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

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 C(2)C(1)Im-C8SO4) saturated with univalent sulfate salts in the temperature range 10 degrees C-90 degrees C. This ionic liquid presents a lyotropic mesophase about room temperature when mixed with about a 20 wt% of water. The salts used are lithium, sodium and potassium sulfates, and the amount of water varies from 16 wt% to 22 wt% 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 x 10(-6)), while in the aqueous mixtures prepared it varies, going from 200 x 10(-6) for Na up to 700 x 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

Liquid-solid-liquid phase transition hysteresis loops in the ionic conductivity of ten imidazolium-based ionic liquids

This paper extends previous ionic conductivity measurements to the phase transition between liquid to solid states, where we present measurements for increasing and decreasing temperatures. Data show the existence of hysteresis loops for some ILs, while others do not present any transition (at least it is not measurable). Seven of the studied ILs have 1-ethyl-3-methyl imidazolium (EMIM) as a common cation, which allows us to observe the anion influence, and four IL compounds have tetrafluoroborate (BF 4-) as common anion and the 1-alkyl-3-methyl imidazolium as cation with four different alkyl chains (ethyl, decyl, dodecyl and hexadecyl). The hysteresis loop differs in shape and amplitude for the different IL compounds presented here, thus some of them present a closed hysteresis loop, without any jump in the value of ionic conductivity, while other ILs have a sharp decrease of the conductivity at a given temperature, and so they present open hysteresis loops. Those hysteresis loops are presented here for the first time and it is shown that they are different for the studied ILs. These measurements demonstrate the capacity of the ILs to maintain its liquid state characteristics (being so in a super cooled liquid state) when temperature decreases well below its melting point (up to 60K for some compounds). We explain it as resilience of the pseudolattice structure already present in liquid state to become a rigid crystalline structure, typical of these ILs at solid state. © 2012 Elsevier B.V