Effect of anion size on the size of polar aggregates in imidazolium-based ionic liquids

Room temperature ionic liquids (RTILs) are interesting due to their expanding use as solvents in fields such as catalysis and electrochemistry. Their properties are affected by ionic aggregation, yet structural understanding is difficult to achieve. This contribution extends a model we recently presented for estimating inter-ionic distances and the sizes of polar aggregates from scattering data for pyrrolidinium-based RTILs to a series of imidazolium-based RTILs with anions of increasing size, 1-methyl-3-propylimidazoliumX (C1C3ImidX, with X^- = Cl^-, Br^-, BF₄^-, PF₆^-, OTf^-, TFSI^-). The model predicts the positions of maxima and minima in anion-anion and cation-cation radial distribution functions, g(r), for the entire series within 6.0% of those calculated directly from molecular dynamics simulations. This reinforces our previous conclusion that distances within "polar scattering domains" responsible for the charge alternation peak in the total structure factor, S(Q), are quantitatively related to interionic distances within polar aggregates in these RTILs. The half-lengths defined in our previous work to represent a measure of polar aggregate size are found to fall within the first minimum in anion-anion or cation-cation g(r) for these RTILs. This implies that half-lengths encompass the first coordination shell, they are approximately one nanometer in size, and thus polar aggregates are approximately a nanometer in size. Half-lengths also increase approximately linearly as anion size increases.