SMALL-ANGLE NEUTRON-SCATTERING OF CONCENTRATED ADHESIVE-HARD-SPHERE DISPERSIONS

Small-angle neutron-scattering experiments were performed on concentrated dispersions of sterically stabilized silica particles dispersed in benzene, where the particles show an effective attraction on lowering the temperature. In this study, the system was cooled down from 52-degrees-C, where almost-hard-sphere behavior is observed, to just above the phase transition temperature at 33-degrees-C in seven steps. At each temperature, scattered intensities were measured at four volume fractions up to 0.28. The constancy of the particle form factor allowed data analysis at the level of structure factors. Experimental data were fitted with model calculations using the Percus-Yevick solution for Baxter's adhesive hard-sphere potential, taking the particle size polydispersity into account. A quite satisfactory model description of the experimental structure factors could be obtained. At lower temperatures and higher volume fractions, the fits are even quantitative. The obtained stickiness parameter is directly related to sample temperature, and it does not depend on volume fraction. These observations confirm the model of a short-ranged attraction between the particles, with a well depth governed by enthalpic and entropic contributions to the chain-chain and chain-solvent interactions.