Structural Relaxation and H Bonding in Isomeric Octanols and Their LiCl Solutions by Calorimetry

Abstract

Isomeric octanols show a variety of dielectric behavior. At low temperatures, some behave as nonpolar liquids, others as polar liquids. This results from the extent of steric hindrance for intermolecular H bonding when the CH3 group is in the proximity of the OH group in the seven-membered −C−C− chain. Differential scanning calorimetric studies of nine isomeric octanols and LiCl solutions of two of them are reported here. These are discussed in terms of structural relaxation, glass transition temperature Tg, configurational contribution to the heat capacity ΔCp, and distribution of structural relaxation times parameter β. Tg of the isomers of octanols lies between 148 and 168 K, ΔCp between 35 and 82 kJ/mol, and β between 0.33 and 0.93. Those octanols that behave dielectrically as nonpolar liquids have a broader distribution of relaxation times, or lower β, than those isomers that behave dielectrically as highly polar liquids. Addition of 5 mol % LiCl increases Tg and ΔCp in the former case but decreases both in the latter case, and the distribution of relaxation times broadens in both cases. An analysis in terms of configurational contributions to Cp shows that steric hindrance of the OH group determines the H-bonded motifs formed in liquid octanols near their glass−liquid transition temperature.