Impact of electrostatics on the adsorption of microgels at the interface of Pickering emulsions.

The importance of electrostatics on microgel adsorption at a liquid interface is studied, as well as its consequence on emulsion stabilization. In this work, poly(N-isopropylacrylamide) (pNIPAM) microgels bearing different numbers of charges and various distribution profiles are studied, both in solution and at the oil-water interface of emulsion drops. Charged microgels are compared to neutral ones, and electrostatic interactions are screened by adding salt to the aqueous solution. In solution, electrostatics has a significant impact on microgel swelling, as induced by the osmotic pressure exerted by mobile counterions in the gel network. At the interface of drops, microgels pack in a hexagonal array, whose lattice parameter is independent of the number of charges and range of electrostatic interactions. Microgel morphology and packing are ruled only by the adsorption of the pNIPAM chain at the interface. Conversely, decreasing the charge density of microgels by the protonation of the carboxylic groups leads to unstable emulsions, possibly as a result of the impact of hydrogen bonding on microgel deformability.

Impact of pNIPAM microgel size on its ability to stabilize Pickering emulsions.

We study the influence of the particle size on the ability of poly(N-isoprolylacrylamide) microgels to stabilize direct oil-in-water Pickering emulsions. The microgel size is varied from 250 to 760 nm, the cross-linking density being kept constant. The emulsion properties strongly depend on the stabilizer size: increasing the particle size induces an evolution from dispersed drops and fluid emulsions toward strongly adhesive drops and flocculated emulsions. In order to get insight into this dependency, we study how particles adsorb at the interface and we determine the extent of their deformation. We propose a correlation between microgel ability to deform and emulsion macroscopic behavior. Indeed, as the microgels size increases, their internal structure becomes more heterogeneous and so does the polymeric interfacial layer they form. The loss of a uniform dense layer favors bridging between neighboring drops, leading to flocculated and therefore less handleable emulsions.

Pickering emulsions stabilized by soft microgels: influence of the emulsification process on particle interfacial organization and emulsion properties.

This work reports a new evidence of the versatility of soft responsive microgels as stabilizers for Pickering emulsions. The organization of microgels at the oil-water interface is a function of the preparation pathway. The present results show that emulsification energy can be used as a trigger to modify microgel deformation at the oil-water interface and their packing density: high shear rates bring strong flattening of the microgels, whereas low shear rates lead to dense monolayers, where the microgels are laterally compressed. As a consequence, the resulting emulsions have opposite behavior in terms of flocculation, which arises from bridging between neighboring drops and is strongly dependent on their surface coverage. This strategy can be applied to any microgel which can sufficiently adsorb at low shear rates, i.e. small microgels or lightly cross-linked ones. The control of the organization of microgels at the interface does not only modify emulsion end-use properties but also constitutes a new tool for the development of Janus-type microgels, obtained by chemical modification of the adsorbed microgels.

Origin and control of adhesion between emulsion drops stabilized by thermally sensitive soft colloidal particles.

We used soft microgels made of poly(N-isopropylacrylamide) (pNIPAM) of variable cross-linking degrees and the same colloidal size to stabilize oil-in-water Pickering emulsions. The extent of droplet flocculation increased and the resistance of the emulsions to mechanical stresses decreased as the cross-linking density was augmented. Large flat films were separating the droplets, and we could measure the adhesion angle at the junction with the free interfaces through several microscopy methods. The size of the flat films and the values of the angles were reflecting strong adhesive interactions between the interfaces as a result of microgel bridging. In parallel, cryo-SEM imaging of the thin films allowed a precise determination of their structure. The evolution of the adhesion angle and of the film structure as a function of microgels cross-linking density provided interesting insights into the impact of particle softness on film adhesiveness and emulsion stability. We exploited our main findings to propose a novel route for controlling the emulsions end-use properties (flocculation and stability). Owing to particle softness and thermal sensitivity, the interfacial coverage was a path function (it depended on the sample "history"). As a consequence, by adapting the emulsification conditions, the interfacial monolayer could be trapped in a very dense and rigid configuration, providing improved resistance to bridging flocculation and to flow-induced coalescence.

Water-in-oil emulsions stabilized by water-dispersible poly(N-isopropylacrylamide) microgels: understanding anti-Finkle behavior.

