Communication: Infrared spectroscopy of salt-water complexes

Synergistic effects of starch and carrageenan from Kappaphycus alvarezii in composite film formation: Physicochemical and degradable properties

Rising concerns around plastic pollution from single-use plastic (SUPs), especially food packaging, have driven interest in sustainable alternatives. As such, algae biomass has gained attention for bioplastic production due to algae's rapid growth and abundant polysaccharides. This research focuses on extracting carrageenan from Kappaphycus alvarezii, extensively cultivated in Sabah, Malaysia, and utilizing it in combination with starch and glycerol to develop algae-based films. The physicochemical properties and degradation rate of these films were evaluated, revealing that the addition of carrageenan enhanced overall thermal stability meanwhile increasing water solubility, water content but reducing the degradation rate and swelling degree. This is primarily due to the crystalline structures of carrageenan, which provide a more rigid arrangement compared to the network of starch polymers. However, the incorporation of starch into the blends has enhanced the elongation and surface morphology, resulting in more balanced properties. Overall, these carrageenan films displayed impressive thermal, mechanical, and biodegradability characteristics, establishing their viability as substitutes for conventional plastics.

Water-Induced Dissociative Mechanism of Carboxylate and Divalent Calcium Ions Revealed by IR Laser Spectroscopy

The dissociation of carboxylate and divalent calcium ions is investigated at the molecular level in microsolvation experiments by gradually increasing the number of water molecules around the ions. IR photodissociation (IRPD) laser spectroscopy of H2-tagged (Ca2+, AcO-)(H2O)n=8-21 clusters in the ν(CO2-) spectral range combined with RI-B97-D3-BJ-abc/TZVPPD frequency calculations is used to identify the type of ion pairs involved in this process. These results reveal that the ion dissociation follows a multistep mechanism involving in particular pseudobridged monodentate contact ion pairs (CIPs), which are found to be the first intermediate species formed from bidentate CIPs along the ion dissociation path. Altogether, structural assignments suggest a sequence of simple reactions in the first coordination shell of the carboxylate group, leading us to propose two possible dissociation paths. The appearance threshold of monodentate structures is measured at n = 10, with that of solvent-shared ion pairs (SIPs) being potentially at n = 18. By showing in detail how solvation progressively takes over from the ionic interaction in shaping these supramolecular structures, this study can serve as a reference for solving ion-pairing/dissociation problems.

Benchmark computational investigations for the basic model of the salt-water complex: NaCl(H2O) and its anion NaCl(H2O). Phys Chem Chem Phys 2023;25:27215-27229. [PMID: 37791409 DOI: 10.1039/d3cp03421f] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The microsolvation of salts in water is a fundamental physicochemical process. In this work, the aqueous salt complex NaCl(H2O) and its anion NaCl(H2O)- were investigated using comprehensive calculations, including the costly and accurate CCSD(T)-F12a and focal point analysis (FPA) methods. For the neutral NaCl(H2O), three isomers exist, two of which are mirror-symmetric with almost identical structures and their corresponding anions are also mirror-symmetric. For the NaCl(H2O)- anion, there are four isomers. Several transition states are found for the first time. The structural rearrangements of neutral NaCl(H2O) and NaCl(H2O)- anions are mainly caused by breaking and forming of the hydrogen bonds and the enhancement and weakening of interactions between Na and O atoms. The distributions of the anion complexes from 15-300 K are computed and compared to recent experimental results. The analysis of the intermolecular weak interactions shows the weak van der Waals interactions between Na and O atoms, as well as hydrogen bonding between H and Cl. Moreover, the theoretically predicted anion photoelectron spectra are assigned and analyzed in detail, and they agree with experimental spectra satisfactorily. The Na-Cl stretching vibrational mode dominates the vibrational structure in both anion spectra with some minor contributions from the intermolecular motions between H2O and NaCl.

Importance of Mid-Infrared Spectra Regions for the Prediction of Mastitis and Ketosis in Dairy Cows

Mid-infrared (MIR) spectroscopy is routinely applied to determine major milk components, such as fat and protein. Moreover, it is used to predict fine milk composition and various traits pertinent to animal health. MIR spectra indicate an absorbance value of infrared light at 1060 specific wavenumbers from 926 to 5010 cm−1. According to research, certain parts of the spectrum do not contain sufficient information on traits of dairy cows. Hence, the objective of the present study was to identify specific regions of the MIR spectra of particular importance for the prediction of mastitis and ketosis, performing variable selection analysis. Partial least squares discriminant analysis (PLS-DA) along with three other statistical methods, support vector machine (SVM), least absolute shrinkage and selection operator (LASSO), and random forest (RF), were compared. Data originated from the Austrian milk recording and associated health monitoring system (GMON). Test-day data and corresponding MIR spectra were linked to respective clinical mastitis and ketosis diagnoses. Certain wavenumbers were identified as particularly relevant for the prediction models of clinical mastitis (23) and ketosis (61). Wavenumbers varied across four distinct statistical methods as well as concerning different traits. The results indicate that variable selection analysis could potentially be beneficial in the process of modeling.

