High pressure-temperature proton migration in P-3 brucite [Mg(OH)2]: Implication for electrical conductivity in deep mantle

Hydrous minerals contribute largely to the transport and distribution of water into the mantle of earth to regulate the process of deep-water cycle. Brucite is one of the simplest layered dense hydrous mineral belonging to MgO-SiO2-H2O ternary system, which contains significant amount of water in the form of OH- groups, spanning a wide range of pressure stability. Simultaneously, the pressure (p) and temperature (T) induced mobility of protons within the layered structure of brucite is crucial for consequences on electrical conductivity of the mantle. Using ab initio molecular dynamics (AIMD) simulations, we investigate the diffusion of H in high-pressure trigonal P-3 polymorph of brucite in a combined p-T range of 10-85 GPa and 1250-2000K, relevant to the mantle of earth. The AIMD simulations reveal an unusual pressure-dependence of the proton migration in brucite characterized by maximum H-diffusion in the pressure range of 72-76 GPa along different isotherms. We predict that in the P-3 brucite the H mobility is onset only when a critical hydrostatic pressure is attained. The onset pressure is observed to drop with increasing temperature. The H-diffusion in brucite phase at elevated p-T takes place in such a manner that the process results in the amorphization of the H-sublattice, without disturbing the Mg- and O-sublattices. This selective amorphization yields a pool of highly mobile protons causing a subsequent increment in the electrical conductivity in P-3 brucite. Our calculated values of conductivity are compared with ex-situ geophysical magnetic satellite data indicating that brucite can be present in larger quantities in the lower mantle than previously observed. This hydroxide phase can occur as segregated patches between the dominant constituents e.g., silicates and oxides of the lower mantle and thus can explain the origin of high electrical conductivity therein.Comment: Preliminary draft, 6 figures, presented in Goldschimdt 2023 Conference (Lyon, France), comments are welcom

Recent advances in pesticide formulations for eco-friendly and sustainable vegetable pest management: A review

In order to reduce the loss and maintain the quality of vegetables harvest, pesticides are used together with other pest management techniques during cropping to destroy pests and prevent diseases. However, the use of pesticides during production often leads to the presence of pesticide residues in vegetables after harvest. Higher doses and repeated applications of conventional formulations lead to accumulate pesticide residues in vegetable commodities along with environmental pollution. With the increasing awareness of toxic effects of conventional formulations, there is a significant trend towards switching over from such pesticide formulations using petroleum and organic solvent based constituents to user and environment friendly water based pesticide formulations. The developed world has progressed substantially in this regard to develop eco-friendly formulations which are safer to vegetable and the environment. These formulations would not only replace toxic, non-degradable ingredients/adjuvants of the conventional formulations but also increase the bio-efficacy of the products through incorporating latest technologies including size reduction (Wettable Powder to Suspension Concentrate, Soluble Concentrate to Microemulsion), increased coverage of applied surface area (EC to ME/Nano-formulations), reduced wastage (Dust/WP to Controlled Release Formulations) and dose rates of applied same pesticides to improve food quality with minimum pesticide residues

Excited-state proton transfer from pyranine to acetate in methanol

Excited-state proton transfer (ESPT) of pyranine (8-hydroxypyrene-1,3,6-trisulphonate, HPTS) to acetate in methanol has been studied by steady-state and time-resolved fluorescence spectroscopy. The rate constant of direct proton transfer from pyranine to acetate (k1) is calculated to be ~1 × 109 M-1 s-1. This is slower by about two orders of magnitude than that in bulk water (8 × 1010 M-1 s-1) at 4 M acetate

Ultrafast dynamics in biological systems and in nano-confined environments

Ultrafast chemical dynamics in a nano-confined system is very different from that in a bulk liquid. In this account, we give an overview on recent femtosecond study on dynamics of ultrafast chemical processes in the nanocavity of a biological system. Dynamics in a biological system crucially depends on the location of the fluorescent probe. We show that one can study solvation dynamics in different regions (i.e. spatially resolve) by variation of the excitation wavelength. We discuss two interesting cases of how structure affects dynamics. First, solvation dynamics of two protein folding intermediates of cytochrome c is found to be differ significantly in the ultrafast initial part (< 20 ps). Second, methyl substitution of the OH group in a cyclodextrin is shown to slow down the initial part of solvation dynamics quite dramatically. The most interesting observation is the discovery of the ultraslow component of solvation dynamics which is 100-1000 times slower compared to bulk water. The electron- and proton-transfer processes in a nano-confined system are found to be markedly retarded because of slow solvation and structural constraints. Close proximity of the reactants in a confined system is expected to accelerate dynamics of bi-molecular processes. This is illustrated by ultrafast fluorescence resonance energy transfer (FRET) in ≈ 1 ps time scale between a donor and an acceptor in a micelle. Finally, it is demonstrated that the decay of fluorescence anisotropy provides structural information (e.g. size of a cyclodextrin inclusion complex) and may be used to detect formation of a nano-aggregate. Dynamics of ultrafast chemical processes in a nanocavity is studied using femtosecond emission spectroscopy. Dynamics in a nano-confined system differs markedly from that in a bulk liquid. This has implication in many biological processes

Ultrafast photoinduced electron transfer from dimethylaniline to coumarin dyes in sodium dodecyl sulfate and triton X-100 micelles

The primary steps of photoinduced electron transfer (PET) from N,N-dimethylaniline (DMA) to five coumarin dyes are studied in an anionic micelle [sodium dodecyl sulfate (SDS)] and a neutral micelle [triton X-100 (TX-100)] using femtosecond upconversion. The rate of PET in micelle is found to be highly nonexponential. In both the micelles, PET displays components much faster (~ 10 ps) than the slow components (180-2900 ps) of solvation dynamics. The ultrafast components of electron transfer exhibit a bell-shaped dependence on the free energy change. This is similar to Marcus inversion. The rates of PET in TX-100 and SDS micelle are, in general, faster than those in cetyltrimethylammonium bromide (CTAB) micelle. In the SDS and TX-100 micelle, the Marcus inversion occurs at -ΔG0 ~ 0.7 eV which is lower than that (~ 1.2 eV) in CTAB micelle. Possible causes of variation of PET in different micelles are discussed

Ultrafast electron transfer in a nanocavity. Dimethylaniline to coumarin dyes in hydroxypropyl γ-cyclodextrin

Photoinduced electron transfer (PET) from N,N-dimethylaniline (DMA) to four coumarin dyes (C151, C481, C153, and C480) inside the cavity of hydroxypropyl γ-cyclodextrin (hpCD) is studied using femtosecond upconversion. The rate of PET is found to be nonexponential and to vary significantly with the coumarin dyes. The rate for C481 is 100 times faster than that for C480. The PET rate displays a bell-shaped dependence on the free energy change and thus reveals a Marcus-type inverted region. The anisotropy decay of the four dyes in hpCD are found to be very similar, and hence the observed variation in the rate of PET is not due to variation in rotational diffusion of the acceptors (coumarin dyes)

Slow solvation dynamics of 4-AP and DCM in binary mixtures

In a binary mixture of benzene and dimethylformamide (DMF), solvation dynamics of 4-aminophthalimide (4-AP) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) displays a slow component. As mole fraction of DMF increases from 0.028 to 0.28 the average solvation time (<τs>) for 4-AP decreases from 830 to 450 ps while for DCM it decreases from 450 to 100 ps. In dioxane-water mixtures <τs> for DCM is 250 ps which remains unaffected as mole fraction of water increases from 0.22 to 0.50