A study of the evaporation and condensation of n-alkane clusters and nanodroplets using quantum chemical methods

• Evaporation rate of organics depends more strongly on temperature than on droplet size.
• Upon equilibrium, the relative number of evaporated molecules is independent on the droplet size.
• Free Gibbs solvation energy can be used to estimate free energy of evaporation.

The evaporation rate (γ) of n-alkane molecules in the C8–C27 range from molecular clusters and nanodroplets is analysed using the quantum chemical solvation model (SMD) and the kinetic gas theory, assuming that the system is in a state of thermodynamic equilibrium (evaporation and condensation rates are equal). The droplet size, liquid density, evaporation enthalpy and Gibbs free energy of evaporation are calculated at 300–640 K. The quantum chemical calculations (SMD/HF or SMD/B3LYP methods with the 6-31G(d,p) basis set) are used to estimate changes in the Gibbs free energy during the transfer of a molecule from a liquid medium (clusters or nanodroplets) into the gas phase. The kinetic gas theory is used to estimate the collision rate of molecules/clusters/nanodroplets in the gas phase. This rate depends on partial pressures, temperature, sizes and masses of molecules and clusters/nanodroplets. An increase in the molecular size of evaporated alkanes from octane to heptacosane results in a strong decrease in the values of γ. Preliminary estimates of the evaporation/condensation coefficient, based on the direct analysis of the collisions of individual molecules with molecular clusters, are presented.

The interaction (sticking or reflection) of a molecule with a droplet depends on droplet temperature and the velocity of the molecule.Figure optionsDownload as PowerPoint slide