Water is an essential and at the same time the least explored substance on our planet. Multiple research papers are published every year addressing the properties of water, i.e., its structure, conditions under which water clusters form, as well as its behavior and dynamics when it interacts with different materials. Here we are reviewing some of the studies carried out in this area in recent years.

There are a multitude of approaches and methods for determining and investigating water structure. Scientists from Brazil have proposed a method that enables obtaining quantitative structural information about water using cryogenic electron microscopy (cryo-EM). The paper was published in The Journal of Physical Chemistry Letters. In the past years, Cryo-EM has become a fundamental instrument to determine the structure of a great variety of substances — from inorganic to biological macromolecules. Cryo-EM samples are fast-frozen which ultimately results in their becoming embedded in a unique glassy-water or amorphous ice environment. The local structure of water brought to this state has not been investigated so far. The scientists have examined the pair distribution function (PDF) of water under cryo-EM conditions using electron diffraction data. The PDF gives the probability of finding an atom at a certain distance from another atom. Cryo-EM has been shown to be a powerful tool for investigating the structure of amorphous frozen water. Water under cryo-EM conditions is between low-density amorphous ice and supercooled water, and its structure depends on sample thickness and the freezing time. Cubic ice areas were found to form within thicker Cryo-EM water samples. The proposed technique may considerably contribute to the understanding of water structure, opening new opportunities to investigate its states and interactions with dissolved substances, nanoparticles and biological samples.

The Cryo-EM water experiment. With Cryo-EM, water is in a glassy state, and when exposed to an electron beam, a diffraction pattern can be obtained. Using these data, the scientists have examined the pair distribution function which provides quantitative information about glassy-water structure under Cryo-EM conditions. Refer to the source

Another study which has investigated small water volumes in a hydrophobic solvent has provided the following findings. Tetrahedral structures – low-entropy water clusters – form in such systems following supercooling. Scientists from Japan have investigated small bulk water domains in a hydrophobic solvent over a wide temperature range (235–333 K), including supercooling temperatures. The paper was published in The Journal of Physical Chemistry Letters in April 2020. A model was proposed in which the population of locally favored tetrahedral structures (low-entropy clusters) becomes dominant at temperatures below 250 K, whereas water is a disordered normal-liquid structure at above 300 К. According to the scientists, low-entropy water clusters in a hydrophobic solvent represent a unique water morphology and can be used as a probe material for investigations.

Changes with temperature in the structural organization of water in a hydrophobic solvent. At supercooling temperatures, the population of water locally favored tetrahedral structures becomes dominant. Above 300 K, all local water structures become a disordered normal-liquid structure. Refer to the source

Water structure is investigated not only with temperature changes but also with changes in pressure. There is experimental evidence indicating a pressure dependence of certain dynamic properties of water, attributed to changes in its local structures. Water shows some thermodynamic and dynamical anomalies in liquid and supercooled metastable phases, and the natures of these phases are still hotly debated. In March this year, a paper by Italian scientists was published in the same journal – The Journal of Physical Chemistry Letters. The Kerr effect was used to measure water properties depending on pressure under different conditions (at 273 K from 0.1 to 750 MPa and at 297 K from 0.1 to 1350 MPa), reaching the supercooled metastable phase. The data analysis suggested the presence in the water phase diagram of a so-called crossover area that divides it into two states characterized by different dynamic regimes, which appear to be related to two different structural organizations – dominated by the high-density water and by the low-density water, respectively. According to the researchers, the interpretation of the crossover phenomenon is well consistent with the two-state model of water where low-density and high-density water phases are local transient fluctuations of the network structure.

Measurements of water using the Kerr effect as a function of pressure along two isotherms. The water phase diagram reveals a crossover area that divides two differently structured regions – high-density and low-density forms. Refer to the source.

Old methods are being improved and new methods are being developed for water structure research. In June 2020, the PNAS journal published a paper by a team of scientists from China, who have used a recently developed, hybrid approach to observe structural evolution of water clusters using infrared spectroscopy (IR) with a tunable vacuum ultraviolet free-electron laser. By exploiting this method, the scientists captured infrared spectra of size-selected neutral water clusters, (H₂O)_n, where n = 3-6. Quantum-chemical studies were also carried out in order to better understand the structural and spectral changes in these clusters. Infrared spectroscopic studies of water clusters are critical in understanding the hydrogen-bonding networks in liquid water and ice. The selected approach enabled the scientists to observe striking spectral change in the O-H stretch region from water tetramer to penta- and hexamer, due to appearance of three-dimensional hydrogen-bonding networks. The main features of IR spectra of the pentamer and hexamer (H₂O)_n (n = 5 and 6) span the entire O-H stretching band of liquid water. The experimental results provide a consistent picture for the development of structural diversity of the hydrogen-bonding networks that are responsible for the major features of the structures and properties of condensed-phase water.

Infrared spectra of neutral (H2O)n clusters with n = 3-6 in the O-H stretch region. Refer to the source

Сomparison of the experimental IR spectra of water pentamer (A) and hexamer (B) with those of liquid water (on the bottom). Refer to the source

Using molecular dynamics methods, scientists from the UK have investigated the rheology of water confined between multilayer moving graphene walls. The findings were presented in the Langmuir journal. Water confined by hydrophilic materials shows unique transport properties compared to bulk water, thereby offering new opportunities for the development of nanofluidic devices. When nanoconfined, water may undergo liquid- to solid-phase-like transitions depending on confinement conditions. In the case of water confined by graphene layers, the van der Waals forces are known to deform the graphene layers, whose bending leads to further nonuniform confinement effects. The UK scientists have examined rheological water characteristics using a range of different slit widths and velocity strain rates. It was found that under subnanometer confinement, water loses the rotational symmetry of a Newtonian fluid, transforming under such conditions into ‘ice’. This structure is completely insensitive to the applied shear force and behaves like a frozen slab sliding between the graphene walls. For flows at large velocity strain rates in moderate to large slits between the graphene walls, water is in the liquid state and reveals shear thinning. In this case, water exhibits a constant slip length on the wall, which is typical of liquids in the vicinity of hydrophobic surfaces. The results of this study may be a benchmark for further research into the properties of water or other substances subjected to extreme confinement and shear forces, and they can be used in the development of nanofluidic devices, such as high-throughput membranes, or for correct interpretation of atomic force microscopy findings.

Atomistic structure of water confined between moving graphene walls (3.8 nm and 0.8 nm gaps; respectively, and 23.68×10¹⁰ s^-1 velocity strain rate). Refer to the source

Since the interaction of water with materials is influenced by its properties and structure, it is no surprise that this can be used in water treatment processes. Scientists from Switzerland have investigated how the properties of water may be used for its purification from nanoparticles using coagulation. Their findings were published in February this year in Water Environment Research. Intensive use of engineered nanoparticles results in their release into aquatic systems and consequently into drinking water resources. Two conventional coagulants, polyaluminum chloride and iron chloride, were added to bottled mineral and Lake Geneva waters. The coagulants were examined for the efficiency of removing TiO₂, CeO₂ and polystyrene nanoparticles from water. TiO₂ nanoparticles are already used in food and cosmetics industries and for photovoltaic power generation, while CeO₂ nanoparticles are now used as fuel additives, in electronic devices and in cosmetics. Nanoparticle coagulation was shown to depend on water parameters such as hardness, pH, and the presence of natural organic matter. Furthermore, polyaluminum chloride was found generally more efficient compared with FeCl3, whereas polystyrene nanoparticles were harder to remove compared with TiO₂ and CeO₂.