Mesophases in Nearly 2D Room-Temperature Ionic Liquids

TL;DR

This study uses computer simulations to demonstrate that long-chain ionic liquids confined in nearly 2D conditions can form mesophases, which may influence their lubricating properties.

Contribution

It reveals the conditions under which mesophases form in nearly 2D ionic liquids, highlighting the role of chain length and confinement.

Findings

Mesophases form in long-chain RTILs under high confinement.

Shorter-chain RTILs do not form in-plane mesophases.

Layering of charge and tails occurs in surface-adsorbed systems.

Abstract

Computer simulations of (i) a [C12mim][Tf2N] film of nanometric thickness squeezed at kbar pressure by a piecewise parabolic confining potential reveal a mesoscopic in-plane density and composition modulation reminiscent of mesophases seen in 3D samples of the same room-temperature ionic liquid (RTIL). Near 2D confinement, enforced by a high normal load, relatively long aliphatic chains are strictly required for the mesophase formation, as confirmed by computations for two related systems made of (ii) the same [C12mim][Tf2N] adsorbed at a neutral solid surface and (iii) a shorter-chain RTIL ([C4mim][Tf2N]) trapped in the potential well of part i. No in-plane modulation is seen for ii and iii. In case ii, the optimal arrangement of charge and neutral tails is achieved by layering parallel to the surface, while, in case iii, weaker dispersion and packing interactions are unable to bring aliphatic tails together into mesoscopic islands, against overwhelming entropy and Coulomb forces. The onset of in-plane mesophases could greatly affect the properties of long-chain RTILs used as lubricants.