Conformation and Reactivity
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.
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.
and complex formation between polyelectrolytes and nanoparticles
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.
complex structure and extended tail formation.
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.
Velocities of Fractal
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
flow through a 2d CCA Aggregate
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.
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
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:
rationalization, characterization, and design of new flocculants
to be used in waste water treatment processes
behavior of polyelectrolytes in solutions and the
polyelectrolyte-nanoparticle supramolecular complex formation
to investigate nanostructured objects, possible complexation
of biopolymer with nanoparticles
association of colloids (inorganic particles, biopolymers,
fulvics structures...) via aggregation and flocculation
sedimentation rate of fractal aggregates to better understand
the circulation of the colloidal material in water columns
modelling of interaction forces between colloidal matter
by improving solid/liquid interface descriptions
processes in solution and at interfaces
concentrated polymer/particle solutions for the investigation
of porous systems
characterization and reactivity
all cases a major goal is to relate molecular and supramolecular
processes to the macroscopic ones.
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).
and Associated Members of the Research Group
Céline (Dr - ICS)
Fabrice (PhD Student)
Daniel (PhD Student)
Marianne (PhD Student)
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:email@example.com
processes for the rational design of polymeric flocculants.
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.
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
Modeling of interactions between natural colloids.
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.
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.