Adhesive hard sphere colloidal dispersions. I. Diffusion coefficient as a function of well depth

When dispersed in a marginal solvent like benzene, sterically stabilized silica particles exhibit an effective attraction. This attraction is due to the interaction of solvent molecules with the stabilizing chains and is modeled by a square-well potential; the well has a depth of ε/kT=L(θ/T−1), σ≤r≤σ+Δ. L is an interaction parameter, θ the theta temperature of the chain–solvent pair, σ the particle diameter and Δ the width of the square well. An expression for the diffusivity of adhesive hard spheres is derived with the help of the general Stokes–Einstein equation. The model allows scaling to one master plot of experimentally determined diffusion coefficients for samples of six volume fractions, at 20 different temperatures and for each sample at three scattering angles. Additional results obtained with the model are discussed as well. Phase separation in the abovementioned silica dispersions occurs at a well depth of about 5 kT.