Water characteristics depend on the ionic environment. Thermodynamics and modelisation of the aquo ions

We have characterized the aquo ion in solution by five parameters: the charge q and the corresponding crystallographic radius R-c of the ion with the coordination number N, the radius R-w of the water molecule in the aquo complex and the number H of water molecules in the second hydration shell. A consistent set of these parameters has been derived from literature data. From an analysis of experimental determinations of the cation-oxygen distance and evaluation of electrostriction phenomena, we have concluded that the radius R-w of the water molecule depends on the charge of the central ion and on its crystallographic radius. The different ion-water interactions and previous models are discussed, and we propose an ionic model and a general relation which gives the free hydration energy, DeltaG(hyd), of a monatomic ion with ionic character. It depends on the five fundamental characteristics of the aquo ion and reproduces the experimental data with standard deviation less than 0.3%. In the case of lanthanides, it is shown that the model including average values of the oxygen-cation distance, or different distances, taking into account the local structure of the aquo ion, gives similar thermodynamic data. The proposed model gives also the possibility to deduce properties of the ions such as consistent values of the coordination numbers. (C) 2001 Elsevier Science B.V. All rights reserved.