Dielectric absorption of liquid normal alkanes in the microwave and far infrared regions

The dielectric absorption of liquid n-alkanes has been investigated as a function of frequency and temperature by means of interferometric and resonator techniques in the microwave range and Fourier spectrophotometry in the far infrared band. The large amount of permittivity data now available in a wide frequency range (9 to 6000 GHz) permits a satisfactory evaluation of relaxation parameters. The obtained dielectric principal relaxation times (1.6 ps for n-pentane to 8.8 ps for n-dodecane at room temperature) have been compared with other characteristic times taken from the literature for n-alkanes and cognate aliphatic chains. The absorption in the microwave range is possibly caused either by a reorientational relaxation process of weak permanent dipoles attached to the molecule ends or by a translational relaxation process of temporary dipoles induced in the molecule by the local electric field evoked by surrounding neighbours. An estimation shows that, in the latter case, dipole moments are generated which are of amounts comparable with the measured values (about 0.07 D at room temperature), and the lifetimes of which are given by the measured dielectric relaxation time.