Complexation of hydrogen and alkali cations with 18Crown6 in 25 and 50% (v/v) methanol-water mixtures

Abstract

The formation constants of complexes of K⁺, Na⁺ and H₃O⁺ with 18Crown6 (18C6) (<em>K</em>_Kc<em>, K</em>_Nac and <em>K</em>_Hc respectively) in 25 and 50% (v/v) methanol-water mixtures have been determined by competitive ion exchange technique. K⁺/H⁺ and Na⁺/H⁺ exchanges on Dowex 50WX8 both in the absence and presence of 18C6 have been studied in these media. The values of <em>K</em>_Kc<em>, K</em>_Nac (212 ± 6 and 14 ± 2 in 25% and 2400 ± 200 arid 236 ± 23 in 50% (v/v) methanol-water mixtures), are greater than those in pure aqueous medium. This behaviour is consistent with the generally observed enhancement of stability constants in media of lower dielectric constant. The values of <em>K</em>_Hc for H₃O⁺- 18C6 derived from the two exchange systems are in good agreement with each other. The <em>K</em>_Hc comes out to be 0.5 ± 0.5, in 25% (v/v) methanol-water medium which is lower than that in pure aqueous medium. On the other hand <em>K</em>_Hc in 50% (v/v) methanol-water mixture is 66 ± to. This unusual behaviour in methanol-water mixture of low methanol content is explained in terms of structure of methanol-water mixtures of low methanol content. This also provides an instance where medium structure plays a dominant role in determining the stability constant. <em>K</em>_Hc in 50% (v/v) methanol- water mixture follows the normal trend as observed for alkali metal cations and is consistent with normal behaviour of methanol-water mixtures in this composition range. The free energies of transfer of K⁺, Na⁺ and H₃O⁺-18C6 complexes from water to 25 and 50% (v/v) methanol-water mixtures also have been computed.