Effects of added salts on adsorbed film and premicelle formation of crown ether surfactants

• Dodecanoyloxymethyl- (C11Φ5) and octanoyloxymethyl-15-crown-5 (C7Φ5) were synthesized.
• Surface tension vs. concentration curves had two break points in all cases.
• Formation of premicelle with aggregation number of 2-4 is suggested.
• CMC is the highest for Na+ that fits crown ring the best among C11Φ5-salt systems.

Two crown ether surfactants, dodecanoyloxymethyl- (C11Φ5) and octanoyloxymethyl-15-crown-5 (C7Φ5), were synthesized and the concentration dependence of surface tension of their aqueous solutions was measured in the absence and the presence of various salts to study the effect of coexistent cation on adsorbed film and aggregation behavior of crown ether surfactants. In the absence of salt, the surface tension vs. concentration curves had two break points for both C11Φ5 and C7Φ5. The break point at the higher concentration (m0) was confirmed to be attributed to the micelle formation by the solubilization of water-insoluble dye and the concentration dependence of proton chemical shifts. Another break point at the lower concentration (mI) would be caused by the formation of premicellar aggregates. For C11Φ5 in the presence of KCl two break points were also observed. At about 0.05 mol kg−1 KCl, m0 and mI were minimized. The cause would be that the salting-out effect of KCl dominates below 0.05 mol kg−1 but above it the repulsive interaction between charged head groups of C11Φ5 complex with K+ becomes larger than it. Two break points were also observed for C11Φ5 in the presence of LiCl, NaCl, CsCl and NH4Cl of 0.22 mol kg−1. It has been found that the critical micelle concentration (m0) are the highest for Na+ having an ionic diameter comparable to the hole size of 15-crown-5 ring and decrease as an ionic diameter of added cation departs from the hole size. The aggregation number of premicelle, which was estimated from the surface excesses at mI and m0 on the assumption of two step micellization, has been about 4 in the absence of salt, about 3 for Li+ hydrated strongly and about 2 for all other cations.

Figure optionsDownload as PowerPoint slide