Measurements of Liquid Diffusion Coefficients in Transparent Model Systems

Abstract

For analyses and modeling of solidification phenomena, as for planar breakdown, cellular or dendritic growth, either constrained or otherwise, the surface energies and liquid diffusion coefficients are probably the most difficult quantities to determine precisely: this contribution raises some questions about the latter.

The question arising is whether values of D _L derived from solidification (or melting) analyses should be expected to give the same results as those obtained from transport measurements in single phase liquids, from convection free transport experiments, from electrical conductivities in ionic solutions, and more recently, from nuclear magnetic resonance experiments.

Under truly steady state conditions the volume change attending solidification or melting should not affect the atomic/molecular diffusion process for an incompressible fluid. However, the derived data and most other direct methods yield a chemical or mutual diffusion coefficient, D, which is related to the intrinsic coefficients for each component by D = X ₁ D ₂ + X ₂ D ₁ where X ₁ and X ₂ are the molar fractions of components #1 and #2. Because the values of D ₁ and D ₂ differ in most systems, there occurs some differential mass transport as evidenced by Kirkendall effect in solids or by osmosis in liquids and it seems possible that this may also apply to events at the solidification or melting front.

Some recent data are presented for intrinsic diffusion coefficients in the transparent model systems ammonium chloride-water, succinonitrile-water and acetone using nuclear magnetic resonance (the pulsed gradient spin echo experiment). These data are compared with those obtained by other methods and with data derived from solidification/melting experiments.

In the system NH ₄ Cl — H ₂ O the NMR results are limited to the NH ₄ ⁺ ion which are in good agreement with those from other single phase measurements, but are not compatible with those derived from dendrite analysis. In the succinonitrile base systems the NMR intrinsic data for the solutes, water or acetone, are in fair agreement with other measured and derived chemical diffusion coefficients but the intrinsic coefficients for succinonitrile are significantly lower.