Vapour–liquid interfacial properties of n-alkanes

The vapour–liquid interfacial properties of linear alkanes (n-hexane, n-heptane, n-octane, n-nonane, and n-decane) were studied via the square gradient theory (SGT) and by molecular dynamics (MD) simulations. The objectives were the determination of interfacial density profiles, surface tensions, molecular orientations, and conformations via order parameters and radii of gyration, the self-diffusion coefficients and free energy profiles. The Soave-Redlich-Kwong (SRK) and Perturbed-Chain SAFT (PC-SAFT) equations of state (EoSs) were combined with SGT to correlate experimental surface tensions and to predict density profiles. SGT-EoSs and MD interfacial density profiles and interface thicknesses are very similar, especially at low reduced temperatures. MD simulations revealed that not only does the density change throughout the interface but also the average molecular orientation and conformation change. The interface can be roughly divided into three main regions: around the Gibbs dividing surface the molecules, on average, lie at the interface; and in the two regions close to the bulk phases they favour a non-parallel orientation. With respect to the self-diffusion coefficients, three different zones were also observed: close to the bulk phases, the diffusivity is a nonlinear function of the local density, whereas in the middle zone it is linear and coincides with the quasi-linear portion of the density profile. Free energy profiles were obtained via umbrella sampling technique, showing an increase of energy at the interfacial region. These profiles show a similar behaviour as the density profiles.