IR spectra of water droplets in no man’s land and the location of the liquid-liquid critical point

Isotope effects on the structural transformation and relaxation of deeply supercooled water

We have examined the structure of supercooled liquid D2O as a function of temperature between 185 and 255 K using pulsed laser heating to rapidly heat and cool the sample on a nanosecond timescale. The liquid structure can be represented as a linear combination of two structural motifs, with a transition between them described by a logistic function centered at 218 K with a width of 10 K. The relaxation to a metastable state, which occurred prior to crystallization, exhibited nonexponential kinetics with a rate that was dependent on the initial structural configuration. When the temperature is scaled by the temperature of maximum density, which is an isostructural point of the isotopologues, the structural transition and the non-equilibrium relaxation kinetics of D2O agree remarkably well with those for H2O.

Breakdown of the Stokes-Einstein Relation in Supercooled Water/Methanol Binary Mixtures: Explanation Using the Translational Jump-Diffusion Approach

A recent experiment has directly checked the validity of the Stokes-Einstein (SE) relation for pure water, pure methanol, and their binary mixtures of three different compositions at different temperatures. The effect of composition on the nature of breakdown of the SE relation is interesting. While in the majority of the systems, an increasing SE breakdown is observed with decreasing temperature, the breakdown is already significant at higher temperatures for the equimolar mixture. Violations of the SE relation in pure supercooled water at different temperatures and pressures have been previously explained using the translational jump-diffusion (TJD) approach, which provides a fundamental molecular basis, by directly connecting the SE breakdown with jump-diffusion of the molecules. We have used the same TJD approach for explaining the SE breakdown for the methanol/water binary mixtures of compositions studied in the experiment over a wide range of temperatures between 220 K and 300 K. We have understood that the jump-diffusion is the key responsible factor for the SE breakdown. The maximum jump-diffusion contribution gives rise to the early SE breakdown observed for the equimolar mixture observed in the experiment. This study, therefore, provides molecular insight into the SE breakdown for the supercooled water/methanol binary mixture, as found in the experiment.

The anomalies and criticality of liquid water

The origin of water's anomalies has been a matter of long-standing debate. A two-state model, dating back to Röntgen, relies on the dynamical coexistence of two types of local structures-locally favored tetrahedral structure (LFTS) and disordered normal-liquid structure (DNLS)-in liquid water. Phenomenologically, this model not only explains water's thermodynamic anomalies but also can rationalize the existence of a liquid-liquid critical point (LLCP) if there is a cooperative formation of LFTS. We recently found direct evidence for the coexistence of LFTS and DNLS in the experimental structure factor of liquid water. However, the existence of the LLCP and its impact on water's properties has remained elusive, leaving the origin of water's anomalies unclear. Here we propose a unique strategy to locate the LLCP of liquid water. First, we make a comprehensive analysis of a large set of experimental structural, thermodynamic, and dynamic data based on our hierarchical two-state model. This model predicts that the two thermodynamic and dynamical fluctuation maxima lines should cross at the LLCP if it exists, which we confirm by hundred-microsecond simulations for model waters. Based on recent experimental results of the compressibility and diffusivity measurements in the no man's land, we reveal that the two lines cross around 184 K and 173 MPa for real water, suggesting the presence of the LLCP around there. Nevertheless, we find that the criticality is almost negligible in the experimentally accessible region of liquid water because it is too far from the LLCP. Our findings would provide a clue to settle the long-standing debate.

Reversible structural transformations in supercooled liquid water from 135 to 245 K

A fundamental understanding of the unusual properties of water remains elusive because of the limited data at the temperatures and pressures needed to decide among competing theories. We investigated the structural transformations of transiently heated supercooled water films, which evolved for several nanoseconds per pulse during fast laser heating before quenching to 70 kelvin (K). Water’s structure relaxed from its initial configuration to a steady-state configuration before appreciable crystallization. Over the full temperature range investigated, all structural changes were reversible and reproducible by a linear combination of high- and low-temperature structural motifs. The fraction of the liquid with the high-temperature motif decreased rapidly as the temperature decreased from 245 to 190 K, consistent with the predictions of two-state “mixture” models for supercooled water in the supercritical regime.

"On-The-Fly" Calculation of the Vibrational Sum-Frequency Generation Spectrum at the Air-Water Interface

In the present work, we provide an electronic structure based method for the "on-the-fly" determination of vibrational sum frequency generation (v-SFG) spectra. The predictive power of this scheme is demonstrated at the air-water interface. While the instantaneous fluctuations in dipole moment are obtained using the maximally localized Wannier functions, the fluctuations in polarizability are approximated to be proportional to the second moment of Wannier functions. The spectrum henceforth obtained captures the signatures of hydrogen bond stretching, bending, as well as low-frequency librational modes.

