The evaporation rate, free energy, and entropy of amorphous water at 150 K

Supercooled Liquid Water Diffusivity at Temperatures near the Glass Transition Temperature

Isotopically layered amorphous solid water films were used to measure the diffusivity of deeply supercooled liquid water near the glass transition. The films, composed of separate H218O and H216O layers, were grown by vapor deposition at low temperature and then heated to observe the intermixing of the isotopic layers. Very slow heating rates (as low as 10-4 K/s) were used to decouple the diffusion and crystallization processes to ensure that the observed intermixing occurred at temperatures that were well-separated from the onset of crystallization. Numerical simulations of the desorption spectra were used to extract the translational diffusivities. The diffusivities obtained in this paper are consistent with translational liquid-like motion at temperatures near and above the proposed Tg of 136 K. These findings support the idea that the melt of amorphous water, above its glass transition temperature is thermodynamically continuous with normal supercooled liquid.

Nonequilibrium Vibrational Distribution of Polyatomic Molecules Desorbed from an Ice Surface

Broadband rotational spectroscopy is coupled with buffer gas cooling to examine the population in low-frequency vibrational modes of isopropyl cyanide following temperature-programmed desorption from a neat ice. The measurements permit determination of the nascent vibrational temperature which is validated using analogous measurements of a room temperature sample. It is found that the two lowest frequency vibrational modes yield distinct vibrational temperatures which are significantly lower than the temperature of desorption. This is evidence of nonequilibrium dynamics in sublimation. Although long predicted, this is the first such evidence of this behavior for polyatomic molecules.

Medium-density amorphous ice unveils shear rate as a new dimension in water's phase diagram

Recent experiments revealed a new amorphous ice phase, medium-density amorphous ice (MDA), formed by ball-milling ice Ih at 77 K [Rosu-Finsen et al., Science 379, 474-478 (2023)]. MDA has density between that of low-density amorphous (LDA) and high-density amorphous (HDA) ices, adding to the complexity of water's phase diagram, known for its glass polyamorphism and two-state thermodynamics. The nature of MDA and its relation to other amorphous ices and liquid water remain unsolved. Here, we use molecular simulations under controlled pressure and shear rate at 77 K to produce and investigate MDA. We find that MDA formed at constant shear rate is a steady-state nonequilibrium shear-driven amorphous ice (SDA), that can be produced by shearing ice Ih, LDA, or HDA. Our results suggest that MDA could be obtained by ball-milling water glasses without crystallization interference. Increasing the shear rate at ambient pressure produces SDAs with densities ranging from LDA to HDA, revealing shear rate as a new thermodynamic variable in the nonequilibrium phase diagram of water. Indeed, shearing provides access to amorphous states inaccessible by controlling pressure and temperature alone. SDAs produced with shearing rates as high as 106 s-1 sample the same region of the potential energy landscape than hyperquenched glasses with identical density, pressure, and temperature. Intriguingly, SDAs obtained by shearing at ~108 s-1 have density, enthalpy, and structure indistinguishable from those of water "instantaneously" quenched from room temperature to 77 K over 10 ps, making them good approximants for the "true glass" of ambient liquid water.

Coverage-dependent desorption kinetics of water on a well-ordered alumina thin film surface

We have developed an experimental and analytical setup for thermal desorption spectroscopy of solid water films on surfaces. We obtain the coverage-dependent desorption kinetics of water molecules from a well-defined ultra-thin alumina/NiAl(110) surface in the coverage range of 0-2 monolayers. We use a novel deconvolution technique to eliminate the pumping delay of water vapor in the vacuum system, which has previously hindered the accurate estimation of desorption kinetic parameters, such as activation energy and pre-exponential factor. The coverage-dependent Arrhenius analysis reveals that the desorption activation energy decreases with increasing coverage in the sub-monolayer range, indicating that the water-water interaction is not attractive. We also find that the pre-exponential factor for the second layer is higher than that for the sub-monolayer. We explain this difference in terms of transition state theory and propose that entropic effects play a significant role in water desorption kinetics.

