Salt Effect on the Aggregation Behavior of 1-Decyl-3-methylimidazolium Bromide in Aqueous Solutions

Understanding of the specific salt effect on the aggregation behavior of ionic liquids (ILs) is relevant to multiple applications. In this work, the influence of a series of 15 salts on the aggregation behavior of [C10mim]Br in aqueous solutions has been investigated by conductivity, fluorescence, and dynamic light scattering. It was shown that NaCl, NaBr, NaI, CH3CO2Na, NaSCN, NaNO3, NaBrO3, NaClO3, C6H5COONa, Na2CO3, Na2SO4, Na2C4H4O6, and Na3CH5O7 have salting-out effect, whereas FeBr3 and AlBr3 have salting-in effect on the aggregation of [C10mim]Br in aqueous solutions. The effect of anions of the added sodium salts on the critical aggregation concentration (CAC), degree of anionic binding (β), and aggregation number (N agg) of the IL basically follows the Hofmeister series, and the CAC values decrease but the β and N agg values increase with increasing concentration of the salts. Hydrophobicity of the anions is suggested to play important roles in the salt effect on the aggregation of [C10mim]Br in aqueous solutions. Furthermore, the IL aggregates were found to grow slowly as the increase of the salt concentrations under studied static conditions, and resulting in the increased aggregation number of the IL. These results are expected to be useful in the applications of ionic liquids.