Dielectric absorption in aliphatic long-chain esters in the solid state

Abstract

Further dielectric measurements of aliphatic long-chain esters are reported and the dielectric rate process parameters calculated. These are combined with other results to determine the relationship between the enthalpy of activation ΔH and the molecular chain-length n. If the esters are classified according to the position of the polar group in the chain, then each class gives a separate relationship between ΔH and n. In each case, ΔH approaches a maximum as n increases, presumably because chain-twisting plays an increasing role in dipole orientation. A further result is that in each class of ester the enthalpy difference ΔH₀ between equilibrium positions of the dipoles decreases with increasing chain-length n. Taking this in conjunction with the evidence on chain-twisting it appears that ΔH₀ originates mainly in the end-to-end interaction between molecules in adjacent layers. Comparing the observed magnitudes of the dielectric absorption with values calculated by means of Onsager's equation it is concluded that the equilibrium positions of the rotating dipoles are less than 180° apart.