Nucleation Processes of Alkali Halide Crystals

In aqueous solutions of low concentration, when theories of ionic conductivities are applicable, no ion pairs will be formed in the case of the lithium and sodium halides at room temperature. Even in 13.9 mol (kg H20) 1 LiCl aqueous solution where the molar ratio of LiCl to H20 is 1 4, essentially no ion pairs between Li+ and Cl- ions are formed around 25°C, according to an MD simulation (28). Formation of the 1 1 ion pair between Li+ and Cl in aqueous solution is, however, concluded in 18.5 mol (kg H20) 1 aqueous solution where the nLiCI h2o molar ratio is 1 3, which is close to the saturation concentration of LiCl in water (29). Formation of the 1 1 Li+ Cl ion pair has been suggested by a neutron diffraction method (30), but the data derived from such measurements were not in good agreement with the simulation results. No evidence has been found for ion-pair formation between Li+ and I ions at 20 and 50°C in 2.78 and 6.05 mol (kg H20) 1 aqueous lithium iodide using the solution X-ray diffraction method (31). 

In almost saturated aqueous solutions of NaCl [6.18 mol (kg H20) ] and KC1 [4.56 mol (kg H20) ], about 30 and 60 mol%, respectively, of the ions form contact ion pairs according to X-ray diffraction measurements (26). The Na+-Cl and K+-C1 bond lengths have been evaluated to be 282 and 315 pm. The ion pairs, as well as free ions, are certainly hydrated, and the average hydration numbers of the Na+ and Cl ions in the NaCl solution are 4.6 and 5.3, respectively, and those of K+ and Cl ions in the KC1 solution are both 5.8. No diffraction measurement has been carried out yet, however. 

Potassium fluoride has a higher solubility than NaCl and KC1 in water. An MD simulation has been examined for 16.15 mol (kg H20) 1 KF aqueous solution at about 25°C [the concentration of a saturated solution is 17.50 mol (kg H20) 1 at 25°C] and reveals the formation of ionic clusters that contain more than 1 1 ion pairs. The average 

Ion-pair formation is more clearly seen in cesium halide solutions than in other alkali halide solutions discussed previously, because of weak hydration of cesium ions. The formation of contact ion pairs between Cs+ and F ions has been suggested by Szasz and Heinzinger (33) in 2.2 mol dm-3 aqueous solution. 

According to the MD simulations examined for cesium iodide solutions of 2.78 mol dm-3 (Csl H20 = 1 20) at 25 and 64°C and 5.56 mol dm-3 (Csl H20 = 1 10) at 64°C (34), higher ion pairs than 1 1 form in the 2.78 mol dm-3 solution, but formation is suppressed as the temperature is raised. Thus, ion-pair formation reactions between cesium and iodide, both of which are weakly hydrated, must be exothermic, although no thermodynamic data are available in the literature. The increase in concentration at a given temperature (64°C) enhances the formation of the ionic aggregates from raCsI = 1.07 to 1.80. 

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