Structural evolution of salt-free aqueous Laponite dispersions: A study based on low-field NMR relaxometry and rheological investigations

The combination of Low-Field NMR relaxometry (LF-NMR) and rheological analyses was used to investigate the kinetics of aging and structural evolution of salt-free aqueous Laponite dispersions with concentrations ranging from 0.33 to 2.00 wt%. The examination of LF-NMR results reveals that the rate of nanoclay delamination increases with increasing Laponite concentration while its degree decreases. Furthermore, the LF-NMR results were interpreted together with rheological results to assess the kinetics of processes occurring on different length scales reflecting on the time evolution of the three-dimensional structure. The structural evolution is described by the definition and comparison of three different characteristic times. The LF-NMR decay time (t(r)) designates the time required for Laponite delamination. The induction time (t(i)) indicates when interparticle edge-face interactions become effective enough for incipient networking. The gelation time (t(g)) corresponds to sufficiently high degree of interparticle connectivity for the development of a macroscopic gel behavior. The comparison of t(i) and t(g), on one hand, and t(r), on the other hand, allows to conclude that for low concentration Laponite dispersions nanoclay delamination is a prerequisite for the formation of the new dispersion structure with a macroscopic gel behavior. Interestingly, the development of the new structure, based on orientation of clay disks into edge-to-face pattern and mutual interparticle attractive interactions, starts when the delamination process is still running.