Thermodynamics Research Center / ThermoML | Fluid Phase Equilibria

ABSTRACT

This work presents a detailed phase equilibria study on the aqueous solutions of two hydrophilic ILs that are solid at room temperature: 1-ethyl-3-methylimidazolium bromide ([EMIM][Br]) and 1-ethyl-3-methylimidazolium tosylate ([EMIM][TOS]). The melting temperatures of the pure ILs were measured to discriminate between existing literature data that show some disparity. Solid-liquid equilibrium (SLE) data for (water + [EMIM][Br]), so far lacking in the literature, were determined by a dynamic method over the major portion of the entire composition range. Using a chilled mirror dew point technique, extensive and accurate vapor-liquid equilibrium (water activity) measurements were further conducted for the aqueous solutions of both ILs. The temperature and concentration dependences of the activity data were correlated simultaneously by an extended NRTL model, whereby establishing a common description of pertinent equilibria and related thermodynamic behavior of these systems. Good performance of this model was verified against other experimental data from the literature and this work. In particular, correct predictions of experimental SLE data proved the model to extrapolate well beyond the temperature range of data underlying the correlation. Excess thermodynamic properties calculated from the model revealed that these systems exhibit large negative deviations from Raoult's law, a strongly exothermic mixing and, except for (water + [EMIM][TOS]) in the water-rich region, positive excess entropies. Comparing the two ILs, the observed deviations from ideality are always more pronounced for [EMIM][Br], which indicates that [Br]- is a more hydrophilic anion than [TOS]-. The observed pattern of thermodynamic behavior was rationalized on the molecular level by means of simple MD simulations. They showed that while there are more H-bonds formed between water and [TOS]-, these bonds are weaker than those between water and [Br]-.