Abstract
Our previous work showed that the critical micelle concentration (CMC) of dodecanoyloxymethyl-18-crown-6 (C11Φ6) in the presence of 0.22 mol kg-1 alkali metal ion was the highest for K+ having an ionic diameter comparable to the hole size of 18-crown-6 ring and decreased as an ionic diameter of added cation departed from the hole size. In this study, we used octanoyloxymethyl-18-crown-6 (C7Φ6) having the higher CMC than C11Φ6, measured electric conductivities of aqueous C7Φ6 solutions in the presence of alkali metal ions and evaluated stability constants of C7Φ6/alkali metal ion complexes to confirm that the dependence of CMC of crown ether surfactant on the ionic diameter of alkali metal ion is attributed to the selective cation inclusion of its crown ring. As a result, it was found that the stability constant of C7Φ6/alkali metal ion complex was the largest for K+ and decreased as a diameter of cation departed from the hole size as expected. C7Φ6 exhibited the larger stability constant and the lower cation selectivity than 18-crown-6, suggesting that the oxygen atom of carbonyl group in joint part between crown ring and alkyl chain of C7Φ6 participates in the complexation with alkali metal ion.
