While glassy materials can be made from virtually every class of liquid (metallic, molecular, covalent, and ionic), to date, formation of glasses in which structural units impart porosity on the nanoscopic level remains undeveloped. In view of the well-established porosity of metal–organic frameworks (MOFs) and the flexibility of their design, we have sought to combine their formation principles with the general versatility of glassy materials. Although the preparation of glassy MOFs can be achieved by amorphization of crystalline frameworks, transparent glassy MOFs exhibiting permanent porosity accessible to gases are yet to be reported. Here, we present a generalizable chemical strategy for making such MOF glasses by assembly from viscous solutions of metal node and organic strut and subsequent evaporation of a plasticizer–modulator solvent. This process yields glasses with 300 m2/g internal surface area (obtained from N2 adsorption isotherms) and a 2 nm pore–pore separation. On a volumetric basis, this porosity (0.33 cm3/cm3) is 3 times that of the early MOFs (0.11 cm3/cm3 for MOF-2) and within range of the most porous MOFs known (0.60 cm3/cm3 for MOF-5). We believe the porosity originates from a 3D covalent network as evidenced by the disappearance of the glass transition signature as the solvent is removed and the highly cross-linked nanostructure builds up. Our work represents an important step forward in translating the versatility and porosity of MOFs to glassy materials.

Physicochemical properties, ionicity, and fragility for protic ionic liquids (PILs) based on the protonation of the extremely fragile molecular liquid decahydroisoquinoline (DHiQ) by various Brønsted acids have been studied. The ionicity was evaluated using the Walden plot diagnostic, while the m-fragility (slope of Tg-scaled Arrhenius plot at Tg) was quantitatively measured by the Moynihan–Wang–Velikov variable scan rate, differential scanning calorimetry, method. DHiQ-derived PILs prove to cover the whole range of IL ionicities from poor IL to good IL, and even superionic, assessed from the Walden plot, depending on the choice of Brønsted acid. We find that the superfragile character of the parent DHiQ becomes completely suppressed upon conversion to ionic liquid, the initial value m = 128 sinking to m = 45–91 for the ionic liquid. Such values are in the intermediate to fragile range. The DHiQ-based PIL showing superionic behavior, anion [HSO4-], proves to be the case with the lowest m value (m = 45) so far reported for either aprotic or protic ILs. Both low fragility and dry proton conductivity can be attributed to an extended hydrogen bond network that is set up by the hydrogensulfate anion. The good DHiQ PILs have m values similar to those reported for typical aprotic ILs (m = 60–80), while the poor DHiQ PILs in which proton transfer from acid to base is weak show some memory of the parent fragility. Thus, a correlation of ionicity with m-fragility is characteristic of this system. A range of noncrystallizing, and also nonglassforming, behavior is observed in this series of compounds, suggesting a possible test for ideal glassformer existence.

We show that, with appropriate doping, the ethersulfone-based electrolytes that we earlier reported to have 5.6 V electrochemical windows but poor cycling performance in Li-ion cells, can succeed. We show they can be used to make cells that, at C/20 charge/discharge rates, have cycling performance equal to those with the standard LiPF6-carbonate electrolytes. A difference in performance that develops at higher C is removed by substituting an ethersulfone with one fewer methyl groups, for lower viscosity.

This Account covers research dating from the early 1960s in the field of low-melting molten salts and hydrates,which has recently become popular under the rubric of “ionic liquids”. It covers understanding gained in the principal author’s laboratories (initially in Australia, but mostly in the U.S.A.) from spectroscopic, dynamic, and thermodynamic studies and includes recent applications of this understanding in the fields of energy conversion and biopreservation. Both protic and aprotic varieties of ionic liquids are included, but recent studies have focused on the protic class because of the special applications made possible by the highly variable proton activities available in these liquids.

We report the reversible thermal unfolding/refolding, and long period stabilization against aggregation and hydrolysis, of >200 mg ml–1 solutions of lysozyme in ionic liquid-rich, ice-avoiding, solvents.