Time-dependent vibrational sum-frequency generation spectroscopy of the air-water interface

Vibrational sum-frequency generation spectroscopy is a powerful method to study the microscopic structure and dynamics of interfacial systems. Here we demonstrate a simple computational approach to calculate the time-dependent, frequency-resolved vibrational sum-frequency generation spectrum (TD-vSFG) of the air-water interface. Using this approach, we show that at the air-water interface, the transition of water molecules with bonded OH modes to free OH modes occurs at a time scale of $$\sim$$ ~ 3 ps, whereas water molecules with free OH modes rapidly make a transition to a hydrogen-bonded state within $$\sim$$ ~ 2 ps. Furthermore, we also elucidate the origin of the observed differential dynamics based on the time-dependent evolution of water molecules in the different local solvent environments.

Vibrational echo spectroscopy of aqueous sodium bromide solutions from first principles simulations

A theoretical study of the time-dependent vibrational echo spectroscopy of sodium bromide solutions in deuterated water at two different concentrations of 0.5 and 5.0 M and at temperatures of 300 and 350 K is presented using the method of ab initio molecular dynamics simulations. The instantaneous fluctuations in frequencies of local OD stretch modes are calculated using time-series analysis of the simulated trajectories. The third-order polarization and intensities of three pulse photon-echo are calculated from ab initio simulations. The timescales of vibrational spectral diffusion are determined from the frequency time correlation functions (FTCF) and short-time slope of three pulse photon echo (S3PE) calculated within the second-order cumulant and Condon approximations. It is found that under ambient conditions, the rate of vibrational spectral diffusion becomes slower with increase in ionic concentration. Decay of S3PE calculated for different systems give timescales, which are in close agreement with those of FTCF and also with the results of experimental time-dependent vibrational spectroscopic experiments. © 2019 Wiley Periodicals, Inc.

Temperature dependence of the ultrafast vibrational echo spectroscopy of OD modes in liquid water from first principles simulations

The temperature dependence of the vibrational spectral diffusion of OD modes in liquid water is investigated through calculations of vibrational echo spectral observables from first principles molecular dynamics.

Common microscopic structural origin for water's thermodynamic and dynamic anomalies

Water displays a vast array of unique properties, known as water's anomalies, whose origin remains subject to hot debate. Our aim in this article is to provide a unified microscopic physical picture of water's anomalies in terms of locally favored structures, encompassing both thermodynamic and dynamic anomalies, which are often attributed to different origins. We first identify locally favored structures via a microscopic structural descriptor that measures local translational order and provide direct evidence that they have a hierarchical impact on the anomalies. At each state point, the strength of thermodynamic anomalies is directly proportional to the amount of locally favored structures, while the dynamic properties of each molecule depend on the local structure surrounding both itself and its nearest neighbors. To incorporate this, we develop a novel hierarchical two-state model. We show by extensive simulations of two popular water models that both thermodynamic and kinetic anomalies can be almost perfectly explained by the temperature and pressure dependence of these local and non-local versions of the same structural descriptor, respectively. Moreover, our scenario makes three unique predictions in supercooled water, setting it apart from other scenarios: (1) Presence of an "Arrhenius-to-Arrhenius" crossover upon cooling, as the origin of the apparent "fragile-to-strong" transition; (2) maximum of dynamic heterogeneity around 20 K below the Widom line and far above the glass transition; (3) violation of the Stokes-Einstein-Debye relation at ∼2T _g, rather than 1.2T _g typical of normal glass-formers. These predictions are verified by recent measurement of water's diffusion at very low temperatures (point 1) and discoveries from our extensive simulations (points 2-3). We suggest that the same scenario may generally apply to water-like anomalies in liquids tending to form locally favored structures, including not only other important tetrahedral liquids such as silicon, germanium, and silica, but also metallic and chalcogenide liquids.

Vibrational Spectroscopy of Water with High Spatial Resolution

The ability to examine the vibrational spectra of liquids with nanometer spatial resolution will greatly expand the potential to study liquids and liquid interfaces. In fact, the fundamental properties of water, including complexities in its phase diagram, electrochemistry, and bonding due to nanoscale confinement are current research topics. For any liquid, direct investigation of ordered liquid structures, interfacial double layers, and adsorbed species at liquid-solid interfaces are of interest. Here, a novel way of characterizing the vibrational properties of liquid water with high spatial resolution using transmission electron microscopy is reported. By encapsulating water between two sheets of boron nitride, the ability to capture vibrational spectra to quantify the structure of the liquid, its interaction with the liquid-cell surfaces, and the ability to identify isotopes including H2 O and D2 O using electron energy-loss spectroscopy is demonstrated. The electron microscope used here, equipped with a high-energy-resolution monochromator, is able to record vibrational spectra of liquids and molecules and is sensitive to surface and bulk morphological properties both at the nano- and micrometer scales. These results represent an important milestone for liquid and isotope-labeled materials characterization with high spatial resolution, combining nanoscale imaging with vibrational spectroscopy.

