Interaction between ionic and nonionic surfactants in the adsorbed film and micelle. 3. Sodium dodecyl sulfate and tetraethylene glycol monooctyl ether

The surface tension of the aqueous solutions of sodium dodecyl sulfate (SDS ) and tetraethylene glycol monooctyl ether (C8E4) was measured as a functio n of the total molality of the surfactants and the composition of C8E4 at c onstant temperature under atmospheric pressure. The results of the surface tension measurement were analyzed by our thermodynamic procedure. Both the phase diagram of adsorption and th at of micelle formation were found to ha ve an azeotropic point, and the stronger energetic stabilization was sugges ted. The extent of this nonideal mixing was estimated quantitatively in ter ms of the excess Gibbs energy, (g) over cap (H,E), in the adsorbed film and of that in the micelle, (g) over cap (M,E). The dependence of (g) over cap (H,E) on the surface tension was also utilized to elucidate the mechanism of the attraction between the different species in these aggregates in deta il. By combining the results of the present study with that of the previous one, we concluded that the attractive interaction acting in the adsorbed f ilm and micelle is the indirect interaction between dodecyl sulfate ions (D S-) and the ether oxygen atom of C8E4 molecules through the Na countereatio ns. Sodium ions interact with the DS- ion and C8E4 simultaneously, probably through the electrical attraction.