Aging and rheology of salt-free aqueous Laponite dispersions

Laponite dispersions have been widely studied for their appealing industrial applications as well as for their interesting and peculiar properties. The anisotropy of the Laponite nanoparticles, combined with the competition between attractive and repulsive interactions originates the complexity of the phase diagram of such colloidal systems. Depending both on nanoclay and salt concentrations, different final arrested states (gel, repulsive or attractive glass) can be attained after quite different aging times. The present study concerns the aging evolution of salt-free aqueous dispersions with Laponite concentrations ranging from 0.33 to 2.0 % wt/wt. Rheological measurements were performed under linear and nonlinear conditions after different days from sample preparation, also in combination with Low-Field NMR relaxometry tests. The time evolution of the most significant rheological parameters (zero shear viscosity, yield stress and storage modulus) is similar for all the systems examined. The sigmoidal profiles of the individual data sets can be superposed onto a master curve through a double shifting procedure, using a proper coordinate frame. Accordingly, the effects of concentration on aging kinetics of Laponite dispersions are synthetically described by the asymptotical values of the rheological parameters and the characteristic induction time. Additionally Low-Field NMR tests were carried out to evaluate the dependence of the relaxation time (T2)) of the water hydrogens on laponite concentration and its evolution with aging time and to recognize possible correlations between NMR relaxation and rheological parameters of nanoclay dispersions.