Vapour–liquid coexistence of natural phenolic monoterpenoid, thymol: comparison with structural isomer, carvacrol

Abstract

Thymol, a naturally occurring structural isomer of carvacrol, is known to exhibit medicinal properties. Since vapour–liquid equilibria data are critical for the design of efficient separation processes, theoretical methods and molecular simulations in conjunction with a simple united atom force field are employed to predict the coexistence properties; also at high temperatures where experimental data are scarce. Process design simulators used to model separation processes employ the equation of state approach which requires as input the critical state properties and acentric factor. Here, these inputs are computed using structure–property relations for use with the theoretical approach and are also determined from the more fundamental molecular simulations to be Tc = 70216 K, Pc = 336 bar, Vc = 48637 cc/mol and ω = 0.459. The structure of the coexisting liquid phase has been investigated using the radial distribution functions and number integrals to determine the extent of association present. The simple united atom force field captures the differences in the phase behaviour of thymol with that of carvacrol that arise due to steric effects of the relative placement of the hydroxyl group with respect to the methyl and isopropyl groups on the aromatic ring.