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 Å. 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 Å 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 Å and r ≈ 11 Å.

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×2H2O 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. Fig.1 A schematic phase diagram of NaCl aqueous solution.Above the melting point and in low concentration of NaCl the solution is in liquid phase. At higher concentrations, saturation leads to NaCl crystal or NaCl×2H2O hydrates formation (shown on the right). Upon decreasing the temperature below the melting point, ice is formed pushing out the salt, in a process called brine rejection. When decreasing further the temperature below 251.9 K, ice and NaCl×2H2O crystals coexist. These authors contributed equally to this work.

Modulating, by design, the electronic properties of nanoscale objects that are water-compatible is at the forefront to develop redox-responsive materials capable of interfacing and probing living organisms. In the present work, we report the synthesis and structural and electronic characterizations of two novel water-soluble naphthalene diimide (NDI) oligomers that differ in the nature of the flexible chemical linkages tethering the πconjugated cores. Ground-state electronic absorption spectroscopy experiments, performed as a function of oligomer concentration in an aqueous medium, reveal negligible interactions between oligomer chains but significant interactions among NDI units that comprise a single oligomer chain. Contrasting the monomer controls, which demonstrate well-resolved first and second reduction processes, the NDI oligomers exhibit complex redox processes suggesting the screening of injected charges in the oligomer chains. We show that the flexible chemical linkages offer a synthetic handle to stabilize the first reduction potentials by more than 325 mV when compared to monomer controls. Because the electronic structures of these water-soluble oligomers are characterized by low-lying reduction potentials (E 0/-= −0.025 V vs SCE), biologically relevant sacrificial electron donors, sodium ascorbate and sodium hydrosulfide, possess sufficient driving force to incrementally n-dope the NDI oligomers. Further supported by spectroelectrochemical measurements and time-dependent density functional theory (TD-DFT) calculations, the spectroscopic properties of the reduced NDI units indicate the emergence of novel spectroscopic states diagnostic of radical π−anion interactions. The existence of delocalized electron spin density is unraveled by electron paramagnetic resonance spectroscopy and is shown to be dependent on the strength of the sacrificial electron donors used to n-dope the NDI oligomer chains. Our results not only deliver new insights into the spectroscopic signatures that allow for tracking π−anion interactions in NDI-derived superstructures but also provide new avenues to tune electron spin properties in water-soluble nanoscale objects.