INTERFACE OF MIXED MICELLES FORMED OF ANIONIC CATIONIC AND IONIC NONIONIC SURFACTANTS - ELECTRON-SPIN STUDIES

Electron spin echo modulation (ESEM) and electron spin resonance of x-doxylstearic acids spin probes (x-DSA, x = 5, 7, 10, 12, and 16) in mixed micellar solutions of sodium dodecylsulfate (SDS) and dodecyltrimethylammonium chloride (DTAC) or mixed micelles of SDS, DTAC and hexakis(ethyleneglycol)monododecyl ether (C-12E6), have been studied as a function of the mixed micelle composition in H2O and D2O. Analysis of the ESR parameters shows a strong decrease of the polarity and a strong increase of the microviscosity of the surface of SDS/DTAC mixed micelle, as compared to pure micelles. ESEM effects due to x-DSA interaction with water deuteriums give direct evidence that changes in the surface hydration of the mixed micelle are present. It is also shown in the SDS/C-12E6 or DTAC/C-12E6 mixed micelles that the polar headgroups ot the anionic and cationic surfactants are located in the ethylene oxide region of C-12E6. The average location of SDS and DTAC polar headgroups in the ethylene oxide region of C-12E6 is reported. These results provide an explanation at the molecular level of the different thermodynamical behavior found for mixed micelles composed of anionic-cationic, anionic-nonionic and cationic-nonionic surfactants.