Stabilization of polymer colloid dispersions with pH-sensitive poly-acrylic acid brushes

Polyelectrolyte brushes are widely used for surface modification of nano-and colloidal particles because of their ability to dramatically change their conformation, hydrophobicity, polarity, charge, etc., as a response to smooth variations in environmental conditions. In this work, we have studied experimentally the stability behavior of polymer colloids with grafted poly-acrylic acid (PAA) surface brushes. We have measured the Fuchs stability ratio (W) as a function of electrolyte concentrations at different pH. It is observed that at pH  8), since most of the carboxylic groups are ionized, the colloidal stability is much higher than that at pH ~ 5. However, the W values are basically the same with 1% and 2% PAA, implying that the contribution of the ionized AA in the two cases is practically the same. This experimental evidence indicates that only the ionized AA groups in the outer region of long brushes contribute to colloidal stability, thus supporting the hypothesis of local electroneutrality in the inner region of long brushes (LEA

Tracking of fluorescently labeled polymer particles reveals surface effects during shear-controlled aggregation

Surface chemistry is believed to be the key parameter affecting the aggregation and breakage of colloidal suspensions when subjected to shear. To date, only a few works dealt with the understanding of the role of the physical and chemical properties of the particles’ surface upon aggregation under shear. Previous studies suggested that surface modifications strongly affect polymer particles’ adhesion, but it was very challenging to demonstrate this effect and monitor these alterations upon prolonged exposure to shear forces. More importantly, the mechanisms leading to these changes remain elusive. In this work, shear-induced aggregation experiments of polymer colloidal particles have been devised with the specific objective of highlighting material transfer and clarifying the role of the softness of the particle’s surface. To achieve this goal, polymer particles with a core–shell structure comprising fluorescent groups have been prepared so that the surface’s softness could be tuned by the addition of monomer acting as a plasticizer and the percentage of fluorescent particles could be recorded over time via confocal microscopy to detect eventual material transfer among different particles. For the first time, material exchange occurring on the soft surface of core–shell polymer microparticles upon aggregation under shear was observed and proved. More aptly, starting from a 50% labeled/nonlabeled mixture, an increase in the percentage of particles showing a fluorescent signature was recorded over time, reaching a fraction of 70% after 5 h

A first update on mapping the human genetic architecture of COVID-19

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Kinetic Analysis of the Catalytic Reduction of 4‑Nitrophenol by Metallic Nanoparticles

We present a study on the catalytic reduction of 4-nitrophenol (Nip) to 4-aminophenol (Amp) by sodium borohydride (BH₄^–) in the presence of metal nanoparticles in aqueous solution. This reaction which proceeds via the intermediate 4-hydroxylaminophenol has been used abundantly as a model reaction to check the catalytic activity of metallic nanoparticles. Here we present a full kinetic scheme that includes the intermediate 4-hydroxylaminophenol. All steps of the reaction are assumed to proceed solely on the surface of metal nanoparticles (Langmuir–Hinshelwood model). The discussion of the resulting kinetic equations shows that there is a stationary state in which the concentration of the intermediate 4-hydroxylaminophenol stays approximately constant. The resulting kinetic expression had been used previously to evaluate the kinetic constants for this reaction. In this stationary state there are isosbestic points in the UV/vis-spectra which are in full agreement with most published data. We compare the full kinetic equations to experimental data given by the temporal decay of the concentration of Nip. Good agreement is found underlining the general validity of the scheme. The kinetic constants derived from this analysis demonstrate that the second step, namely the reduction of the 4-hydroxylaminophenol is the rate-determining step