Interactions water-ion

As discussed in Section I.3(i), AX indicates the variation in the work function of a metal as an interface is created by bringing a solid and a liquid in contact. In principle, it should be possible to compare AX values with A values measured directly in gas phase experiments. This is the aim of UHV synthesis of the electrochemical double layer868 in which the electrode interface is created molecule by molecule, starting with the bare metal surface. It is thus possible to obtain evidence of ion-water interactions that can be envisaged from electrochemical measurements but that are not directly demonstrable. Wagner55 has given a recent comprehensive review of electrochemical UHV experiments. 

E Exby(MX). The first term is related to the ion-water interactions of the components of the mixture and the higher order terms are related to ion-ion interactions. Combining equations (50) and (51) one obtains... 

The ion-water interactions are very strong Coulomb forces. As the hydrated ion approaches the solution/metal interface, the ion could be adsorbed on the metal surface. This adsorption may be accompanied by a partial loss of coordination shell water molecules, or the ion could keep its coordination shell upon adsorption. The behavior will be determined by the competition between the ion-water interactions and the ion-metal interactions. In some cases, a partial eharge transfer between the ion and the metal results in a strong bond, and we term this process chemisorption, in contrast to physisorption, which is much weaker and does not result in substantial modification of the ion s electronic structure. In some cases, one of the coordination shell molecules may be an adsorbed water molecule. hi this case, the ion does not lose part of the coordination shell, but some reorganization of the coordination shell molecules may occur in order to satisfy the constraint imposed by the metal surface, especially when it is charged. 

This overall process can be considered as composed of two parts (1) separation of ions from the lattice (breaking ion-ion bonds in the lattice), and (2) interaction of the ions with water molecules (hydration). Both processes involve ion-water interaction (Fig. 2.10). During crystal dissolution, the two processes are occurring simultaneously. Thus, we can write for the heat of solvation of a salt... 

Ion-Dipole Model In this model ion-dipole forces are the principal forces in the ion-water interaction. The result of these forces is orientation of water molecules in... 

In this section we link our discussion in two previous sections dealing with structure of water and with ion-water interaction. In the discussion on ion-water interaction it was shown that ions in water arrange their immediate neighboring water dipoles into a local structure of the primary water of hydration. Between this local structure and the bulk water is the nonstructured secondary water of hydration. Thus, the presence of ions in water will change the number of water molecules in both the structured and unstructured regions. Any decrease in the number of water molecules in a cluster will result in a corresponding decrease in the value of g and thus a decrease in the dielectric constant of water [Eq. (2.4)]. 

The Ion-Dipole Model. In this model ion-dipole forces are the principal forces in the ion-water interaction. The result of these forces is orientation of water molecules in the immediate vicinity of an ion (Fig. 2.11). One end of the water dipole is attached electrostatically to the oppositely charged ion. The result of this orienting force is that a certain number of water molecules in the immediate vicinity of the ion are preferentially oriented, forming a primary hydration shell of oriented water molecules. These water molecules do not move independently in the solution. Rather, the ion and its primary water sheath is a single entity that... 

In this discussion we link two previous sections (1) structure of water and (2) ion-water interaction. In the discussion on ion-water interaction, it was shown that ions in water arrange their immediate neighboring water dipoles into a local structure of the primary water of hydration. Between this local structure and the bulk... 

Aqvist, J. (1990), Ion-Water Interaction Potentials Derived from Free Energy Perturbation Simulations,... 

The problem of polarization of solvent molecules, as well as solutes in a solution, is a serious one when MD simulations are applied to electrolyte solutions. Attempts have been made to combine ab initio calculations with the usual MD simulation process in which the polarization of water molecules caused by ion-water interactions is estimated at each step of the MD calculations, and thus, we can expect that... 

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