A Journey Through Water: A Scientific Exploration of The Most Anomalous Liquid on Earth

The ever increasing demand for clean water has prompted the world to consider water scarcity in a serious way. Some regions in the world are already at the brink of war over the ownership of major water resources, and it is feared that the situations may become worse. The marginalised people living in the impoverished regions of the world are struggling to obtain clean water, non−availability of which puts their life in utmost misery. Despite the fact that technological innovation provides some solution to this matter, water’s growing demand surpasses what technology can offer. A joint approach unifying various facets of human life is necessary to overcome the issue, and hence they are discussed in detail. It must be appreciated that several organisations including the U. N., representing all nations around the globe, is taking proactive steps to curb this problem by setting up various committees to study the matter in depth and taking appropriate measures to decentralise the resources to all. With the development of robust computer simulation methods, and water models, it is now possible to study water at microscopic level. Together with state−of−the−art experimental techniques the properties of water can be unravelled further. This is expected to have tremendous impact upon improving the quality of water refinement process since most of them are fundamentally of a chemical nature.

Liquid is one the three principal states of matter and its properties are known to be intermediate between gaseous and solid phases. Several types of intermolecular forces, categorised into long range and short range, play important roles in defining liquid structure. Long range forces are of three types, namely electrostatic, induction and dispersion, whilst the short range forces are of quantum chemical nature, due to exchange of electrons. A wide range of materials, including elements, oxides, mixtures of salts and dilute acids, are known to form glasses, which are non−crystalline, amorphous matter. Methods such as lattice theories have been devised long time back to understand the structure of liquids. Several other theories have been put forward as well in order to explain complex behaviour of liquids at lower temperature such as formation of highly viscous glassy materials. Most notable theoretical propositions include Adam – Gibbs theory, Mode Coupling Theory and Energy Landscape theory. Inherent Structure (IS) analysis is a powerful tool to identify the fundamental structures of the system under investigation, and to obtain a pictorial characterisation of the energetics between strong and fragile glasses. Relaxation times exhibit two distinct kinetics, alpha and beta relaxations, which can be properly explained by Mode Coupling Theory. Aqueous solutions of sugars and alkali salts such as lithium chloride are known to be good glass formers, which require only low cooling rates in order to form glasses, bypassing crystallisation.

A wide range of experimental techniques has been developed and applied for investigating matter at high resolution. Scattering experiments are considered as powerful tools for structure elucidation of liquids including normal water and supercooled water. Employing techniques such as Differential Scanning Calorimetry (DSC), one can record the temperature of phase changes, the glass transition temperature. Quasi Elastic Neutron Scattering (QENS) spectral analysis suggests distinct relaxation behaviour of diffusive motions of water molecules. Nuclear Magnetic Spectroscopy (NMR) is very useful tool in elucidating molecular structures of systems including liquids and aqueous solutions. By Compton Scattering and NMR techniques, estimation of average number of hydrogen bonds has been achieved to a considerable level of accuracy. Extensive studies have been made on water clusters using a sophisticated spectroscopic technique namely Far Infra−Red Vibration−Rotation−Tunneling (VRT) spectroscopy. Optical Kerr Spectroscopy has been employed to investigate the relaxation process at femtosecond and picosecond levels. Properties such as compressibility and diffusion coefficient have been experimentally measured by simple capillary tube techniques. Electron microscopic techniques have become invaluable tools to obtain high resolution of molecular structure materials. Electron microscopic techniques equipped with better resolution can yield further information regarding the microstructure of materials including liquids.

