of the solute concentrations, suggesting that they represent an intrinsic property of water. These findings support theories that LDA and HDA ice are thermodynamically distinct and that they are continuously connected to two different liquid states of water.

Pressure-induced transformations in LiCl-H2O at 77 K

A systematic study of the properties of high-density amorphous ice (HDA) in the presence of increasing amounts of salt is missing, especially because it is challenging to avoid ice crystallization upon cooling the pressurized liquid. In order to be able to study HDA also in the presence of small amounts of salt, we have investigated the transformation behaviour of quenched aqueous LiCl solutions (mole fraction x < 0.25) upon pressurization in a piston-cylinder setup at 77 K. The sample properties were characterized by in situ dilatometry under high pressure conditions and after recovery by ex situ powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) at ambient pressure. Two regimes can be identified, with a rather sharp switch at about x = 0.12. At x < 0.12 the samples show the phenomenology also known for pure water samples. They are composed mainly of hexagonal ice (Ih) and experience pressure-induced amorphization to HDA at P > 1 GPa. The observed densification is consistent with the idea that a freeze concentrated LiCl solution of x = 0.14 (R = 6) segregates, which transforms to the glassy state upon cooling, and that the densification is only due to the Ih → HDA transition. Also the XRD patterns and DSC scans are almost unaffected by the presence of the segregated glassy LiCl solution. Upon heating at ambient pressure HDA experiences the polyamorphic transition to low-density amorphous ice (LDA) at ∼120 K, even at x ∼ 0.10. Based on the latent heat evolved in the transition we suggest that almost all water in the sample transforms to an LDA-like state, even the water in the vicinity of the ions. The glassy LiCl solution acts as a spectator that does not shift the transformation temperature significantly and experiences a glass-to-liquid transition at ∼140 K prior to the crystallization to cubic ice. By contrast, at x > 0.12 the phenomenology completely changes and is now dominated by the salt. Hexagonal ice no longer forms upon quenching the LiCl solution, but instead LDA forms. A broad pressure-induced transformation at >0.6 GPa can be attributed to the densification of LDA, the glassy LiCl solution and/or glassy hydrates.

Structural characterization of eutectic aqueous NaCl solutions under variable temperature and pressure conditions

The structure of amorphous NaCl solutions produced by fast quenching is studied as a function of pressure, up to 4 GPa, by combined neutron diffraction experiments and classical molecular dynamics simulations. Similarly to LiCl solutions the system amorphizes at ambient pressure in a dense phase structurally similar to the e-HDA phase in pure water. The measurement of the static structure factor as a function of pressure allowed us to validate a new polarizable force field developed by Tazi et al., 2012, never tested under non-ambient conditions. We infer from simulations that the hydration shells of Na(+) cations form well defined octahedra composed of both H2O molecules and Cl(-) anions at low pressure. These octahedra are gradually broken by the seventh neighbour moving into the shell of first neighbours yielding an irregular geometry. In contrast to LiCl solutions and pure water, the system does not show a polyamorphic transition under pressure. This confirms that the existence of polyamorphism relies on the tetrahedral structure of water molecules, which is broken here.

Low-density liquid water is the mother of ice: on the relation between mesostructure, thermodynamics and ice crystallization in solutions

Predicting the temperature and extent of ice freezing in aqueous solutions is crucial for areas as diverse as cryobiology and materials design. It has long been recognized that the thermodynamics of liquid water controls the temperature and kinetics of ice crystallization. Parameterizations of the freezing temperatures in terms of the water activity of the solution have been successfully established, but the fundamental origin of the thermodynamic control of the non-equilibrium crystallization of ice has remained elusive. Here we use large-scale molecular simulations to elucidate the relationship between the structure, thermodynamics, and ice crystallization temperatures for solutions of mW water and a strongly hydrophilic solute that mimics LiCI ions. Fast cooling of solutions with up to 20 mol% ions results in the formation of nanosegregated glasses with domains of low-density amorphous ice and an ion-rich vitrified solution. Slow cooling of the mixtures results in nucleation and growth of ice within the domains of four-coordinated liquid water. The temperature of crystallization Tx coincides with the temperature of appearance of nanoscopic domains of four-coordinated liquid water in the mixture, T(L). We use the insight provided by the simulations to derive a thermodynamic expression for the crystallization temperature as a function of the water activity, T(X)(a(W)), analogous to the dependence of the melting temperature, T(m)(a(W)). The simple expression derived in this work provides a good account of the experimental freezing temperatures of water and the well-known steepest dependence of Tx on solute concentration compared to that of T(m).

