Vapor-deposited amorphous ice, so-called amorphous solid water, exhibits complex structural and morphological transformations upon heating. A network of micropores, present at the deposition temperature (80 K), collapses at 100-145 K, and a glass transition takes place simultaneously above 120 K. Here, we separate the two processes by allowing the micropores to collapse upon heating, which is monitored by small-angle x-ray scattering experiments. The combined micropore collapse and glass transition dynamics are studied using x-ray photon correlation spectroscopy. After cooling back down and heating a second time, we see remaining pores collapsing only near T-g. Our analysis reveals both diffusive and ballistic processes attributed to pore collapse dynamics. Fast processes (similar to 100 & Aring;(2)/s) occur only when both micropore collapse and glass transition are simultaneously at play. In other words, both processes impact on each other and lead to a speed-up. The glass transition dynamics mainly features a slow diffusive process with a diffusion coefficient of around 1 & Aring;(2)/s and lower. This value is in nice agreement with other work on thin and on bulk samples.

This work constitutes part II of a series on vibrational spectroscopy of formic acid, focusing on the molecular interactions of its carboxyl group during dimerization. Such interactions drive the earliest steps of clustering in organic-mediated atmospheric new particle formation (NPF), and are difficult to access with the mass spectrometric methods typically used to study NPF. While theoretical thermochemistry provides binding free energies for various cluster conformers, these conformers have not been experimentally probed to date, and it is unclear which conformers ultimately participate in atmospheric organic clustering. Vibrational spectroscopy using matrix-isolation provides a route to experimentally investigate the conformers of small clusters. To identify simple conformers in the clustering of the carboxyl group, we use the model system of the formic acid monomer and dimer in cryogenic matrices in combination with ab initio calculations. We find that: (1) efficient harmonic frequency calculations, even at a low level of density functional theory, reproduce both the direction and magnitude of the experimentally observed dimer-formation frequency shifts across different conformers. (2) Matrix effects effectively cancel out when investigating such shifts. (3) The most reliable region to study these shifts is the νCO and νC-O regions, supported secondarily by the broader and less intense νOH and δoopCOH regions. (4) Under cryogenic matrix-isolation conditions, thermodynamically less favorable conformers appear with relatively high abundance, suggesting that the matrix serves as a kinetic trap. Altogether, we can capture structural information about short-lived conformers and thus demonstrate their experimental existence. This supports the common practice of including less favorable conformers in ab initio computational nucleation models, as they are considered intermediates in atmospheric organic clustering. Our insights from this model system of formic acid pave the way for future studies of organic clustering analyzed in cryogenic matrices to investigate the earliest steps of sub-nanometer NPF.

Freezing and lyophilization are commonly used methods for stabilizing pharmaceutical and biochemical formulations. However, these processes can introduce a variety of freezing-induced stresses that may lead not to stabilization, but rather to the destabilization of active molecules. One of the most significant of these stresses is freezing-induced acidity change, which has been shown to cause protein aggregation, loss of structural integrity, and increased chemical reactivity. While buffers are routinely used in liquid formulations to minimize pH fluctuations, several studies have demonstrated that certain buffers not only fail to maintain pH during freezing but may actively contribute to acidity shifts. In this study, we investigate the effects of cooling rate, initial pH, mannitol concentration, and lyophilization on acidity in glycine and L-histidine buffer systems in the solid state (both frozen and lyophilized) using UV-VIS spectroscopy and differential scanning calorimetry. Our results indicate that the freezing of amino acid buffers causes a slight increase in pH (basification); however, changes in acidity are not solely the consequence of freezing as they also occur during lyophilization. Notably, in L-histidine with mannitol at pH 7, lyophilization induces acidification of up to 4 units - opposite to the direction observed during freezing. Furthermore, we explore the correlation between vitrification of the freeze-concentrated solution and freezing-induced acidity changes, as quantified using the Hammett acidity function (H2-). These findings may inform the rational design of more robust stabilization strategies.

