A multiscale coarse-graining study of the liquid/vacuum interface of room-temperature ionic liquids with alkyl substituents of different lengths

Multiscale coarse-grained (MS-CG) molecular dynamics simulations were performed for the liquid-vacuum interface of room-temperature ionic liquids having alkyl substituents of different lengths. Surface properties, such as the number density profile, the orientational ordering, and surface tension, were calculated. Both CG site and electron number density profiles show that, for the ionic liquids with a long enough alkyl chain, a unique multilayer ordering occurs. In contrast, ionic liquids with a shorter chain exhibit an interfacial structure consistent with a monolayer ordering. Detailed analysis indicates that such mono/multilayer ordering results from the strong electrostatic interactions among the polar groups, and the collective short-range interactions among the nonpolar groups. Such surface behavior may be interpreted as the two-dimensional manifestation of bulk spatial heterogeneity in ionic liquids (Wang, Y.; both, G. A. J. Am. Chem. Soc. 2005, 127, 12192). The orientational behavior of the cation shows that the alkyl chain tends to align parallel to the surface normal, while the aromatic ring is preferentially perpendicular to the surface normal for all the underlying species. An analysis for the surface electron density demonstrates that the cation plays a key role in determining the surface electron density oscillations, while the anion only marginally affects the enhancement. The surface tension is seen to monotonically decrease and approaches a constant as the chain length increases, in agreement with Langmuir theory and experiments.