The surface tension of aqueous solutions of dodecylammonium chloride (DAC) and tetraethylene glycol monooctyl ether (C8E4) was measured as a function of the total molality of surfactants and the composition of C8E4 at constant temperature under atmospheric pressure. The results of the surface tension measurements were analyzed by our thermodynamic procedure, and phase diagrams of adsorption and micelle formation were drawn. The miscibility of the surfactants and their intermolecular interaction in both the adsorbed film and the micelles were examined. Deviation from ideal mixing was observed and an attractive interaction was suggested. Furthermore, the phase diagram of micelle formation was shown to have an azeotropic point, and a stronger energetic stabilization was suggested in the mixed micelle formation than in the adsorbed film at the cmc. This difference suggests that the structures of both the surfactant molecules and the aggregates significantly affect the intermolecular interaction between surfactants.

The surface tension of the aqueous solutions of sodium dodecyl sulfate (SDS) and tetraethylene glycol monooctyl ether (C8E4) was measured as a function of the total molality of the surfactants and the composition of C8E4 at constant temperature under atmospheric pressure. The results of the surface tension measurement were analyzed by our thermodynamic procedure. Both the phase diagram of adsorption and that of micelle formation were found to have an azeotropic point, and the stronger energetic stabilization was suggested. The extent of this nonideal mixing was estimated quantitatively in terms of the excess Gibbs energy, ĝ H,E, in the adsorbed film and of that in the micelle, ĝ M,E. The dependence of ĝ H,E on the surface tension was also utilized to elucidate the mechanism of the attraction between the different species in these aggregates in detail. By combining the results of the present study with that of the previous one, we concluded that the attractive interaction acting in the adsorbed film and micelle is the indirect interaction between dodecyl sulfate ions (DS-) and the ether oxygen atom of C8E4 molecules through the Na+ countercations. Sodium ions interact with the DS- ion and C8E4 simultaneously, probably through the electrical attraction.

The aqueous solutions of hydrochloric acid−tetraethylene glycol monooctyl ether (C8E4) and sodium chloride−C8E4 mixtures were investigated to examine the interaction between inorganic ions and nonionic surfactants both in the adsorbed film and micelle. Their surface tension was measured as a function of the total molality of the inorganic electrolyte and C8E4 mixture and the composition of C8E4. The results of surface tension measurement were analyzed by our thermodynamic procedure, and the phase diagrams of adsorption and micelle formation were drawn. By comparing the phase diagrams of these systems, it was been that the Na+ ions do but Cl- ions do not interact with the ethylene oxide group of the C8E4 molecule. This finding leads us to the conclusion that the attractive interaction observed in the dodecylammonium chloride−C8E4 system reported previously is caused by ion−dipole interaction or hydrogen bonding between the dodecylammonium ion and oxygen atom of the ethylene oxide group of the C8E4 molecule.