Rejuvenating clay nanocomposites through nanospheres

The results of experimental studies on the effect of added nanoparticles to a suspension of colloidal platelets (disc-shaped clays) are presented. In the absence of additives, clay particles will rapidly aggregate and settle. The addition of nanoparticles (silica nanospheres 22 nm in diameter or less) and salt results in a gel transition of the clay suspension to a viscoelastic solid. The location of this sol/gel transition is largely a function of the nanoparticle size, and as the nanoparticles used become smaller, the lower the threshold concentration of nanoparticles becomes. The concentration of salt needed to produce the effect seems unaffected by other parameters.;Microscopically, the gelled samples exhibit a highly porous and ordered microstructure whereas suspensions that do not gel show only a random arrangement of the plates with no structure. The size of the pores is strongly correlated with suspension parameters. As the concentration of nanospheres or salt is increased or the nanosphere diameter decreased, the average pore size decreases.;Rheologically, the gelled samples develop significant yield stresses, sometimes large enough such that the measuring equipment's capabilities are exceeded. Also the gels are observed to have at least two time scales for gelation; a very short time scale and a much longer one, which is consistent with other clay gel systems.;The proposed mechanism is that the silica nanospheres adsorb to the clay platelet surface, completely saturating it. Upon the addition of salt, the electrostatic repulsion between platelets is diminished and an overall van der Waals attraction between platelets results; thus the nanospheres act as the "glue" which holds the platelets together. This mechanism is supported by stability, microstructural and rheological results, as well as additional experiments involving heating, sonication, microelectrophoresis and nanoparticle and clay substitution in addition to algebraic and trigonometric estimations.;The proposed mechanism explains virtually all of the experimental results, including the observed highly porous microstructure, the development of yield stresses, and the observed gelation time scales; the unique rejuvenating capabilities of the clay gels, as well as the effects that sonication and heat have upon the system. Also, algebraic estimations of complete surface saturation of the nanospheres agree well with the observed sol/gel transitions and other experimental indications of complete surface saturation.