Solvation Layer of Antifreeze Proteins Analyzed with a Markov State Model

Three structurally very different antifreeze proteins (AFPs) are studied, addressing the question as to what extent the hypothesized preordering-binding mechanism is still relevant in the second solvation layer of the protein and beyond. Assuming a two-state model of water, the solvation layers are analyzed with the help of molecular dynamics simulations together with a Markov state model, which investigates the local tedrahedrality of the water hydrogen-bond network around a given water molecule. It has been shown previously that this analysis can discriminate the high-entropy, high-density state of the liquid (HDL) from its more structured low-density state (LDL). All investigated proteins, regardless of whether they are an AFP or not, have a tendency to increase the amount of HDL in their second solvation layer. The ice binding site (IBS) of the antifreeze proteins counteracts that trend, with either a hole in the HDL layer or a true excess of LDL. The results correlate to a certain extent with recent experiments, which have observed ice-like vibrational (VSFG) spectra for the water atop the IBS of only a subset of antifreeze proteins. It is concluded that the preordering-binding mechanism indeed seems to play a role but is only part of the overall picture.

Heating- and pressure-induced transformations in amorphous and hexagonal ice: A computer simulation study using the TIP4P/2005 model

We characterize the phase behavior of glassy water by performing extensive out-of-equilibrium molecular dynamics simulations using the TIP4P/2005 water model. Specifically, we study (i) the pressure-induced transformations between low-density (LDA) and high-density amorphous ice (HDA), (ii) the pressure-induced amorphization (PIA) of hexagonal ice (I_h), (iii) the heating-induced LDA-to-HDA transformation at high pressures, (iv) the heating-induced HDA-to-LDA transformation at low and negative pressures, (v) the glass transition temperatures of LDA and HDA as a function of pressure, and (vi) the limit of stability of LDA upon isobaric heating and isothermal decompression (at negative pressures). These transformations are studied systematically, over a wide range of temperatures and pressures, allowing us to construct a P-T phase diagram for glassy TIP4P/2005 water. Our results are in qualitative agreement with experimental observations and with the P-T phase diagram obtained for glassy ST2 water that exhibits a liquid-liquid phase transition and critical point. We also discuss the mechanism for PIA of ice I_h and show that this is a two-step process where first, the hydrogen-bond network (HBN) is distorted and then the HBN abruptly collapses. Remarkably, the collapse of the HB in ice I_h occurs when the average molecular orientations order, a measure of the tetrahedrality of the HBN, is of the same order as in LDA, suggesting a common mechanism for the LDA-to-HDA and I_h-to-HDA transformations.

Coherent X-rays reveal the influence of cage effects on ultrafast water dynamics

The dynamics of liquid water feature a variety of time scales, ranging from extremely fast ballistic-like thermal motion, to slower molecular diffusion and hydrogen-bond rearrangements. Here, we utilize coherent X-ray pulses to investigate the sub-100 fs equilibrium dynamics of water from ambient conditions down to supercooled temperatures. This novel approach utilizes the inherent capability of X-ray speckle visibility spectroscopy to measure equilibrium intermolecular dynamics with lengthscale selectivity, by measuring oxygen motion in momentum space. The observed decay of the speckle contrast at the first diffraction peak, which reflects tetrahedral coordination, is attributed to motion on a molecular scale within the first 120 fs. Through comparison with molecular dynamics simulations, we conclude that the slowing down upon cooling from 328 K down to 253 K is not due to simple thermal ballistic-like motion, but that cage effects play an important role even on timescales over 25 fs due to hydrogen-bonding.

Probing the OH Stretch in Different Local Environments in Liquid Water

We use resonant inelastic X-ray scattering (RIXS) to resolve vibrational losses corresponding to the OH stretch where the X-ray absorption process allows us to selectively probe different structural subensembles in liquid water. The results point to a unified interpretation of X-ray and vibrational spectroscopic data in line with a picture of two classes of structural environments in the liquid at ambient conditions with predominantly close-packed high-density liquid (HDL) and occasional local fluctuations into strongly tetrahedral low-density liquid (LDL).