Tuning physical properties and mesomorphic structures in aqueous 1-ethyl-3-methylimidazolium octylsulfate rigid-gel with univalent salt doping

Abstract

We report measurements of density, viscosity and electrical (ionic) conductivity of three aqueous mixtures of 1-ethyl-3-methyl imidazolium octyl-sulfate ([EMIm][OSO4] or C2C1Im-C8SO4) saturated with univalent sulfate salts in the temperature range 10°C–90°C. This ionic liquid presents a lyotropic mesophase about room temperature when mixed with about a 20wt% of water. The salts used are lithium, sodium and potassium sulfates, and the amount of water varies from 16wt% to 22wt% depending on the given mixture. As a reference, we have also prepared and characterized four aqueous IL mixtures (without salt addition) in the same water concentration range. The solubility of the different salts is scarce in the pure ILs (less than 20×10−6), while in the aqueous mixtures prepared it varies, going from 200×10−6 for Na up to 700×10−6 for Li. As expected, we find that with the addition of the salts, both density and viscosity increase while the ionic conductivity decreases. However, the extent of these effects depends very much on the nature of the salt and the amount of water (which also influence the solubility of the salt). The measured results are rationalized in terms of water content and salt solubility in the ternary mixtures. We observed that the transition from the liquid to the rigid-gel phases takes some temperature degrees, where the sample is in a stable quasi-gel state. Those transition temperatures, from the liquid to the quasi-gel states, and from the quasi-gel to rigid-gel states are related to the nature of the salt metal present in the mixture for samples with the same water content.