Computer modelling is a powerful enterprise for the investigation of matter at atomic and molecular levels, and has generally been accepted as a supplementary tool to traditional experimental methods. Its advantages over real experiments are primarily exemplified by its portability and cost effectiveness. Monte Carlo and Molecular Dynamics methods are two principal techniques that have gained a great level of popularity among various computer simulation methods. Numerous mathematical models, popularly known as Forcefields, have been developed in order to investigate water computationally. The application of computer simulation methods is limited by the choice of parameters that define the intra and inter molecular interactions within the framework of Forcefields. Ab−initio forcefields are expected to overcome the limitations of other types of water models. Concept of ensemble provides a theoretical basis for deriving physical properties by significantly reducing number of particles in a system. Mathematical devices such as Periodic Boundary Conditions (PBC) bypass the inconsistencies in simulations. Density Functional Theory (DFT) and Wave Function methods are two important classes of quantum chemical methods for investigating matter at electronic level. Born−Oppenheimer approximation provides a fruitful means for separating electronic and nuclear motions, which reduces the complexity of quantum calculations to a great extent. Hartree−Fock (HF) method is the most fundamental wave function procedure for calculating the energy of multi−electronic systems. On the contrary, Density Functional Theory (DFT) is based on the estimation of electron density, which can be validated by experimental means. Combined electronic and classical approaches are increasingly becoming popular in the scientific community.

Structural elucidation of water is so fundamental in understanding its roles as a solvent as well as a reagent in facilitating multifarious chemical reactions. The internal structure of water molecule is very “simple” to explain yet the physical and chemical properties of this liquid remains to be elusive in spite of tremendous theoretical and experimental efforts till date. Several propositions have been made in order to account for water structure, in particular its enigmatic hydrogen bonding environment that accounts for its exceptional properties. The concept of uniform distribution of tetrahedral network in water has been emerged from various experimental investigations. Water structure as equilibria of large number of clusters formed by varying number of water molecules has also been proposed based on computer simulations and Raman spectroscopy. Percolation model provides a quantitative picture of hydrogen bonding in liquids. The nature of hydrogen bond is dynamic in nature, spurring sporadic changes in its local structure, which can effectively be probed by various spectroscopic and scattering techniques. Local structure of water molecules is influenced by thermodynamic changes, most notably in temperature and density. Both computational experimental findings reveal that density plays a vital role in determining average number of hydrogen bonds a water monomer can have across wide temperature domain. More importantly, water undergoes a cascade of morphological changes upon alteration in temperature, which is still a fascinating subject for many researchers.

The roles of two low temperature and non−crystalline forms of water, (supercooled and glassy water) are very pivotal in supporting the existence of several microorganisms below 0°C, although they are very metastable with respect to the stable crystalline form of water, ice. In the supercooled regime, the hydrogen bond lifetime of a single hydrogen bond and water clusters are found to be significantly higher than in higher temperatures. Diffusion coefficient and configurational entropy show a distinct maximum at density 1.15g/cm3. Two inter−convertible forms of supercooled water, known as Low Density Liquid (LDL) and High Density Liquid (HDL), are found to coexist at temperatures below the freezing point of water. If water is cooled at very fast rate, it becomes glassy, the most profound form of water in the universe, bypassing the formation of ice. Polyamorphism is one of the characteristics observed in glassy water. Glass transition temperature in water has sparked debate in the scientific community. Different experimental procedures as well as water models produced varying values for the glass transition temperatures in water. It has been experimentally monitored and computationally simulated the transition between the two glassy phases of water, HDA and LDA. The transition is terminated at a critical point, according to Liquid−Liquid Critical Point (LLCP) theory. The concept of strong and fragile glasses is very powerful tool in furthering our understanding of the dynamics of glassy materials. It is interesting to note that a transition from strong to fragile occurs in water.

