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 char ge 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 com plex and the number H of water molecules in the second hydration shell. A c onsistent set of these parameters has been derived from literature data. Fr om an analysis of experimental determinations of the cation-oxygen distance and evaluation of electrostriction phenomena, we have concluded that the r adius R-w of the water molecule depends on the charge of the central ion an d 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 hydr ation energy, DeltaG(hyd), of a monatomic ion with ionic character. It depe nds on the five fundamental characteristics of the aquo ion and reproduces the experimental data with standard deviation less than 0.3%. In the case o f lanthanides, it is shown that the model including average values of the o xygen-cation distance, or different distances, taking into account the loca l 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.