Supercooled solutions of sodium perchlorate in TIP4P/2005 water: The effect of martian solutes on thermodynamics and structure

We study the thermodynamic behavior of sodium perchlorate solutions in supercooled water through molecular dynamics numerical simulations. These solutions are of special interest because of the recent experimental results that led to hypothesize the presence of liquid water in perchlorate solutions beneath the Martian soil. We model water using the TIP4P/2005 potential. The results we obtain for solutions with concentrations 1.63 and 15.4 wt% are in agreement with those of a system undergoing a liquid-liquid phase transition where the liquid-liquid critical point shifts to slightly higher temperatures and lower pressures. The structure of the system is also analyzed, and we come to the conclusion that, even at the highest concentration considered, water retains its anomalous behavior.

Kinetics and Mechanisms of Pressure-Induced Ice Amorphization and Polyamorphic Transitions in a Machine-Learned Coarse-Grained Water Model

Water glasses have attracted considerable attention due to their potential connection to a liquid-liquid transition in supercooled water. Here we use molecular simulations to investigate the formation and phase behavior of water glasses using the machine-learned bond-order parameter (ML-BOP) water model. We produce glasses through hyperquenching of water, pressure-induced amorphization (PIA) of ice, and pressure-induced polyamorphic transformations. We find that PIA of polycrystalline ice occurs at a lower pressure than that of monocrystalline ice and through a different mechanism. The temperature dependence of the amorphization pressure of polycrystalline ice for ML-BOP agrees with that in experiments. We also find that ML-BOP accurately reproduces the density, coordination number, and structural features of low-density (LDA), high-density (HDA), and very high-density (VHDA) amorphous water glasses. ML-BOP accurately reproduces the experimental radial distribution function of LDA but overpredicts the minimum between the first two shells in high-density glasses. We examine the kinetics and mechanism of the transformation between low-density and high-density glasses and find that the sharp nature of these transitions in ML-BOP is similar to that in experiments and all-atom water models with a liquid-liquid transition. Transitions between ML-BOP glasses occur through a spinodal-like mechanism, similar to ice crystallization from LDA. Both glass-to-glass and glass-to-ice transformations have Avrami-Kolmogorov kinetics with exponent n = 1.5 ± 0.2 in experiments and simulations. Importantly, ML-BOP reproduces the competition between crystallization and HDA→LDA transition above the glass transition temperature T_g, and separation of their time scales below T_g, observed also in experiments. These findings demonstrate the ability of ML-BOP to accurately reproduce water properties across various regimes, making it a promising model for addressing the competition between polyamorphic transitions and crystallization in water and solutions.

Thickness dependent homogeneous crystallization of ultrathin amorphous solid water films

The crystallization mechanism and kinetics of amorphous materials are of paramount importance not only in basic science but also in the application field because they are closely related to their thermal stability. In the case of amorphous nanomaterials, thermal stability distinctively different from that of bulk materials often emerges. Despite intensive studies in the past, a thorough understanding of the stability at the molecular level has not been reached particularly on how crystallization processes depend on size and are influenced by their surface and interface. In this article, we report the film-size-dependent crystallization of thermally relaxed nonporous ASW ultrathin films on a Pt(111) surface as a benchmark system of amorphous molecular films. The crystallization processes at the surface and interior of the ASW ultrathin films are monitored simultaneously with thermal desorption and infrared reflection absorption, respectively, as a function of the film thickness. Here, we demonstrate that the crystallization is initiated solely by "homogeneous nucleation" irrespective of the film thickness while the crystallization rate remarkably depends on the thickness; the rate of 5-layer (∼1.5 nm) ASW films is one order of magnitude higher than that of 20-layer (∼6 nm) films. Moreover, we found a clear correlation between the film-thickness-dependent crystallization kinetics and microscopic structural disorder associated with the broad distribution of hydrogen-bond lengths between water molecules.

Millimeter/Submillimeter Spectroscopic Detection of Desorbed Ices: A New Technique in Laboratory Astrochemistry

A new laboratory technique has been developed that utilizes gas-phase, direct-absorption millimeter and submillimeter spectroscopy to detect and identify desorbed species from interstellar and cometary ice analogues. Rotational spectroscopy is a powerful structure-specific technique for detecting isomers and other species possessing the same mass that are indistinguishable with mass spectrometry. Furthermore, the resultant laboratory spectra are directly comparable to observational data from far-infrared and millimeter telescopes. Here, we present the proof-of-concept measurements of the detection of thermally desorbed H₂O, D₂O, and CH₃OH originating in a solid film created at low temperature (∼12 K). The surface binding energy of H₂O is reported and compared to results from traditional techniques, including mass spectrometry and quartz-crystal microbalance measurements of mass loss. Lastly, we demonstrate that this technique can be used to derive thermodynamic values including the sublimation enthalpy and entropy of H₂O.

