Ion Associated Hydrates

The electrolyte concentration is very important when it comes to discussing mechanisms of ion transport. Molar conductivity-concentration data show conductivity behaviour characteristic of ion association, even at very low salt concentrations (0.01 mol dm ). Vibrational spectra show that by increasing the salt concentration, there is a change in the environment of the ions due to coulomb interactions. In fact, many polymer electrolyte systems are studied at concentrations greatly in excess of 1.0 mol dm (corresponding to ether oxygen to cation ratios of less than 20 1) and charge transport in such systems may have more in common with that of molten salt hydrates or coulomb fluids. However, it is unlikely that any of the models discussed here will offer a unique description of ion transport in a dynamic polymer electrolyte host. Models which have been used or developed to describe ion transport in polymer electrolytes are outlined below. 

Electrolytes contain ions in more or less solvated (hydrated) forms and solvent molecules however, undissociated molecules or ion associations, and so on may also be present. The composition of... 

Hydrated ion — Ion associated with water molecules in aqueous solution. Since an ion has an electric charge, it... 

In the mean spherical approximation (MSA) treatment of the ion association in aqueous solutions, the linearity of the relative permittivity and of the hydrated cation diameters with the electrolyte concentration was taken into account and a good fit of the experimental activity and osmotic coefficient was obtained [72-75]. The MSA model was elaborated on the basis of cluster expansion considerations involving the direct correlation function the treatment can deal with the many-body interaction term and with a screening parameter and proved expedient for the interpretation of experimental results concerning inorganic electrolyte solutions [67,75-77]. 

In considering various computational approaches to solvation, it must first be understood that the ion-water association alone offers a great range of behavior as far as the residence time of water in a hydration shell is concerned. Certain ions form hydrates with lifetimes of months. However, for the ions that are nearly always the goal of computation (ions of groups lA and HA in the Periodic Table and halide ions), the lifetime may be fractions of a nanosecond. 

Electrolytes for which the concentration is less than lO Mcan usually be dealt with by the Debye-Huckel limiting law. Utilize the Debye-Huckel theory extended by allowance for ion size and also for removal of some of the active solvent into the ion s primary solvation shell to calculate the activity coefficient of 5 M NaCland 1M LaClj solutions (neglecting ion association or complexing). Take the total hydration number at the 5 M solution as 3 and at the 1 M solution as 5. Take r,- as 320 pm. 

In the development of the theory of Debye and Hiickel, it was assumed that two main changes had to be made in the ion size (represented by the distance of closest approach a) and the diminution of the available waters due to hydration. Ion association was taken into account also. 

In a parallel series of developments, starting with the Mayer theory and continuing with the so-called mean spherical approximation, the effects of hydration and ion association were arbitrarily removed from consideration, in spite of their undeniable presence in nature. 

The reduction of hydrophobic atmosphere around the complex ions with increasing charge may be directly linked to the increases of the hydration entropy and enthalpy. Therefore, if the electrostatic field around the complex ion is reduced by the presence of the anion through the ion association, the hydrophobic atmosphere is enhanced, followed by the decreases of the hydration entropy and enthalpy of the complexion. If such decrease in or. . "is... 

The momentary association of simple ions is a well-known phenomenon that has been treated in a number of ways. For example, the ion association constant of Bjerrum has received much experimental support. However, the association of simple electrolytes is considered to be shortlived and has been included in the Debye-Hiickel electrostatic theory as correction constants to the concentration. On the contrary, the hydration of the ions may be long-lived. This may be accounted for by considering additionally the ionic interaction ... 

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