Acetate ion and its interesting solvation shell structure and dynamics

Green process for xylo-oligosaccharide production from acetic acid hydrolysis of sugarcane bagasse by an integrated membrane technology and activated carbon adsorption

Xylooligosaccharides (XOS), consisting 2-6 xylose residues, are a new type of prebiotic and functional oligosaccharides, and can usually be produced from the xylan-riched lignocellulosic biomass by acetic acid (HAc) hydrolysis, while the waste HAc was a problem to the environment. In this study, the main aim was to recover and reuse the waste HAc in XOS production. First, it was found that a temperature of 190 °C and a hydrolysis time of 60 min were favorable for XOS production by HAc hydrolysis, and the by-products xylose and furfural were the main inhibitors, hindering the reuse of the waste HAc. Then, xylose can be successfully decreased below to its inhibition concentration (i.e., 0.05 g/L) by a two-stage NF90 membrane process under 25 °C. After diafiltering the NF90 retentate using a triploid volution of a HCl-solution of pH 2.37, the total HAc recovery rate reached 82.87%. Moreover, after the recovered HAc solution was further treated with 25 g/L activated carbon under 25 °C and pH regulator with glacial HAc to 2.37 wherein the furfural concentration was below 0.015 g/L, the recovered HAc solution can be successfully reused in XOS production without affecting the XOS production. Thus, in the developed integrated process, it replaced 82.87% of the pristine HAc with the recovered HAc and simplified extraction process of XOS by HAc hydrolysis on an industrial scale.

Uncovering the binding nature of thiocyanate in contact ion pairs with lithium ions

Ion pair formation is a fundamental molecular process that occurs in a wide variety of systems, including electrolytes, biological systems, and materials. In solution, the thiocyanate (SCN-) anion interacts with cations to form contact ion pairs (CIPs). Due to its ambidentate nature, thiocyanate can bind through either its sulfur or nitrogen atoms, depending on the solvent. This study focuses on the binding nature of thiocyanate with lithium ions as a function of the solvents using FTIR, 2D infrared spectroscopy (2DIR) spectroscopies, and theoretical calculations. The study reveals that the SCN- binding mode (S or N end) in CIPs can be identified through 2DIR spectroscopy but not by linear IR spectroscopy. Linear IR spectroscopy shows that the CN stretch frequencies are too close to one another to separate N- and S-bound CIPs. Moreover, the IR spectrum shows that the S-C stretch presents different frequencies for the salt in different solvents, but it is related to the anion speciation rather than to its binding mode. A similar trend is observed for the anion bend. 2DIR spectra show different dynamics for N-bound and S-bound thiocyanate. In particular, the frequency-frequency correlation function (FFCF) dynamics extracted from the 2DIR spectra have a single picosecond exponential decay for N-bound thiocyanate and a biexponential decay for S-bound thiocyanate, consistent with the binding mode of the anion. Finally, it is also observed that the binding mode also affects the line shape parameters, probably due to the different molecular mechanisms of the FFCF for N- and S-bound CIPs.

Binary Mixtures of Choline Acetate and Tetrabutylammonium Acetate with Natural Organic Acids by Vibrational Spectroscopy and Molecular Dynamics Simulations

We present a study of several mixtures obtained by the mixing of two organic acetate-based salts (choline acetate, ChAc, or tetrabutylammonium acetate, TBAAc) with three different natural organic acids (ascorbic acid, AA, citric acid, CA, and maleic acid, MA). The structures of the starting materials and of the mixtures were characterized by infrared spectroscopy (FT-IR) and classic molecular dynamics simulations (MD). The thermal behavior was characterized by differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA). The obtained mixtures, especially the ChAc-based ones, strongly tend to vitrify at low temperatures and are stable up to 100-150 °C. The FTIR measurements suggest the formation of a strong H-bond network: the coordination between acids and ChAc or TBAAc takes place by the donation of the H-bond by the acids to the oxygen of the acetate anion, which acts as an acceptor (HBA). The comparison with MD analysis indicates that acids predominantly exploit their more acidic hydrogens. In particular, we observe the progressive shift of νC═O and νOH when the ratios of acids increase. The structural differences between the two studied cations influence the spatial distribution of the components in the mixture bulk phases. In particular, the analysis of the theoretical structure function I(q) of the TBAAc-based systems shows the presence of important prepeaks at low q, a sign of the formation of apolar domain, due to the nanosegregation of the alkyl chains.

FTIR spectroscopy and molecular level insight of diluted aqueous solutions of acetic acid

Aqueous solutions of acetic acid (AA) have been intensively explored for decades with a particular attention addressed to the hydrogen bond network generated by COOH group at different concentrations. In majority of studies conducted so far the envelope originated from νCO is decomposed into two bands assigned to differently hydrated monomers: the one presumably to AA···H2O, and another one to AA···(H2O)2. In order to examine if species other than the mentioned monomers produce this spectral signature, we performed computational and FTIR spectroscopic study of AA in aqueous solutions. Dilute solutions of deuterated acetic acid (CD3COOD) in D2O and in C2Cl4 as a reference were prepared (c0 = 0.001, 0.01 and 0.1 mol dm-3) as well as of deuterated sodium acetate (CD3COONa) in D2O. CD3COOD in 0.1 mol dm-3 solution in D2O displays a feature that separated in two signals with maxima at 1706 cm-1 and 1687 cm-1. A combined DFT and molecular dynamics study performed in this work showed the assignation of those spectral bands to be a more complex problem than previously thought, with syn-anti isomerism and hydration contributing to the experimentally observed broad νCO envelope.

