Effect of the relative humidity and isomeric structure on the physical properties of pyridinium based-ionic liquids

• Physical properties of dried and saturated pyridinium-based ILs.
• Viscosity data were fitted to the VFT equation.
• The effect of the humidity on the physical properties was analyzed.
• The influence of the ionic structure on the studied properties was evaluated.

In this work, the effect of the relative humidity (water coming from the atmosphere and pure water) and isomeric structure on the physical properties of pyridinium-based ionic liquids (ILs) was studied. For that purpose, physical properties (density, speed of sound, refractive index and viscosity) of dried and hydrated ILs were measured as a function of temperature at atmospheric pressure. The hydration process was carried by two ways: (a) leaving the pure ILs for several days in open Petri dishes under controlled temperature and humidity conditions; and (b) experimental determination of the liquid–liquid equilibrium of binary mixtures {pure water + ionic liquid} at T = 298.15 K. From these data, the influence of the humidity coming from the atmosphere and pure water on the physical properties of ILs was studied. The density, speed of sound and refractive index data were properly fitted to a linear equation, while viscosity data were fitted to Vogel–Fulcher–Tamman (VFT) equation. Besides, these data were used to calculate the thermal expansion coefficient, the isentropic compressibility and the molar refraction. Moreover, the effect of the number and position of the alkyl substituents of the ionic liquid on the physical properties was also analyzed and discussed. Finally, a comparison with literature data reported for the studied ILs was also carried out.