EFFECTS OF PARTICLE-SIZE AND POLYDISPERSITY ON THE DEPLETION AND STRUCTURAL FORCES IN COLLOIDAL DISPERSIONS

The effective pair interaction (potential of the mean force) between l arge hard spherical colloidal particles in a dispersion containing sma ll hard sphere particles has been studied theoretically. We calculated the effective interaction from the total correlation function by solv ing the Ornstein-Zernike equation. For a simple binary mixture of part icles, the calculated effective interaction between two large (colloid al) particles is found to be oscillatory in general, including both th e Asakura-Oosawa type attractive depletion well and the repulsive ener gy barrier (which is caused by the formation of small particle layers between these two large particles). A systematic study was conducted t o examine the relationship between the effective interaction potential and the controllable parameters like the particle concentration and t he size ratio. The existence of the structural energy barrier is expec ted to have dual effects on the stability of the large particle disper sion in general: stabilizing in a short time scale and destabilizing i n a long time scale. The effect of the polydispersity of small particl es on both depletion and the structural force between large particles is also addressed in our study, and we found that polydispersity affec ts the structural energy barrier more than it does the attractive depl etion well. In comparison with the depletion/structuring force measure d from the force apparatus experiments, we found that our theoretical results are in good agreement with the experiment.