The effective dipole moments of isobutyric acid in the liquid state and dilute solutions in methanol

Abstract

The dipole moments of isobutyric acid (I) were determined in the liquid state (μ₁) and dilute solutions in methanol (μ₁) at 20–50°C. The permittivity of I in the liquid state was found to increase as the temperature grew, and the permittivity of solutions of I was lower than that of pure methanol; it decreased as the concentration of I and the temperature of solutions increased. The effective dipole moments of I were calculated using the Onsager polarization theory for the pure liquid and the Buckingham statistical polarization theory for solutions with various acid concentrations in methanol. The small μ₁ (∼0.8 D) and higher μ₂ (∼3.0 D) values compared with the dipole moment of I in the gas phase μ₀ (1.9 D) were analyzed as determined by the character of acid-acid, acid-solvent, and solvent-solvent intermolecular interactions, a key role in which was played by H-bonds. An analysis of the dipole moments of I in methanolic solutions led us to conclude that the μ₁ and μ₂ values corresponded to the dipole moments of associates and solvates comprising like and unlike molecules linked by intermolecular H-bonds. Their stoichiometry changed as the temperature increased.