Effect of Additives on the Phase Behavior of SDS/CTAB/H2O Systems

The effect of additives on the phase behavior of aqueous solutions of sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) has been studied in detail for extended applications. The results showed that both the widths of an anionic two-phase aqueous system (ATPSa) region with SDS in excess and a aqueous cationic two-phase system (ATPSc) region with CTAB in excess are very narrow, less than 1% in absence of any additives. However, phase regions of ATPS move toward the corresponding isotropic single phase regions and become wider in the presence of inorganic additives such as NaBr in the aqueous surfactant solutions. This is because the addition of inorganic salt increases the concentration of inorganic counter-ions and therefore the electrostatic interactions between counter-ions and surfactant aggregates. Meanwhile, the organic salts, sodium salicylate and tetrabutylammonium bromide, have been found to have a greater capacity to expand the range of ATPS because the electrostatic interactions between organic ions and aggregates were adjustable through the deformation of the electron cloud of organic ions. Organic additives with different properties can effectively extend the range of either ATPSc or ATPSa. Wherein, sodium salicylate can expand ATPSc efficiently and tetrabutylammonium bromide can expand ATPSa significantly.

Electrostastic control of spontaneous curvature in catanionic reverse micelles

By means of small-angle neutron scattering and conductivity measurements, we study the microstructure of octylammoniumoctanoate/octane/water catanionic reverse microemulsions with an excess of anionic or cationic surfactant. Increasing the surface charge makes the microemulsion able to incorporate much more water than in the neutral case, up to 10 water molecules per surfactant. Even with charges in the surfactant film, wormlike micelles are present in the microemulsion domain. Along water dilution lines, the classical rod-to-sphere transition due to the minimization of the curvature energy of the rigid surfactant film is observed. When temperature is decreased, a re-entrant phase transition associated with the liquid-gas equilibrium of attractive cylinders is observed. Using the framework of the Tlusty-Safran theory, attraction could originate from junctions between wormlike reverse micelles. In any case, the spontaneous curvature of the catanionic surfactant film depends on both the temperature and the net charge, whatever the sign of the latter.

Phase behavior, topology, and growth of neutral catanionic reverse micelles

The ternary catanionic system octylammoniumoctanoate/octane/water is studied by combined SANS, light scattering, conductivity, and phase diagram approach in the water-poor microemulsion region. The sphere-to-cylinder growth and branching depends on the concentration, the water-to-surfactant ratio, and the temperature. The unidimensional growth leads to a network of interconnected wormlike micelles. Like most studied linear nonionic surfactants, in this true catanionic system at equimolarity of anionic and cationic surfactant, the curvature toward water increases with temperature, making connections between cylinders less frequent.