Physical properties of aqueous mixtures of the ionic 1-ethl-3-methyl imidazolium octyl sulfate: A new ionic rigid gel

• We present the phase diagram of the system EMIM-octyl sulfate + water.
• We show that a rigid gel phase appears for certain concentrations and temperatures.
• That gel presents a smectic-like phase, being really a rigid gel crystal.
• The gel is as electrical conductive as the liquid in spite of the viscosity change.
• Density does not change at the (liquid + gel) transition.

We report the existence of hydrophobically driven lyotropic rigid gel phase in aqueous mixtures of the ionic liquid (IL) 1-ethyl-3-methyl imidazolium octyl sulfate (EMIM-OS), and we characterize the physical properties of the gel phase by means of density, electrical conductivity and viscosity measurements. Also we include polarized microscopy images, showing the existence of a crystalline-like gel phase with the cations and anions (mainly the alkyl chain of this last ones) ordered in a fashion similar to a liquid crystal, which is induced by the presence of the water molecules. As was pointed out briefly in a previous paper, some of the mixtures of that system under goes a (liquid + gel) phase transition only in a quite narrow concentration interval: for ionic liquid molar fraction, xIL, from 0.09 to 0.5. Below xIL = 0.09 and above xIL = 0.5, the mixtures crystallize at low temperature becoming a hard solid crystal. Here we report the complete phase diagram of this binary system, as well as measurements of the temperature behavior of several physical properties around the (liquid + gel) and (liquid + crystal) transitions. Curiously enough, we did not detect any change in the temperature dependence of the measured magnitudes at the (liquid + gel) phase transition, but for viscosity, even in the case of the electrical conductivity. Viscosity increases enormously at the transition going from less than 0.3 Pa · s three degrees above the liquid to gel transition temperature to more than 20 Pa · s three degrees below it, indicating a deep change in the mass transport in the system as molecular diffusion is frozen out. No similar change is registered in the charge transport mechanism nor in density from liquid to gel phases in agreement with previous theoretical predictions of pseudolattice structure of ILs in their liquid phase.

Phase diagram indicating three transitions temperatures: (liquid + gel) (dot symbols), (liquid + crystal) (square symbols) and (crystal + liquid) (triangle symbols) for the aqueous EMIM-OS mixtures.Figure optionsDownload as PowerPoint slide