Chul Hee Cho

Understanding all of water’s anomalies with a nonlocal potential

Though water in its liquid state is of central importance to a vast number of problems in the sciences, in engineering, and in health and environmentally related areas, it remains a mysterious and misunderstood material. It seems to violate the very basis of our 20th century scientific training—starting from first principles. For this reason, one of the most important of all scientific problems remains in a chaotic state of understanding, stifling progress in many other fields. Many scientists feel that this old problem must surely have been solved, or for other reasons is simply not worth worrying about, and continue to visualize water in incorrect ways. However, it is noteworthy that not until our own recent work was any one of the roughly ten anomalous properties of this liquid understood at a molecular level. In fact, it is rather easy to show that every intermolecular potential model currently used for water has the same basic flaw—too great a dominance by Coulomb terms. This flaw prevents such models from providing a realistic picture of water’s anomalous properties. Since water is more and more becoming considered not simply a solvent but an integral part of chemical and biological systems, it is now urgent to determine its true character. One has first to create a “working model” that is consistent with water’s anomalies. Next, because the “first principle” understanding of water extends only from the monomer to nanoscale clusters, a new intermolecular potential, whose consequences extend beyond nanoscale, is proposed. The success of this new potential in helping to explain all the anomalies supports the view that our new “working model” can provide a valid molecular-level description of this material.

ISOSBESTIC POINTS IN LIQUID WATER : FURTHER STRONG EVIDENCE FOR THE TWO-STATE MIXTURE MODEL

Three different types of isosbestic points observed for liquid water are dealt with in this paper—temperature induced Raman, temperature induced structural, and pressure induced structural isosbestic points. These isosbestic points leave no doubt as to the precise two-state outer-neighbor mixture model description of this important substance from supercooled temperatures up to at least 40 °C. New pressure-dependent Raman experiments are suggested that will help confirm further this most simple idea that has already been shown to be quantitatively consistent with the temperature- and pressure-dependent properties of liquid water, including all its anomalies.

Role of Hydration Water in Protein Unfolding

In this paper, following our work on the two-state outer neighbor mixed bonding model of water, it is proposed that polar groups promote the formation of the low density ice Ih-type bonding in their neighborhood, whereas nonpolar groups tend to promote the higher density ice II-type structure. In a protein, because of the large numbers of exposed polar and nonpolar groups, large changes in the neighboring water structure can occur. These changes, of course, depend on whether the protein is in its native or its unfolded state and will be shown here to have a direct impact on the thermodynamics of protein unfolding at both high and low temperatures. For example, it is known that the polar hydration entropies become rapidly more negative with increasing temperature. This very unusual behavior can be directly related to the promotion in the outer bulk liquid of the more stable Ih-type bonding at the expense of II-type bonding by polar groups of the protein. In contrast, nonpolar groups have an opposite effect on the thermodynamics. It is the delicate balance created by these outer hydration contributions, mixed with ordinary thermodynamic contributions from the inner hydration shell and those from hydrogen-bond and van der Waals forces within the protein molecule itself that is responsible for both heat and cold denaturation of proteins.

Liquid water and biological systems: the most important problem in science that hardly anyone wants to see solved

The main emphasis of this paper is the design of a water model that gives the correct temperature-dependent density. Water interaction models currently used have been highly oversimplified and are presently incapable of producing, over even a modest range of temperature and pressure, the properties of real water. The new feature is a modified second-neighbour non-hydrogen-bonded interaction to match those known to exist in the moderately dense ice polymorphs or the high density amorphous solid. Combined dynamically in the liquid with ordinary tetrahedral bonding, the more dense metastable structure tends to grow in with increasing temperature because of the entropic driving force, creating the density maximum. With this new model, more realistic structural and dynamic properties of liquid water near surfaces, particularly biologically important macromolecules and membranes, can be expected in future work.

Molecular-level description of temperature and pressure effects on the viscosity of water

The steep non-Arrhenius temperature dependence at low temperatures of the shear viscosity of water and its backwards-sounding increased fluidity under pressure for temperatures below 33 °C are two of the anomalies of this liquid that have been known for a very long time. The purpose of the present paper is to show how these two important characteristics of water emerge quantitatively from an explicit two-state outer-neighbor mixture model that we have used to explain many other properties of this substance. It will be shown here that both of these viscosity anomalies are directly related to the steep variations with temperature and pressure of the fractional compositions of ice-Ih-type bonding and ice-II-type bonding in the two-state mixture. This compositional dependence has already been obtained in earlier work from the variations of the density and the isothermal compressibility of water with temperature. The viscosity analysis presented here thus helps to unify further all the properties of this liqui...

Temperature and pressure effects on the structure of liquid water

Abstract Structural studies of liquid water have been carried out for some time. However, no model has emerged that fully explains all the currently known features present. The radial-distribution function or the closely related pair-correlation function, used as interpretive tools for local structure, are known to be both pressure and temperature sensitive, indicating that the local structure of liquid water changes as a function of these two variables. This would be expected. However, it is the details and subtleties of these structural changes that have not been understood. In this article, it is shown how non-hydrogen-bonded second neighbor considerations can be used to explain these structural changes. By use of an outer structure two-state mixture model, which has been successful in explaining all the anomalies of water, the detailed features of the structure functions and their sensitivity to temperature and pressure can be reproduced. In particular, the isochoric temperature differential X-ray scattering results of Bosio, Chen and Teixeira emerge from the analysis, giving new insights into the experiments.

Mixture model description of the T-, P dependence of the refractive index of water

In this paper the temperature/pressure dependence of the refractive index of liquid water is analyzed using the two-state outer-neighbor mixed bonding structural model. So far, this theoretical model has been successful in reproducing, usually within the experimental uncertainty, the temperature/pressure dependence of the density, the viscosity, and the oxygen–oxygen pair correlation functions, in addition to the isothermal compressibility and isotope effects of this important substance. The philosophy of the present paper is to use the high accuracy of refractive index measurements to further test this model. It is shown that a very simple linear dependence with respect to temperature and pressure of the specific refractions LI and LII of the two contributing structural components in this two-state model is sufficient to give better than 5-decimal-point agreement with the experimental refractive index data at low pressures and temperatures between about −10 °C and +70 °C. The maximum in the refractive in...

Water anomalies and the double-well Takahashi model

Abstract Water in its liquid state has many anomalous properties. It is only recently that a molecular-level explanation of these anomalies was discovered through recognition that a thermal competition takes place between the inner and outer second-neighbor structure in liquid water. This essential feature, which sets water off from all other liquids, is captured here in a double-well Takahashi model. The availability of an exact solution of this model allows one to understand how the various anomalies arise as the temperature and pressure of the system are varied. It is expected that this study will give useful leads in future computer molecular dynamics studies of real water using new water potentials that take into account this important outer structure characteristic.

Response to “Comment on ‘Mixture model description of the T-, P dependence of the refractive index of water’ ” [J. Chem. Phys. 115, 7795 (2001)]

The valid range of the emphasized experimental data and need, in very precise comparisons, to compensate for the very small temperature shifts resulting from revisions in the accepted standard for the International Temperature Scale over the years, are acknowledged. However, it is emphasized that these corrections do not invalidate the approach and that the physical motivation underlying the mixture model analysis leads to physically meaningful fitting parameters; a result not found in any previous treatment.