FRICTION CONSTANTS FOR TETRA-N-PROPYLAMMONIUM TETRAFLUOROBORATE

The diffusion of tetra-n-propylphosphonium tetrafluoroborate into its ammonium homolog was measured by a capillary technique between 252 and 269°. The cation diffusion constant, which is 0.86 × 10-5 cm2/sec at 252°, is an approximation to the self-diffusion coefficient of the cation. An estimate of the error in this approximation is made from the electrical conductances of the salts. The conductances of both the phosphonium and arsonium salts are reported. From the diffusion coefficient and the electrical conductance, the cation-cation and cation-anion friction constants are calculated. They are four to five times as large as the friction constants for KCl; however, the ratio of the friction constants for ions of the same relative size is about the same for Pr4NBF4 and KCl. Although KCl has nearly twice the coulomb energy, the coulomb potential does not appear to make significant contributions to the friction constants. The viscosity of Pr4NBF4, estimated from the friction constants by a hard-sphere model, is closer to the measured value than a similar calculation yields for KCl. However, the effects of the smaller coulomb potential in the organic salt cannot be separated from the effects caused by its having larger, polyatomic ions at twice the hard-sphere density of KCl. © Copyright, 1969, by the American Chemical Society.