Volatility of Amorphous Solid Water

Amorphous solid water is probably the most abundant form of solid water in the universe. Its saturation vapor pressure and thermodynamic properties, however, are not well known. We have investigated the saturation vapor pressure over vapor-deposited amorphous ice at temperatures between 133 and 147 K using a novel experimental method. The new method determines the absolute vapor pressures and the sublimation rates by measuring the mass growth rates of ice-covered nanoparticles under supersaturated water vapor conditions. We find that the vapor pressure of amorphous solid water is up to a factor of 3 higher than that predicted by current parameterizations, which are based in part on calorimetric measurements. We demonstrate that the calorimetric measurements can be reconciled with our data by acknowledging the formation of nanocrystalline ice as an intermediate ice phase during the crystallization of amorphous ice. As a result, we propose a new value for the enthalpy of crystallization of amorphous solid water of Δ H = 2312 ± 227 J/mol, which is about 1000 J/mol higher than the current consensus. Our results shine a new light on the abundance of water ice clouds on Mars and mesospheric clouds on Earth and may alter our understanding of ice formation in the stratosphere.

Temperature-dependent Shape Changes of Ice Nanoclusters on Ag(100)

We investigate the influence of the annealing temperature on the evolution of the ice nanoclusters' geometry by means of low-temperature scanning tunneling microscopy. The clusters, grown at 110 K on Ag(100), gradually increase in height and their shape becomes more compact during annealing at 120 K, 125 K, and 130 K. The increase in height indicates an upward mass transport and reflects a stronger water-water than water-surface bonding. The change in shape, quantitatively expressed as an increase in fractal dimension, is driven by a reduction of the total energy of the step edge. The significant changes in geometry induced by a relatively mild temperature increase underline the importance of temperature for the shape and all properties influenced by this shape of hydrogen-bonded clusters of water ice.

Hypercooling Temperature of Water is about 100 K Higher than Calculated before

For deeply supercooled liquids the transition from a two-stage freezing process to complete solidification in just one freezing step occurs at the hypercooling temperature, a term that seems to be almost unknown in water research; to our knowledge, it has only been mentioned by Dolan et al. for high-pressure ice. The reason for the absence of this expression may be that the best estimate to be found in the literature for the hypercooling temperature of water is about -160 °C (113 K). This temperature is far below the limit of experimentally realizable degrees of supercooling near -40 °C (233 K), which marks the homogeneous nucleation temperature T_H of common pure water; in fact, it is even below the glass-transition temperature (133 K). Here we show that, surprisingly, a more thorough analysis taking into account the temperature dependence of the heat capacities of water and ice as well as of the enthalpy of freezing shows that the hypercooling temperature of water is about -64 °C or 209 K, almost 100 K higher than estimated before. One of the most exciting consequences is that existing experiments are already able to reach these degrees of supercooling, and it is our prediction that a transition in the freezing behavior occurs at these temperatures.

Preparation-Dependent Orientation of Crystalline Ice Islands on Ag(111)

We observe the transformation of fractal ice islands grown at 96 K to compact ones annealed at 118 K and compare those to compact islands grown directly at 118 K. The low-temperature grown islands form a four bilayer high wetting layer. The annealing causes a crystallization and reshaping of the islands and a substantial increase in height and roughness in particular at higher coverage. Moreover, it leads to a dewetting of the ice film. The islands grown at the higher temperature show qualitative similarities to the annealed ones at smaller nucleation density. However, their orientation with respect to the surface differs by 30° as compared to the annealed islands.

