Quantitative analysis of free water in ionic liquid-water mixtures

Abstract

The purpose of this paper is to determine the amount of water in ionic liquid aqueous solutions that does not form hydrogen bonds (that is to say, free water). Here, the amount of free water was determined in mixtures of water and four ionic liquids based on the imidazolium cation: 1-Butyl-3methylimidazolium acetate, 1-Butyl-3methylimidazolium bromide, 1-Butyl-3methylimidazolium chloride, and 1–3, dimethyl-imidazolium chloride. Their ionic liquid mass fraction was between 0% and 80%. The amount of free water in the mixtures was determined from the concentration profiles obtained by analysing the near infrared spectra of the mixtures between 800 and 1070 nm using multivariate curve resolution-alternating least squares. The absorption band characteristic of the OH- group in the water is present in the spectral region considered. The analysis was done at three temperatures: 298.15, 313.15 and 333.15 K. The major conclusions obtained from a comparative analysis of the results are these: a) the length of the alkyl chain significantly affects the hydrophobicity of the cations when the molality of the ionic liquid in the solutions is higher than 1.435 mol/kg. b) for the solutions with the same cation, the amount of free water in the chloride solutions is lower than in the acetate and bromide solutions when the temperature is lower than 333.15 K. At this temperature, the capacity of acetate and bromide solutions to interact with water is the same. Between 298.15 and 333.15 K, the ionic liquid concentration at which there is no free water in the solutions ranges between 62.70% and 59.59% for the 1–3, dimethylimidazolium chloride, 66.72% and 87.75% for the 1-Butyl-3methylimidazolium chloride, 69.76% and 78.36% for the -1-Butyl-3methylimidazolium bromide and between 69.77% and 78.26% for the 1-Butyl-3methylimidazolium acetate. So, the ionic liquid with the greatest capacity to retain water is the 1–3, dimethylimidazolium chloride.