Département de chimie organique

 

Group of Environmental Physical Chemistry, Institute F.-A. Forel, Earth and Environmental Sciences Section

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Teaching

Research Activities

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

 

 

 

 

 

 

Philosophy

The fate and transport of contaminants in ecosystems, such as trace metal elements and pesticides, introduced by rivers or by atmospheric inputs in waters by human activities, needs to be understood to evaluate their long-term impacts as well as their influences and effects on biota. With industrialization, their fluxes in aquatic systems and accumulation in soils and sediments have continuously increased in the past decades.

Numerical modelling is performed, in conjunction with experimental studies in order to understand the factors controlling the behaviour at a microscopic, mesoscopic and macroscopic length scale of colloids, inorganic particles, biopolymers, polyelectrolytes, and concentred mixtures containing polymers and nanoparticles. Most of these complex systems have important and direct applications in environmental chemistry, analytical chemistry, waste water treatment, adsorption processes, and nanoparticle toxicity.

Research Interests

Monte Carlo, statistical generation methods, Brownian dynamics, Lattice Boltzmann modeling as well as experimental techniques such as light scattering, particle counting, fluorescence spectroscopy are used to get an insight into:

  • the rationalization, characterization, and design of new flocculants to be used in waste water treatment processes
  • the behavior of polyelectrolytes in solutions and the polyelectrolyte-nanoparticle supramolecular complex formation to investigate nanostructured objects, possible complexation of biopolymer with nanoparticles
  • the association of colloids (inorganic particles, biopolymers, fulvics structures...) via aggregation and flocculation processes
  • the sedimentation rate of fractal aggregates to better understand the circulation of the colloidal material in water columns
  • mathematical modelling of interaction forces between colloidal matter by improving solid/liquid interface descriptions
  • micellization processes in solution and at interfaces
  • concentrated polymer/particle solutions for the investigation of porous systems
  • nanoparticle characterization and reactivity

In all cases a major goal is to relate molecular and supramolecular processes to the macroscopic ones.

Collaborations: Centre Universitaire Informatique (CUI), AQUA+TECH specialties SA, UniFR, EPFL, the Swiss Federal Laboratories for Materials Testing and Research, Centre Européen des Géosciences de l'Environnement (CEREGE), Institut Charles Sadron (ICS) and LAGEP (University of Lyon).

Members and Associated Members of the Research Group
  • STOLL Serge (MER)
  • ARNOLD Céline (Dr - ICS)
  • CARNAL Fabrice (PhD Student)
  • PALOMINO Daniel (PhD Student)
  • ULRICH Serge (Dr)
  • SEIJO Marianne (PhD Student)

Address. Institut Forel, Section des Sciences de la Terre et de l'Environnement, Faculté des Sciences, Université de Genève, 13 rue des Maraichers, CH-1205 Geneva / Switzerland
Tel. ++ 41 22 379 64 27 Email:serge.stoll@unige.ch

Flocculation processes for the rational design of polymeric flocculants.

From an environmental point of view in natural aquatic systems, aggregation processes involving colloids such as biopolymers, humic and fulvic acids and inorganic particles play an essential role in removing pollutants from the water column. Unfortunately these processes are still poorly understood in comparison of their importance for the environment. Actually synthetic organic polymers are used in waste water treatment as polymeric flocculants in order to retrieve the maximum quantities of particulate matter of the incoming water before releasing it into natural aquatic systems. Their optimal dosage is difficult to determine due to the lack in the understanding of the mechanisms involved during the flocculation processes. To get an insight into these complex processes, we are using well characterized polymeric flocculants and colloidal suspension mixtures for the determination of the optimal conditions and rational use of polymeric flocculants. Kinetic aggregation rates constants, windows of effective use and aggregates fractal dimension Df are investigated by comparing different architectures of cationic synthetic polymers. The ionic strength is today under consideration to determine the effect of the presence of salt, which is an important parameter in natural waters, on the polymer efficiency.

Right: Stable natural colloidal suspension. Left; colloidal suspension after addition of a polymeric floculant (two minutes). The colloidal fraction has rapidly coagulated and been removed by sedimentation from water.

Mathematical Modeling of interactions between natural colloids.

Aggregate formation and sedimentation are amongst the most important processes in aquatic systems, not only for the effective operation of water treatment, but also for the prediction of sediment fluxes and trace compound residence times in natural waters. However, current mathematical models used to simulate trace compound circulation usually do not take into account coagulation-sedimentation processes and, when coagulation-sedimentation is considered, the coagulating material is described in a simplistic way, most often as impermeable spheres by using relationships directly derived from Smoluchowski theory and Stokes' law.
While factors such as particle size and concentration have received some attention, little importance has been given so far to (i) the heterogeneity of the colloidal material, i.e mainly inorganic particles, biopolymers and fulvics, present at variable relative concentrations depending on the system considered, (ii) the use of a unique value for the collision efficiency of the system irrespective of the individual sticking probabilities between the different entities present, and (iii) the fractal character of the aggregates formed.
The nature of this project is fundamental with the aim of improving existing coagulation-sedimentation models for surface waters by focusing on the adequate parametrisation of collision efficiencies and fractal dimensions. In collaboration with Prof. J. Buffle and Dr. M. Filella we plan to employ well-established, analytical and computational schemes for (i) calculating the forces acting between the colloids at the miscroscopic level, and (ii) simulating aggregate and floc formation at the mesoscopic scale, in order to (iii) extract the best parameters to be used in macroscopic models dealing with colloid aggregate formation and circulation.

Fulvic Acid- hematite structures obtained at different ionic strengths: a) 5x10-4 mol.L-1, b) 1x10-3 mol.L-1, c) 1.10-2 mol.L-1, d) 5x10-2 mol.L-1, e) 1x10-1 mol.L-1. f =0.002, pH=8, T=25°C. The hematite particle is the central particle and fulvic acids are the small ones. At low ionic strength (5x10-4 mol.L-1), only a monolayer of FA is observed at the hematite surface. By increasing the ionic strength, not only the number of adsorbed Fulvic Acid, but also the thickness of adsorbed FA increases.

webmaster: S. Stoll

 

 

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