SANS investigation of the microstructures in catanionic mixtures of SDS/DTAC and the effect of various added salts.

The aggregation behavior of two common ionic surfactants with opposite charges, sodium dodecylsulfate (SDS) and dodecyltrimethylammonium chloride (DTAC), was studied for their mixtures and for different added salts. The aggregates formed were characterized by means of small-angle neutron scattering (SANS), at two temperatures: 25 and 50 degrees C (below and above the "Krafft point" of the catanionic salt) and at two overall concentrations (50 and 200 mM). Results have been compared to the well-known SDS+DTAB system. Similar results are found, showing that with no excess of salt the nature of the counter-ion, bromide or chloride, has no dominant effect on this mixture of oppositely charged surfactants. Further SANS experiments were carried out to check the effect of ions on the pure surfactants, the ions being chosen to mimic the head group of the paired surfactant in the catanionic mixture. Tetramethylammonium chloride (TMACl) was added to SDS and sodium methylsulfate (SMS) to DTAC. Their effects were compared to NaCl, which was included in this study as a reference, and explained in terms of competition between the behavior of simple ions (screening) and that of a "binding ion" (attachment to the micellar surface). Apparently the effect of salt addition to these two surfactants is clearly strongly ion-specific.