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.

https://science.sciencemag.org/content/sci/369/6510/1490.full.pdf

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

Water is the essential liquid on earth since it not only plays vital roles in living systems but also has a significant impact on our daily life from various industrial applications to earth’s clim...

Direct Evidence in the Scattering Function for the Coexistence of Two Types of Local Structures in Liquid Water.

Based on recent experimental data that can be interpreted as indicating the presence of specific structures in liquid water, we build and optimize two structural models which we compare with the available experimental data. To represent the proposed high-density liquid structures, we use a model consisting of chains of water molecules, and for low-density liquid, we investigate fused dodecahedra as templates for tetrahedral fluctuations. The computed infrared spectra of the models are in very good agreement with the extracted experimental spectra for the two components, while the extracted structures from molecular dynamics (MD) simulations give spectra that are intermediate between the experimentally derived spectra. Computed x-ray absorption and emission spectra as well as the O–O radial distribution functions of the proposed structures are not contradicted by experiment. The stability of the proposed dodecahedral template structures is investigated in MD simulations by seeding the starting structure, and remnants found to persist on an ∼30 ps time scale. We discuss the possible significance of such seeds in simulations and whether they can be viable candidates as templates for structural fluctuations below the compressibility minimum of liquid water.

/pdf/a-proposal-for-the-structure-of-high-and-low-density-4526i6z9b3.pdf

A proposal for the structure of high- and low-density fluctuations in liquid water.

/pdf/drivers-challenges-and-emerging-technologies-for-35ob0m49.pdf

Drivers, challenges, and emerging technologies for desalination of high-salinity brines: A critical review

Liquids generally become more ordered upon cooling. However, it has been a long-standing debate on whether such structural ordering in liquid water takes place continuously or discontinuosly: continuum vs. mixture models. Here, by computer simulations of three popular water models and analysis of recent scattering experiment data, we show that, in the structure factor of water, there are two overlapped peaks hidden in the apparent "first diffraction peak", one of which corresponds to the neighboring O-O distance as in ordinary liquids and the other to the longest periodicity of density waves in a tetrahedral structure. This unambiguously proves the coexistence of two local structural motifs. Our findings not only provide key clues to settle long-standing controversy on the water structure but also allow experimental access to the degree and range of structural ordering in liquid water.

Distinct signature of two local structural motifs of liquid water in the scattering function

Studying the freezing of saltwater on a molecular level is of fundamental importance for improving freeze desalination techniques. In this study, we investigate the freezing process of NaCl solutions using a combination of X-ray diffraction and molecular dynamics simulations (MD) for different salt-water concentrations, ranging from seawater conditions to saturation. A linear superposition model reproduces well the brine rejection due to hexagonal ice Ih formation and allows us to quantify the fraction of ice and brine. Furthermore, upon cooling at T = 233 K, we observe the formation of NaCl·2H2O hydrates (hydrohalites), which coexist with ice Ih. MD simulations are utilized to model the formation of NaCl crystal hydrates. From the simulations, we estimate that the salinity of the newly produced ice is 0.5% mass percent (m/m) due to ion inclusions, which is within the salinity limits of fresh water. In addition, we show the effect of ions on the local ice structure using the tetrahedrality parameter and follow the crystallite formation using the ion coordination parameter and cluster analysis.

/pdf/brine-rejection-and-hydrate-formation-upon-freezing-of-nacl-yfxe97exis.pdf

Brine rejection and hydrate formation upon freezing of NaCl aqueous solutions.

Wide angle x-ray scattering of supercooled water down to 234.8 K was studied using high energy x rays at the European Synchrotron Radiation Facility. The oxygen-oxygen pair distribution function (PDF) was calculated from the scattering pattern out to the 5th peak at an intermolecular distance, r ≈ 11 A. We observe that the 4th peak and the 5th peak in the PDF increase in height upon supercooling. We also observe that the 4th peak position (r4) shifts to shorter distances upon supercooling consistent with previous studies, but we see a more rapid change at the lowest temperature. The running oxygen-oxygen coordination number is calculated for 5 different temperatures, and an isosbestic point at riso = 3.31 ± 0.05 A was found corresponding to a coordination number of 4.39 ± 0.15. The comparison of the PDF of the coldest water with that of amorphous ice shows distinct differences. We propose that there are 5-member pentamer rings in low density liquid-like structures giving rise to the sharp correlations at r ≈ 9 A and r ≈ 11 A.

