MIXED MICELLES OF SDS C12E6 AND DTAC/C12E6 SURFACTANTS/

Electron spin echo modulation (ESEM) of x-doxylstearic acid spin probe s (x-DSA, x = 5, 7, 10, 12, and 16) in mixed micellar solutions of ion ic and nonionic surfactants has been studied as a function of the doxy l position along the alkyl chain of the stearic acid spin probe and of the mixed micellar composition. The mixed micellar systems investigat ed were sodium dodecyl sulfate (SDS) or dodecyltrimethylammonium chlor ide (DTAC) with hexakis(ethylene glycol) monododecyl ether (C12E6), se lectively deuterated along the poly(ethylene glycol) group (C12D6) or along the alkyl chain ((CD)12E6) in H2O and D2O. The average probe con formation and probe location in the pure surfactants and in the mixed micelles are reported as a function of the doxyl position, x. Modulati on effects due to the interactions of the probe unpaired electron with deuterium in D2O give direct evidence that the hydration is maximized for an equimolar mixture of SDS/C12E6 MiXed micelles. It is also foun d that the polar head groups of SDS and DTAC surfactants are located i n the ethylene oxide region of the C12E6 surfactant. A comparative ana lysis of the deuterium modulation depth, arising from deuteriums locat ed in the alkyl chain or in the ethylene oxide groups of the nonionic surfactant, shows that SDS polar head groups are located at the surfac e of the mixed micelle, close to the second ethylene oxide group of C1 2E6, while DTAC polar head groups are located deeper inside the mixed micelle, at the 5th-6th ethylene oxide group of the nonionic surfactan t. These results provide an explanation at the molecular level of the different thermodynamical behavior found for mixed micelles of anionic -nonionic and cationic-nonionic surfactants.