Hard-Sphere Hydrodynamics as Reference State in Velocity Cross-Correlation Analysis of Real Systems

The ability of hard-sphere hydrodynamics to model translational motions in real systems is discussed. The possibility to split the excluded volume effect on mutual diffusion into thermodynamic and hydrodynamic contributions is established by using accurate experimental activity and mutual diffusion data. The combination of hard-sphere theory at the second order of volume fraction for mutual diffusion, intradiffusion, and thermodynamic properties allows calculation of velocity cross-correlation coefficients (VCC), herein proposed as new standard coefficients. The comparison of these new standards with real VCCs, corresponding to several aqueous solutions of conventional solutes and macromolecules, tests the applicability of this procedure. The new VCC analysis scales the excluded volume effect and allows more meaningful discussion about correlation of translational motion in solution, especially in the case of the cross-term solute -solvent VCCs in macromolecule-solvent mixtures. Possible extension to analytically quantify correlation between internal motions of biopolymers is briefly discussed.