Reynolds, J. G.; Graham, T. R.; Pearce, C. I. Ion hydration controls self-diffusion in multicomponent aqueous electrolyte solutions of NaNO2-NaOH-H2O. Journal of Molecular Liquids. 2022. 360, 119441. DOI: 10.1016/j.molliq.2022.119441
Determining the diffusive properties of multicomponent electrolyte mixtures is challenging because the compositional space is large. Here, this complexity was accounted for in aqueous mixtures of sodium hydroxide (NaOH) and sodium nitrite (NaNO2) up to saturation with the help of Zavitsas’ model. The model calculates the quantity of water bound to electrolytes and equates the mole fraction of remaining (free) water to the water activity. The amount of free water was linearly correlated with the self-diffusion coefficients for both proton (1H) and sodium (23Na) over the whole concentration range. Furthermore, both the translational diffusion coefficients of water and sodium ions, and the amount of unbound water were monotonically related to the width of the Raman-active deformation band (v2) and symmetric stretching band (v1) of the nitrite oxyanion in NaNO2 solutions. Thus, similar correlations exist between rotational reorientation times of nitrite oxyanions and the concentration of free water. These findings collectively suggest that diffusion through bulk water may be faster than diffusion around electrolytes. At higher concentrations the self-diffusion rates are slower because some of the water is bound to electrolytes and there is less bulk water through which to diffuse.