Motions of Molecules in Liquids: Viscosity and Diffusivity

The fluidity of a simple liquid is proportional to its degree of expansion over the volume, V 0 , 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 — V 0 )/ V 0 ] where η is the viscosity and V is the molal volume. Values of B are the same for normal paraffins from C 3 H 8 to C 7 H 16 and then decrease progressively as the paraffin lengths increase. Values for other liquids, C 6 H 6 , CCl 4 , P 4 , CS 2 , CHCl 3 , 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 V 0 and can be correlated for temperature dependence, and for solute-solvent interrelations.