Structural stability and polarisation of ionic liquid films on silica surfaces

We used molecular dynamics simulations to investigate the effect of disorder of the hydroxylated amorphous silica surface on the structure of 8 nm IL films formed from two ionic liquids featuring the same cation 1-butyl-3-methyl-imidazolium or [BMIM], paired with bis(trifluoromethanesulphonyl)amide [NTF2] and tetrafluoroborate [BF4] anions. Several silica surfaces were modelled to estimate the effect of their atomic-scale configuration on the solid-liquid interface and the results are compared to those simulated on the crystalline cristobalite surface. Using strongly polar surfaces, we could also evaluate the response of the ILs to the electric field externally controlled or generated by charged defects in the silica film. We found that the structure of the liquids becomes weaker away from the interface and more susceptible to electric field. Our simulations show that [BMIM][BF4] has a large intrinsic dipole originating at the interface, resilient to external fields, while the polarisation of [BMIM][NTF2] can be more easily controlled.