Colloidal aggregation with mobile impurities

The structure and aggregation kinetics of diffusion-limited cluster-cluster three-dimensional monomeric aggregates and gels is investigated as a funct ion of the molar fraction of two types of impurities. In one case the impur ities are allowed to aggregate among themselves whereas in the other the im purities are mobile monomers that remain as such during the whole aggregati on process. Computer simulations are performed on a simple cubic lattice fo r which the functionality of the aggregating particles is effectively 6. Th e first type of impurity shows a decrease in the fractal dimension when com pared to that of a one component system at the same concentration. As a con sequence of this decrease, the gelation concentration is lowered, At higher concentrations a gelling to nongelling transition was observed. In the non gelling regime the colloidal aggregates are kept apart by the impurity clus ters, developing a local ordering. For the monomeric type of impurity, at l arge impurity molar fractions, a nonstructured nongelling phase appears at high enough concentration, in which the colloidal aggregates are kept apart by the sea of mobile impurities that inhibits the formation of a gel. Smal ler molar fractions of mobile monomeric impurities strongly affect both the fractal dimension and the kinetics of the aggregating colloid.