Energetic and Entropic Contributions to the Surface Energy of a Free-Standing Thin Film of n-Tetracontane

The surface free energy, γ μ, calculated from the chemical potential, μ, and the contribution of the internal energy to the surface energy, γ U, are compared for a molecular model of a free-standing thin film of n -tetracontane. The coarse-grained chains are represented on a high-coordination lattice with a step length of 2.5 Å. Each occupied site on this lattice represents a two-carbon unit of n -tetracontane. The configurations of the coarse-grained chains are governed by two types of energies. Short-range intramolecular conformations are weighted using first- and second-order interactions in the classic rotational isomeric state model of Abe et al. for polyethylene ( J. Am. Chem. Soc . 1966, 88, 631). Intermolecular, as well as long-range intramolecular, interactions are weighted with a discretized Lennard-Jones potential energy function, truncated after the third shell. The contribution of these two energies to γ U of the amorphous free-standing thin film is 22 erg/cm 2 at 400 K. The chemical potential of the chains in the thin film is evaluated using the chain increment method proposed by Kumar et al. ( Phys. Rev. Lett . 1991, 66, 2935). The analysis of the chemical potential specifies γ μ = 20 erg/cm 2 at 400 K. The estimates of γ U and γ μ at this temperature, T, differ by about 10%. The small difference in these two values implies that the energetic contributions dominate the entropic contributions to γ μ of this amorphous free-standing thin film. Both γ U and γ μ decrease, at nearly the same rate, when T of the simulation rises above 400 K. With a reduction in T below 390 K, crystallization of the thin film occurs. The value of γ U increases as T falls below 390 K, reaching 32 erg/cm 2 at 298 K. The usual implementation of the chain increment method for calculating μ becomes unreliable when crystals appear in the thin film.