Ion-solvent interaction. Structural aspects of ion-solvent interaction in aqueous solutions: a suggested picture of water structure

Abstract

In addition to the direct action of the ionic charge on water as a dielectric medium, ions may exert an influence on the equilibrium between the ice-like and non-ice-like forms which are present in room-temperature water. This provides a way of accounting for experimental results in a variety of areas, including entropy, heat capacity, temperature of maximum density, tracer self-difhsion, thermal conductivity, and dielectric relaxation, as well as viscosity and ionic mobility and their temperature coefficients. The tetrabutyl ammonium cation acts as a structure-promoter in the same way as do non-polar soIutes, amino acids and fatty acid anions. These various effects seem explicable in a straightforward manner in terms of a new picture of water as consisting of flickering clusters of hydrogen-bonded molecules, in which the co-operative nature of cluster formation and relaxation is related to the partially covalent character which is postulated for the hydrogen bond.Liquid water has long been known to possess distinctive structural features which are roughly describable by the statement that it retains a certain degree of similarity or analogy to ice. The amount of this " ice-like-ness " may be altered by changes in temperature and pressure and, as has also been known for a long time,l alterations which are presumably comparable (e.g., shifts in the temperature of maximum density) may also be evoked by the presence of ionic solutes. There is therefore nothing very new in inquiring into the ways in which such structural changes may influence, or may, in their turn, be studied by inferences from, observable thermodynamic and kinetic properties of ionic solutions. There have, however, been a number of advances in this field in recent times, and the subject is currently of some interest. In discussing it we must remember that we are trying to get at effects over and above those which the ions are expected to produce as charged spheres in a dielectric medium, even those connected with the discrete molecular nature of the medium, such as dipole saturation in the strong field near an ion. THE SIMPLE MODEL FOR SMALL IONSAmong the last-mentioned effects, which have no apparent necessary connection with the special character of water, is the immobilization of the dipolar solvent molecules which are nearest neighbours to the ions themselves.2 About a spherical ion of radius 2-3 8, in a medium of uniform dielectric constant 80, the field strength is of the order of 106 volts/cm and, except by Gurney,3 as noted below, it seems to be generally agreed that in aqueous solutions of ions not larger than Cs+ and I-the nearest-neighbour water molecules are always essentially immobilized by direct ion-dipole interaction. This idea and the related one of electrostriction have been invoked 4 to explain the small or negative values which salt solutions display of the solute partial molal volumes, heat capacities, compressibilities, etc. It also leads to a simple explanation of the influence of LiCl, 133

Inquiries into the Nature of Free Energy and Entropy in Respect to Biochemical Thermodynamics

“…The molar heat capacity of the liquid at constant atmospheric pressure is also particularly large and is close to twice that of the solid near the normal freezing point and to twice that of the vapor near the normal boiling point [15]. The above-noted expectations are clearly consistent with these experimental findings and with numerous experimental and theoretical observations suggesting that the particulate constituents of liquid water consist largely of labile clusters of molecules, the concentration and average size of which depend on temperature and pressure [15][16][17][18].…”

Energetics and cooperativity in three-center hydrogen bonding interactions. I. Diacetamide-X dimers (X=HCN, CH3OH)

“…13 This energy-based definition of cooperativity has been traditionally used in the discussion of cooperative phenomena in intermolecular H-bonded systems. 14 Other definitions for the cooperativity have been used in the literature 15 because the effects of cooperativity can be manifested in properties other than the energetics. For instance, several groups have shown that a quantitative treatment of cooperativity effects in intermolecular and intramolecular H bonds can be achieved in terms of the relative vibrational frequency shifts undergone by the X-H group involved in the hydrogen bonding.…”

Capillary Dynamics of Water/Ethanol Mixtures

“…Each sp 3 hybridized oxygen atom can form four approximately tetrahedrally disposed hydrogen bonds in liquid water 37 . Similar to water, ethanol can be regarded as having a sp 3 hybridized tetrahedral oxygen 38 .Since the oxygen atom of an ethanol molecule carries one proton and two lone pairs of electrons, it might form three hydrogen bonds with its neighbors -another ethanol molecule or a water molecule 38 .…”