Stability Constants of Complexes of Crown Ether Surfactant with Several Kinds of Alkali Metal Cations

Abstract

In the previous study we showed that the critical micelle concentration (CMC) of dodecanoyloxymethyl-18-crown-6 (C11 phi 6) in the presence of 0.22 mol kg(-1) alkali metal cation 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. To investigate the cause of these observations, we evaluated stability constants of C11 phi 6/alkali metal cation complexes from electric conductivity data in this work. The stability constant of C11 phi 6/alkali metal cation complex was found to be the largest for Rb(+). Excluding the case of K(+), in which the obtained stability constant was much smaller than the expected value although the reason is unknown, the stability constant decreased with the deviating of the cationic diameter from the hole size of the crown ether ring as previously reported for octanoyloxymethyl-18-crown-6 having the higher CMC. The larger stability constant and the lower cation selectivity than 18-crown-6 were observed, suggesting that the oxygen atom of carbonyl group in joint part between crown ring and alkyl chain of C11 phi 6 participates in the complexation with alkali metal ion.