Measurement of the self-diffusion constant of aqueous calcium nitrate solutions by an NMR spin–echo technique

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Abstract
The self‐diffusion constant of concentrated aqueous solutions of calcium nitrate has been measured using an NMR spin‐echo technique. Solutions with mole ratios of water to calcium nitrate of 4 (the fused tetrahydrate), 6, 8, and 10 were studied over an extensive temperature range including the supercooled regime. Diffusion data were also obtained on solutions with a water to salt mole ratio of 18.5, 55.5, and 222 over a narrower temperature range. The NMR ’’spin‐echo’’ diffusion measurement system which was designed and fabricated in this laboratory incorporated provision for both steady and pulsed gradient diffusion measurements. The system provides a method for improved stability of the widths and amplitudes of the magnetic gradient pulses compared to earlier instrumentation. The improved stability was accomplished with development of a special pulse generator and current control circuitry. The diffusion data were represented over its entire range by the empirical Vogel–Tamman–Fulcher (VTF) equation. The fitting parameters were determined and the values of T0D were found to approach that of water for concentrations higher than R=4. For purposes of comparison with viscosity and conductivity data, fitting of the VTF equation was made for the data taken in the same temperature intervals. Results show an almost Arrhenius behavior for the diffusion process in contrast to the viscosity and conductivity. It appears, from these data, that the water has a mobility which at room temperature is about a factor of 3 higher than the mobility for the other transport properties and which does not tend to ’’freeze out’’ as fast as the glass transition temperature Tg is approached. This high mobility may be attributed to either the motion of the entire water molecule independent of the cation hydration complex, or to the cooperative proton transport where the water is partially dissociated by the strong electrical field of the calcium ions (Grotthuss mechanism).

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