Selective scandium ion capture through coordination templating in a covalent organic framework

The use of coordination complexes within covalent organic frameworks can significantly diversify the structures and properties of this class of materials. Here we combined coordination chemistry and reticular chemistry by preparing frameworks that consist of a ditopic (p-phenylenediamine) and mixed tritopic moieties-an organic ligand and a scandium coordination complex of similar sizes and geometries, both bearing terminal phenylamine groups. Changing the ratio of organic ligand to scandium complex enabled the preparation of a series of crystalline covalent organic frameworks with tunable levels of scandium incorporation. Removal of scandium from the material with the highest metal content subsequently resulted in a 'metal-imprinted' covalent organic framework that exhibits a high affinity and capacity for Sc3+ ions in acidic environments and in the presence of competing metal ions. In particular, the selectivity of this framework for Sc3+ over common impurity ions such as La3+ and Fe3+ surpasses that of existing scandium adsorbents.

BoxCARS 2D IR spectroscopy with pulse shaping

BoxCARS and pump-probe geometries are common implementations of two-dimensional infrared (2D IR) spectroscopy. BoxCARS is background-free, generally offering greater signal-to-noise ratio, which enables measuring weak vibrational echo signals. Pulse shapers have been implemented in the pump-probe geometry to accelerate data collection and suppress scatter and other unwanted signals by precise control of the pump-pulse delay and carrier phase. Here, we introduce a 2D-IR optical setup in the BoxCARS geometry that implements a pulse shaper for rapid acquisition of background-free 2D IR spectra. We show a signal-to-noise improvement using this new fast-scan BoxCARS setup versus the pump-probe geometry within the same configuration.

Ultrafast vibrational dynamics of aqueous acetate and terephthalate

We study the vibrational population relaxation and mutual interaction of the symmetric stretch ( ν s) and antisymmetric stretch ( ν as) vibrations of the carboxylate anion groups of acetate and terephthalate ions in aqueous solution by femtosecond two-dimensional infrared spectroscopy. By selectively exciting and probing the ν s and ν as vibrations, we find that the interaction of the two vibrations involves both the anharmonic coupling of the vibrations and energy exchange between the excited states of the vibrations. We find that both the vibrational population relaxation and the energy exchange are faster for terephthalate than for acetate.

Role of Hydration in Magnesium versus Calcium Ion Pairing with Carboxylate: Solution and the Aqueous Interface

The binding of group II metal cations such as Ca²⁺ and Mg²⁺ has been largely categorized as electrostatic or ionic using carboxylate symmetric and asymmetric stretching frequency assignments that have been historically used with little regard for the solvation environment of aqueous solutions. However, given the importance of these cations and their binding mechanisms related to biological function and in revealing surface enrichment factors for ocean to marine aerosol transfer, it is imperative that a deeper understanding be sought to include hydration effects. Here, infrared reflection-absorption and Raman spectra for surface and solution phase carboxylate binding information, respectively, are compared against bare (unbound) carboxylate and bidentate Zn²⁺:carboxylate spectral signatures. Spectral non-coincidence effect analysis, temperature studies, and spectral and potential of mean force calculations result in a concise interpretation of binding motifs that include the role of mediating water molecules, that is, contact and solvent-shared ion pairs. Calcium directly binds to the carboxylate group in contact ion pairs where magnesium rarely does. Moreover, we reveal the dominance of the solvent-shared ion pair of magnesium with carboxylate at the air-water interface and in solution.

Structure of aqueous sodium acetate solutions by X-Ray scattering and density functional theory

The structure of aq. sodium acetate solution (CH3COONa, NaOAc) was studied by X-ray scattering and density function theory (DFT). For the first hydrated layer of Na+, coordination number (CN) between Na+ and O(W, I) decreases from 5.02 ± 0.85 at 0.976 mol/L to 3.62 ± 1.21 at 4.453 mol/L. The hydration of carbonyl oxygen (OC) and hydroxyl oxygen (OOC) of CH3COO− were investigated separately and the OC shows a stronger hydration bonds comparing with OOC. With concentrations increasing, the hydration shell structures of CH3COO− are not affected by the presence of large number of ions, each CH3COO− group binds about 6.23 ± 2.01 to 7.35 ± 1.73 water molecules, which indicates a relatively strong interaction between CH3COO− and water molecules. The larger uncertainty of the CN of Na+ and OC(OOC) reflects the relative looseness of Na-OC and Na-OOC ion pairs in aq. NaOAc solutions, even at the highest concentration (4.453 mol/L), suggesting the lack of contact ion pair (CIP) formation. In aq. NaOAc solutions, the so called “structure breaking” property of Na+ and CH3COO− become effective only for the second hydration sphere of bulk water. The DFT calculations of CH3COONa (H2O)n=5–7 clusters suggest that the solvent-shared ion pair (SIP) structures appear at n = 6 and become dominant at n = 7, which is well consistent with the result from X-ray scattering.

Physiological Calcium Concentrations Slow Dynamics at the Lipid-Water Interface

Phospholipids can interact strongly with ions at physiological concentrations, and these interactions can alter membrane properties. Here, we describe the effects of calcium ions on the dynamics in phospholipid membranes. We used a combination of time-resolved ultrafast two-dimensional infrared spectroscopy and molecular dynamics simulations. We found that millimolar Ca2+ concentrations lead to slower fluctuations in the local environment at the lipid-water interface of membranes with phosphatidylserine. The effect was only observed in bilayers containing anionic phosphatidylserine; membranes composed of only zwitterionic phosphatidylcholine did not experience a slowdown. Local water dynamics were measured using the ester groups as label-free probes and were found to be up to 50% slower with 2.5 mM Ca2+. Molecular dynamics simulations show that Ca2+ primarily binds to the carboxylate group of phosphatidylserines. These findings have implications for apoptotic and diseased cells in which phosphatidylserine is exposed to extracellular calcium and for the biophysical effects of divalent cations on lipid bilayers.