We describe an electrospray technique for in situ preparation, for differential scanning calorimetry study, of samples of molecular liquids quenched into the glassy state on extremely short time scales (hyperquenched). We study the cases of a hydrogen-bonded liquid, propylene glycol, PG and a Van der Waals liquid, di-n-butyl phthalate DBP. Using a fictive temperature method of obtaining the temperature dependence of enthalpy relaxation, we show that the electrospray method yields quenching rates of ∼105 K/s, while the more common method, dropping a sealed pan of sample into liquid nitrogen, yields only 120 K/s. These hyperquenched samples start to relax, exothermically, far below the glass temperature, at a temperature (0.75Tg) where the thermal energy permits escape from the shallow traps in which the system becomes localized during hyperquenching. This permits estimation of the trap depths, which are then compared with the activation energy estimated from the fictive temperature of the glass and the relaxation time at the fictive temperature. The trap depth in molar energy units is compared with the ‘height of the landscape’ for PG, the quasi-lattice energy of the liquid based on the enthalpy of vaporization, and the single molecule activation energy for diffusion in crystals. The findings are consistent with the mechanism of relaxation invoked in a current model of relaxation in glassforming liquids. In the case of di-n-butyl phthalate we investigate the additional question of sub-Tg annealing effects. We find the ‘shadow’ glass transition, (an annealing prepeak) seen previously only in multicomponent mineral and metallic glasses. The phenomenon is important for understanding microheterogeneities in viscous liquid structures.

Although the majority of glasses in use in technology are complex mixtures of oxides or chalcogenides, there are numerous examples of pure substances—‘glassformers’—that also fail to crystallize during cooling. Most glassformers are organic molecular systems, but there are important inorganic examples too1, 2, such as silicon dioxide and elemental selenium (the latter being polymeric). Bulk metallic glasses can now be made3; but, with the exception of Zr50Cu50 (ref. 4), they require multiple components to avoid crystallization during normal liquid cooling. Two-component ‘metglasses’ can often be achieved by hyperquenching, but this has not hitherto been achieved with a single-component system. Glasses form when crystal nucleation rates are slow, although the factors that create the slow nucleation conditions are not well understood. Here we apply the insights gained in a recent molecular dynamics simulation study5 to create conditions for successful vitrification of metallic liquid germanium. Our results also provide micrographic evidence for a rare polyamorphic transition preceding crystallization of the diamond cubic phase.

We report the successful application of low-melting inorganic salts with protonated cations (e.g. ammonium) as electrolytes in fuel cells operating in the temperature range 100–200 °C, where even with unoptimized electrodes, cell performance is comparable to that of the phosphoric acid fuel cell operating with optimized electrodes in the same temperature range, while open circuit voltages, and efficiencies at low current densities, can be much better—and there is no need for humidification or pressure to sustain performance.

We describe the preparation and characterization of a glassy form of the moderately good glassformer PbGeO3, by mechanical damage, and compare its properties with those of the normal melt-quenched glass and the crystal. The damage-formed glass exhibits a DSC thermogram strikingly similar to that of a hyperquenched glass, implying that it forms high on the energy landscape. The final glass transition endotherm occurs within 4 K (0.006Tg) of that of the melt-quenched glass, but crystallization occurs at a lower temperature, as if pre-nucleated. In particular, we have studied the low frequency vibrational dynamics of the alternatively prepared amorphous states in the boson peak region, and find the damage-formed glass boson peak to be almost identical in shape to, but more intense than, that of the normal melt-formed glass, as previously found for hyperquenched glasses. In view of the quite different preparation procedures, this similarity would seem to eliminate equilibrium liquid clusters as a source of the boson peak vibrations, but leaves plausible a connection to force constant fluctuations or to specific vitreous state defects.

Abstract

Aqueous solutions are generally assumed to be superior electrolytic conductors because of the unique dielectric and fluid properties of water. We show that their conductivities can be matched by liquid electrolytes that contain no solvent. These are proton transfer salts that are liquid at ambient temperature. The high conductivities are due to the high fluidity and ionicity rather than some sort of Grotthus mechanism, although in certain cases a mobile proton population may make a non-negligible contribution. The highest conductivities have been obtained when both cations and anions contain protons. At 25°C, values of >150 millisiemens per centimeter (mS cm^–1) appear possible; at 100°C, 470 mS cm^–1 has been measured. Because of the combination of high ionicity and proton exchange kinetics with low vapor pressure, the systems we describe also make excellent fuel cell electrolytes.

Although liquids normally crystallize on cooling, there are members of all liquid types (including molecular, ionic and metallic) that supercool and then solidify at their glass transition temperature, Tg. This continuous solidification process exhibits great diversity within each class of liquid—both in the steepness of the viscosity–temperature profile, and in the rate at which the excess entropy of the liquid over the crystalline phase changes as Tg is approached. However, the source of the diversity is unknown. The viscosity and associated relaxation time behaviour have been classified between ‘strong’ and ‘fragile’ extremes, using Tg as a scaling parameter1, but attempts to correlate such kinetic properties with the thermodynamic behaviour have been controversial2, 3. Here we show that the kinetic fragility can be correlated with a scaled quantity representing excess entropy, using data over the entire fragility range and embracing liquids of all classes. The excess entropy used in our correlation contains both configurational and vibration-related contributions. In order to reconcile our correlation with existing theory and simulations, we propose that variations in the fragility of liquids originate in differences between their vibrational heat capacities, harmonic and anharmonic, which we interpret in terms of an energy landscape. The differences evidently relate to behaviour of low-energy modes near and below the boson peak.

