Soft ionic atmosphere model for molar conductivity, diffusion coefficient and viscosity in concentrated electrolytes

A novel approach using a soft ionic atmosphere model for the diffusion of ions in concentrated aqueous electrolytes is developed to quantify molar conductivity (\(\Lambda\)), diffusion coefficient (D), and relative viscosity (\(\eta _{\text {r}}^*\)). The entropy-driven expansion of the ionic atmosphere in the concentrated electrolyte is characterized through average ion size (\({\overline{r}}_{\text {H}}\)), ionic screening length for point particle ions (\(l_{\text {D}}\)) and a hardness exponent (\(\gamma\)). The radius \((l_{\text {s}})\) of expanded ionic sphere for finite size ions: \(l_{\text {s}}= l_{\text {D}}(1+ ({\overline{r}}_{\text {H}} /l_{\text {D}})^3)\). \(l_{\text {s}}\) circumvents the limitations of the classical Debye screening length \((\kappa ^{-1})\) in concentrated electrolytes. This model leads to a power law dependence of \(\Lambda\), D and \(\eta _{\text {r}}^*\) on \(l_{\text {s}}\). The extent of the hardness of the ionic atmosphere is characterized by an exponent \(\gamma\), which is characteristic of an electrolyte solution and lies between 0.2–0.8. The expansion of the ionic sphere increases with concentration causing enhancement of the effective size of ions, resulting in the reduction in diffusion coefficient and molar conductivity. The model captures the experimental molar conductivity data for the fifteen salts in the aqueous medium.

Graphical abstract