Dynamics, morphology, and structure of glassy water are highly relevant for cryochemical techniques, in particular for cryo-electron microscopy. Here, we study the structural dynamics of a deposit consisting of thousands of micrometer-sized glassy water droplets during and after droplet coalescence using x-ray photon correlation spectroscopy at the micro- and meso-scale. We cover the temperature range from 94 to 161 K, encompassing droplet coalescence, the glass transition, and crystallization to ice I. Our experimental protocol involves heating beyond the coalescence regime, followed by recooling and reheating beyond crystallization, which allows us to disentangle the dynamics of coalescence from those associated with the glass transition. During coalescence, we observe an irreversible ballistic process in the temperature range between 130 and 145 K, with characteristic velocities of ∼0.1-0.2 Å s-1. In addition, samples that are not annealed below 125 K exhibit a q-independent mode (q0) at 130-145 K, which only appears while coalescence is progressing. We regard this to be a collective relaxation connected to a mobile surface layer at the droplet interfaces. After coalescence is complete, we observe significant diffusive dynamics. In particular, we find a sharp increase in diffusivity to ∼2 Å2 s-1 at around 148 K, indicating the onset of pronounced diffusive motion. From these results, we conclude droplet coalescence is primarily governed by ballistic, non-diffusive dynamics below ∼136 K, whereas strongly heterogeneous diffusive dynamics emerge at higher temperatures. We associate the abrupt increase in diffusivity after coalescence with the bulk glass-to-liquid transition.

This work constitutes part I of a series on vibrational spectroscopy of formic acid. As the simplest carboxylic acid, formic acid and its cyclic dimer are well-established models for investigating vibrational signatures of hydrogen-bonded organic acids. This has led to numerous highly specialized studies on formic acid, rendering the field increasingly fragmented. Here we demonstrate how a straightforward combination of experimental and theoretical approaches can provide a complete description of the vibrational structure of formic acid. Using matrix-isolation Fourier-transform infrared (MI-FTIR) spectroscopy with argon and neon as hosts, we record mid-IR spectra of HCOOH, HCOOD, DCOOH, and DCOOD and assign all fundamental bands, as well as numerous combination bands, overtones, and resonances. Matrix-induced frequency shifts in neon average about 2 cm-1, with maximum shifts of 6 cm-1, making neon a close approximation to the gas phase IR spectrum. Anharmonic vibrational calculations based on a high-quality potential energy surface (PES) reproduce gas phase reference data with mean absolute deviations of ∼2 cm-1 for the monomer and ∼4 cm-1 for the dimer. Taken together, the calculations approach spectroscopic accuracy for the matrix-isolation data. We demonstrate this accuracy for the trans-formic acid monomer and its deuterated isotopocules (HCOOH, HCOOD, DCOOH, DCOOD) as well as for the cyclic non-polar dimer in C2h symmetry ((HCOOH)2, (DCOOD)2), using both vibrational perturbation theory (VPT2) and vibrational configuration interaction theory (VCI). Our results reconnect the diverse threads of formic acid IR spectroscopy and establish a framework for interpreting matrix-, isotope-, and cluster-induced frequency shifts.

Although ice polymorphs commonly feature orientational order and disorder, it is difficult to grasp the nature of partial order. In this study, we report on the hydrogen ordering of ice V using calorimetry at ambient pressure with an isothermal annealing approach. H2O/D2O isotopic substitution underlines the existence of the partially ordered intermediate state between ice XIII (below 113 K) and ice V (above 120 K), which exhibits a large isotope effect on the enthalpy of hydrogen disordering. Combined with the observation of two-staged time evolution of hydrogen order and the significant deuteration-induced slowdown of the ordering kinetics by a factor of 15-60, we propose that this intermediate state bears dynamic disorder. This reflects mutual conversions of ordered configurations taking place, i.e., domain fluctuations between differently ordered configurations. This finding raises a new perspective to characterize partial order, leading to the potential application toward frustrated functional materials.