A nanosecond pulsed laser heating system for studying liquid and supercooled liquid films in ultrahigh vacuum

A pulsed laser heating system has been developed that enables investigations of the dynamics and kinetics of nanoscale liquid films and liquid/solid interfaces on the nanosecond time scale in ultrahigh vacuum (UHV). Details of the design, implementation, and characterization of a nanosecond pulsed laser system for transiently heating nanoscale films are described. Nanosecond pulses from a Nd:YAG laser are used to rapidly heat thin films of adsorbed water or other volatile materials on a clean, well-characterized Pt(111) crystal in UHV. Heating rates of ∼10(10) K/s for temperature increases of ∼100-200 K are obtained. Subsequent rapid cooling (∼5 × 10(9) K/s) quenches the film, permitting in-situ, post-heating analysis using a variety of surface science techniques. Lateral variations in the laser pulse energy are ∼±2.7% leading to a temperature uncertainty of ∼±4.4 K for a temperature jump of 200 K. Initial experiments with the apparatus demonstrate that crystalline ice films initially held at 90 K can be rapidly transformed into liquid water films with T > 273 K. No discernable recrystallization occurs during the rapid cooling back to cryogenic temperatures. In contrast, amorphous solid water films heated below the melting point rapidly crystallize. The nanosecond pulsed laser heating system can prepare nanoscale liquid and supercooled liquid films that persist for nanoseconds per heat pulse in an UHV environment, enabling experimental studies of a wide range of phenomena in liquids and at liquid/solid interfaces.

Desorption and crystallisation of binary 2-propanol and water ices adsorbed on graphite

Strong interactions between 2-propanol and water ice cause marked changes in the crystallisation kinetics and desorption of water.

Peeling the astronomical onion

This work presents a study of water mobility on interstellar dust grain analogues at temperatures as low as 18 K. The work indicates that water forms pure domains rather than covering the entire grain, thereby leaving bare dust grain surfaces available on which other molecules can adsorb as well as themselves providing surfaces for further adsorption from the interstellar gas.

Molecular interactions with ice: molecular embedding, adsorption, detection, and release

The interaction of atomic and molecular species with water and ice is of fundamental importance for chemistry. In a previous series of publications, we demonstrated that translational energy activates the embedding of Xe and Kr atoms in the near surface region of ice surfaces. In this paper, we show that inert molecular species may be absorbed in a similar fashion. We also revisit Xe embedding, and further probe the nature of the absorption into the selvedge. CF4 molecules with high translational energies (≥3 eV) were observed to embed in amorphous solid water. Just as with Xe, the initial adsorption rate is strongly activated by translational energy, but the CF4 embedding probability is much less than for Xe. In addition, a larger molecule, SF6, did not embed at the same translational energies that both CF4 and Xe embedded. The embedding rate for a given energy thus goes in the order Xe > CF4 > SF6. We do not have as much data for Kr, but it appears to have a rate that is between that of Xe and CF4. Tentatively, this order suggests that for Xe and CF4, which have similar van der Waals radii, the momentum is the key factor in determining whether the incident atom or molecule can penetrate deeply enough below the surface to embed. The more massive SF6 molecule also has a larger van der Waals radius, which appears to prevent it from stably embedding in the selvedge. We also determined that the maximum depth of embedding is less than the equivalent of four layers of hexagonal ice, while some of the atoms just below the ice surface can escape before ice desorption begins. These results show that energetic ballistic embedding in ice is a general phenomenon, and represents a significant new channel by which incident species can be trapped under conditions where they would otherwise not be bound stably as surface adsorbates. These findings have implications for many fields including environmental science, trace gas collection and release, and the chemical composition of astrophysical icy bodies in space.

Volume crossover in deeply supercooled water adiabatically freezing under isobaric conditions

The irreversible return of a supercooled liquid to stable thermodynamic equilibrium often begins as a fast process which adiabatically drives the system to solid-liquid coexistence. Only at a later stage will solidification proceed with the expected exchange of thermal energy with the external bath. In this paper we discuss some aspects of the adiabatic freezing of metastable water at constant pressure. In particular, we investigated the thermal behavior of the isobaric gap between the molar volume of supercooled water and that of the warmer ice-water mixture which eventually forms at equilibrium. The available experimental data at ambient pressure, extrapolated into the metastable region within the scheme provided by the reference IAPWS-95 formulation, show that water ordinarily expands upon (partially) freezing under isenthalpic conditions. However, the same scheme also suggests that, for increasing undercoolings, the volume gap is gradually reduced and eventually vanishes at a temperature close to the currently estimated homogeneous ice nucleation temperature. This behavior is contrasted with that of substances which do not display a volumetric anomaly. The effect of increasing pressures on the alleged volume crossover from an expanded to a contracted ice-water mixture is also discussed.