Sudden switchover between the polyamorphic phase separation and the glass-to-liquid transition in glassy LiCl aqueous solutions

Lithium chloride aqueous solutions (LiClaq solutions) below 10 mol.% are vitrified by cooling from room temperature to 77 K at 0.3 GPa. We examine the solvent state of the glassy sample and its transformation by heating at 1 atm using low-temperature differential scanning calorimetry and Raman spectroscopy. This experimental study suggests strongly that the solvent state of the glassy LiClaq solution closely relates to the state of high-density amorphous ice. Moreover, we reconfirm that the separation into the low-density amorphous ice and the glassy highly concentrated LiClaq solution occurs in the glassy dilute LiClaq solution at ∼130 K, not the glass-to-liquid transition which is commonly observed in the glassy LiClaq solution above ∼10 mol.%. In order to interpret the sudden switchover between the glass-to-liquid transition and the phase separation at ∼10 mol.%, we propose a state diagram of LiClaq solution which connects with a polyamorphic state diagram of pure water and discuss a possibility that the electric field induces a polyamorphic transition of water.

Structural study of low concentration LiCl aqueous solutions in the liquid, supercooled, and hyperquenched glassy states

Neutron diffraction experiments on a solution of LiCl in water (R = 40) at ambient conditions and in the supercooled and hyperquenched states are reported and analyzed within the empirical potential structure refinement framework. Evidence for the modifications of the microscopic structure of the solvent in the presence of such a small amount of salt is found at all investigated thermodynamic states. On the other hand, it is evident that the structure of the hyperquenched salty sample is similar to that of pure low density amorphous water, although all the peaks of the radial distribution functions are broader in the present case. Changes upon supercooling or hyperquenching of the ion's hydration shells and contacts are of limited size and evidence for segregation phenomena at these states does not clearly show up, although the presence of water separated contacts between ion of the same sign is intriguing.

Nanophase segregation in supercooled aqueous solutions and their glasses driven by the polyamorphism of water

We use large-scale molecular dynamics simulations to investigate the phase transformation of aqueous solutions of electrolytes cooled at the critical rate to avoid the crystallization of ice. Homogeneous liquid solutions with up to 20% moles of ions demix on cooling producing nanophase segregated glasses with characteristic dimensions of phase segregation of about 5 nm. The immiscibility is driven by the transformation of water to form a four-coordinated low-density liquid (LDL) as it crosses the liquid-liquid transformation temperature T(LL) of the solution. The ions cannot be incorporated into the tetrahedral LDL network and are expelled to form a solute-rich water nanophase. The simulations quantitatively reproduce the relative amounts of low and high-density liquid water as a function of solute content in LiCl glasses [Suzuki and Mishima, Phys. Rev. Lett. 2000, 85, 1322-1325] and provide direct evidence of segregation in aqueous glasses and their dimensions of phase segregation.

Phase separation in dilute LiCl-H2O solution related to the polyamorphism of liquid water

When an emulsified 4.8 mol % LiCl-H2O solution was cooled under a pressure of 0.35 or 0.45 GPa and decompressed to 0.1 GPa at 142 K, slightly above its glass transition temperature (approximately 140 K at 0.1 GPa), its volume increased suddenly. This was regarded as an appearance of the low-density amorphous ice in the liquid solution as suggested by x-ray and Raman measurements, and this appearance corresponded to the high-to-low-density polyamorphic transition of pure H2O. Hysteresis was considered to accompany this volumetric change. The hysteresis of the liquid transition proves its first-order nature and, as for the solution, this suggests that the transition is a polyamorphic phase separation.

