RETENTION THEORY OF SEDIMENTATION FIELD-FLOW FRACTIONATION AT FINITE-CONCENTRATIONS

In field-flow fractionation (FFF) of concentrated suspensions, particle interactions result in deviations of the equilibrium concentration profile and velocity profile from their respective exponential and parabolic infinite dilution limits. Following the Einstein diffusion formalism, the sedimentation equilibrium concentration profile of a concentrated suspension-is derived for hard spheres; The velocity profile is calculated by taking into account the concentration dependence of the viscosity. The retention factor is then found to depend on two parameters, the basic FFF parameter, as usual, and the average volume fraction of the suspension. While the effects of deviations of the concentration and velocity profiles on the retention factor act in opposite directions, it is found that the former is dominating iind that the retention factor increases with increasing concentrations of the suspension. The predictions of this retention model are found in good agreement with available experimental data on concentrated particulate suspensions in sedimentation and other FFF techniques. Methods for taking van der Waals and electrostatic particle-particle interactions as well as particle-wall interactions into account in the model are given. Experimental FFF data on concentrated polymer solutions are discussed at the light of the model as well as its application to other modes of retention.