Manifestation of hydration of Na+ and Cl- ions in the IR spectra of NaCl aqueous solutions in the range of 2750-4000 cm-1

This work is aimed at the study at studying the influence of the interaction of solvate shells on the profiles of the IR spectra of sodium chloride solutions in the 2750-4000 cm^-1 range. The IR spectra of distilled water and sodium chloride solutions were obtained with the limit (0.356 g per 100 g of water) and 50 % of the limit (0.178 g per 100 g of water) concentrations at a temperature of 21˚. Theoretical methods based on the use of the DFT approach with the XLYP exchange-correlation potential are used to calculate the profiles of the IR spectra of clusters containing 9 water molecules per one NaCl molecule at the limit concentrations of the solution. In the case when the cluster contained a NaCl molecule, the spectra were calculated for interacting and non-interacting solvate shells in which the number of H₂O molecules varied from 3 to 6. The expansion of the experimental band profile on a basis containing the profiles of the theoretical bands made it possible to study the features of NaCl hydration with a change in the concentration of solutions. It was found that the IR spectrum band is formed mainly by interacting Na⁺ and Cl^- solvation shells, each containing 4 H₂O molecules, while the ninth H₂O molecule provides the bond between the solvated ions. As the salt concentration increases, the contribution of the solvation shells to the band profile increases too. The agreement reached in the positions and profiles of experimental and theoretical water bands at different solution concentrations substantiates the adequacy of the theoretical description of NaCl hydration. Theoretical studies explained the effect of a decrease in the band width, an increase in the peak intensity, and a shift of its maximum toward higher wavenumbers with increasing solution concentration.

Theoretical Investigation of the Structures and Energetics of (MX)-Ethanol Complexes in the Gas Phase

The structures and energy of alkali halide salt (MX) complexes with ethanol have been investigated in this work. The core of this study is to explore the effect of ion size on the interactions between solvent and solute. LiF and KBr as monovalent salts with different sizes of inion and cation have been chosen to explore this difference in addition to various physical properties. Three complexes of each LiF and KBr with ethanol taking the formula MX(CH3CH2OH)n (n=1-3), were studied. Ab-initio calculations have been performed to optimize the chemical structures of these complexes and explore the possible structures, isomers, and their corresponding IR spectra using Density functional theory (DFT/ B3LYP). 6-311G** were chosen as basis sets for these calculations. The geometry evaluations, energy searches, vibrational frequency calculations, and each complex's binding energy were also theoretically extracted in this study. The minimum energy structures were calculated, and different isomers were found. The presence of Ionic hydrogen bonds (IHBs) was observed and proposed to be the main binding between the MX salt and ethanol. Also, the infrared vibrational bands in the OH stretching region were recorded for the minimum structures, and the determined red-shift was at about 400 cm-1. In addition, the binding energy calculations found a gradual rise in the BE value with every additional ethanol molecule added to MX salt.

Impact of anion polarizability on ion pairing in microhydrated salt clusters

Despite longstanding interest in the mechanism of salt dissolution in aqueous media, a molecular level understanding remains incomplete. Here, cryogenic ion trap vibrational action spectroscopy is combined with electronic structure calculations to track salt hydration in a gas phase model system one water molecule at a time. The infrared photodissociation spectra of microhydrated lithium dihalide anions [LiXX'(H2O) n ]- (XX' = I2, ClI and Cl2; n = 1-3) in the OH stretching region (3800-2800 cm-1) provide a detailed picture of how anion polarizability influences the competition among ion-ion, ion-water and water-water interactions. While exclusively contact ion pairs are observed for n = 1, the formation of solvent-shared ion pairs, identified by markedly red-shifted OH stretching bands (<3200 cm-1), originating from the bridging water molecules, is favored already for n = 2. For n = 3, Li+ reaches its maximum coordination number of four only in [LiI2(H2O)3]-, in accordance with the hard and soft Lewis acid and base principle. Water-water hydrogen bond formation leads to a different solvent-shared ion pair motif in [LiI2(H2O)3]- and network formation even restabilizes the contact ion pair motif in [LiCl2(H2O)3]-. Structural assignments are exclusively possible after the consideration of anharmonic effects. Molecular dynamics simulations confirm that the significance of large amplitude motion (of the water molecules) increases with increasing anion polarizability and that needs to be considered already at cryogenic temperatures.