Intermediate range O-O correlations in supercooled water down to 235 K

A two-component stapling strategy is used to covalently tether a new class of water-soluble supramolecular polymers built from bay-functionalized perylene bisimide (PBI) units. By leveraging a novel combined strategy where excitonic coupling and fluorescence data are exploited as spectroscopic reporters, structural design principles are established to form light-harvesting superstructures whose ground-state electronic properties are not sensitive to solvation environments. Moreover, we interrogate the structural properties of stapled superstructures by capitalizing on the drastic changes in fluorescence quantum yields against parent supramolecular assemblies. In essence, our work shows that the combination of excitonic coupling measurements and photoluminescence experiments delineates a more accurate understanding of the design principles required to limit the degree of structural defects and magnify short- and long-range electronic couplings between redox-active units in this new class of solvated nanoscale objects. These results highlight that the fragile conformation of non-covalent assemblies, which are regulated by weak secondary interactions, can be preserved by post-assembly modification of preformed supramolecular polymers. These synthetic and spectroscopic principles can in turn be codified as experimental handles to parameterize the optoelectronic properties of light-harvesting nanoscale objects.

Covalent post-assembly modification of π-conjugated supramolecular polymers delivers structurally robust light-harvesting nanoscale objects.

Studying the freezing of saltwater on a molecular level is of fundamental importance for improving freeze desalination methods. Here, we study the freezing process of NaCl solutions using a combination of x-ray diffraction and molecular dynamics simulations for different salt-water concentrations, ranging from seawater conditions to saturation. A linear superposition model can reproduce well the brine rejection due to hexagonal ice Ih formation and allows us to quantify the fraction of recovered ice and brine. Furthermore, upon further cooling we observe the formation of NaCl$\cdot$2H$_2$O hydrates, which coexist with ice Ih in the form of micro-pockets. The molecular dynamics simulations are utilized to model the formation of NaCl crystallites and quantify ion-water cross-correlations and ion inclusion within the ice Ih lattice. From the simulation we estimate that the salinity of the newly produced ice is 0.5% mass percent (m/m) due to ion inclusions, which is within the salinity limits of fresh water. This study demonstrates a sensitive method of detecting salt trapped inside the ice in the form of crystallites and provides a valuable insight to improve freeze desalination efficiency from a molecular perspective.

Brine rejection and hydrate formation during freeze desalination.

Studying the freezing of saltwater on a molecular level is of fundamental importance for improving freeze desalination techniques. Here, we investigate the freezing process of NaCl solutions using a combination of x-ray diffraction and molecular dynamics simulations (MD) for different salt-water concentrations, ranging from seawater conditions to saturation. A linear superposition model reproduces well the brine rejection due to hexagonal ice Ih formation and allows us to quantify the fraction of ice and brine. Furthermore, upon cooling at T = 233 K we observe the formation of NaCl$\cdot$2H$_2$O hydrates (hydrohalites), which coexist with ice Ih. MD simulations are utilized to model the formation of NaCl crystallites. From the simulations we estimate that the salinity of the newly produced ice is 0.5% mass percent (m/m) due to ion inclusions, which is within the salinity limits of fresh water. In addition, we show the effect of ions on the local ice structure using the tetrahedrality parameter and follow the crystalite formation by using the ion coordination parameter and cluster analysis.

/pdf/brine-rejection-and-hydrate-formation-upon-freezing-of-nacl-3p5voffkdx.pdf

Ifigeneia Tsironi

Papers

Brine rejection and hydrate formation upon freezing of NaCl aqueous solutions.

Intermediate range O-O correlations in supercooled water down to 235 K

Covalent post-assembly modification of π-conjugated supramolecular polymers delivers structurally robust light-harvesting nanoscale objects.

Brine rejection and hydrate formation during freeze desalination.

Brine rejection and hydrate formation upon freezing of NaCl aqueous solutions