R. Martínez-García

CLUSTER-SIZE DISTRIBUTION IN COLLOIDAL AGGREGATION MONITORED BY SINGLE-CLUSTER LIGHT SCATTERING

The aggregation of polymer colloids is studied in processes induced at high ionic concentration. Cluster-size distributions are measured with a single-cluster light scattering instrument constructed by our team. A brief description of the instrument is given and its proper performance is checked. The results are interpreted with the framework of Smoluchowskis equation. The rate constant for dimer formation is measured and compared with the theoretical prediction. Viscous interaction is taken into account to explain the differences. The temporal evolution of the number-average mean cluster size and the time-independent scaling function are determined from the cluster-size distributions. Dynamic scaling is observed even for small clusters and short aggregation times. Parameters λ and μ from Van Dongen and Ernsts classification scheme for homogeneous kernels are estimated and the aggregation mechanism is identified.

Dynamic scaling concepts applied to numerical solutions of Smoluchowski's rate equation

Smoluchowski’s equation is widely applied to describe the time evolution of the cluster-size distribution during aggregation processes. Analytical solutions for this equation, however, are known only for a very limited number of kernels. Therefore, numerical methods have to be used for describing the time evolution of the cluster-size distribution. In this work, we present a novel self-consistent method for solving Smoluchowski’s equation for any homogeneous kernel. The method considers dynamic scaling to be valid but does not need to assume a given form for the scaling distribution Φ(x). Moreover, the scaling distribution Φ(x) is obtained as a natural result of the algorithm. Due to the implementation of dynamic scaling concepts, the algorithm converges almost immediately with a minimal calculation effort. Comparing calculated size distributions with the corresponding analytical solutions shows the validity of the method. The method is then used to fit experimental data for diffusion limited aggregation....

Simulated Reversible Aggregation Processes for Different Interparticle Potentials: The Cluster Aging Phenomenon

The kinetics of reversible 3D aggregation processes was studied for different interparticle potentials by means of simulations. In previous work ( Phys. ReV. E 2002, 65, 031405), 1 freely diffusing particles were considered that aggregate whenever a collision occurs but disintegrate only with a single given breakup probability. The DLVO theory, however, predicts also interparticle potentials showing two minima of different depths separated by an energetic barrier. Hence, two different kind of bonds, primary and secondary ones, can be formed and should be treated separately. In the present work, this behavior was implemented by considering bonds with different breakup probabilities. The data obtained from simulations were compared with the stochastic solutions of the corresponding master equation. For this purpose, the Brownian kernel was employed together with novel fragmentation kernels. The agreement between the simulations and the kinetic description was found to be quite satisfactory. Moreover, studying the time evolution of the bond population showed that cluster aging appears as a natural consequence of the employed model.

Electrosuperficial properties of calcium oxalate hydrates: Crystal growth and ionic reactivity

Abstract It is shown in this work the way the presence of certain chemical species in the synthesis medium of CO crystal modifies the crystalline form that has been obtained and their electrosuperficial properties. For the crystals obtained in the presence of citrate ions, the changes in their ζ-potential in aqueous solution were studied when several ionic species were added to the dispersion medium.

The kinetics of irreversible aggregation processes

A sticking probability model for irreversible aggregation processes is developed. It allows a kernel capable of describing not only the diffusion-limited and reaction-limited aggregation regimes but also the whole transition region to be deduced. According to the definition given by van Dongen and Ernst, the kernel establishes λ = 0 for the entire range of sticking probabilities. The model presented gives further insight into the detailed aggregation mechanism for slow aggregation processes.