We show first that when the excitations described by the `bond lattice' model, and its `defect' model relatives, are allowed to interact, this simple class of model predicts the possibility of first-order transitions between viscous and fluid liquid states. Looking for support of this prediction, we examine the behavior of a series of `tetrahedral' liquids, all of which are famous for one or another aspect of their behavior. We show that when diffusivity data for these liquids, SiO2, BeF2, water, and liquid Si, are plotted on a reduced temperature scale with glass transition temperature as the scaling temperature, a systematic pattern is revealed in which a mild deviation, in the case of BeF2, from the `strong' extreme of the normal strong/fragile glass-former pattern becomes a (contentious) strong deviation in the case of water and develops finally into the predicted first-order transition deviation in the case of liquid Si. Unfortunately, in the latter two cases, the systematic strengthening of the anomalous character is associated with a decrease in the temperatures of occurrence, such that in each case they fall below the melting point. The corresponding competition with crystallization makes their observation difficult. Our account of the phenomenology, therefore, depends heavily on computer simulation studies, but extensive links to experimental results are given. We relate our findings to the recent observations that the amorphous states of Si and water are unique among glassy systems in showing little or no trace of the very low temperature (glassy state as opposed to liquid state) anomalies formerly considered `ubiquitous' among glassy systems. We interpret this to mean that systems with strong cooperativity in their excitations are able to access the lower minima on their respective configuration space potential energy hypersurfaces and thereby to reach states, which are close to the ideal of the `perfect glass'. In this state the residual entropy is near zero, and the defect-related boson peak and two-level tunneling system excitations are weak or absent. Such systems require unusual routes to access their glassy states and their properties are more closely related to those of crystals than to those of ordinary glasses. A new designation may be required. The range of such systems is large, embracing all the 3:5 and many 2:6 semiconductors.

If crystallization can be avoided when a liquid is cooled, it will typically form a glass. Near the glass transition temperature the viscosity increases continuously but rapidly with cooling. As the glass forms, the molecular relaxation time increases with an Arrhenius-like (simple activated) form in some liquids, but shows highly non-Arrhenius behaviour in others. The former are said to be 'strong' liquids, and the latter 'fragile'1,2. Here we show that the fragility of a liquid can be determined from purely thermodynamic data (as opposed to measurements of kinetics) near and below the melting point. We find that for most liquids the fragilities estimated this way are consistent with those obtained by previous methods and by a new method (ref. 3 and K.I., C.A.A. and C.T.M., unpublished data) at temperatures near the glass transition. But water is an exception. The thermodynamic method indicates that near its melting point it is the most fragile of all liquids studied, whereas the kinetic approach indicates that near the glass transition it is the least fragile. We propose that this discrepancy can be explained by a fragile-to-strong transition in supercooled water near 228 K, corresponding to a change in the liquid's structure at this point.

The phenomenology of electrical relaxation in superionic glasses and their melts, and in salt-in-polymer electrolytes is reviewed, and then recent observations on nuclear spin lattice relaxation of mobile species like 7Li+ are examined in the same conceptual frame work are examined. To shed light on the origin of difference found in superionic glasses between the correlation times, τc, for fluctuations causing spin lattice relaxation and those, τσ, allowing conductivity relaxation, 7Li nuclear spin resonance was studied across a binary liquid system of [polypropylene oxide + Li salt(s)]. In the polymer-rich solutions, the two correlation times are found to have the same value but above the (salt-in-polymer) (ppolymer-in-salt) boundary, a gap starts to open up. The value of T1 at its minimum is found to be almost independent of solution composition, implying that the Li+ cations find comparable sites as polymer replaces salt in the solution, To observed the behavior in polymer-free salt melts, salt mixtures had to be used to gain access to the viscous liquid regime. The study of such liquid salts reveals a difference in τc and τσ of 1.1 orders of magnitude, comparable to that found in glassy superionics. The results are discussed in terms of the serial decoupling of different relaxation modes observed in fragile liquids during cooling towards the glass transition, and are used to support the assertion that conductivity in the ‘polymer-in-salt’ electrolytes is dominated by Li+ migration.

We report the reversible thermal unfolding/refolding, and long period stabilization against aggregation and hydrolysis, of >200 mg ml–1 solutions of lysozyme in ionic liquid-rich, ice-avoiding, solvents.