Sol phase and sol–gel transition in SnO₂ colloidal suspensions

The effect of concentration on the structure of SnO₂ colloids in aqueous suspension, on their spatial correlation and on the gelation process was studied by small angle x-ray scattering (SAXS). The shape of the experimental SAXS curves varies with suspension concentration. For diluted suspensions ([SnO₂] ≤ 0.13 mol L⁻¹), SAXS results indicate the presence of colloidal fractal aggregates with an internal correlation length ξ ≃ 20 Å, 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 concentrations ([SnO₂] = 0.16, 0.32, and 0.64 mol L⁻¹), the colloidal aggregates are larger (ξ = 24 Å) and exhibit a certain degree of spatial correlation between them. The pair correlation function corresponding to the sol with the highest concentration (0.92 mol L⁻¹) reveals a rather strong short range order between aggregates, characteristic of a fluid-like structure, with an average nearest-neighbor distance between aggregates d₁ = 125 Å and an average second-neighbor distance d₂ = 283 Å. The pair distribution function remains essentially invariant during the sol-gel transition, suggesting that gelation involves the formation of a few points of connection between the aggregates resulting in a gel network constituted by essentially linear chains of clusters.