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Polyelectrolyte Conformation and Reactivity Since several years polyelectrolytes have attracted experimentally and theoretically much attention because of their ionizable groups which may be associated with a large range of compounds including charged polymers (polymeric flocculants), macroions such as micelles and biomacromolecules (proteins, DNA, and polysaccharides found in natural systems). The range of parameters influencing polyelectrolyte conformation, chemical reactivity and complexation processes is nowadays not well understood and stimulates research in this field. In the vicinity of polyelectrolyte chains, explicit counterions interact strongly with the chain backbone leading to a rich conformational behavior. To fill the gap between theory and experiments, Monte Carlo simulations are communly used. The acid/base properties of weak polyelectrolyte chains surrounded by explicit counterions and salt particles is investigated by focusing on the influence of explicit ions on the protonation/deprotonation process and associated conformational changes. Several competing effects are observed. On the one hand, the electrostatic repulsions along the chains increase with pH which favor the chain expansion. On the other hand, explicit ions accumulate close to the polyelectolyte backbone which limit the intramolecular interactions. In general, deprotonation process is facilitated by the presence of oppositely charged ions in solution. Salt valency also constitutes a parameter of main importance. Monovalent salt leads to the formation of stretched structures (Fig. a) at high pH values while collapsed strucutres (Fig. b) are observed with trivalent salt. The presence of explicit ions then influences strongly the final chain conformations and may not be neglected when multivalent salt is considered.

Macromolecules and complex formation between polyelectrolytes and nanoparticles Understanding the complexation processes between nanoparticles and polyelectrolytes is an essential aspect in many branches of nanotechnology, nanoscience, chemistry, environmental impact of nanoparticles, biology to describe processes such as nanoparticle stabilization/destabilization and dispersion, water treatment, microencapsulation, complexation with biomolecules for example, and evolution of the interface of many natural and synthetic systems. In view of the complexity of such processes, applications are often based on empirical or semiempirical observations rather than on predictions based on theoretical or analytical models. The complex formation between an isolated weak polyelectrolyte and an oppositely charged nanoparticle is investigated using Monte Carlo simulations with screened Coulomb potentials in the grand canonical ensemble. Polyelectrolyte/nanoparticle complex structure and extended tail formation. The roles of the nanoparticle surface charge density, solution pH and ionic concentration Ci are systematically investigated. The phase diagrams of complex conformations are also presented. It is shown that the polyelectrolyte conformation at the surface of the nanoparticle is controlled by the attractive interactions with the nanoparticle but also by the repulsive interaction between the monomers. To bridge the gap with experiments titration curves are calculated. We clearly demonstrate that an oppositely charged nanoparticle can significantly modify the acid-base properties of a weak polyelectrolyte.

Settling Velocities of Fractal Aggregates The structure and hydrodynamic properties of fractal aggregates cover a wide range of phenomena in colloid and polymer science, biophysics and material engineering. Fractal dimensions of aggregates influence their hydrodynamic radii and hence translational friction coefficients which control transport properties such as sedimentation rate and diffusion coefficients. Sedimentation rate of aggregated material in aquatic systems are amongst the most important processes, not only for the rational design or the effective operation of water treatment but also for the prediction of suspended matter diffusion, sediment fluxes and particle residence times in aquatic systems. The hydrodynamic properties of fractal aggregates are in most cases very complex and ill understood. Numerical techniques based on computer simulations are hence expected to play an important role for solving such a problem. Lattice Boltzmann systems and their extensions are very efficient modeling techniques, leading to fast and simple computer simulations. They contrast with the traditional scientific computing approach which mostly consists of solving partial differential equations. In this project we employ this novel, well-established, computational scheme for simulating solid-fluid suspension interaction to obtain quantitative informations on the hydrodynamics properties of 3D fractal aggregates and finally calculate settling velocities. Results should be directly applicable to understand the transport, circulation and fate of colloids and associated trace metals in aquatic systems. Fluid flow through a 2d CCA Aggregate