Effects of the surroundings and conformerisation ofn-dodecane molecules on evaporation/condensation processes

Abstract

The evaporation/condensation coefficient () and the evaporation rate () for n-dodecane vs. temperature, gas pressure, gas and liquid density, and solvation effects at a droplet surface are analysed using quantum chemical DFT calculations of several ensembles of conformers of ndodecane molecules in the gas phase (hybrid functional ωB97X-D with the cc-pVTZ and cc-pVDZ basis sets) and in liquid phase (solvation method SMD/ωB97X-D). It is shown that  depends more strongly on a number of neighbouring molecules interacting with an evaporating molecule at a droplet surface (this number is estimated through changes in the surface Gibbs free energy of solvation) than on pressure in the gas phase or conformerisation and cross-conformerisation of molecules in both phases. Thus, temperature and the surrounding effects at droplet surfaces are the dominant factors affecting the values of  for n-dodecane molecules. These values are shown to be similar (at reduced temperatures T/T c < 0.8) or slightly larger (at T/T c > 0.8) than the values of  calculated by the MD FF methods. This endorses the reliability of the previously developed classical approach to estimation of

Kinetic and MD modelling of automotive fuel droplets heating and evaporation: recent results

“…To obtain the values of this coefficient, using this analysis, one would need to repeat these calculations for a wide range of angles of attack, orientation of molecules and energies for various conformers and various conditions of clusters and nanodroplets. Since this is not currently feasible, an alternative approach to calculating this coefficient, taking into account quantum chemical effects, is described in the next section, following [11,7].…”

“…It was shown that the most accurate expression for the condensation coefficient (assumed to be equal to the evaporation coefficient) is the one averaged over the states of various conformers transferred between two phases. This was given by the following formula [11]:…”

“…In the analysis originally presented in [11] and summarised in [7] the transition state theory (TST) was used. This theory was based on a QC DFT approach taking into account the conformerisation of n-dodecane molecules (approximation of Diesel fuel).…”

Molecular Conformational Manifolds between Gas-Liquid Interface and Multiphasic

“…However, a novel hybrid method of the quantum transition state theory and classical kinetic gas theory (QTST-CKGT) could make this connection in a new way without requiring the evaporation/condensation coefficient [3,4]. In Reference [1], it was assumed that the evaporation of n-dodecane is an enthalpic process with no entropy change in the temperature range of 298.15-648.15 K, as well as no energy barrier over the interfacial layers. However, recently the basis of the entropy-enthalpy competition mechanism [2] and the importance of confinement effects alongside the molecular conformational changes at the interface [3,4] were established during the evaporation/condensation process.…”

“…Recently, a model was developed by Gun'ko et al [1] to estimate the evaporation/condensation coefficient (β) and evaporation rate (k) of n-dodecane based on the quantum mechanics solvation model SMD and the Gibbs free energy of evaporation (∆G ev (T)). SMD is based on the solute electron density, the dielectric constant, and the atomic surface tension [2].…”