A self-consistent approach to the Lamm sedimentation equation is proposed for describing the diffusion induced by a centrifugal (or gravitational) field in a two-component condensed system, particularly for a high solute concentration or a high centrifugal field. In this approach, the driving force inducing the diffusion is expressed by introduction of the effective mass of the surrounding mixture (solvent and solutes) which causes the buoyant force. It is self-consistently represented as a function of concentration in place of the constant value in the Lamm equation. The theory is constructed on the basis of nonequilibrium thermodynamics and the Nernst-Einstein relation considering the effect of concentration change. The resulting diffusion equation is presented in a nonlinear form instead of the linear one of the Lamm equation. The concentration profile in equilibrium state is found by solving the nonlinear diffusion equation. It is shown that the nonlinear effect is significant with increase either in the solute concentration or in the centrifugal field.

• Received 18 April 1988

DOI:https://doi.org/10.1103/PhysRevA.38.4149