Contribution of nonhydrodynamic interactions to the concentration dependence of the friction factor of the mutual diffusion coefficient

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Abstract
The mutual diffusion coefficient Dm of a Brownian particle may be expressed as the ratio of a thermodynamic factor K and a friction factor fm, K and fm both being dependent on the macroparticle concentration. The contribution to fm of direct (e.g., hard sphere, electrostatic) interactions is here estimated to first order in the macroparticle concentration. In contrast to our earlier calculation [J. Chem. Phys. 67, 4690 (1977)] in which the direct interaction contribution Δfm to the Stokes’ Law drag coefficient was obtained from a fluctuation–dissipation equation. Δfm is here obtained directly from a mechanical argument based on the equations of motion of the macroparticles in solution. ΔfD is explicitly evaluated for the case of particles which interact through a weak Gaussian potential