SMALL-ANGLE NEUTRON-SCATTERING INVESTIGATION OF LONG-RANGE CORRELATIONS IN SILICA AEROGELS - SIMULATIONS AND EXPERIMENTS

Numerical simulations of gel formation by off-lattice diffusion-limite d cluster-cluster aggregation (DLCA) of identical spherical particles in a cubic box are performed. Both the correlation function g (r) and the small-angle scattering function S (q), which is related to the Fou rier transform of g (r) - 1, are calculated for the resulting gel stru cture which is made of connected fractal clusters. In addition to the short-range features already described in a previous paper, it is foun d that g (r) goes through a minimum before tending to unity for large r values. As a consequence the scattering function S (q) exhibits a ma ximum at small q values. After multiplying S (q) by the form factor P (q), the intensity curve I (q) is calculated and its behavior is found to be in good agreement with small-angle neutron-scattering experimen ts on silica aerogels of various densities. In contrast with previous approaches, the low-q maximum in the I (q) curve is here directly quan titatively accounted for, without introducing any phenomenological ad hoc term to describe the intercluster correlations.