Temperature dependencies of Henry's law constants and octanol/water partition coefficients for key plant volatile monoterpenoids

To model the emission dynamics and changes in fractional composition of monoterpenoids from plant leaves, temperature dependencies of equilibrium coefficients must be known. Henry's law constants (Hpc, Pa m3 mol−1 and octanol/water partition coefficients (KOW, mol mol−1) were determined for 10 important plant monoterpenes at physiological temperature ranges (25-50 °C for Hpc and 20-50 °C for KOW). A standard EPICS procedure was established to determine Hpc and a shake flask method was used for the measurements of KOW. The enthalpy of volatilization (ΔHvol) varied from 18.0 to 44.3 kJ mol−1 among the monoterpenes, corresponding to a range of temperature-dependent increase in Hpc between 1.3- and 1.8-fold per 10 °C rise in temperature. The enthalpy of water-octanol phase change varied from −11.0 to −23.8 kJ mol−1, corresponding to a decrease of KOW between 1.15- and 1.32-fold per 10 °C increase in temperature. Correlations among physico-chemical characteristics of a wide range of monoterpenes were analyzed to seek the ways of derivation of Hpc and KOW values from other monoterpene physico-chemical characteristics. Hpc was strongly correlated with monoterpene saturated vapor pressure (Pv), and for lipophilic monoterpenes, ΔHvol scaled positively with the enthalpy of vaporization that characterizes the temperature dependence of Pv Thus, Pv versus temperature relations may be employed to derive the temperature relations of Hpc for these monoterpenes. These data collectively indicate that monoterpene differences in Hpc and KOW temperature relations can importantly modify monoterpene emissions from and deposition on plant leaves.