(166e) Intrinsic Analysis of Fluid Interfaces Involving Ionic Liquids

Significant progress in our molecular level understanding of fluid interfaces has recently been achieved by the development of algorithms allowing for an intrinsic analysis of interfaces. These algorithms work by identifying the instantaneous location of the interface, at the atomic level, for each molecular configuration [1] and then computing properties relative to this location [2]. Such a procedure eliminates the broadening of the interface caused by the thermally induced capillary waves and reveals the underlying intrinsic features of the system. Here we applied such an intrinsic analysis to systems including ionic liquids (ILs), focusing in particular on the most commonly studied imidazolium-based ILs [3,4]. First we examined in detail the molecular organisation at vapour/liquid interfaces of ILs, systematically changing individual parameters like cation chain length, anion size and temperature. In all systems, we observe pronounced layering at the interface, alternating non-polar with ionic regions. The outermost regions of the surface are populated by alkyl chains, which are followed by a dense and tightly packed layer formed of oppositely charged ionic moieties. Increasing the cation chain length promotes orientations in which the chain is pointing into the vapour, thus increasing the coverage of the surface with alkyl groups. Larger anions promote a disruption of the dense ionic layer, increasing the orientational freedom of cations and increasing the amount of free space.

Subsequently, we examined the liquid/liquid interfaces of imidazolium ILs with liquids of varying polarity (namely cyclohexane and water) [5]. Such systems pose additional technical challenges due to the mutual miscibility of both components. To overcome this, we combine a distance-based cluster search algorithm with the ITIM intrinsic analysis method [6] to distinguish the two liquid phases and to identify atoms residing at the instantaneous surface. In contrast to the well structured vapour/liquid surface, density correlations, ionic associations, and orientational preferences are all weakened at liquid/ liquid interfaces, this effect being much more pronounced when the other species is water. In such systems we observe a drastic reduction in the presence of the cation at the surface and an increase of appearance of polar moieties leading to decreased apolar character of the interface. Anion−cation associations are reduced and partially replaced by water−anion and rarely also water−cation associations. Our work demonstrates the power of molecular simulations, coupled with intrinsic analysis, for the screening and design of ILs for interfacial applications.

[1] Jorge, M.; Jedlovszky, P.; Cordeiro, M. N. D. S. J. Phys. Chem. C 114 (2010) 11169.

[2] Jorge, M.; Hantal, G.; Jedlovszky, P.; Cordeiro, M. N. D. S. J. Phys. Chem. C 114 (2010) 18656.

[3] Hantal, G.; Cordeiro, M. N. D. S.; Jorge, M. Phys. Chem. Chem. Phys. 113 (2011) 21230.

[4] Hantal, G.; Voroshylova, I.; Cordeiro, M. N. D. S.; Jorge, M. Phys. Chem. Chem. Phys. 14 (2012) 5200.

[5] Hantal, G.; Sega, M.; Kantorovich, S. S.; Schröder, C.; Jorge, M. J. Phys. Chem. C 119 (2015) 28448.

[6] Pártay, L. B.; Hantal, G.; Jedlovszky, P.; Vincze, A.; Horvai, G. J. Comput. Chem. 29 (2008) 945.