Emulsions were prepared using poly(N-isopropylacrylamide) microgels as thermoresponsive stabilizers. The latter are well-known for their sensitivity to temperature: they are swollen by water below the so-called volume phase transition temperature (VPTT = 33 °C) and shrink when heated above it. Most of the studies reported in the literature reveal that the corresponding emulsions are of the oil-in-water type (O/W) and undergo fast destabilization upon warming above the VPTT. In the present study, whereas O/W emulsions were obtained with a wide panel of oils of variable polarity and were all thermoresponsive, water-in-oil (W/O) emulsions were found only in the presence of fatty alcohols and did not exhibit any thermal sensitivity. To understand the peculiar behavior of emulsions based on fatty alcohols, we investigated the organization of microgels at the oil-water interface and we studied the interactions of pNIPAM microgels with octanol. By combining several microscopy methods and by exploiting the limited coalescence process, we provided evidence that W/O emulsions are stabilized by multilayers of nondeformed microgels located inside the aqueous drops. Such behavior is in contradiction with the empirical Finkle rule stating that the continuous phase of the preferred emulsion is the one in which the stabilizer is preferentially dispersed. The study of microgels in nonemulsified binary water/octanol systems revealed that octanol diffused through the aqueous phase and was incorporated in the microgels. Thus, W/O emulsions were stabilized by microgels whose properties were substantially different from the native ones. In particular, after octanol uptake, they were no longer thermoresponsive, which explained the loss of responsiveness of the corresponding W/O emulsions. Finally, we showed that the incorporation of octanol modified the interfacial properties of the microgels: the higher the octanol uptake before emulsification, the lower the amount of particles in direct contact with the interface. The multilayer arrangement was thus necessary to ensure efficient stabilization against coalescence, as it increased interface cohesiveness. We discussed the origin of this counterexample of the Finkle's rule.

In vitro effects of nanoparticles on renal cells.

BACKGROUND: The ability of nanoparticles to cross the lung-blood barrier suggests that they may translocate to blood and to targets distant from their portal of entry. Nevertheless, nanotoxicity in organs has received little attention. The purpose of this study was to evaluate nanotoxicity in renal cells using in vitro models. Various carbon black (CB) (FW2-13 nm, Printex60-21 nm and LB101-95 nm) and titanium dioxide (TiO2-15 and TiO2-50 nm) nanoparticles were characterized on size by electron microscopy. We evaluated theirs effects on glomerular mesangial (IP15) and epithelial proximal tubular (LLC-PK1) renal cells, using light microscopy, WST-1 assay, immunofluorescence labeling and DCFH-DA for reactive oxygen species (ROS) assay. RESULTS: Nanoparticles induced a variety of cell responses. On both IP15 and LLC-PK1 cells, the smallest FW2 NP was found to be the most cytotoxic with classic dose-behavior. For the other NPs tested, different cytotoxic profiles were found, with LLC-PK1 cells being more sensitive than IP15 cells. Exposure to FW2 NPs, evidenced in our experiments as the most cytotoxic particle type, significantly enhanced production of ROS in both IP15 and LLC-PK1 cells. Immunofluorescence microscopy using latex beads indicated that depending on their size, the cells internalized particles, which accumulated in the cell cytoplasm. Additionally using transmission electronic microscope micrographs show nanoparticles inside the cells and trapped in vesicles. CONCLUSION: The present data constitute the first step towards determining in vitro dose effect of manufactured CB and TiO2 NPs in renal cells. Cytotoxicological assays using epithelial tubular and glomerular mesangial cell lines rapidly provide information and demonstrated that NP materials exhibit varying degrees of cytotoxicity. It seems clear that in vitro cellular systems will need to be further developed, standardized and validated (relative to in vivo effects) in order to provide useful screening data about the relative toxicity of nanoparticles.

Physicochemical characteristics and bronchial epithelial cell cytotoxicity of Folpan 80 WG(R) and Myco 500(R), two commercial forms of folpet.

BACKGROUND: Pesticides, in particular folpet, have been found in rural and urban air in France in the past few years. Folpet is a contact fungicide and has been widely used for the past 50 years in vineyards in France. Slightly water-soluble and mostly present as particles in the environment, it has been measured at average concentration of 40.1 mug/m3 during its spraying, 0.16-1.2 mug/m3 in rural air and around 0.01 mug/m3 in urban air, potentially exposing both the workers and the general population. However, no study on its penetration by inhalation and on its respiratory toxicity has been published. The objective of this study was to determine the physicochemical characteristics of folpet particles (morphology, granulometry, stability) in its commercial forms under their typical application conditions. Moreover, the cytotoxic effect of these particles and the generation of reactive oxygen species were assessed in vitro on respiratory cells. RESULTS: Granulometry of two commercial forms of folpet (Folpan 80WG(R) and Myco 500(R)) under their typical application conditions showed that the majority of the particles (>75%) had a size under 5 mum, and therefore could be inhaled by humans. These particles were relatively stable over time: more than 75% of folpet remained in the particle suspension after 30 days under the typical application conditions. The inhibitory concentration (IC50) on human bronchial epithelial cells (16HBE14o-) was found to be between 2.89 and 5.11 mug/cm2 for folpet commercial products after 24 h of exposure. Folpet degradation products and vehicles of Folpan 80 WG(R) did not show any cytotoxicity at tested concentrations. At non-cytotoxic and subtoxic concentrations, Folpan 80 WG(R) was found to increase DCFH-DA fluorescence. CONCLUSION: These results show that the particles of commercial forms of folpet are relatively stable over time. Particles could be easily inhaled by humans, could reach the conducting airways and are cytotoxic to respiratory cells in vitro. Folpet particles may mediate its toxicity directly or indirectly through ROS-mediated alterations. These data constitute the first step towards the risk assessment of folpet particles by inhalation for human health. This work confirms the need for further studies on the effect of environmental pesticides on the respiratory system.