Surface structure of long-chain ionic liquids: Temperature and chain-length evolution

Hypothesis: The surface-parallel depth-decaying layering at the room-temperature ionic liquid (RTIL)/air interface should exhibit a thermal behavior different from bulk's locally-layered structure. The absence of an (attractive) upper-half-space of matter above the surface should compress the layer spacing and provide for a larger thermal variation compared to the bulk. At the same time, the flat and sharp liquid surface is expected to align the layers macroscopically, and thus enhance the layering depth beyond the range of the bulk's local layering. Experiments: Temperature (T)-dependent X-ray reflectivity (XRR) measurements off the free surfaces of a homologous series of model RTILs [C_nmim][NTf₂] (C_n, even n=12−18, where n is the carbon number of the cationic hydrocarbon chain). Such combined (n,T) surface structure evolution is not currently available for any aprotic homologous RTIL series. Findings: The surface layering exhibits an anomalous thermal contraction, as does the bulk, rather than a conventional expansion, with surface layer spacings falling below those of the bulk by 5-25%, and exhibiting larger contraction-increases with increasing T. A 10-35% increase in the spacing-normalized layering range was also found at the surface compared to the bulk for all n. Surprisingly, a hardening of the surface layers against T-variations is found for increasing n, shown by the decrease of the surface spacing's thermal slope |∂d_s(T)/∂T| with increasing n, while the bulk softens, with its |∂d_b(T)/∂T| increasing with n. These bulk/surface differences originate in the effects hypothesized above.