The UV-Vis spectra of H2CO3 are investigated in a combined experimental and theoretical approach. A sample of solid H2CO3, prepared by electron irradiation of water-carbon dioxide ice, shows characteristics of both amorphous and crystalline H2CO3 in the infrared spectrum. To rationalize the experimentally observed redshift in the UV-Vis spectra between monomer and bulk H2CO3, a systematic computational study is devised using time-dependent density functional theory. H2CO3 is investigated from the monomer to (H2CO3)n clusters, with n up to 66; in addition regular oligomer arrangements derived from previously proposed ambient-pressure H2CO3 crystal structures are also examined. The calculations explain the UV-Vis absorption of solid carbonic acid, which is redshifted by ≈2 eV and ≈5 eV compared to the experimentally observed adiabatic ionization energy of the H2CO3 monomer. It is highlighted how these shifts emerge due to 1) increasing cluster size, 2) nonplanar arrangements, and 3) noncovalent interactions between H2CO3 chains and sheets. The study aims to establish spectrum-to-structure relationships and serves as computational reference data for astrochemical applications in the absence of experimental laboratory data of H2CO3 oligomers.

We identify hydrogen ordering in H2O ices spectroscopically in the near-infrared (NIR) range (10 000-4000 cm-1/1-2.5 mu m) based on the example of ices V/XIII. Previously it was thought that hydrogen ordering can only be revealed on the basis of lattice phonons, i.e., intermolecular vibrations. Here we show differences in the overtone spectrum of the intramolecular OH-stretching vibration. This makes NIR spectroscopy the first remote sensing method that is sensitive to different orientations of the water dipoles within ice. As such it will allow for future observations of the hydrogen order of ices in space by the James-Webb Space Telescope or the JUICE mission.

The process of merging bubbles or droplets with one another or with a continuous phase is known as coalescence.⁠1 It takes place from the microscale, e.g., in technological applications such as contrast-enhanced ultrasound in medicine, to the macroscale, e.g., in planet and star formation. Coalescence of water droplets is an important process in nature, e.g., in Earth’s troposphere for growth of raindrops⁠2. Usually coalescence is studied at ambient temperature, but even in clouds it takes place in the temperature range down to 235 K, where water is supercooled and metastable with respect to crystalline ice.⁠3 In space, transport of molecules between water interfaces takes place at even harsher conditions in the deeply supercooled or glassy state, down to 10 K. This is for example the case for grainy amorphous ice covering interstellar dust particles in molecular clouds⁠4. Yet, it is currently unknown whether droplet coalescence occurs in deeply supercooled water at all and if so at what time scale. Here we observe that micrometer-sized glassy water droplets coalesce between ~123 and ~140 K upon slow heating on the time scale of hours to minutes based on small-angle X-ray scattering as well as scanning electron microscopy experiments. Droplet interfaces start to vanish close to water’s first glass transition temperature, indicating that water molecules experience translational motion across the droplet interfaces even under cryo-conditions. This means that glassy low-density water turns into a viscous liquid at its glass transition temperature, ruling out the possibility of an orientational glass transition⁠5 or point defect dynamics⁠6. The latter cases would not lead to a supercooled liquid but to a solid with rotational disorder. This finding helps to resolve the debate of whether amorphous ice is thermodynamically continuously connected to deeply supercooled liquid water in terms of a glass transition⁠7–12 and provides us with an ultraslow-motion observation of the processes of coalescence at high viscosities. The idea in coalescence theory13 that the viscous force arrests the droplet interfaces does not hold at 125 K, where water is of ultrahigh viscosity, but coalescence takes place.

The replacement of conventional fuels with sustainable and renewable green fuels such as biodiesel to protect the environment is an important focus of today's research and a strong area of interest among students. Here, we develop a research-oriented experiment in the context of renewable fuels aimed at introducing fundamental concepts in physical chemistry, organic chemistry, and analytical chemistry to undergraduate chemistry students. Specifically, the students first carry out the organic chemistry task of base-catalyzed transesterification of rapeseed oil to biodiesel. Second, they execute the analytical chemistry task of observing the time dependent, temperature-controlled increase of fatty acid methyl ester concentration using gas chromatography based on flame ionization detection (GC-FID). Finally, the students carry out the physical chemistry task of extracting kinetic properties from the data, including the concepts of rate constants, reaction order, activation energies, catalysts, and mathematical modeling. These data reveal the transesterification is either a pseudo first or second order reaction, where the kinetics change a few minutes after the start of the reaction. The activation energies of 51 kJ mol-1 (pseudo first order reaction) and 52 kJ mol-1 (second order reaction) determined here are similar to literature values for similar oils and similar catalysts. Here, we have developed an inquiry-based, research-oriented experiment emphasizing deductive reasoning, which arises in the context of renewable fuels aimed at undergraduate students.

