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, IµOH, 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, αo, of the dimeric acid into free molecules in solution. αo 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, αo 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 IµOH 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.