Diffusion flux describes the transport of species in mixtures. The flux is driven by concentration gradients, temperature gradient, and pressure gradient. Unlike many other processes, there are some basic differences between diffusion flux in binary and multicomponent mixtures (more than two species). As an example, in a binary mixture the flux from molecular diffusion is always from higher to lower concentration. This may not be the case in multicomponent mixtures. There are also basic differences between diffusion coefficients in binary and multicomponent mixtures in terms of stability and criticality.

Diffusion coefficients determine the diffusion fluxes. Thermal and molecular diffusion coefficients can only be determined from measurements. Pressure diffusion do not have their own phenomenological coefficients and can therefore be computed from the thermal and molecular diffusion coefficients.

A thermogravitational column is a rectangular slab with a height h much greater than its width L. The column can be open or filled with porous media. After saturating the column with a mixture, a constant temperature gradient is established across the column. This results in a horizontal flux due to thermal diffusion. The temperature gradient also causes convective bulk flow and the coupled effect of diffusion and convection leads to a vertical separation of the components in the mixture. The steady state separations in the column can be related to the thermal diffusion coefficients. The only reported measurement of thermal diffusion coefficients in a non-electrolyte ternary mixture was made with a thermogravitational column.

More recently, it has been demonstrated that both molecular and thermal diffusion coefficients in binary mixtures can be determined from the transient separation of components in a thermogravitational column. In this work, we demonstrate that the technique can be extended to determine the molecular and thermal diffusion coefficients of multicomponent mixtures.