No abstract

A simple sticking probability model is used for deducing a kernel capable to describe the kinetics of computer-simulated irreversible aggregation processes. Not only the diffusion-and reaction-limited aggregation regimes were fitted but also the whole transition region. The deduced kernel establishes λ = 0 for the entire range of sticking probabilities and helps to understand how irreversible cluster-cluster aggregation works.

The thermal-induced changes in molecular magnets based on Prussian blue analogues, M(3)[Fe(CN)(6)](2).xH(2)O (M = Mn, Co, Ni, Cu, Zn, and Cd), were studied from infrared, X-ray diffraction, thermo-gravimetric, Mössbauer, and magnetic data. Upon being heated, these materials loose the crystalline water that enhances the interaction between the metal centers, as has been detected from Mössbauer spectroscopy data. At higher temperatures, a progressive decomposition process takes place, liberating CN(-) groups, which reduces the iron atom from Fe(III) to Fe(II) to form hexacyanoferrates(II). The exception corresponds to the cobalt compound that undergoes an inner charge transfer to form Co(III) hexacyanoferrate(II). In the case of zinc ferricyanide, the thermal decomposition is preceded by a structural transformation, from cubic to hexagonal. For M = Co, Ni, Cu, and Zn the intermediate reaction product corresponds to a solid solution of M(II) ferricyanide and ferrocyanide. For M = Mn and Cd the formation of a solid solution on heating was not detected. The crystal frameworks of the initial M(II) ferricyanide and of the formed M(II) ferrocyanide are quite different. In annealed Mn(II) ferricyanide samples, an increasing anti-ferromagnetic contribution on heating, which dominates on the initial ferrimagnetic order, was observed. Such a contribution was attributed to neighboring Mn(II) ions linked by aquo bridges. In the anhydrous annealed sample such interaction disappears. This effect was also studied in pure Mn(II) ferrocyanide. The occurrence of linkage isomerism and also the formation of Ni(III), Cu(III), and Zn(III) hexacyanoferrates(II) were discarded from the obtained experimental evidence.

Two limiting regimes for colloidal particle aggregation are well described in the literature: diffusion-limited cluster aggregation and reaction-limited cluster aggregation. Between these two limiting regimes, a vast transition region is expected. In this paper, the transition region is studied by means of static and dynamic light scattering. Therefore, a system of latex particles is aggregated at different electrolyte concentrations. The time dependence of the average diffusion coefficient is fitted considering the Brownian kernel and the kernel proposed by Schmitt et al. [Phys. Rev. E 62, 8335 (2000)]. The first fits the experimental data only at high electrolyte concentrations while the latter, which considers multiple cluster-cluster contacts, is found to fit the complete set of experimental data. Copyright 2001 Academic Press.

Reversible aggregation processes were simulated for systems of freely diffusing sticky particles. Reversibility was introduced by allowing that all bonds in the system may break with a given probability per time interval. In order to describe the kinetics of such aggregation-fragmentation processes, a fragmentation kernel was developed and then used together with the Brownian aggregation kernel for solving the corresponding kinetic master equation. The deduced fragmentation kernel considers a single characteristic lifetime for all bonds and accounts for the cluster morphology by averaging over all possible configurations for clusters of a given size. It became evident that the simulated cluster-size distributions could be described only when an additional fragmentation effectiveness was considered. Doing so, the stochastic solutions were in good agreement with the simulated data.