Surface tension and surface orientation of perfluorinated alkanes

M Tsige and GS Grest, JOURNAL OF PHYSICAL CHEMISTRY C, 112, 5029-5035 (2008).

DOI: 10.1021/jp710678w

Molecular dynamics simulations using two all-atom force fields, the OPLS force field of Jorgensen et al. and the recently developed force field of Borodin et al. (exp-6), have been used to study the equilibrium liquid-vapor interface properties of perfluorinated alkanes. Both force fields predict similar liquid densities and are in excellent agreement with existing experimental data for short chains. The OPLS force field offers better agreement with experimental data for surface tension at lower molecular weights than the exp-6 force field, which overpredicts it by as much as 9%. However, for longer chains the OPLS force field predicts higher surface tension than the exp-6 force field. Also, the OPLS substantially overpredicts the melting temperature confirming a recent suggestion that this force field may not be transferable to longer perfluorinated alkane chains. The orientation of the chains at the surface is found to be strongly dependent on both chain length and temperature. On average, the chain segments tend to orient perpendicular to the surface with the -CF3 end segments at the surface.

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