The thermodynamic properties of S-lactic acid

Abstract

The enthalpies of combustion and formation of S-lactic acid at 298.15 K, Δc H m o (cr.) = −1337.9 ± 0.8 and Δf H m o (cr.) = −700.1 ± 0.9 kJ/mol, were determined by calorimetry. The temperature dependence of acid vapor pressure was studied by the transpiration method, and the enthalpy of its vaporization was obtained, Δvap H o(298.15 K) = 69.1 ± 1.0 kJ/mol. The temperature and enthalpy of fusion, T m (330.4 K) and Δm H o(298.15 K) = 14.7 ± 0.2 kJ/mol, were determined by differential scanning calorimetry. The enthalpy of formation of the acid in the gas phase was obtained. Ab initio methods were used to perform a conformational analysis of the acid, calculate fundamental vibration frequencies, moments of inertia, and total and relative energies of the stablest conformers. Thermodynamic properties were calculated in the ideal gas state over the temperature range 0–1500 K. A thermodynamic analysis of mutual transformation processes (the formation of SS- and RS(meso)-lactides from S-lactic acid and the racemization of these lactides) and the formation of poly-(RS)-lactide from S-lactic acid and SS- and RS(meso)-lactides was performed.