Anion–Caffeine Interactions Studied by 13C and 1H NMR and ATR–FTIR Spectroscopy

Electrolyte selection toward efficient photoelectrochemical glycerol oxidation on BiVO4

Glycerol, a primary by-product of biodiesel production, can be oxidized into various value-added chemicals, significantly enhancing the techno-economic value of photoelectrochemical (PEC) cells. Several studies have explored various photoelectrode materials and co-catalysts, but the influence of electrolytes on PEC glycerol oxidation has remained relatively unexplored despite its significance. Here, we explore the impact of various acidic (pH = 2) electrolytes, namely NaNO3, NaClO4, Na2SO4, K2SO4, and KPi, on PEC glycerol oxidation using nanoporous thin film BiVO4 as a model photoanode. Our experimental findings reveal that the choice of electrolyte anion and cation significantly affects the PEC performance (i.e., photocurrent, onset potential, stability, and selectivity towards value-added products) of BiVO4 for glycerol oxidation. To explain this interesting phenomenon, we correlate the observed performance trend with the ion specificity in the Hofmeister series as well as the buffering capacity of the electrolytes. Notably, NaNO3 is identified as the optimal electrolyte for PEC glycerol oxidation with BiVO4 when considering various factors such as stability and production rates for glycerol oxidation reaction (GOR) products, surpassing the previously favored Na2SO4. Glycolaldehyde emerges as the most dominant product with ∼50% selectivity in NaNO3. The general applicability of our findings is confirmed by similar observation in electrochemical (EC) GOR with a polycrystalline platinum anode. Overall, these results emphasize the critical role of electrolyte selection in enhancing the efficiency of EC/PEC glycerol oxidation.

Salt Effects on Caffeine across Concentration Regimes

Salts affect the solvation thermodynamics of molecules of all sizes; the Hofmeister series is a prime example in which different ions lead to salting-in or salting-out of aqueous proteins. Early work of Tanford led to the discovery that the solvation of molecular surface motifs is proportional to the solvent accessible surface area (SASA), and later studies have shown that the proportionality constant varies with the salt concentration and type. Using multiscale computer simulations combined with vapor-pressure osmometry on caffeine-salt solutions, we reveal that this SASA description captures a rich set of molecular driving forces in tertiary solutions at changing solute and osmolyte concentrations. Central to the theoretical work is a new potential energy function that depends on the instantaneous surface area, salt type, and concentration. Used in, e.g., Monte Carlo simulations, this allows for a highly efficient exploration of many-body interactions and the resulting thermodynamics at elevated solute and salt concentrations.

The chaotropic effect of ions on the self-aggregating propensity of Whitlock's molecular tweezers

Molecular tweezers feature the first class of artificial receptors to pique the interest of researchers and emerge as an effective therapeutic candidate. The exceptional structure and exquisite binding specificity of tweezers establish this overall class of receptors as a promising tool, with abundant applications. However, their inclination to self-aggregate by mutual π-π stacking interactions of their aromatic arms diminishes their efficacy as a therapeutic candidate. Therefore, following up on sporadic studies, since the discovery of the Hofmeister series, on the ability of ions to either solvate (salting-in) or induce aggregation (salting-out) of hydrophobic solutes, the notions of ion-specificity effects are utilized on tweezer moieties. The impacts of three different aluminum salts bearing anions Cl^-, ClO₄^- and SCN^- on the self-association propensity of Whitlock's caffeine-pincered molecular tweezers are investigated, with a specific emphasis placed on elucidating the varied behavior of the ions on the hydration ability of tweezers. The comparative investigation is conducted employing a series of all-atom molecular dynamics simulations of five tweezer molecules in pure water and three salt solutions, at two different concentrations each, maintaining a temperature of 300 K and a pressure of 1 atm, respectively. Radial distribution functions, coordination numbers, and SASA calculations display a steady reduction in the aggregation proclivity of the receptor molecules with an increase in salt concentration, as progressed along the Hofmeister series. Orientational preferences between the tweezer arms reveal a disruptive effect in the regular π-π stacking interactions, in the presence of high concentrations of ClO₄^- and SCN^- ions, while preferential interactions and tetrahedral order parameters unveil the underlying mechanism, by which the anions alter the solubility of the hydrophobic molecules. Overall, it is observed that SCN^- exhibits the highest salting-in effect, followed by ClO₄^-, with both anions inhibiting tweezer aggregation through different mechanisms. ClO₄^- ions impart an effect by moderately interacting with the solute molecules as well as modifying the water structure of the bulk solution promoting solvation, whereas, SCN^- ions engage entirely in interaction with specific tweezer sites. Cl^- being the most charge-dense of the three anionic species experiences stronger hydration and therefore, imparts a very negligible salting-in effect.

