| Abstract
| - Multiscale coarse-grained (MS-CG) molecular dynamics simulations were performed for the liquid−vacuuminterface 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 CGsite and electron number density profiles show that, for the ionic liquids with a long enough alkyl chain, aunique multilayer ordering occurs. In contrast, ionic liquids with a shorter chain exhibit an interfacial structureconsistent with a monolayer ordering. Detailed analysis indicates that such mono/multilayer ordering resultsfrom the strong electrostatic interactions among the polar groups, and the collective short-range interactionsamong the nonpolar groups. Such surface behavior may be interpreted as the two-dimensional manifestationof bulk spatial heterogeneity in ionic liquids (Wang, Y.; Voth, 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 surfacenormal, while the aromatic ring is preferentially perpendicular to the surface normal for all the underlyingspecies. An analysis for the surface electron density demonstrates that the cation plays a key role in determiningthe surface electron density oscillations, while the anion only marginally affects the enhancement. The surfacetension is seen to monotonically decrease and approaches a constant as the chain length increases, in agreementwith Langmuir theory and experiments.
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