Application of polyamorphism in water to spontaneous crystallization of emulsified LiCl‐H2O solution

Aqueous solutions are widely explained by the hydration or the bound waterfree water notion. Amorphous polymorphism (polyamorphism) in pure water, which is presently under vigorous discussion, may provide a different view over the solutions. Here, I changed pressure, P, temperature, T, and concentration, C, of emulsified LiCl-H2O solutions and studied their freezing by detecting its heat evolution. It was experimentally indicated that the homogeneous nucleation of low-density crystalline ice I (phase Ih or Ic), in pure water and in solutions, connects to the polyamorphic transition of high-density amorphous ice (HDA) to low-density amorphous ice (LDA). Thus, the polyamorphism of water relates to the phase behavior of aqueous solution. In accordance with the recent simulation result, the nucleation was thought to occur in two stages: the appearance of the LDA-like state and the crystallization. Usefulness of the polyamorphic point of view about the solutions was seen.

The glass-to-liquid transition of the emulsified high-density amorphous ice made by pressure-induced amorphization

Emulsified high-density amorphous ice, made by pressure-induced amorphization of emulsified ice Ih, was decompressed at about 160 K. The onset of an endothermic event was observed around 0.4 GPa during the decompression. This is consistent with existence of the glass transition to a liquid state, implying the close relationship between melting and amorphization.

Two distinct raman profiles of glassy dilute LiCl solution

We make glass of dilute LiCl aqueous solution by cooling micrometer-sized droplets of the solution extremely quickly and measure the Raman spectra of the glass. It is found that the OH stretching vibration mode of the glass of dilute solution is composed of the OH stretching vibration mode of pure glassy water and that of the glass of solvent water in the highly concentrated solution. This is consistent with the possibility of the existence of two distinct glassy states of water in dilute LiCL solution at low temperature.

Vitreous domains in an aqueous ribose solution

Aqueous solutions containing d-ribose demonstrate the ability to form more than one vitreous domain when exposed to low temperatures. Differential scanning calorimetry revealed two glass transitions (at Tgs of -63 and -43 degrees C) upon cooling and warming at a constant rate of 5 degrees C.min-1. The bulk water of the solution crystallizes at -18 degrees C (Tc). Heat capacity and enthalpy changes, and the derivatives for each thermal event, are calculated. Relaxation studies on the observed Tgs produced anticipated overshoots characteristic of the presence of glassy states.

Beyond water activity: recent advances based on an alternative approach to the assessment of food quality and safety

Water, the most abundant constituent of natural foods, is a ubiquitous plasticizer of most natural and fabricated food ingredients and products. Many of the new concepts and developments in modern food science and technology revolve around the role of water, and its manipulation, in food manufacturing, processing, and preservation. This article reviews the effects of water, as a near-universal solvent and plasticizer, on the behavior of polymeric (as well as oligomeric and monomeric) food materials and systems, with emphasis on the impact of water content (in terms of increasing system mobility and eventual water "availability") on food quality, safety, stability, and technological performance. This review describes a new perspective on moisture management, an old and established discipline now evolving to a theoretical basis of fundamental structure-property principles from the field of synthetic polymer science, including the innovative concepts of "water dynamics" and "glass dynamics". These integrated concepts focus on the non-equilibrium nature of all "real world" food products and processes, and stress the importance to successful moisture management of the maintenance of food systems in kinetically metastable, dynamically constrained glassy states rather than equilibrium thermodynamic phases. The understanding derived from this "food polymer science" approach to water relationships in foods has led to new insights and advances beyond the limited applicability of traditional concepts involving water activity. This article is neither a conventional nor comprehensive review of water activity, but rather a critical overview that presents and discusses current, usable information on moisture management theory, research, and practice applicable to food systems covering the broadest ranges of moisture content and processing/storage temperature conditions.