In this work, we focus on the low-temperature behavior of concentrated aqueous solutions of cesium chloride and discover two hydrates of CsCl. We employ four different methods, namely, (i) simple cooling at rates between 0.5 and 80 K s-1, (ii) simple cooling followed by pressurization, (iii) hyperquenching at 106 to 107 K s-1, and (iv) hyperquenching followed by pressurization. Depending on the method, different types of phase behaviors are observed, which encompass crystallization involving freeze-concentration, pressure-induced amorphization, full vitrification, and polyamorphic transformation. The CsCl hydrates discovered in our work cold-crystallize above 150 K upon heating after ultrafast vitrification (routes iii and iv) and show melting temperatures below the eutectic temperature of 251 K. We determine the composition of these hydrates to be CsCl5H2O and CsCl6H2O and find evidence for their existence in ESEM, calorimetry, and X-ray diffraction. The dominant and less metastable hydrate is the hexahydrate, where the pentahydrate appears as a minority species. We also reveal the birthplace for the CsCl hydrates, namely, the freeze-concentrated solution (FCS) formed upon cold-crystallization of the fully glassy solution (from iii and iv). The spongy FCS produced upon cooling of the liquid (from i and ii) is incapable of crystallizing CsCl-hydrates. By contrast, the FCS produced upon heating the glassy solution (from iii and iv) shows tiny, fine features that are capable of crystallizing CsCl-hydrates. Our findings contradict the current knowledge that alkali chlorides only have hydrates for the smaller cations Li+ and Na+, but not for the larger cations K+, Rb+, and Cs+ and pave the way for future determination of CsCl-hydrate crystal structures. The pathway to metastable crystalline materials outlined here might be more generally applicable and found in nature, e.g., in comets or on interstellar dust grains, when glassy aqueous solutions crystallize upon heating.

AbstractIce XIX is a partly hydrogen-ordered polymorph related to disordered ice VI, similar to ice XV. We here investigate the order–order–disorder sequence ice XIX→ice XV→ice VI based on calorimetry at ambient pressure both for D2O and H2O-ice XIX. From these data we extract configurational entropy differences between ice XIX, ice XV and ice VI. This task is complex because, unlike for all other ices, the order–disorder transition from ice XIX to ice VI takes place in two steps via ice XV. Even more challenging, these two steps take place in an overlapping manner, so that careful separation of slow kinetics is necessary. This is evidenced best by changing the heating rate in calorimetry experiments: For fast heating experiments the second step, disordering of ice XV, is suppressed because the first step, formation of ice XV from ice XIX, is too slow. The transient state ice VI‡ that is initially produced upon ice XIX decay then does not have enough time to convert to ice XV, but remains disordered all along. In order to tackle the challenge to determine the entropy difference between ice XIX and VI as well as the entropy difference between ice XV and VI we employ two different approaches that allow assessing the impact of kinetics on the entropy change. “Single peak integration” defines a kinetically limited result, but “combined peak integration” allows estimation of the true thermodynamic values. Our best estimate for the true value shows ice XIX to be much more ordered than ice XV (25 ± 3% vs 9 ± 4% of the Pauling entropy). For D2Oice XIX samples we obtain 28% of order, but only when a small number of fast H-isotope defects are used. In the second part we use these results to estimate the location of the ice XIX phase boundary both for protiated and deuterated ice XIX. The initial Clapeyron slope at ambient pressure is determined from the combination of neutron powder diffraction volume differences and calorimetry entropy differences data to be 21 K GPa−1 with an order–disorder transition temperature To-d(0.0 GPa) = 103 ± 1 K. An in situ bracketing experiment at 1.8 GPa yields To-d(1.8 GPa) = 116 ± 3 K, i.e., the phase boundary slope flattens at higher pressures. These data allow us to determine the region of thermodynamic stability of ice XIX in the phase diagram and to explain the surprising isotope shift reversal at 1.6 GPa compared to 0.0 GPa, i.e., why D2O-ice XIX disorders at lower temperatures than H2O-ice XIX at 1.6 GPa, but at higher temperatures at ambient pressures.

