Scattering of disklike particle suspensions: Evidence for repulsive interactions and large length scale structure from static light scattering and ultra-small-angle neutron scattering

We present static light scattering results for Laponite suspensions. Analyzing our data using the Zimm. technique, we measure a second virial coefficient of 6.0 x 10(-23) m(3) at an ionic strength of 10(-4) M. Both results agree well with recent theories and computations for the thermodynamics of hard disks. Our data support the conclusion that the classical Onsager result underestimates the repulsive contribution in discotic systems. We also present neutron scattering results at very low q which conclusively confirm the existence of large-scale structure in Laponite gels above the isotropic-nematic transition.