LIQUID VAPOR INTERFACES OF ALKANE OLIGOMERS - STRUCTURE AND THERMODYNAMICS FROM MOLECULAR-DYNAMICS SIMULATIONS OF CHEMICALLY REALISTIC MODELS

Molecular dynamics simulations are used to predict the equilibrium liquid-vapor interface structure and surface tension of n-alkanes decane and eicosanes, C10H22 and C20H42, respectively. The model treats each methyl and methylene group as one united Lennard-Jones atom. Realistic bond lengths, bond angle potentials, and torsional potentials are included in the simulation. The simulations predict that although the total mass density profile is monotonic, the density profile of the chain centers of mass and central segments is strongly peaked. The outer edge of the surface is dominated by chain ends. At constant temperature, the interface width as defined by the total mass density profile decreases with increasing chain length. The chains are flattened in the outer regions of the surface but slightly elongated below in the region corresponding to the peak in the center of mass density profile. The computed surface tensions and liquid coexistence densities show the correct trends, although the values of surface tensions are significantly overestimated by this model, as is the case with Lennard-Jones models of noble gases.