Interaction between saponite and cationic, zwitterionic and nonionic surfactants

Saponite is a commercially available synthetic clay mineral that can easily be dispersed in water to a transparent solution. With increasing concentration the solutions show a sol-gel transition around 2 wt.%. The gels are rather stiff and have a high yield value. The saponite particles are present in the suspension as very thin bilayers with a thickness of about 10 Angstrom and a diameter of about 500 Angstrom. The bilayers have a relatively high negative charge density of about 0.9 meq g(-1). The particles in the suspensions were characterized by electric birefringence measurements and the signals show large birefringence anomalies. The adsorption of cationic (tetradecyltrimethylammonium bromide, TTMABr), zwitterionic (tetradecyldimethylamine oxide, TDMAO) and nonionic surfactants on the clay mineral particles was studied by surface tension, conductivity, pH and electrophoretic mobility measurements. TTMABr binds on the clay particles for C << CMC. With the binding of TTMABr on the particles, their charge density is reduced and the particles flocculate. The flocculated particles cannot be redispersed by excess TTMABr. The adsorption results for TDMAO are very different. With increasing concentration of the surfactant one observes first flocculation, and then redispersion to a transparent single phase. The pH and surface tension measurements show that TDMAO is mainly adsorbed in the zwitterionic form and only a fraction in the protonated form. The electrophoretic mobility of the particles in the dispersed and redispersed states remains always negative. In the redispersed state at saturation, the clay mineral particles bind 1 g TDMAO per g saponite. With increasing concentration of the nonionic surfactants C12E7.8 and C12E4.8 one observes a single phase, a coagulation region, redispersion to a single phase, two liquid phases, and finally again one liquid phase. The nonionic surfactants start to bind on the hydrophilic clays far below their CMC. In the saturated state the clays adsorb as much nonionic surfactant as zwitterionic surfactant.