Slow liquid dynamics near solid surfaces: Insights from site-resolved studies of ionic liquids in silica confinement

We perform molecular dynamics simulations of ionic liquids that are confined between amorphous silica surfaces and composed of 1-butyl-3-methylimidazolium [C₄mim] cations and tetrafluoroborate [BF₄], hexafluorophosphate [PF₆], or bis(trifluoromethylsulfonyl)imide [TFSI] anions. Near the silica surfaces, we observe strong slowdowns of ion dynamics, which involve mainly two layers of ions and amount to about two orders of magnitude, depending on temperature and anion type. For a detailed investigation of the slow interfacial dynamics, we determine the adsorption sites of the various anion species on the amorphous silica surfaces and ascertain the repopulation dynamics of these sites. The analysis reveals that the mean residence times show a broad distribution, where anions stay longer at sites, which provide better opportunities for hydrogen bonding. Furthermore, the mean residence times follow Arrhenius laws, providing access to site-specific activation energies E_i. The distributions G(E_i) have Gaussian shape with mean values from ∼0.40 eV for TFSI to ∼0.48 eV for PF₆ and standard deviations of about 0.31 eV. Thus, the amorphous silica surfaces impose static and disordered energy landscapes to the neighboring liquid, which have considerable ruggedness and, in this way, substantially hinder ion rearrangements. We discuss that qualitatively similar situations are expected for all kinds of confined liquids.