Role of long-range repulsive interactions in two-dimensional colloidal aggregation:: Experiments and simulations

A theoretical model for the interaction between colloidal particles trapped at the air-water interface is proposed in order to explain experimental aggregation results. Kinetic and structural aspects of 2D aggregation processes point out the long-range nature of the particle interactions. These interactions have been modeled by means of monopolar and dipolar repulsive forces, which depend on the monopole and dipole surface fractions at the emergent part of the colloidal particles, f(mon) and f(dip), respectively. Brownian dynamics simulations have been used to fit the model to experiment results using the fractal dimension d(f) and the kinetics exponent z as comparative parameters. Simulation results show that dipolar interaction controls aggregation at high subphase salt concentration whereas the monopolar interaction determines aggregation at low salt concentrations. Moreover, results show that f(mon) is the main parameter controlling kinetics in 2D aggregation and, hence, a critical coagulation concentration (CCC) can be defined from the salt concentration at which the monopole fraction becomes zero, f(mon) = 0.