  We study dilute aqueous solutions of charged disk-like mineral particles
(Laponite) by a combination of static light scattering (SLS) and small-angle
x-ray scattering (SAXS). Laponite solutions are known to form gels above a
certain critical concentration that must be described as non-equilibrium
states. Here we focus on the investigation by SLS and SAXS at concentrations
below gelation (c < 0.016 g/L) and at low concentrations of added salt (0.001
and 0.005 mM). Thus, we have obtained the scattering function of single
Laponite platelets as well as the structure factor describing their interaction
at finite concentration. A detailed analysis of the combined sets of data
proves that the solutions are in a well-defined equilibrium state. Moreover,
this analysis demonstrates the internal consistency and accuracy of the
scattering functions obtained at finite concentrations. We find that Laponite
particles interact through an effective pair potential that is attractive on
short range but repulsive on longer range. This finding demonstrates that
Laponite solutions exhibit only a limited stability at the concentration of
added salt used herein. Raising the ionic strength to 0.005 mM already leads to
slow flocculation as is evidenced from the enhanced scattering intensity at
smallest scattering angles. All data strongly suggest that the gelation
occurring at higher concentration is related to aggregation.
