Characterization and comparison of hydrophilic and hydrophobic room temperature ionic liquids incorporating the imidazolium cation

A series of hydrophilic and hydrophobic 1-alkyl-3-methylimidazolium room te mperature ionic liquids (RTILs) have been prepared and characterized to det ermine how water content, density, viscosity, surface tension, melting poin t, and thermal stability are affected by changes in alkyl chain length and anion. In the series of RTILs studied here, the choice of anion determines water miscibility and has the most dramatic effect on the properties. Hydro philic anions (e.g., chloride and iodide) produce ionic liquids that are mi scible in any proportion with water but, upon the removal of some water fro m the solution, illustrate how sensitive the physical properties are to a c hange in water content. In comparison, for ionic liquids containing more hy drophobic anions (e.g., PF((6)- and N(SO2CF3)(2)(-)), the removal of water has a smaller affect on the resulting properties. For a series of 1-alkyl-3 -methylimidazolium cations, increasing the alkyl chain length from butyl to hexyl to octyl increases the hydrophobicity and the viscosities of the ion ic liquids increase, whereas densities and surface tension values decrease. Thermal analyses indicate high temperatures are attainable prior to decomp osition and DSC studies reveal a glass transition for several samples. ILs incorporating PF6- have been used in liquid/liquid partitioning of organic molecules from water and the results for two of these are also discussed he re. On a cautionary note, the chemistry of the individual cations and anion s of the ILs should not be overlooked as, in the case of certain conditions for PF6- ILs, contact with an aqueous phase may result in slow hydrolysis of the PF6- with the concomitant release of HF and other species.