Fragile-to-strong crossover coupled to the liquid-liquid transition in hydrophobic solutions

Using discrete molecular dynamics simulations we study the relation between the thermodynamic and diffusive behaviors of a primitive model of aqueous solutions of hydrophobic solutes consisting of hard spheres in the Jagla particles solvent, close to the liquid-liquid critical point of the solvent. We find that the fragile-to-strong dynamic transition in the diffusive behavior is always coupled to the low-density-high-density liquid transition. Above the liquid-liquid critical pressure, the diffusivity crossover occurs at the Widom line, the line along which the thermodynamic response functions show maxima. Below the liquid-liquid critical pressure, the diffusivity crossover occurs when the limit of mechanical stability lines are crossed, as indicated by the hysteresis observed when going from high to low temperature and vice versa. These findings show that the strong connection between dynamics and thermodynamics found in bulk water persists in hydrophobic solutions for concentrations from low to moderate, indicating that experiments measuring the relaxation time in aqueous solutions represent a viable route for solving the open questions in the field of supercooled water.

Breaking Through the Glass Ceiling: Recent Experimental Approaches to Probe the Properties of Supercooled Liquids near the Glass Transition

Experimental measurements of the properties of supercooled liquids at temperatures near their glass transition temperatures, Tg, are requisite for understanding the behavior of glasses and amorphous solids. Unfortunately, many supercooled molecular liquids rapidly crystallize at temperatures far above their Tg, making such measurements difficult to nearly impossible. In this Perspective, we discuss some recent alternative approaches to obtain experimental data in the temperature regime near Tg. These new approaches may yield the additional experimental data necessary to test current theoretical models of the dynamical slowdown that occurs in supercooled liquids approaching the glass transition.

Water confined in MCM-41: a mode coupling theory analysis

In this paper we analyze molecular dynamics simulation results on supercooled water in a MCM-41 pore in order to test the mode coupling theory. A layer analysis must be performed for water in the pore in order to exclude the contribution of water bound to the strongly hydrophilic surface. Upon supercooling a range of temperatures is reached where the liquid follows the mode coupling theory. From the power law behavior of the relaxation times extracted from the Kohlrausch-William-Watts fit to the self-intermediate scattering function, we obtain the crossover temperature T(C) and the γ exponent of the theory. The time-temperature superposition principle is also satisfied. A fit to the von Schweidler law yields a coefficient b from which all the other parameters of the theory have been calculated. In particular, we obtained the same value of γ as extracted from the power law fit to the relaxation times, in agreement with the requirements of the theory. For very low temperatures, the mode coupling theory no longer holds as hopping processes intervene and water turns its behavior to that of a strong liquid.

Thermal and nonthermal physiochemical processes in nanoscale films of amorphous solid water

Amorphous solid water (ASW) is a disordered version of ice created by vapor deposition onto a cold substrate (typically less than 130 K). It has a higher free energy than the crystalline phase of ice, and when heated above its glass transition temperature, it transforms into a metastable supercooled liquid. This unusual form of water exists on earth only in laboratories, after preparation with highly specialized equipment. It is thus fair to ask why there is any interest in studying such an esoteric material. Much of the scientific interest results from the ability to use ASW as a model system for exploring the physical and reactive properties of liquid water and aqueous solutions. ASW is also thought to be the predominant form of water in the extremely cold temperatures of many interstellar and planetary environments. In addition, ASW is a convenient model system for studying the stability of amorphous and glassy materials as well as the properties of highly porous materials. A fundamental understanding of such properties is invaluable in a diverse range of applications, including cryobiology, food science, pharmaceuticals, astrophysics, and nuclear waste storage, among others. Over the past 15 years, we have used molecular beams and surface science techniques to probe the thermal and nonthermal properties of nanoscale films of ASW. In this Account, we present a survey of our research on the properties of ASW using this approach. We use molecular beams to precisely control the deposition conditions (flux, incident energy, and incident angle) and create compositionally tailored, nanoscale films of ASW at low temperatures. To study the transport properties (viscosity and diffusivity), we heat the amorphous films above their glass transition temperature, T(g), at which they transform into deeply supercooled liquids prior to crystallization. The advantage of this approach is that at temperatures near T(g), the viscosity is approximately 15 orders of magnitude larger than that of a normal liquid. As a result, the crystallization kinetics are dramatically slowed, increasing the time available for experiments. For example, near T(g), a water molecule moves less than the distance of a single molecule on a typical laboratory time scale (∼1000 s). For this reason, nanoscale films help to probe the behavior and reactions of supercooled liquids at these low temperatures. ASW films can also be used for investigating the nonthermal reactions relevant to radiolysis.

