Dielectric Study of the Pressure Dependence of Intermolecular Association in Isomeric Octyl Alcohols

Abstract

The equilibrium dielectric constant and density of 2-octanol, 3-octanol, 2-methyl-3-heptanol, and 5-methyl-3-heptanol have been measured as a function of pressure to 4 kbar from about 100°C to the freezing temperature. The isobaric temperature dependence and the isothermal pressure dependence of the dielectric constant are both sensitive functions of temperature, pressure, and the isomeric nature of the alcohol. Analysis of the data in terms of Kirkwood's dipole correlation factor clearly reveals the differences between isomers. It is proposed that the principal effect of pressure is to perturb the liquid structure. A model of intermolecular association involving monomeric, linear chain, and ring-dimer species is developed and thermodynamic parameters characterizing the various intermolecular associative equilibria are evaluated. ΔH of association is nearly independent of pressure. Thus, application of pressure favors formation of linear chains primarily because of entropic factors. The nature of the short-range order in these liquids is discussed.