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Title: Chemical Hydration Numbers


Author: Jacob Kielland*


Affiliation: Porsgrunn, Norway


Journal of Chemical Education, September, 1937





For research at atomic and molecular scale, hydration numbers are very important because of the interaction of the solute with the bipolar solvent. The water molecules surround the solute which changes the transport properties of the solute through the membrane. In this paper, hydration numbers of thirty two organic compounds and gases have been computed in water. The method used to find the hydration number is entropy deficiency method.

The total entropy of hydration is an integrated effect of all the water molecules surrounding the solute and this is different from the constant entropy deficiency per molecule of water. The distinction between total and chemical hydration is clear from the following table 1.



Table 1. Chemical and total hydration numbers of some ions

Like ions, non-electrolytes (polar and non-polar) have also the hydration numbers. To obtain the hydration number, divide the entropy difference between aqueous ion and gaseous ion by six. This is shown in the following table. It was found that the hydration number of higher aliphatic alcohols lie in the range of monovalent ions and the striking result showed that inert gases have also hydration numbers corresponding to least hydrated ions. Since non-polar and even inert gases also have hydration numbers, so they must be present for ions. It was also found that the hydration numbers of aliphatic compounds were found approximately additive (last column in table 2).















Important Note: Hydration numbers are important in molecular dynamics simulations. In order to choose the minimum pore size, the size with corresponding hydration number must be chosen instead of the size of the ion.

Reviewer: Aamir Alaud Din

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