Constante d'acidité

Résumé

vignette|350px|L'acide acétique, un acide faible, donne un proton (ion hydrogène, ici surcoloré en vert) à l'eau dans une réaction d'équilibre donnant l'ion acétate et l'ion hydronium (code couleur : rouge=oxygène, noir=carbone, blanc=hydrogène). En chimie, une constante d'acidité ou constante de dissociation acide, Ka, est une mesure quantitative de la force d'un acide en solution. C'est la constante d'équilibre de la réaction de dissociation d'une espèce acide dans le cadre des réactions acido-basiques. Plus cette constante Ka est élevée, plus la dissociation des molécules en solution est grande, et donc plus fort est l'acide. L'équilibre chimique de la dissociation acide peut être écrit de façon symbolique \rm AH \rightleftharpoons A^- + H^+ où AH est un acide générique se dissociant en A−, appelée sa base conjuguée, et en ion hydrogène H+, communément appelé « proton », qui, si la réaction s'effectue en solution aqueuse n'existe que sous la forme d'ion hydronium, H3O

Source officielle

https://fr.wikipedia.org/wiki/Constante_d'acidit%C3%A9

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In this work, alkaline hydrogen evolution reaction (HER) processes of three typical nickel-based electrocatalysts [i.e., Ni, alpha-Ni(OH)(2), and beta-Ni(OH)(2)] were investigated to probe critical factors that determine the activity and durability. The HER activity trend was observed as Ni >>alpha-Ni(OH)(2) > beta-Ni(OH)(2), likely attributed to a synergy between metallic Ni and Ni(OH)(2) components on the Ni surface and fast water dissociation kinetics on the alpha-Ni(OH)(2) surface. With the HER proceeding, the metallic Ni surface, however, gradually became alpha-Ni(OH)(2), and alpha-Ni(OH)(2) surface ultimately transformed into beta-phase, leading to a dramatic activity decrease of Ni electrodes. Therefore, Ni electrodes were coated with alpha-Ni(OH)(2) nanosheets to slow down the nickel hydroxylation and optimize the surface ratio of Ni(OH)(2) to metallic Ni. This simple coating procedure enhanced both activity and durability of Ni electrocatalysts.

WILEY-V C H VERLAG GMBH2022

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Water Orientation at the Anatase TiO2 Nanoparticle Interface: A Probe of Surface pK(a )Values

Marie Bischoff, Arianna Marchioro

The acid-base properties of surfaces significantly influence catalytic and (photo)electrochemical processes. Estimation of acid dissociation constants (pK(a )values) for colloids is commonly performed through electroanalytical techniques or spectroscopic methods employing label molecules. Here, we show that polarimetric angle-resolved second harmonic scattering (AR-SHS) can be used as an all-optical, label-free probe of colloid surface pK(a) values. We apply AR-SHS to dispersions of 100 nm anatase TiO(2 )particles to extract surface potential and surface susceptibility, a measure of interfacial water orientation, as a function of pH. The surface potential follows changes in surface charge density, while the interfacial water orientation inverts at pH similar to 4.8, similar to 6, and similar to 7.6. As the variation in bulk pH modifies the populations of Ti-OH2+, Ti-OH, and Ti-O- interfacial groups, a change in water orientation reports on the ratio of protonated/deprotonated species. Such observation allows for pK(a) evaluation from plots of surface susceptibility versus pH. A Nerstian trend in the surface potential is additionally demonstrated.

AMER CHEMICAL SOC2022

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Systematic model reduction captures the dynamics of extrinsic noise in biochemical subnetworks

Barbara Bravi

We consider the general problem of describing the dynamics of subnetworks of larger biochemical reaction networks, e.g., protein interaction networks involving complex formation and dissociation reactions. We propose the use of model reduction strategies to understand the "extrinsic" sources of stochasticity arising from the rest of the network. Our approaches are based on subnetwork dynamical equations derived by projection methods and path integrals. The results provide a principled derivation of different components of the extrinsic noise that is observed experimentally in cellular biochemical reactions, over and above the intrinsic noise from the stochasticity of biochemical events in the subnetwork. We explore several intermediate approximations to assess systematically the relative importance of different extrinsic noise components, including initial transients, long-time plateaus, temporal correlations, multiplicative noise terms, and nonlinear noise propagation. The best approximations achieve excellent accuracy in quantitative tests on a simple protein network and on the epidermal growth factor receptor signaling network. Published under license by AIP Publishing.

AMER INST PHYSICS2020

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