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

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 300-640K. 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.