Molecular evaporation and condensation of liquid n-alkane films

Energetic, structural, and dynamical properties of solid-to-liquid and liquid-to-vapor interfaces and molecular evaporation and condensation processes from high-temperature liquid n-C6Ht4 and n-Ct6Hs4 films were investigated with molecular dynamics simulations. For hexadecane all evaporation events occurred via monomers while for hexane, evaporation of monomers as well as dimers were observed. For both alkane liquids the molecular evaporation mechanism is found to be sequential in nature, starting with an end segment of a molecule leaving the surface and subsequently the evaporation of the molecule occurs via sequential “dragging” of the rest of the molecule. The condensation coefficients of vapor molecules onto the liquid surface are estimated as -0.9 for hexane and -1 for hexadecane. Evaporation is accompanied by significant molecular conformational changes. In hot liquid n-CteHs, the tram (t) and gauch (g+ and g-) dihedral conformations are distributed as (t,g + ,g -) = (66,17,17) while in the vapor the distribution is almost uniform with a large decrease in the fraction of tram conformations, i.e., (33,31,36). On the other hand, for the shorter alkane hot liquid (t,g+ ,g-)=(72,14,14) while in the vapor the fraction of trans conformations is increased, i.e., (83,10,7). These results are discussed in light of theoretical treatments of evaporation processes.