Motions of Molecules in Liquids: Viscosity and Diffusivity

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Science  29 Oct 1971:
Vol. 174, Issue 4008, pp. 490-493
DOI: 10.1126/science.174.4008.490


The fluidity of a simple liquid is proportional to its degree of expansion over the volume, V0, at which its molecules are so crowded as to inhibit self-diffusion and viscous (as distinguished from plastic) flow. The equation of proportionality is 1/η = B[(V — V0)/V0] where η is the viscosity and V is the molal volume. Values of B are the same for normal paraffins from C3H8 to C7H16 and then decrease progressively as the paraffin lengths increase. Values for other liquids, C6H6, CCl4, P4, CS2, CHCl3, and Hg, appear to vary with repulsive forces. liquids can be moderately fluid when expanded by less than 10 percent; this shows the unreality of some theoretical treatments of the liquid state. Diffusivity begins from the temperature at which V equals V0 and can be correlated for temperature dependence, and for solute-solvent interrelations.