Transition from water to ice is very crucial in many natural and artificial processes on which our lives depend. No other substance exhibits more crystalline forms than ice, the solid phase of water. Several ice polymorphs are found to exist in pairs, corresponding to high temperature proton disordered state and low temperature proton ordered state. Hexagonal ice is the dominant form of ice at ambient conditions. Ice X is highly symmetrical ice polymorph with hydrogen atoms exactly positioned equidistant to the two adjacent oxygen atoms. Five and seven membered rings of water molecules are observed in ice XII. The orientation of hydrogen bonds plays important roles in assigning the geometries of various forms of ice as in the case of normal and supercooled waters. The largest hydrogen bond bending is observed in ice VI. Orientations of hydrogen atoms result in Bjerrum and ionisation defects in ice crystals, which are responsible for dielectric effects. Auto ionisation, leading to the generation of hydronium and hydroxyl ions in water, promotes ionisation defects. Catalytic properties of ice are found to be remarkable in large number of reactions. TIP4P water model and its variants seem to be the popular models for simulating ice phase of water. Rotational motion of oxygen−hydrogen bonds is responsible for the destruction of ice lattice as temperature increases, leading to the melting of ice. Ice exhibits excellent electrical, optical, mechanical, thermal & surface properties. Thanks to its exceptional thermal properties, ice has been successfully employed as a better alternative to the traditional air cooling systems.

Water beyond its boiling temperature has been investigated by numerous theoretical and experimental tools including classical and quantum simulations, Neutron Scattering, Nuclear Magnetic Resonance, Infra−Red, and a more recent Tetrahertz Vibrational−Rotational−Tunnelling spectroscopic technique. Water becomes Super Critical Water (SCW) when it reaches temperature 647 K with critical pressure and density at which vapour and liquid phases coexist. SCW has been found to have exceptional properties such that many chemical reactions can be efficiently carried out without the presence of catalysts, and hence is considered as a better alternative to many of the traditional reagents, which are currently being used in organic synthesis. It is also effective in biofuel production and in burying toxic waste products by converting them to water and CO2. Simulations and experiments point into dramatic changes in the intermolecular structure of water at elevated temperatures signalled by depletion of tetrahedral hydrogen bonding network, which gives rise to higher population of water clusters than found in ambient water, with varying size and geometry. Cyclic water isomers are found to be stable up to clusters containing five water molecules, whereas three dimensional cage structures are found to be stable in higher analogues. Density along with temperature plays a vital role in determining diffusive properties of SCW. It has been demonstrated by numerous experiments and computer simulations that a proportional increase in hydrogen bonding is observed as density increases. On the contrary, diffusive motion of water molecules is retarded upon a hike in density.

Numerous anomalies of water have been reported in the literature. Anomalous behaviour of liquid water is so striking when it is supercooled below the melting temperature of ice, Tm. Several physical properties have been found to be diverging in the supercooled liquid phase, including isobaric heat capacity, isothermal compressibility, relaxation time and thermal expansion coefficient. Interestingly hydrogen bond life times show a divergence at this temperature, indicating its connection to these singularities. Liquid water exhibits both density maximum and minimum, the latter has been discovered by a recent Small Angle Neutron Scattering (SANS) experiments, considered to be two of its most notable thermodynamic anomalies. Unlike other liquids, translational and diffusive motions in water exhibit contrasting behaviour and product of these two diffusive constants is found to be insensitive to temperature and density. Formation of water clusters of varying sizes dictates the nature of diffusion in supercooled water. Several propositions have been made in order to account for water’s anomalies, which include Liquid−Liquid Critical Point theory, Singularity Free hypothesis, Critical Point−Free hypothesis and Stability Limit conjecture. In bulk phase, water shows its most of the anomalies. In addition, it exhibits several other anomalous characters when confined to nanoscale geometries and is near to macromolecular surface. It has to be noted that in the vicinity of non−polar solutes the strength and lifetimes of water network increases.

Recent advances in computing and development of sophisticated experimental techniques have enabled us to make giant leaps in understanding the microstructure of water. The structure of water in normal temperature range is still shrouded in Continuum-Mixture model controversy while new characterisation methods are reported on yearly basis, leaving water an interesting and controversial theme for ever. A multifaceted approach, amalgamating social, economic, political, geographical and technological aspects, is required to alleviate the issues related to the scarcity of water to considerable extent. Centres have been established for performing cutting edge research on water across the globe in order to develop efficient technologies in response to chronic water scarcity. Molecular scientists can greatly contribute to the technological advances that could allay the problems related to fresh water, and influence the policy makers at various organisation levels.