Size, Shape, and Charge of Salt-Free Catanionic Microemulsion Droplets: A Small-Angle Neutron Scattering and Modeling Study

The formation and microstructure of a novel microemulsion based on a salt-free catanionic surfactant have been examined by considering the hexadecyltrimethylammonium octylsulfonate (TASo)-decane-D2O system and using small-angle neutron scattering and self-diffusion NMR. With focus on the emulsification failure boundary, o/w discrete droplets have been observed and characterized for all of the studied microemulsion range. The evaluation of the experimental data was facilitated by using structure factors of a model system composed of charged particles interacting with a screened Coulomb potential. Furthermore, a more simplified model involving a charge regulation mechanism has been employed. Both approaches support the view that the droplets are mainly spherical, fairly monodisperse, and charged. The net charge of the surfactant film is a consequence of the partial dissociation of the short-chain counterpart, owing to its higher solubility. We have further quantified how the droplet charge varies with volume fraction and, from that dependence, explained the unusual phase behavior of the TASo-water system, a seldom found coexistence of two lamellar liquid-crystalline phases in a binary system. This coexistence is quantitatively modeled in terms of a fine balance between the attractive and repulsive colloidal forces acting within the system.