The ideas of the first and second papers in this series, which make it possible to interpret entropy data in terms of a physical picture, are applied to binary solutions, and equations are derived relating energy and volume changes when a solution is formed to the entropy change for the process. These equations are tested against data obtained by various authors on mixtures of normal liquids, and on solutions of non-polar gases in normal solvents. Good general agreement is found, and it is concluded that in such solutions the physical picture of molecules moving in a ``normal'' manner in each others' force fields is adequate. As would be expected, permanent gases, when dissolved in normal liquids, loosen the forces on neighboring solvent molecules producing a solvent reaction which increases the partial molal entropy of the solute. Entropies of vaporization from aqueous solutions diverge strikingly from the normal behavior established for non-aqueous solutions. The nature of the deviations found for non-polar solutes in water, together with the large effect of temperature upon them, leads to the idea that the water forms frozen patches or microscopic icebergs around such solute molecules, the extent of the iceberg increasing with the size of the solute molecule. Such icebergs are apparently formed also about the non-polar parts of the molecules of polar substances such as alcohols and amines dissolved in water, in agreement with Butler's observation that the increasing insolubility of large non-polar molecules is an entropy effect. The entropies of hydration of ions are discussed from the same point of view, and the conclusion is reached that ions, to an extent which depends on their sizes and charges, may cause a breaking down of water structure as well as a freezing or saturation of the water nearest them. Various phenomena recorded in the literature are interpreted in these terms. The influence of temperature on certain salting-out coefficients is interpreted in terms of entropy changes. It appears that the salting-out phenomenon is at least partly a structural effect. It is suggested that structural influences modify the distribution of ions in an electrolyte solution, and reasons are given for postulating the existence of a super-lattice structure in solutions of LaCl3 and of EuCl3. An example is given of a possible additional influence of structural factors upon reacting tendencies in aqueous solutions.

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

The finding of Wetlaufer et al., that addition of urea to water increases the (mole fraction) solubility of hydrocarbon gases (except methane) while making them dissolve with a smaller evolution of heat, is interpreted as a primarily statistical phenomenon. For this purpose, it is treated in terms of a skeleton model in which not only is water represented as a two-species mixture of dense and bulky constituents but the dissolved hydrocarbon is represented as dissolving separately in these constituents, as if it were distributed between two phases, i.e., between a quasiclathrate solution in the bulky constituent and a quasilattice or “regular” solution in the dense one. Added urea is pictured as being able to enter only one of the solutions, the quasilattice one in the dense water, with the result that it acts as a structure breaker. A statistical-thermodynamic analysis of this model leads to equations for the chemical potentials and the partial molal enthalpies and entropies of the hydrocarbon and the urea solutes, and it is found that very simple assumptions suffice to give calculated values for log γ and L̄ of urea, in binary aqueous solutions, in good agreement with experiment. The choice of parameters to fit the results of the Wetlaufer transfer experiments (of hydrocarbon from water to urea-water) leads to calculated values for the partial molal enthalpies and entropies of the hydrocarbons in their hypothetical binary solutions in the bulky (quasiclathrate solution) and dense (quasilattice solution) waters and comparison of these quantities with properties of appropriate real physical systems makes them appear to be acceptable. One inference from this successful result is that the hypothesis of Frank and Evans, that the “extra” negativeness of the S̄2 for hydrocarbon gases in water arises from a structure shift in the latter, may not be unique, but that a similar effect might also arise from the fact that water received some of the hydrocarbon solute into a different type of environment from what it would find in a “normal” solvent.

An "apparatus" is described in which, in a thought experiment, the whole of a sample of fluid may be subjected to a uniform electrostatic field under controlled conditions. The work done in such a process can be written into fundamental thermodynamic equations so as to enable the field strength E to be a variable of state in a way which is essentially symmetrical with the way in which P and T are variables of state. This enables a variety of differential coefficients to be obtained, some of which appear to be new. A discussion is given of the relationship between "electrostriction pressure" (as derived in electrostatics) and thermodynamic pressure. Application of the formalism to systems of more than one component is illustrated by discussion of the change in composition produced by charging the plates of a small condenser immersed in a large volume of binary fluid mixture. The customary representation of the "energy density" in a field as equal to ElK/87r is discussed, and found to be limited in formal validity when the field is nonuniform, when the charging process takes place under conditions which do not hold the volume constant, or when dielectric saturation is taken into account. 1