SHAPE INVESTIGATION OF MIXED MICELLES BY SMALL-ANGLE NEUTRON-SCATTERING

Several mixtures of the zwitterionic surfactant tetradecyldimethylamine oxide (TDMAO; which normally forms uncharged rodlike micelles) and the anionic sodium dodecyl sulfate (SDS; which normally forms charged globular micelles) were studied by small angle neutron scattering. The aim was to investigate the mixing behavior of these surfactants on one hand and the influence of surfactant charge on the micellar shape on the other hand. A contrast variation experiment on a 60 mM solution with the mixing ratio TDMAO:SDS = 8:2 was performed. The fact that the SDS chains were protonated or deuterated, respectively, provided further information about the internal structure of the micelles. Experimental data are in good accordance with a model of elongated micelles consisting of a core of surfactant chains surrounded by a shell of headgroups while the two surfactants mix homogeneously without any significant enrichment of SDS at the end caps. Taking homogeneous mixing for granted, the shape and surface charge of the micelles were determined for other mixing ratios. A three-axes ellipsoid was chosen as a model for the shape of the micellar core. For the axial ratios a:b:c we found values of 1:(1-2):(1-6) depending on the mixing ratio (1:9 to 9:1). We used the Hayter-Penfold model for globular macroion solutions as well as a semiphenomenological approach according to Farsaci for an attempt at describing the structure factor S(Q) and the micellar interactions.