Molecular dynamics simulations of discotic ionic liquid crystals: comparison of a full-charge and a scaled-charge force field

We investigated the effect of using either a full-charge force field (FF) or a scaled-charge FF in the classical non-polarizable molecular dynamics (MD) simulation of discotic ionic liquid crystals (ILCs). We considered the tetrafluoroborate salts of three different gallic acid derivatives, namely, 3,4,5-tris(dodecyloxy)-<i>N,N,N</i>-trimethylbenzenamonium (1[BF₄]), 3,4,5-tris(dodecyloxy) benzyltrimethylammonium (2[BF₄]) and 3,4,5-tris(dodecyloxy)benzyltriethylammonium (3[BF₄]), also experimentally investigated in the literature, to prepare polymeric membranes for water desalination. The three cations all possess three alkyl chains, each containing 12 carbon atoms, attached to the phenyl ring <i>via</i> an ether linkage, making the hydrophobic part of the ion pair very significant. It is now well accepted that for simple ionic liquids (ILs), normally possessing one short alkyl chain on the ionic core, scaled-charge FFs perform much better than full-charge FFs in classical non-polarizable MD simulations by partly recovering the missing polarizability and charge transfer effects (which lowers the effective charge). However, it is unclear whether the same beneficial effect can also be found in ILCs, particularly discotics, which normally bear several and much longer alkyl chains. Therefore, it might be expected that the electrostatic contribution to phase stability is weaker for such systems and, as a consequence, the effect of charge scaling is less important. The results of our simulations clearly show that the scaling of charge instead has a very significant and favourable effect for the simulations of ILCs, although the volume fraction of the hydrophobic regions in discotic systems (hardly affected by the scaling) is significantly larger than that of the ionic region.