Partition Coefficients of Alkaloids in Biphasic Ionic-Liquid-Aqueous Systems and their Dependence on the Hofmeister Series

Abstract

The aim of this work is to test the potential of hydrophobic phosphonium-based ionic liquids for the extraction of caffeine and nicotine from aqueous phases through the determination of the alkaloids' partition coefficients. It was found that the caffeine partitioning for the ionic-liquid-rich phase increases with the ionic liquid hydrogen-bonding accepting capability (or water content), while for nicotine a nearly opposite behavior was observed. In addition, both the influence of the ionic concentration of the aqueous solution (ranging from 0.0 mol · kg⁻¹ to 3.0 mol · kg⁻¹), and the salt nature (with K- and Na-based salts), in the partitioning of caffeine for the ionic-liquid-rich phase were investigated. The influence of the inorganic salt nature in the alkaloid partitioning for the ionic-liquid-rich phase closely follows the Hofmeister series.

Keywords:

• alkaloid
• caffeine
• Hofmeister series
• ionic liquid
• nicotine
• partition coefficient

ACKNOWLEDGEMENTS

The authors would like to acknowledge Cytec Industries for providing all the phosphonium-based ionic liquids used in this work, and the financial support provided by FCT (Portugal) through projects PTDC/QUI/71331/2006, PTDC/QUI/72903/2006, PTDC/QUI/QUI/101794/2008 and PTDC/EQU-EPR/104554/2008, and through the Post-Doctoral grant SFRH/BPD/41781/2007 of Mara G. Freire.