We present laboratory spectra of pure amorphous and crystalline H _2 O ices in the near-infrared (NIR, 1–2.5 μ m/10,000–4000 cm ^−1 ) at 80–180 K. The aim of this study is to provide spectroscopic reference data that allow remotely accessing ice properties for icy objects such as icy moons, cometary ice, or Saturn rings. Specifically, we identify new spectral markers for assessing three important properties of ices in space: (i) porosity/fluffiness, (ii) bulk density of amorphous ice, and (iii) cubicity in crystalline ice. The analysis is based on the first OH-stretching overtone (2 ν _OH ) and the combinational band at 5000 cm ^−1 /2 μ m, which are potent spectral markers for these properties. By comparison of vapor-deposited, microporous amorphous solid water, pore-free low-, high-, and very-high-density amorphous ice, we are able to separate the effect of (bulk) density from the effect of porosity on NIR-spectra of amorphous ices. This allows for clarifying a longstanding inconsistency about the density of amorphous ice vapor-deposited at low temperatures, first brought up by Jenniskens & Blake. Direct comparison of NIR spectra with powder X-ray diffractograms allows us to correlate spectral features with the number of cubic stacking layers in stacking-disordered ice I _sd , ranging from fully cubic ice I _c to fully hexagonal ice I _h . We show that exposure times for instruments on the James Webb Space Telescope are in the hour range to distinguish these properties, demonstrating the usefulness of the neglected NIR spectral range for identifying ices in space.

Water's anomalous behavior is often explained using a two-liquid model, where two types of water, high-density liquid (HDL) and low-density liquid (LDL), can be separated via a liquid-liquid phase transition (LLPT) at low temperature. Mixtures of water and the ionic liquid hydrazinium trifluoroacetate were suggested to also show an LLPT but with the advantage that there is no rapid ice crystallization hampering its observation. It remains controversial whether these solutions exhibit an LLPT or are instead associated with complex phase separation phenomena. We here show detailed low-temperature calorimetry and diffraction experiments on aqueous solutions containing hydrazinium trifluoroacetate and other similar ionic liquids, all at a solute mole fraction of x = 0.175. Hydrazinium trifluoroacetate, ammonium trifluoroacetate, ethylammonium trifluoroacetate and hydrazinium pentafluoropropionate all boast exothermic transitions unrelated to crystallization as well as remarkable structural changes upon cooling into the glassy state. We propose a model inspired by micelle formation and decomposition in surfactant solutions, which is complemented by MD simulations and allows rationalizing the rich phase behavior of our mixtures during cooling. The fundamental aspect of the model is the hydrophobic nature of fluorinated anions that enables aggregation, which is reversed upon cooling and culminates in the remarkable exothermic first-order transition observed at low temperature. That is, we assign the first-order transition not to an LLPT but to phase-separations similar to the ones when falling below the Krafft temperature. All other solutions merely show simple vitrification behavior. Still, they exhibit distinct differences in liquid fragility, which is decreased continuously with decreasing hydrophobicity of the anions. This might enable the systematic tuning of ionic liquids with the goal of designing aqueous solutions of specific fragility. The hydrophobic nature of small perfluorinated anions causes aggregation in the liquid and phase-separation upon cooling. The latter is causes an exothermic first-order transition that was previously confused with a liquid-liquid phase transition in water.

