Effect of alkyl chain length of protic ionic liquids on dynamic and static properties of alkylammonium nitrate/water mixtures

Abstract

Mixtures of alkylammonium nitrate (AAN) as a protic ionic liquid (IL) with water were investigated with regard to their dynamic (self-diffusion coefficient, viscosity) and static (chemical shift, density, radial distribution function) properties. As a result, the chemical shift of the cation and the solution volume did not change significantly at 0–60 mol% water, which suggests that the liquid structure of AAN was maintained and water had entered the liquid structure of AAN. In the same concentration range, the chemical shift of the water in AAN showed a downfield shift that was clearly larger than that in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) as an aprotic IL. The microscopic mobility (self-diffusion coefficient) of water also changed more sensitively to the macroscopic mobility (viscosity) in the AAN/water mixture than in the [BMIM][BF4]/water mixture. The mobility of the water in the AAN/water mixture is thus more restricted to a liquid structure than the water in the [BMIM][BF4]/water mixture. These results are considered to be due to the difference in the strength of the hydrogen bonds between AAN and [BMIM][BF4]. On the other hand, the chemical shift of cation and water, the sensitivity of the microscopic mobility of water to the macroscopic mobility of AAN/water mixture, the coordination number of water to cation and water to anion did not change significantly with the alkyl chain length. Therefore, it is expected that water is present near the polar domain composed of ammonium groups and nitrate, which is not easily affected by the alkyl chains.