Ionic hydration and secondary relaxations in vitrified concentrated aqueous solutions

By freezing out the primary electrical relaxation modes of various electrolyte solutions at the glass transition, it has become possible to observe well-defined secondary relaxation processes which appear to bear a direct relation to solvation sheath structures. Dielectric loss tangent data at 1 Hz are reported for a series of vitrified aqueous solutions of Ca(NO₃)₂ and Li₂ZnCl₄. Two resolvable relaxations are observed in most of the spectra, together with a poorly resolved relaxation at high water contents. At low water contents (R < 8, where R is the water to salt mole ratio) two distinct relaxations (β and γ) are observed. The β relaxation occurs at ca. 0.8T_g and is unambiguously associated with cation-bound water. The γ relaxation occurs at lower temperatures but is not correlated with T_g. Its dielectric activity may come from movement of cation-bound anions which is influenced by cation-bound water; as more water becomes cation coordinated with increasing R, directional hydrogen bonding interactions increase the activation energy and the relaxation moves to higher temperatures. In the Ca(NO₃)₂ glasses water has displaced all the anions in the first shell at R ∼ 8 and it merges with the β relaxation. At R values greater than 8 a strong relaxation is observed whose intensity increases steadily with R up to the glass-forming limit (R = 16), suggesting that it involves hydrogen bonding interactions with water not directly bound to cations. The origin of a poorly resolved relaxation seen at high water content in the Ca(NO₃)₂ glasses is not clear, although it could be a displaced β relaxation of cation bound water. Masking of the β relaxation by labile charges is demonstrated for two cases using observations on proton-substituted glasses.