Salt effects on the surface properties of long-chain alkyl substituted crown compounds

The effects of added salts on the surface properties of the aqueous solution of alkyl crown compounds were investigated. In some combinations of crown ethers and salts (NaCl or KCl), the critical micelle concentrations (CMCs) were observed to be raised selectively in contrast with the inevitable lowering of CMCs of polyoxyethylene-type nonionics. The addition of salt selectively raised the CMCs of crown compounds depending on the kind of cation and the ring size of the crown compound. This phenomenon agreed with the tendency for cloud point, and was explained by the salting in effect due to the complexation of crown compounds with cations. It was also found that the addition of salt caused the gradual rise of surface tension above CMC, and the γ values increased with the increase of salt concentration, most remarkably for the 18-crown/KCl system. Furthermore, both KCl and NaCl were observed to reduce the foaming property of the aqueous solution of 18-crown and 15-crown selectively.