Effects of Added Salts on Surface Tension and Aggregation of Crown Ether Surfactants.
Two crown ether surfactants, dodecanoyloxymethyl (C11Ph6) and octanoyloxymethyl 18 crown 6 (C7Ph6) were synthesized and the surface tension dependence on surfactant concentration of their aqueous solutions was measured both in the absence and presence of alkali chlorides to confirm the critical micelle concentration (CMC) is highest for the added cation that have an ionic diameter comparable to the hole size of the crown ether ring and that several break points on the surface tension vs. concentration curves occur for these crown ether surfactants. For C11Ph6 and C7Ph6, in the absence of salt, the surface tension vs. concentration curves had two break points. Using the solubilization of a water insoluble dye as an indicator, we found that the break point at the higher concentration (m0) for C7Ph6 was due to micelle formation. Two break points were also observed for the aqueous solution of C11Ph6 in the presence of NaCl, KCl, RbCl, and CsCl salts at concentrations of 0.22 mol kg( 1) and for C7Ph6 with 0.22 mol kg( 1) KCl added. The CMC (m0) was found to be the highest for solutions containing K( salts because K( has an ionic diameter comparable to the hole size of 18 crown 6 ring. Furthermore, the CMC decreased as the ionic diameters of the added cations deviated from the hole size. The molecular areas at two break points, estimated by the Gibbs adsorption isotherm, except for that at the break point at mI of C7Ph6, were very small for an adsorbed monolayer. Further investigation is required to elucidate the reason for the break point at mI.
Journal of oleo science