Miscibility of binary surfactant mixtures in the adsorbed film and micelle: the effect of size of polar head group

The miscibility and interaction of binary surfactant mixtures in adsorbed films and micelles were examined from the standpoint of the influence of the size of polar head groups by surface tension measurement. The mixtures were as follows; pentaethylene glycol monodecyl ether (C10E5)–pentaethylene glycol monooctyl ether (C8E5), C10E5–tetraethylene glycol monooctyl ether (C8E4), dodecyltrimethylammonium brmide–C8E4, and dodecylammonium chloride–C8E4. It was found that the lack of balance of the hydrophilic group size makes the packing of C10E5 and C8E4 molecules less favorable than that of C10E5 and C8E5 molecules both in the adsorbed film and the micelle. On the other hand, it was found that the interaction between cationic surfactant (DTAB or DAC) and C8E4 is stronger than the average of that between same species both in the adsorbed film and the micelle. The excess Gibbs energies of adsorption and micelle formation were calculated to examine the interaction between C8E4 and cationic surfactant molecules in the adsorbed film and the micelle quantitatively. The results showed that the DAC molecules, which have a smaller head group and higher surface charge density, interact with C8E4 molecules more strongly than DTAB molecules, and the interaction is stronger in micelles than in adsorbed films. These findings provide further supports on our previous view that the strong interaction is due to the surfactant ion–dipole interaction or the hydrogen bond between surfactant cation and oxyethylene group.