Near-IR molar absorption coefficient for the OH-stretching mode of cis-9-octadecenoic acid and dissociation of the acid dimers in the pure liquid state

Abstract

The molar absorption coefficient, ε_OH, at 1445 nm for the stretching mode of the unbonded OH in the carboxy group of cis-9-octadecenoic acid has been successfully determined by examination and correlation of both the absorbance at 1445 nm for a dilute CCl₄ solution of the acid and the degree of dissociation, α_o, of the dimeric acid into free molecules in solution. α_o was obtained from the ratio of the original concentration of the acid to its apparent concentration measured on a molecular weight apparatus. Even in dilute solutions ranging in concentration from 5 × 10^–3 to 2.6 × 10^–2 mol dm^–3, α_o fell within the range 0.35–0.22 at 50 °C, implying that the dissociation of the acid dimers into the monomeric species is not complete. Combination of the ε_OH and the apparent molar absorption coefficient of the OH-vibration mode for the monomeric cis-9-octadecenoic acid in the pure liquid state gave the degree of dissociation, α, of the acid dimer into its monomers in the pure liquid phase as a function of temperature, T. Dissociation of the acid dimer occurs even at room temperature and increases with temperature. The α–T relationship has two break points at 30 and 55 °C. The break-point temperatures correspond to the transition temperatures in the liquid structures of cis-9-octadecenoic acid. At 30 °C the quasi-smectic liquid crystal changes to a more disordered liquid crystal, while at 55 °C the disordered liquid crystal is converted into an isotropic liquid. In addition, thermodynamic properties such as the standard enthalpy and entropy for the dissociation of the cis-9-octadecenoic acid dimer into its monomeric species also suggest that the clusters with the quasi-smectic liquid-crystal structure exist in the pure liquid state below 30 °C and the isotropic liquid structure exists above 55 °C.