  We investigate physical origin of ergodicity breaking in an aqueous colloidal
dispersion of synthetic hectorite clay, LAPONITE(R), by performing dissolution
and rheological experiments with monovalent salt and tetrasodium pyrophosphate
solution. We also study the effect of interface, nitrogen and paraffin oil on
the same. Dissolution experiments carried out for dispersions with both the
interfaces show similar results. However, for samples with nitrogen interface,
all the effects are observed to get expedited in time compared to paraffin oil
interface. When kept in contact with water, 1.5 wt. % and 2.8 wt. % colloidal
dispersion at pH 10 swell at small ages, while do not swell at large ages. The
solution of tetrasodium pyrophosphate, interestingly, dissolves the entire
colloidal dispersion samples with pH 10 irrespective of the clay concentration.
Experiments carried out on colloidal dispersions prepared in water having pH 13
demonstrate no effect of water as well as sodium pyrophosphate solution on the
same suggesting a possibility of the presence of negative charge on edge at
that pH. We believe that all the behaviors observed for samples at pH 10 can be
explained by an attractive gel microstructure formed by edge-to-face contact.
Furthermore, the absence of swelling in old colloidal dispersion at pH 10 and
dissolution of the same by sodium pyrophosphate solution cannot be explained by
merely repulsive interactions. This behavior suggests that attractive
interactions play an important role in causing ergodicity breaking in the
colloidal dispersions at pH 10 at all the ages irrespective of the clay
concentration. We substantiate the presence of fractal network structure formed
by interparticle edge-face association using rheological tools and cryo-TEM
imaging. We also conduct a comprehensive study of the effect of sodium
pyrophosphate in the sol-gel transition of LAPONITE(R) dispersion.
