ORIGINAL ARTICLE Stability of crown-ether complexes with alkali-metal ions in ionic liquid-water mixed solvents K. Popov Æ H. Ro ¨nkko ¨ma ¨ki Æ M. Hannu-Kuure Æ T. Kuokkanen Æ M. Lajunen Æ A. Vendilo Æ P. Oksman Æ L. H. J. Lajunen Received: 9 February 2007 / Accepted: 16 June 2007 / Published online: 24 July 2007 Ó Springer Science+Business Media B.V. 2007 Abstract Stability constants of sodium and cesium ion complexes with 18-crown-6 (18C6) and dibenzo-18- crown-6 (DB18C6) in N-butyl-4-methyl-pyridinium tet- rafluoroborate [BMP][BF 4 ] aqueous solutions were mea- sured using the 23 Na and 133 Cs NMR technique at 23 °C. To the best of our knowledge, the estimated values of stability constants reported in this study are the first such values given for ionic liquid solutions. The cationic ex- change between the free and complexed species is rapid, and only formation of the 1:1 complexes [M(18C6)] + and [M(DB18C6)] + (M = Na + , Cs + ) were observed. The complex formation constants demonstrated a strong dependence on the [BMP][BF 4 ] concentration. For [M(18C6)] + , in solutions with a 0.33–0.70 mole fraction of water in [BMP][BF 4 ], lg K values are found to be more than one unit higher than the lg K values measured in pure aqueous solutions, although no information concerning the influence of [BMP][BF 4 ] on the complex formation selectivity could be observed. DB18C6 com- plexes revealed significantly lower stability under the same conditions. An extrapolation to zero water content gave the lg K = 2.42 for [Cs(18C6)] + in [BMP][BF 4 ]. It was discovered that when added to water, [BMP][BF 4 ] increases the solubility of crown ethers and decreases the solubility of alkali metal nitrates. Complex formation with crown ethers enhances the solubility of alkali metal salts in [BMP][BF 4 ]. Keywords Ionic liquid  Crown-ether  Alkali metal ions  Stability constants  Complexes  23 Na NMR, 133 Cs NMR Introduction Room temperature ionic liquids (RTILs) are gaining wide recognition as successful solvents for a variety of reactions [1, 2] and separation processes [3–6]. Complex formation with either RTIL anions or with dissolved chelating agents is demonstrated to play an important role in cation extraction [4–7]. Meanwhile, almost nothing is known about the numerical values of complex formation constants in RTILs. A single publication [8] reports on a significant increase of stability in the case of Ag + complexes with cryptand 222 in a series of RTILs relative to DMSO. The overall aim of this research is to study the complex for- mation of sodium and cesium ions with 18-crown-6 and dibenzo-18-crown-6 N-butyl-4-methyl-pyridinium tetra- fluoroborate aqueous solutions using the 23 Na and 133 Cs NMR technique. K. Popov Department of Physical and Colloid Chemistry, Moscow State University of Food Technologies, Volokolamskoye Sh.11, Moscow 125080, Russia e-mail: ki-popov@mtu-net.ru K. Popov  A. Vendilo State Research Institute of Reagents and High Purity Substances (IREA), Bogorodskii val 3, Moscow 107258, Russia K. Popov e-mail: ki-popov@mtu-net.ru H. Ro ¨nkko ¨ma ¨ki (&) Finnish Institute of Occupational Health, Chemical Agents Team, Aapistie 1, Oulu 90220, Finland e-mail: hannu.ronkkomaki@ttl.fi M. Hannu-Kuure  T. Kuokkanen  M. Lajunen  P. Oksman  L. H. J. Lajunen Department of Chemistry, University of Oulu, P. O. Box 3000, Oulu 90014, Finland 123 J Incl Phenom Macrocycl Chem (2007) 59:377–381 DOI 10.1007/s10847-007-9340-1