The pressure dependence of the dielectric constant and density of acetonitrile at three temperatures

The static dielectric constant of liquid acetonitrile is reported at 10, 25, and 40°C and at pressures up to 3 kbar. Densities of pure acetonitrile were measured where needed in order to analyze the dielectric measurements using the Kirkwood-Fröhlich equation. The Kirkwood correlation factorg_K was found to be less than unity with a positive temperature but negative pressure coefficient, suggesting an increasing anticorrelation of dipoles with decreased temperature and increased pressure. Theg_K factors are analyzed by a dipole pair-bonding model as outlined by Dannhauser and Flueckinger in which the lowg_K values are attributed to the presence of dimers consisting of completely anticorrelated dipoles. The implication of this model on other measurements, especially MNR relaxation studies, is examined.