Salt Enrichment and Dynamics in the Interface of Supercooled Aqueous Droplets

The interconversion reaction of NaCl between the contact-ion pair (CIP) and the solvent-separated ion pair (SSIP) as well as the free-ion state in cold droplets has not yet been investigated. We report direct computational evidence that the lower is the temperature, the closer to the surface the ion interconversion reaction takes place. In supercooled droplets the enrichment of the subsurface in salt becomes more evident. The stability of the SSIP relative to the CIP increases as the ion-pairing is transferred toward the droplet's outer layers. In the free-ion state, where the ions diffuse independently in the solution, the number density of Cl^- shows a broad maximum in the interior in addition to the well-known maximum in the surface. In the study of the reaction dynamics, we find a weak coupling between the interionic NaCl distance reaction coordinate and the solvent degrees of freedom, which contrasts with the diffusive crossing of the free energy barrier found in bulk solution modeling. The H₂O self-diffusion coefficient is found to be at least an order of magnitude larger than that in the bulk solution. We propose to exploit the enhanced surface ion concentration at low temperature to eliminate salts from droplets in native mass spectrometry ionization methods.

Structural Versatility and Energy Difference of Salt-Water Complex NaCl(H2O) Encoded in Cryogenic Photoelectron Spectroscopy

A weakly bound complex usually has multiple structural isomers with small energy differences. The sophisticated ab initio calculations are the main workhorse for providing theoretical results of different isomers. In contrast, the experimental determination of the energy difference is very rare. We report the energy-difference measurement of a model complex: salt-water complex NaCl(H₂O). We measured the energy difference among the structural isomers of the negatively charged NaCl(H₂O) complex and the neutral counterpart using cryogenic photoelectron spectroscopy. The temperature-dependent photoelectron spectra (15-300 K) revealed that the negatively charged NaCl(H₂O) and the neutral counterpart both have three isomers. The two higher-lying isomers are 186(22) and 481(48) cm^-1, respectively, above the most stable isomer for the negatively charged and 123(10) and 1821(24) cm^-1 for the neutral. These results provide a benchmark for the development of theoretic methods of weakly bound complexes. The experimental technique demonstrated here can be employed to investigate other weakly bound complexes with multiple isomers.

Ion Pair Supramolecular Structure Identified by ATR-FTIR Spectroscopy and Simulations in Explicit Solvent*

The present work uses ATR-FTIR spectroscopy assisted by simulations in explicit solvent and frequency calculations to investigate the supramolecular structure of carboxylate alkali-metal ion pairs in aqueous solutions. ATR-FTIR spectra in the 0.25-4.0 M concentration range displayed cation-specific behaviors, which enabled the measurement of the appearance concentration thresholds of contact ion pairs between 1.9 and 2.6 M depending on the cation. Conformational explorations performed using a non-local optimization method associated to a polarizable force-field (AMOEBA), followed by high quantum chemistry level (RI-B97-D3/dhf-TZVPP) optimizations, mode-dependent scaled harmonic frequency calculations and electron density analyses, were used to identify the main supramolecular structures contributing to the experimental spectra. A thorough analysis enables us to reveal the mechanisms responsible for the spectroscopic sensitivity of the carboxylate group and the respective role played by the cation and the water molecules, highlighting the necessity of combining advanced experimental and theoretical techniques to provide a fair and accurate description of ion pairing.

