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Nature of flocculation and tactoid formation in montmorillonite: the role of pH
M. Segad, T. Åkesson, B. Cabanec and Bo Jönssonb
Phys. Chem. - 17 29608-29615 - DOI: 10.1039/C5CP04007H -
The dissolution and swelling properties of montmorillonite at different pH have been studied, using small angle X-ray scattering (SAXS), imaging and osmotic stress methods combined with Monte Carlo simulations. The acidity of montmorillonite dispersions has been varied as well as the counterions to the net negatively charged platelets. At low pH, Na montmorillonite dissolves and among other species Al3+ is released, hydrated, polymerized and then it replaces the counterions in the clay. This dramatically changes the microstructure of Na montmorillonite, which instead of having fully exfoliated platelets, turns into a structure of aggregated platelets, so-called tactoids. Montmorillonite dispersion still has a significant extra-lamellar swelling among the tactoids due to the presence of very small nanoplatelets.
Filtration of precipitated silica aggregates: Length scales, percolation threshold and yielding behaviour
Deka Moussa Ragueh, Martine Meireles, Bernard Cabane, Jérémie Gummel
- vol . 156 (n° 1) 44137 - : http://dx.doi.org/10.1016/j.seppur.2015.10.005 -
Reinforcing precipitated silica systems have a complex hierarchical structure consisting of a branched network made of connected clusters composed of small silica beads welded together into larger dense aggregates. Here, we study the evolution of such structural features during a filtration process. The typical behaviour is that the cakes formed at constant pressure do not reorganize at local scale during a filtration experiment. Accordingly, the creep resistance of a precipitated silica network is high. Overall, there is a percolation threshold, which appears when the branches are pushed into each other. Once this percolation path is reached, the cake withstands compression over more than a decade of applied pressure. Beyond, it seemed useful to make predictions of the filtration properties knowing the typical length scales – small silica beads, dense aggregates, and consolidation behaviour of the cake. A simple approach introducing the concept of an effective medium approximation into Darcy’s law was tested. This approach treats the network as a pseudo-continuum of porous medium built at two main length scales: the size of dense aggregates and a length scale representing the typical distance between the aggregates. The quality of the fit of experimental filtration rates by this simple model indicates that a description based on a continuous network made of two material phases is accurate.
Fast Magnetic Field-Enhanced Linear Colloidal Agglutination Immunoassay
Aurélien Daynès, Nevzat Temurok, Jean-Philippe Gineys, Gilles Cauet, Philippe Nerin, Jean Baudry, and Jérôme Bibette
Anal. Chem. - 87 (15) 7583–7587 - DOI: 10.1021/acs.analchem.5b00279 -
We present the principle of a fast magnetic field enhanced colloidal agglutination assay, which is based on the acceleration of the recognition rate between ligands and receptors induced by magnetic forces.1 By applying a homogeneous magnetic field of 20 mT for only 7 s, we detect CRP (C-reactive protein) in human serum at a concentration as low as 1 pM for a total cycle time of about 1 min in a prototype analyzer. Such a short measurement time does not impair the performances of the assay when compared to longer experiments. The concentration range dynamic is shown to cover 3 orders of magnitude. An analytical model of agglutination is also successfully fitting our data obtained with a short magnetic pulse.
The mechanism of eccrine sweat pore plugging by aluminium salts using microfluidics combined with small angle X-ray scattering.
Bretagne A, Cotot F, Arnaud-Roux M, Sztucki M, Cabane B, Galey JB.
Soft Matter - 13(20) 3812-3821 - doi: 10.1039/c6sm02510b. -
Aluminium salts are widely used to control sweating for personal hygiene purposes. Their mechanism of action as antiperspirants was previously thought to be a superficial plugging of eccrine sweat pores by the aluminium hydroxide gel. Here we present a microfluidic T junction device that mimics sweat ducts, and is designed for the real time study of interactions between sweat and ACH (Aluminium Chloro Hydrate) under conditions that lead to plug formation. We used this device to image and measure the diffusion of aluminium polycationic species in sweat counter flow. We report the results of small angle X-ray scattering experiments performed to determine the structure and composition of the plug, using BSA (Bovine Serum Albumin) as a model of sweat proteins. Our results show that pore occlusion occurs as a result of the aggregation of sweat proteins by aluminium polycations. Mapping of the device shows that this aggregation is initiated in the T junction at the location where the flow of aluminium polycations joins the flow of BSA. The mechanism involves two stages: (1) a nucleation stage in which aggregates of protein and polycations bind to the wall of the sweat duct and form a tenuous membrane, which extends across the junction; (2) a growth stage in which this membrane collects proteins that are carried by hydrodynamic flow in the sweat channel and polycations that diffuse into this channel. These results could open up perspectives to find new antiperspirant agents with an improved efficacy.

584 publications.