Liquid-liquid coexistence in NaCl aqueous solutions: a simulation study of concentration effects

In this paper we investigate by means of molecular dynamics computer simulations how the hypothesized liquid-liquid critical point of water shifts in supercooled aqueous solutions of salt as a function of concentration. We study sodium chloride solutions in TIP4P water, NaCl(aq), for concentrations c = 1.36 mol/kg and c = 2.10 mol/kg. The liquid-liquid critical point is found up to the highest concentration investigated, and its position in the P-T plane shifts to higher temperatures and lower pressures upon increasing concentration. For c = 2.10 mol/kg it is also located very close to the temperature of maximum density line of the system. The results are discussed and compared with previous results for bulk TIP4P water and for c = 0.67 mol/kg NaCl(aq) and with experimental findings. We observe a progressive shrinkage of the low-density liquid region when the concentration of salt increases; this suggests an eventual disappearance of the liquid-liquid coexistence upon further increase of NaCl concentration.

Dewetting growth of crystalline water ice on a hydrogen saturated Rh(111) surface at 135 K

We investigated the water (D(2)O) adsorption at 135 K on a hydrogen pre-adsorbed Rh(111) surface using temperature programmed desorption and infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum. With increasing the hydrogen coverage, the desorption temperature of water decreases. At the saturation coverage of hydrogen, dewetting growth of water ice was observed: large three-dimensional ice grains are formed. The activation energy of water desorption from the hydrogen-saturated Rh(111) surface is estimated to be 51 kJ/mol. The initial sticking probability of water decreases from 0.46 on the clean surface to 0.35 on the hydrogen-saturated surface. In IRAS measurements, D-down species were not observed on the hydrogen saturated surface. The present experimental results clearly show that a hydrophilic Rh(111) clean surface changes into a hydrophobic surface as a result of hydrogen adsorption.

Site competition during coadsorption of acetone with methanol and water on TiO2(110)

The competitive interaction between acetone and two solvent molecules (methanol and water) for surface sites on rutile TiO(2)(110) was studied using temperature-programmed desorption (TPD). On a vacuum-annealed TiO(2)(110) surface, which possessed ~5% oxygen vacancy sites, excess methanol displaced preadsorbed acetone molecules to weakly bound and physisorbed desorption states below 200 K. In contrast, acetone molecules were stabilized on an oxidized surface against displacement by methanol through formation of acetone diolate species. The behavior of acetone with methanol differs from the interactions between acetone and water which are less competitive. Examination of acetone + methanol and acetone + water multilayer combinations shows that acetone is more compatible in water-ice films than in methanol-ice films, presumably because water has greater potential as a hydrogen-bond donor than does methanol. Acetone molecules displaced from the TiO(2)(110) surface by water are more likely to be retained in the near-surface region, in turn having a greater opportunity to revisit the surface, than when methanol is used as a coadsorbate.

On the reality of the residual entropies of glasses and disordered crystals: counting microstates, calculating fluctuations, and comparing averages

In the course of an on-going debate on whether glasses or disordered crystals should have zero entropy at 0 K, i.e., whether the "residual entropy" assigned to them by calorimetric measurements is real, the view has been expressed by some who hold the zero entropy view that to measure entropy, all or an appreciable number of the microstates that contribute to the entropy must be visited. We show here that the entropy calculated on the basis of the number of microstates visited during any conceivable time of measurement would be underestimated by at least 20 orders of magnitude. We also examine and refute the claim that an ensemble average for glassy systems, which predicts a finite residual entropy, also predicts physically impossible properties. We conclude that calorimetrically measured residual entropies are real.