On the contribution of nonhydrodynamic interactions to the concentration dependence of the drag coefficient of rigid macromolecules

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Abstract
The Stokes’ law drag coefficient for a rigid macromolecule in solution has long been known to depend on the macromolecule concentration. This concentration dependence has generally been interpreted entirely in terms of hydrodynamic interactions between the macromolecules. It is argued here from a fluctuation–dissipation relation that direct (e.g., hard sphere, electrostatic) interactions between macromolecules also contribute to the drag coefficient. An approximate calculation indicates that the contribution of direct intermacromolecular interactions to the drag coefficient can be as important as the contribution of hydrodynamic intermacromolecular interactions.