Stability of colloidal suspensions in a binary liquid mixture in the vicinity of its binodal curve

Suspensions of monodisperse polystyrene spheres with a low surface charge density dispersed in 2‐butoxyethanol/water mixtures become unstable in a certain range of compositions and temperatures in the vicinity of the binodal curve for compositions larger than the critical. In this 1‐fluid phase region of the phase diagram water‐rich concentration fluctuations with long range correlations exist. The colloidal particles flocculate reversibly at a well defined temperature below the temperatures of phase separation. Dynamic light scattering experiments show that approaching the temperature of flocculation the hydrodynamic diameter of the polystyrene sphere increases. Evidence is given to support the assumption that the flocculation is caused by a water rich adsorption layer on the surface of the spheres. This leads to a modification of the interparticle potential and triggers flocculation.