Hydration of Simple Cations and Anions

It is useful to divide the methods of investigating hydration into four groups. First of all, there are the many methods in which one looks at the ionic solution in the equilibrium state for example, one can study its compressibility or one can study heats and entropies of hydration and fit the values to various model systems. 

Another approach involves measuring the transport of the ion concerned under an applied electric field, its mobility. This approach has a unique aspect it provides the individual properties of ions directly so long as they are in dilute solution. 

Finally, there is the theoretical method of approaching ionic solvation including the molecular dynamics simulations. These have become increasingly used because they are cheap and quick. However, MD methods use two-body interaction equations and the parameters used here need experimental data to act as a guide for the determination of parameters that fit. 

Most of the data that one gets from the experimental point of view of hydration concerns the net effect of all the ions in the electrolytes, and it is necessary to pull out the quantities corresponding to each ion separately. This is tricky but possible. For example, one can couple a certain anion with a series of cations of increasing size and extrapolate a plot in such a manner that the effect of the cation becomes negligible and that of the anion is isolated. Then, the individual value for that anion can be used to obtain individual values for any cations that can form salts with this anion. 

Apart from neutron diffraction, what other method distinguishes between the static or equilibrium coordination number and the dynamic solvation number, the number of solvent molecules that travel with an ion when it moves One method is to obtain the sum of the solvation numbers for both cation and anion by using a compressibility approach, assuming that the compressibility of the primary solvation shell is small or negligible, then using the vibration potential approach of Debye to obtain the difference in mass of the two solvated ions. From these two measurements it is possible to get the individual ionic solvation numbers with some degree of reliability. 

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