Liquid-liquid interfacial tensions of ionic liquid systems

A. Queimada1, F. Mota1 and E. Macedo1

1LSRE/LCM -Laboratory of Separation and Reaction Engineering, Faculdade de Engenharia, Universidade do Porto, Porto, Portugal

Keywords: liquid-liquid equilibria, aqueous systems
property: interfacial tension
material: aqueous systems

The field of thermophysical properties of ionic liquids is nowadays one of the major growing areas in thermophysical property measurements. The large number of different ionic liquids that we can obtain solely by mixing all the anions and cations, as well as the multiple potential applications of these solvents due to their tunable physical properties, leads to a considerable effort for knowing or describing their thermophysical properties.

Besides the large amount of data on some thermophysical properties such as solubilities and liquid phase densities, not much information is available on the liquid-liquid interfacial tensions of ionic liquid containing systems, although nowadays some of the most relevant applications of ionic liquids require the knowledge of such data, such as in electrochemistry and liquid-liquid extraction.

In this work we will present liquid-liquid interfacial tensions of different imidazolium ionic liquid systems forming two-liquid phases, such as ionic liquid + water and ionic liquid + alcohol. Measurements will be performed using the pendant drop method by means of a Dataphysics OCA 15+ tensiometer. As interfacial tensions are directly related to the equilibrium phase compositions and densities, both phase equilibria compositions and densities will be determined in advance. To generate the equilibrium phases, ionic liquid and the other liquid phase forming component (water or alcohol) will be mixed in thermostatic glass cells whose temperatures are to be controlled within 0.1 K by a Grant LTC1 circulating water bath. For phase composition determinations a Metrohm Karl-Fischer titrator and Uv-vis or gravimetry determinations will be used.

Phase compositions, densities and interfacial tensions will be determined in the temperature range 278.15 – 343.15 K. To ensure constant temperature for the interfacial tension measurements an optical thermostatized cell will be employed, and temperatures will be recorded using calibrated Pt100 probes connected to an Agilent Data Logger.

The output of this work will then be complemented with sum-frequency vibrational spectroscopy results and molecular dynamics simulations that combined with interfacial tension measurements can provide an almost atomistic description of the interface structure, allowing explaining the trends in interfacial tension with the ionic liquid and temperature.

This work was funded by Fundação para a Ciência e a Tecnologia, project PTDC/EQU-FTT/104195/2008 supported by COMPETE/QREN with funds from FEDER (EU).

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