Small-angle neutron scattering and fluorescence studies of mixed surfactants with dodecyl tails

The mixed micelles formed in solutions of the anionic surfactant sodium dod ecylsulfate (SDS) and the nonionic surfactant dodecylmalono-bis-N-methylglu camide (DBNMG) were studied by surface tension, fluorescence, and small-ang le neutron scattering, Measurements of the critical micelle concentration ( CMC) by surface tension and fluorescence show that these tno surfactants mi s nonideally, The experimental values of the CMCs for surfactant mixtures f or all compositions are less than the ideal prediction proposed by, Clint ( J, Chem, Sec. 71, 1327 (1975)), Regular solution theory (for example, Penfo ld et al,, Langmuir 11, 2498 (1995)) can be used to calculate the compositi on of the micelle at the CMC. The micellar composition thus calculated diff ers significantly from that at higher total surfactant concentrations, the latter being much closer to the stoichiometric composition The polarity of the micellar environment, as perceived by a solubilized fluorescence probe, is identical for both the single-component and the mixed micelles. The sca ttering data show that the mixed micelles are comparable in size to the pur e nonionic micelle and slightly larger than the anionic micelle. Parallel s tudies with deuterated SDS showed no evidence of segregation or local order ing within the mixed micelle. The effective charge on the mixed micelle is determined by the amount of ionic surfactant within the micelle. Addition o f 0.1 M NaCl to these systems has no effect on the CMC, micelle ellipticity , and aggregation number-a behavior very different from that observed for s imple SDS solutions. (C) 1999 Academic Press.