Matching water affinities

However, since this work by Robinson and Harned, no other convincing paper appeared that could prove or — on the contrary — discard this assumption. An alternative, and as we will see, more realistic idea was forwarded by Lyklema in 2003. He applied the simple rule of thumb like seeks like inferred from specific ion effects at solid surfaces to qualitatively explain why the mean activity coefficients of Csl are lower than those of Lil at concentrations of 1M. This is a simple consequence of the more sophisticated model of matching water affinities proposed by Collins, as will be discussed in subsequent chapters. 

In the previous sections we repeatedly mentioned the general rule like seeks like . But what is the reason Kim D. Collins published a landmark paper to elucidate and explain this phenomenon without using sophisticated theories. We quote here his main idea that he summarised in the following rule The law of matching water affinities oppositely charged ions in free solution form inner sphere ion pairs spontaneously only when they have equal water affinities. 

On the other hand, and as it is further discussed in Chap. 11, one should always keep in mind that this concept of matching water affinities is a very simplified one that should only be taken as a rule of thumb. In reality, there is certainly a subtle balance between different states of hydration, and like ions may simply tend to be a little bit more attracted than a combination of hard and soft ions. 

Doubtlessly, the concept of matching water affinities as introduced by Kim D. Collins turns out to be most valuable as a first rule of thumb. That big ions preferentially associate with big counterions whereas small ions associate with small counterions is a useful pictorial qualitative interpretation of many experiments. It is also fairly predictive and can be used when further information is not available. 

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