Interaction between Ions and Solvent

This equation actually describes the interaction between the ion and the first solvation shell. Ions frequently form a very stable aquo complex with H2O molecules, for example an Fe(H2O)6 complex. Besides this inner sphere interaction there is also an outer sphere one, leading to a corresponding arrangement of the H2O dipoles around the ions. The outer sphere interaction is given by the energy required if an ion with the inner solvation shell (radius /j -1- z-soi) is transferred from a vacuum into the solution, as derived by Born using the continuum model  

Evaluations of both contributions yield values of the same order of magnitude, i.e. several hundreds of kJ mole . For an exact evaluation some further contributions must be considered, namely the ion quadrupole Interaction and an additional ion dipole interaction induced by the electric field of the ion. Further details of these derivations are given in ref. [2]. 

In which Afjyjj Is the Avogadro number, n is the number of solution molecules which are in direct contact with the ion, Z is the charge of the ions, is the dipole moment, rj and the radii of the ion and the solution molecules, respec- 

The thermodynamics of solutions and soUd-liquid interfaces can be well described in terms of the chemical and electrochemical potentials of the system. The basic definition of the chemical potential [6] is 

The chemical potential or free energy of a species, the latter being a component in a solid or in a solution, depends on the chemical environment. In the case of a charged species, such as an ion or an electron, we have to consider in addition the electrical energy required for bringing a charge to the site of the species. Accordingly, an electrochemical potential is defined instead of the chemical potential. Both are related by 

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Since the term hydration refers to aqueous solutions only, the word solvation was introduced as a general term for the process of forming a solvate in solution. The terms solvation and heat of solvation were introduced at a time when little or nothing was known about polar molecules. We know now that, when an atomic ion is present in a solvent, the molecular dipoles are subject to the ionic field, whose intensity falls off in 1/r2. We cannot draw a sphere round the ion and say that molecules within this sphere react with the ion to form a solvated ion, while molecules outside do not. The only useful meaning that can now be attached to the term solvation is the total interaction between ion and solvent. As already mentioned, this is the sense in which the term is used in this book. 

Ionic solvation is interaction between ions and solvent molecules that leads to the formation of relatively strong aggregates, the solvated ions. In aqueous solutions the terms ionic hydration and hydrated ions are used as weU. 

In polar media, electron transfer is associated with a marked change in the solvation shell of the species concerned. This strong solvation interaction between ions and solvent dipoles mediates electron transfer between the electrode and an electroactive species, and between two components of a redox system. Fluctuations... 

At the same time Gilkerson (43) expressed the dissociation constant in a very similar form, but included a term Es, which allows for interaction between ions and solvent dipoles, and in particular relates to the difference in solvation energy of free ions and ion pairs... 

In the case of reactions involving complex equilibria the influence of the interaction between ions and solvent molecules is generally characterised by means of large color differences. 

Most papers published on single-crystal-face behavior deal with this situation because most ions adsorb on solid electrodes of sp and sd metals. Adsorption depends on the nature of the ions and the metal, the interaction between ions and solvent in the dl,t the interactions between electrode metal and solvent, and the influences of these interactions on each other. All this exists already for electrodes which are not single-crystal faces, but the situation is complicated by the fact that the charge is distributed at the surface in an uncontrolled way. This is not the case for single-crystal faces for which the strength of adsorption, as well as its variation with charge density at the electrode surface, could depend on the atomic structure of the face which is the electrode. However, despite these complications, some progress has and can be made. 

Stabilization of ions in solution occurs by solvation, which has a nonspecific, polarity , component and a specific component involving a direct (quasi-chemical) interaction between ions and solvent. The specific interaction between a carbocation and a solvent is of a kind that destroys the carbocationic state. Thus, the protonation reaction of equation 2 is correctly written in the form of equation 4, in which XH is the acid ... 

The properties of ions in media of high density are of considerable interest, and the way in which the interactions between ions and solvent molecules affect the macroscopic properties of... 

Another interaction between ions and solvent molecules besides solvation has been identified by Bom (1) and latter by Fuoss (13). A moving ion can orient the solvent dipoles as they move through the solution. Owing to the finite time required for the relaxation of such orientations, a retarding force is exerted on the moving ion. Zwanzig (42) has quantitatively evaluated this eflFect and showed that Equation 1 becomes... 

CES. In the case studied by Treiner and Fuoss it is to be expected that large variations of the specific interactions between ions and solvent molecules as the composition of the solvent is varied may be rxiled out since both substances would interact with the ions through their —CN groups. Besides, all the solvent mixtures had very similar dielectric constants. 

Since most electrolytes are less soluble in nonaqueous solvents than in water, studies on the solvation phenomena of ions in nonaqueous solution are much less frequent than those in aqueous solution. Many alcohols and some nonaqueous solvents such as DMF, DMSO, and acetonitrile (AN) have relative medium dielectric constants with donor and acceptor properties comparable with those of water and many electrolytes can be dissolved in these solvents. Donor and acceptor interactions between ions and solvent molecules can be compared with those with water molecules. 

Some understanding of solvent effects has been provided by comparisons of the same reaction in the gas phase and in solution. Some reactions, however, do not occur at all in the gas phase, and one must then be content with comparing their rates in different solvents. When such comparisons are made it is sometimes found that the solvent does not have much effect on the rate. When, on the other hand, ions are involved as reactants or products, solvents usually have a much greater effect on rates, because of the rather strong electrostatic interactions between ions and solvent molecules. 

In analogy to the general approach presented in Eq. (1), the expression for the excess Gibbs energy developed by Papaiconomou et al consists of two parts the long-range contribution (LR) which corresponds to the electrostatic interactions of ions while all non-Coulombic short-range (SR) interactions between ions and solvent or between two solvent molecules are accounted for in a second part ... 

Equation (2.43) describes the effect of long-range forces and howtheycan be modified by short-range interactions between ions. In a solution, however, short-range interactions between ions and solvent molecules need to be considered, and it has been found that such reactions have an approximate variation which is proportional to the concentration of the ionic medium. Therefore, the expression for the activity coefficient can be extended to... 

Nearly all pit models have been based on transport equations which strictly apply in solutions much more dilute than those usually found in pits, which exceed 1 M and often approach saturation in the metal chloride salt. The fundamental shortcoming of dilute solution transport theory is that it accounts only for interactions between ions and solvent molecules, and not between pairs of ions. Ion-ion interactions are manifested, for example, by deviations of the solution conductivity from values predicted by dilute solution theory, which become appreciable at concentrations as low as 0.01 M." This section will examine specific inaccuracies resulting from the dilute solution approximation, and point out cases where the use of concentrated solution transport models is tractable. Dilute and concentrated solution approaches will be compared in the context of a simple example of a one-dimensional pit with passive sidewalls. The metal and electrolyte solution were taken to be aluminum in 0.1 M NaCl. There are no cathodic reactions or homogeneous reactions in the pit, and the solution composition at the pit mouth is that of the bulk solution. This example was described in more detail in an earlier publication. This example is chosen because of its simplicity and since the behavior of the dilute solution model may be familiar to readers. 

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