Reorientation of D2O in concentrated aqueous solutions of lithium chloride studied by nuclear magnetic relaxation

Abstract

Deuteron spin–lattice T₁ and spin–spin T₂ relaxation times have been measured at 10 MHz in aqueous (D₂O) solutions of lithium chloride as a function of concentration (R= 3.5 to 6.3 mol D₂O per mol LiCl) and temperature (175–320 K). These measurements are analysed using an empirical Cole–Davidson distribution of reorientational correlation times τ₂ for D₂O. The mean values of τ₂ are found to be represented by ₂=(1.24 ± 0.27)× 10^–13 s exp {(701 ± 21) K/(T–T₀)} with T₀ values of 137 K (R= 3.7), 129 K (R= 4.8) and 125 K (R= 6.3). At low temperatures, the ₂ are similar to the corresponding shear relaxation times _s, measured between 149–173 K, while at 298 K, ₂, the dielectric relaxation time _D and the neutron diffusional correlation time τ₀ have similar values. It is, therefore, concluded that the reorientation of the D₂O molecules in these solutions is governed by the structural fluctuations associated with the glass transition. T₁ measurements at 298 K, in the range R= 3 to 10, are interpreted to show that these solutions have similar structures to the corresponding glasses at 100 K : small clusters of Li⁺(H₂O)₄Cl^– with excess water incorporated interstitially and having dynamic properties remarkably similar to those of bulk water. Previous T₁ measurements for water (D₂O) are reanalysed and the ₂ shown to be dominated by a thermodynamic singularity at T_s= 228 K.