New Insights on the Characterization of the Ionic Liquid Crystal 1-Ethyl-3-Methylimidazolium Decylsulfate

The ionic liquid 1-ethyl-3-methylimidazolium decylsulfate [EMIm][DSO₄] is a salt that jellifies when it is mixed with a certain amount of water. In this hydrated state, it is a birefringent hydrogel that can be considered as a highly viscous ionic liquid crystal. In this paper, the liquid crystal phase of [EMIm][DSO₄] and its birefringence dispersion are studied for a sample hydrated by atmospheric water absorption to saturation. Isotropic refractive index dispersion at the isotropic liquid state is also measured. The refractive index at the fast and slow axes of the material is determined by Abbe refractometry while birefringence dispersion and isotropic index dispersion are measured by refractive index spectral broadband interferometry over a broad spectral range at several temperatures. The liquid crystal phase structure is characterized through different experimental techniques and simulations. The study reveals the existence of a lamellar lyotropic liquid crystal phase in the thermal range from 278 to 309 K. The birefringence is achromatic and nontemperature dependent within the experimental resolution with a value of Δn = 4.08(12) × 10–³.