Thermodynamic properties of L-menthol in crystalline and gaseous states

Abstract

Objectives. Menthol causes a cooling sensation and reduces the nerve activity when it is applied locally, ingested, or inhaled. This feature explains its extensive use as both an aromatizer and a flavoring agent in food manufacturing, tobacco industry, cosmetics production, as well as a mild anesthetic and antiseptic in dentistry. This work aimed to perform a comprehensive thermodynamic study of L-menthol in both crystalline and gaseous states.Methods. To determine the combustion energy of L-menthol in the crystalline state, combustion bomb calorimetry was used. The temperature dependence of L-menthol’s heat capacity in the range of 5–370 K and the melting (fusion) parameters were determined using adiabatic calorimetry. Quantum chemical calculations were performed on a standalone virtual machine in the Google Cloud Platform using an eight-core Intel Xeon Scalable Processor (Skylake) with a 2.0 GHz (up to 2.7 GHz at peak load) clock frequency and 8 GB RAM.Results. The energy and enthalpy of L-menthol combustion in the crystalline state were determined, and the standard enthalpy of L-menthol formation in the gaseous state was calculated using the standard enthalpy of sublimation. The standard thermodynamic functions (reduced enthalpy, entropy, and reduced Gibbs energy) of L-menthol in both crystalline and liquid states were obtained based on the smoothed values of heat capacity and melting parameters. The group of isodesmic reactions for the ab initio calculation of the enthalpy of formation for gaseous L-menthol was substantiated. Electronic energy and frequencies of normal modes of the molecules involved in these reactions were calculated using the Gaussian 4 composite quantum chemical method. Further, the sublimation enthalpy of L-menthol was calculated using the extended Politzer equation according to the electrostatic potential model.Conclusions. The first comprehensive thermodynamic study of L-menthol in various states of aggregation was performed, and the values calculated using semiempirical methods were consistent with the experimental values within error limits, which confirms the reliability of the results.