On theory of ionic volumes in dilute aqueous solutions of electrolytes

For purposes of calculation of densities of aqueous solutions of strong electrolytes and of their mixtures, theoretical substantiation of the formerly outlined empirical model of apparent molar ionic volumes φ in solution is presented. According to the model, the hydration sheaths of ions consisting of radially close-packed H2O molecules having effective molar volume V’w the same for all ions, are on their contact with the adjacent structure of liquid water surrounded by a layer of excess voids. This layer can bee substituted in the model by continuous gap of which d0, which again is common to all the ions and is temperature-independent. Using experimental φ values of twenty-seven strong electrolytes together with the data on electrolytic transport of water on ions, V’w = 12 cm3 mol-1 and d0 = (40 ± 2) pm were found for mono- and divalent ions in 1 mol dm-3 solutions, independently of ionic charges and crystallographic radii. The exception are small ions Li+ and Na+, the volumes of which – if interpreted on the basis of the model – correspond to hydration sheaths formed by a cluster of voluminous ice-like structure. An anomaly in this respect has been encountered also in the case of NH+4 ion.