Increasing contact-ion pairing as a supercooled water anomaly. Estimation of the fictive temperature of hyperquenched glassy water

In "dilute" aqueous D(2)O solution of calcium nitrate, contact-ion pairing increases on cooling from 300 to 240 K, despite simultaneous increase of water's dielectric permittivity. This increase in contact-ion pairing was quantified by FT-IR spectroscopic evaluation of nitrate's nu(2) band region. The bound/free nitrate area ratios strongly increase on supercooling in a nonlinear way, in a similar manner as the anomalous molar volume of supercooled water does, which suggests that both anomalies have a common origin in the structural changes of supercooled water. By extrapolation of bound/free nitrate area ratios, we conclude that water and the solute become, on hyperquenching into the glassy state, immobilized between 230 and 200 K. This is important for analysis of the thermodynamic path liquid water takes on quenching into the glassy state, and it gives a low-temperature limit where conformational changes can occur in a biomolecule during quenching of its aqueous solution into the glassy state.