Fenton reaction mechanism generating no OH radicals in Nafion membrane decomposition

Mechanism of Fenton reaction, which is a most widely-used degradation test for organic materials using hydrogen peroxide (H$$_2$$ 2 O$$_2$$ 2 ) and iron (Fe) cations, is revealed for the decomposition of hydrated Nafion membrane. This reaction mechanism has been assumed to generate OH radicals. For a doubly-hydrated Nafion membrane model, Fenton reaction with divalent and monovalent Fe (Fe$$^{2+}$$ 2 + and Fe$$^+$$ + ) cation hydration complexes is explored for experimentally-supported hydration numbers using long-range correction for density functional theory. As a result, it is found that H$$_2$$ 2 O$$_2$$ 2 coordinating to the Fe$$^{2+}$$ 2 + hydration complexes first approaches Nafion side chains in high humidity, then leads to the C–S bond dissociation of the side chain to produce carbonic acid group and sulfonic acid ion. On the other hand, once electron transfer proceeds between iron ions, the O–O bond of the coordinating H$$_2$$ 2 O$$_2$$ 2 is extended, then the C–S bond is dissociated to produce trihydroxymethyl group and sulfur trioxide, which are rapidly transformed to carboxyl group and sulfonic acid ion in aquo. This mechanism is confirmed by the vibrational spectrum analysis of the decomposed product. Collective Nafion decomposition mechanisms also suggest that the decomposition reaction uses the recycle of generated Fe cation hydration complexes under acidic condition near membrane surface.

Effect of fluid saturation on the shear modulus of artificial clay-rich tight sandstones

SUMMARY For many geophysical problems it is important to understand the influence of clay on the elastic behaviour of the rocks. However, this is difficult to measure because the complex petrophysical characteristics of tight sandstones make it challenging to control the clay parameters in natural samples. In this study, we synthesized nine tight sandstones with different clay types and content. Ultrasonic measurements and theoretical simulations were then used to analyse the influence of clay on the elastic modulus of brine saturated samples. We found that the shear modulus of smectite-rich samples decreased drastically after saturation, while the decrease observed in kaolinite-rich samples was relatively low. We propose that the reduction in surface energy caused by surface-particle clay hydration is a common mechanism that leads to shear softening in both kaolinite-rich and smectite-rich samples. However, the contact deformation caused by cation hydration of smectite is the primary mechanism leading to greater shear softening of tight sandstones containing smectite. Although the differential Kuster–Toksöz model is based on idealized pore shapes, a dual-porosity scheme can be used to explain and simulate the shear softening of artificial clay-rich tight sandstones.

Chemistry of cation hydration and conduction in a skeletal muscle ryanodine receptor

Ryanodine receptors (RyRs) are Ca2+-regulated Ca2+ channels of 2.2-megadalton in muscles and neurons for calcium signaling. How Ca2+ regulates ion conduction in the RyR channels remains elusive. We determined a 2.6-Å cryo-EM structure of rabbit skeletal muscle RyR1, and used multiscale dynamics simulations to elucidate cation interactions with RyR1. We investigated 21 potential cation-binding sites that may together rationalize biphasic Ca2+ response of RyR1. The selectivity filter captures a cation hydration complex by hydrogen-bonding with both the inner and outer hydration shells of water molecules. Molecular dynamics simulations suggest that adjacent Ca2+ ions moving in concert along ion-permeation pathway are separated by at least two cation-binding sites. Our analysis reveals that RyR1 has been evolved to favor its interactions with two hydration shells of cations.