Irreversible versus reversible aggregation: Mean field theory and experiments

Abstract

Colloidal aggregation processes arising at different electrolyte concentrations were studied by means of experiments and confronted with theoretical predictions of different kinetic aggregation models. For this purpose, aqueous dispersions of relatively large polystyrene microspheres were chosen as experimental systems. Aggregation was induced by adding KBr electrolyte to the initially stable particle dispersions. During the aggregation processes, the cluster-size distribution was monitored by means of single cluster light scattering. Analyzing the time evolution of the monomer concentration, we found that the processes arising even at moderate electrolyte concentrations cannot be described by pure time-independent irreversible aggregation models. Hence, alternative models such as time-dependent irreversible aggregation and several reversible aggregation models were also tested. The model that considers a time-dependent sticking probability was found to fit the data quite satisfactorily. Nevertheless, the fitted was so slow that it seems not very likely to find such a behavior in real systems. The aggregation-fragmentation models reported in the literature were unable to reproduce the experimental observations. Hence, a more realistic reversible aggregation model was developed. This model accounts also for reenforced or double bonds between the constituent particles. The corresponding fit improved significantly and reached the same quality as the time-dependent model. Moreover, the obtained fitting parameters were in qualitative agreement with the DLVO predictions and so, reversible aggregation seems to be a more reasonable explanation for the experimental data than time-dependent irreversible aggregation. However, no definite statement on the possible secondary bond fragmentation mechanism may be made since both the applied shear stress in the measuring cell and thermal fluctuations can cause weaker bonds to break.

Aggregation kinetics of latex microspheres in alcohol–water media

“…We fit the experimental data using the model given in Section 7 of previous work [23]. This model has the advantage of producing diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation (RLCA) as limiting regimes, as well as accounting for the possibility of breakup events.…”

“…The reversible population balance we are solving [23] also needs a fragmentation kernel as input. We employed the expression [23] (4)…”

“…The quotient between secondary double bond formation and total secondary bond formation is a cluster-size function given by [23] (2)…”

Connecting Irreversible to Reversible Aggregation: Time and Temperature

“…We show that in this model, which disfavors the formation of bondloops, elapsed time during irreversible aggregation -leading to the formation of an extended network -can be formally correlated with equilibrium temperature in reversible aggregation. We also show that it is possible to develop a parameter-free description of the self-assembly kinetics, bringing reversible and irreversible aggregation of loopless branched systems to the same level of understanding as equilibrium polymerization.Several natural and synthetic materials, as well as biological structures, result from the self-assembly of elementary units into branched aggregates and networks [1,2,3,4,5]. This self-assembly process is receiving considerable attention in two fast-growing fields: supramolecular chemistry [1, 2, 3] and collective behavior of patchy and functionalized particles [6,7,8], among the most promising building blocks of new materials [9,10].…”

“…Several natural and synthetic materials, as well as biological structures, result from the self-assembly of elementary units into branched aggregates and networks [1,2,3,4,5]. This self-assembly process is receiving considerable attention in two fast-growing fields: supramolecular chemistry [1,2,3] and collective behavior of patchy and functionalized particles [6,7,8], among the most promising building blocks of new materials [9,10].…”