Surface tension of normal and branched alkanes

We present results illustrating the effects of using explicit summation terms for the r(-6) dispersion term on the interfacial properties of normal and branched alkanes. We study two all-atom force fields, the OPLS force field of Jorgensen et al. and the exponential-6 force field of Smith and co-workers. We find that the OPLS force field offers excellent agreement with experimental data for surface tension at low temperatures, while the Smith force field agrees well at lower molecular weights. Cutting off the dispersion term at a finite distance r(c) can have pronounced effects on interfacial properties, with as much as a 20% reduction in the measured liquid-vapour surface tension for r(c) = 12 angstrom. Our results also suggest the need for long simulation for normal alkanes, as equilibrium values are not reached for nearly 3 ns or more. Examining the effect of molecular weight on the surface tension, we find that the data for both force fields show excellent agreement with the M-w(-2/3) dependence proposed in the literature.