Sol phase and sol-gel transition in SnO2 colloidal suspensions

The effect of concentration on the structure of SnO2 colloids in aqueous su spension, on their spatial correlation and on the gelation process was stud ied by small angle x-ray scattering (SAXS). The shape of the experimental S AXS curves varies with suspension concentration. For diluted suspensions ([ SnO2] less than or equal to 0.13 mol L-1), SAXS results indicate the presen ce of colloidal fractal aggregates with an internal correlation length xi c ongruent to 20 Angstrom, without any noticeable spatial correlation between them. This suggests that the aggregates are spatially arranged without any significant interaction like in ideal gas structures. For higher concentra tions ([SnO2] = 0.16, 0.32, and 0.64 mol L-1), the colloidal aggregates are larger (xi = 24 Angstrom) and exhibit a certain degree of spatial correlat ion between them. The pair correlation function corresponding to the sol wi th the highest concentration (0.92 mol L-1) reveals a rather strong short r ange order between aggregates, characteristic of a fluid-like structure, wi th an average nearest-neighbor distance between aggregates d(1) = 125 Angst rom and an average second-neighbor distance d(2) = 283 Angstrom. The pair d istribution function remains essentially invariant during the sol-gel trans ition, suggesting that gelation involves the formation of a few points of c onnection between the aggregates resulting in a gel network constituted by essentially linear chains of clusters..