Freezing and its application is growing in popularity, yet the understanding of the nonequilibrium transformations and acidity changes that occur in frozen solutions upon thawing have remained relatively unexplored. By contrast to other acids such as nitric acid and sulfuric acid, not even the phase diagram is known fully for hydrochloric acid. Even more importantly, the nonequilibrium transformations upon heating glassy freeze-concentrated solution (FCS) are also not well understood and freeze concentration lacks quantification. This work rectifies the knowledge gap by providing the freeze-concentration factors on the example of hydrochloric acid. For this purpose, we have used differential scanning calorimetry to reveal phase changes upon heating. UV-vis spectroscopy of acid-base indicators is employed to elucidate acidity changes. All the samples reach negative values of the Hammett acidity function from -2.5 to -0.25 after freezing, showing that aqueous HCl can freeze concentrate 7-250,000 times depending on its initial concentration. We observe the glass-to-liquid transition of the freeze-concentrated glassy solution above -140 degrees C and cold crystallization of the ultraviscous FCS to HCl hydrates above -110 degrees C. Cold crystallization leads to basification, whereas acidification accompanies the subsequent melting of the eutectic ice/HCl-hexahydrate. Finally, melting of the ice immersed in solution shows basification caused by the dilution with meltwater. High (1 M) and low (<10 mM) concentrations freeze homogeneously, whereas intermediate concentrations reveal the presence of freeze-concentrated regions of higher and lower concentrations having distinct glass transition and melting temperatures.

Unraveling methanol's infrared spectrum has challenged spectroscopists for a century, with numerous loose ends still to be explored. We engage in this exploration based on experiments of isolating single methanol molecules in solid argon and neon matrices. We report infrared spectra of methanol in its natural isotopic composition and with partial and full deuteration. These experiments are accompanied by calculating wavenumbers involving anharmonicity and mode-coupling based on the vibrational configuration interaction approach. This allows for an unambiguous assignment of all fundamentals and resonances in the mid-infrared spectrum. An increasing degree of deuteration lifts resonances and aids in assigning bands uniquely. It also becomes evident that different notations typically used in chemistry or physics to describe molecular vibration from spectroscopy fail to describe the spectra appropriately. We highlight the shortcomings and suggest a more elaborate analysis using Sankey diagrams to unambiguously identify spectral features. Consequently, we demystify debated resonances occurring from various stretches and deformations of the methyl group.

We here demonstrate that small amounts of LiCl dissolved in water extend the existence window of deeply supercooled liquid water. This shift allows us to observe the isocompositional, sharp liquid-liquid transition (LLT), while this is not possible in pure water. Upon heating at ambient pressure, the hyperquenched and densified glass first turns into the high-density liquid (HDL) and then experiences the LLT to the low-density liquid (LDL) at 137-141 K and ambient pressure. At the LLT the viscosity suddenly jumps up by an order of magnitude, from similar to 4.3 x 109 Pas in HDL to similar to 3.7 x 1010 Pa s in LDL for the 3.1 mol % solution based on our calorimetric analysis. That is, the LLT takes place clearly in the ultraviscous liquid domain at ambient pressure. By contrast, the viscosity of the emerging LDL at the LLT is still in the domain of the soft glass for pure water and for solutions exceeding 5 mol % LiCl that were studied in the past. This is owing to our observation that, first, the LDL's glass transition is lowered by about 8 K to 126 K in the presence of small amounts LiCl, whereas the HDL's glass transition remains at 118 K. Second, the stability of HDL at ambient pressure is increased by high-pressure annealing, shifting the LLT to higher temperatures.

We have investigated pressure-induced amorphization (PIA) of an alcohol clathrate hydrate (CH) of cubic structure type I (sI) in the presence of NH4F utilizing dilatometry and x-ray powder diffraction. PIA occurs at 0.98 GPa at 77 K, which is at a much lower pressure than for other CHs of the same structure type. The amorphized CH also shows remarkable resistance against crystallization upon decompression. While amorphized sI CHs could not be recovered previously at all, this is possible in the present case. By contrast to other CHs, the recovery of the amorphized CHs to ambient pressure does not even require a high-pressure annealing step, where recovery without any loss of amorphicity is possible at 120 K and below. Furthermore, PIA is accessible upon compression at unusually high temperatures of up to 140 K, where it reaches the highest degree of amorphicity. Molecular dynamics simulations confirm that polar alcoholic guests, as opposed to non-polar guests, induce cage deformation at lower pressure. The substitution of NH4F into the host-lattice stabilizes the collapsed state more than the crystalline state, thereby enhancing the collapse kinetics and lowering the pressure of collapse.