Anion-cation contrast of small molecule solvation in salt solutions

The contributions from anions and cations from salt are inseparable in their perturbation of molecular systems by experimental and computational methods, rendering it difficult to dissect the effects exerted by the anions and cations individually. Here we investigate the solvation of a small molecule, caffeine, and its perturbation by monovalent salts from various parts of the Hofmeister series. Using molecular dynamics and the energy-representation theory of solvation, we estimate the solvation free energy of caffeine and decompose it into the contributions from anions, cations, and water. We also decompose the contributions arising from the solute-solvent and solute-ions interactions and that from excluded volume, enabling us to pin-point the mechanism of salt. Anions and cations revealed high contrast in their perturbation of caffeine solvation, with the cations salting-in caffeine via binding to the polar ketone groups, while the anions were found to be salting-out via perturbations of water. In agreement with previous findings, the perturbation by salt is mostly anion dependent, with the magnitude of the excluded-volume effect found to be the governing mechanism. The free-energy decomposition as conducted in the present work can be useful to understand ion-specific effects and the associated Hofmeister series.

Engineered multifunctional sand for enhanced removal of stormwater runoff contaminants in fixed-bed column systems

The degradation of surface water quality in the US is mostly contributed by nonpoint-source pollution, in which stormwater runoff plays a major role. Stormwater runoff pollution is difficult to control due to its diffuse and stochastic loading. In this study, multifunctional AlMg/GO engineered sand synthesized via a simple method was used to address four major categories of runoff contaminants, namely nutrient (phosphate), metal (zinc), organic contaminant (caffeine), and pathogen (E. coli), simultaneously. For chemical contaminants (phosphate, zinc, and caffeine), Freundlich and Thomas models can successfully describe the batch isotherms and breakthrough curves of column flow-through experiments, respectively. Better E. coli retention capacity and antibacterial activity of the engineered sand than that of the raw sand was demonstrated in E. coli retention and revitalization experiments. The engineered sand also showed good performance in actual surface runoff. Based on the results of the column flow-through experiments and the literature-reported typical field conditions and design criteria (e.g. 50 m³ engineered sand for 5000 m² catchment; dissolved concentrations in the runoff: phosphate 0.2 mg/L, zinc 0.3 mg/L, and caffeine 0.0002 mg/L), a preliminary operational lifetime estimation was conducted, which indicated that the engineered sand can maintain its effectiveness for 90% removal of the dissolved phosphate, zinc, and caffeine from stormwater runoff for 81, 15, and >100 years, respectively. The engineered multifunctional sand proved to be a promising solution to future stormwater runoff management.

Salting-in species induced self-assembly of stable MOFs

A general strategy based on the Hoffmeister effect is proposed for the aqueous-phase and mild synthesis of stable metal–organic frameworks.

Metal–organic frameworks (MOFs) are attracting immense research interest despite the fact that their synthesis usually proceeds in organic media or under harsh conditions depending on specific cases. Herein, Hofmeister effect was firstly introduced for the construction of MOFs and thereafter a general salting-in species (SS) induced self-assembly strategy was proposed for the aqueous-phase and mild synthesis of stable MOFs based on a unique “solubilization-mediating” mechanism. The SS not only improved the solubility of organic ligands, but also effectively mediated the mutual proximity of the organic linkers and the inorganic nodes, thus facilitating the crystallization of MOFs under mild conditions. Several typical and highly useful stable MOFs were exemplified owing to the availability of various SS. This strategy could set a framework for the development of more stable MOFs in aqueous phase and drive the large-scale and economic production of MOFs.

Crystal structure of caffeinium triiodide – caffeine (1/1), C16H21I3N8O4

C16H21I3N8O4, monoclinic, P21/n (no. 14), a = 14.7257(7) Å, b = 10.5712(5) Å, c = 16.7501(8) Å, β = 114.408(2)°, V = 2374.4(2) Å3, Z = 4, R gt(F) = 0.0254, wR ref(F 2) = 0.0760, T = 290(2) K.