ANIONIC SURFACTANTS WITH DIVALENT COUNTERIONS OF SEPARATE ELECTRIC CHARGE .2. MICELLAR SIZE AND SURFACE-ACTIVITY

The micelle aggregation number and surface activity of tetradecane-1-sulfonates whose cationic counterion, the 1,1′-(1,ω-alkanediyl)bispyridinium ion (with even-numbered radical chains of up to 14 saturated hydrocarbon groups), is divalent with separate electric charges, were measured, and the effect of the extent of charge separation on micellar size and molecular surface area at the air/water interface was examined. The micelle aggregation number decreases, with the number of anionic surfactant ions and increasing charge separation, from 74 to 33 and increases again to 67 passing through a minimum around 8 CH2 groups. On the other hand, the surface excess from the surface tension measurement monotonically decreases with increasing charge separation. The flat molecular area of the divalent counterions on the basis of the CPK atomic model becomes larger around the eight CH2 group separation than the molecular surface area from the surface excess, indicating a vertical loop at an air/solution interface of the counterions of bolaform type. This result substantiates a folding of the alkanediyl group with more than 8 CH2 groups and its penetration into the micellar interior. It also agrees with what can be expected from the critical micelle concentration (cmc) data. The degree of counterion association to micelle was found to be more than 0.8 for all the surfactants. © 1990 American Chemical Society.