Comparison of the Microsolvation of CaX2 (X = F, Cl, Br, I) in Water: Size-Selected Anion Photoelectron Spectroscopy and Theoretical Calculations

To understand the microsolvation of alkaline-earth dihalides in water and provide information about the dependence of solvation processes on different halides, we investigated CaBr₂(H₂O)_n^-, CaI₂(H₂O)_n^-, and CaF₂(H₂O)_n^- (n = 0-6) clusters using size-selected anion photoelectron spectroscopy and conducted theoretical calculations on these clusters and their neutrals. The results are compared with those of CaCl₂(H₂O)_n^-/0 clusters reported previously. It is found that the vertical detachment energies (VDEs) of CaCl₂(H₂O)_n^-, CaBr₂(H₂O)_n^-, and CaI₂(H₂O)_n^- show a similar trend with increasing cluster size, while the VDEs of CaF₂(H₂O)_n^- show a different trend. The VDEs of CaF₂(H₂O)_n^- are much lower than those of CaCl₂(H₂O)_n^-, CaBr₂(H₂O)_n^-, and CaI₂(H₂O)_n^-. A detailed probing of the structures shows that a significant increase of the Ca-X distance (separation of Ca²⁺-X^- ion pair) in CaCl₂(H₂O)_n^-/0, CaBr₂(H₂O)_n^-/0, and CaI₂(H₂O)_n^-/0 clusters occurred at about n = 5. However, for CaF₂(H₂O)_n^-/0, no abrupt change of the Ca-F distance with the increasing cluster size has been observed. In CaCl₂(H₂O)₆^-/0, CaBr₂(H₂O)₆^-/0, and CaI₂(H₂O)₆^-/0, the Ca atom coordinates directly with 5 H₂O molecules. However, in CaF₂(H₂O)_n^-/0, the Ca atom coordinates directly with only 2 or 3 H₂O molecules. The similarity or differences in the structures and coordination numbers are consistent with the fact that CaCl₂, CaBr₂, and CaI₂ have similar solubility, while CaF₂ has much lower solubility.

IR Spectroscopy of the Cesium Iodide-Water Complex

There has been much interest in I^-(H₂O) as a simple model for a hydrated iodide ion. Here we explore how this fundamental ion-solvent interaction is modified by the presence of a counterion, specifically Cs⁺. This has been achieved by forming the CsI(H₂O) complex in superfluid helium nanodroplets and then probing this system using infrared spectroscopy. The complex retains the ionic hydrogen bond between the I^- and a water OH group seen in I^-(H₂O), but the Cs⁺ ion substantially alters the anion-water interaction through formation of a cyclic Cs⁺-O-H-I^- bonding motif. As with I^-(H₂O), the OH stretching band derived from the hydrogen-bonded OH group shows substructure, splitting into a clear doublet. However, in contrast to I^-(H₂O), where a tunneling splitting arising from hydrogen atom exchange plays a role, the doublet we observe is attributed solely to an anharmonic vibrational coupling effect.

Shifting formic acid dimers into perspective: vibrational scrutiny in helium nanodroplets

A metastable dimer of formic acid has been prepared inside superfluid helium nanodroplets and examined using IR spectroscopy and quantum chemical calculations.

Probing Elusive Cations: Infrared Spectroscopy of Protonated Acetic Acid

Protonated carboxylic acids, (RCOOH)H⁺, are the initial intermediates in acid-catalyzed (Fischer) esterification reactions. However, the identity of the isomeric form has been debated. Surprisingly, no optical spectra have been reported for any isomer of the protonated carboxylic acid monomer, despite it being a fundamental organic cation. Here, we address these issues by using a new approach to prepare cold He-tagged cations of protonated acetic acid (AA), which entails electron ionization of helium nanodroplets containing metastable dimers of AA. The protonated species is subsequently probed using infrared photodissociation spectroscopy, and following a comparison with calculations, we identify the two isomers whose roles in Fischer esterification are debated. These are the carbonyl-protonated E, Z isomer and the metastable hydroxyl-protonated isomer. Our technique provides a novel approach that can be applied to other elusive ionic species.

Identification of ion pairs in solution by IR spectroscopy: crucial contributions of gas phase data and simulations

In a context where structure elucidation of ion pairs in solution remains a contemporary challenge, this work explores an original approach where accurate gas phase spectroscopic data are used to refine high level quantum chemistry calculations of ion pairs in solution, resulting in an unprecedented level of accuracy in vibrational frequency prediction. First, gas phase studies focus on a series of isolated contact ion pairs (M⁺, Ph-CH₂-COO^-, with M = Li, Na, K, Rb, Cs) for which conformer-selective IR spectra in the CO₂^- stretch region are recorded. These experiments reveal the interactions at play in isolated contact ion pairs, and provide vibrational frequencies enabling us to assess the accuracy of the theoretical approach used, i.e., mode-dependent scaled harmonic frequency calculations at the RI-B97-D3/dhf-TZVPP level. This level of calculation is then employed on large water clusters embedding either a free acetate ion or its contact or solvent-shared pairs with a sodium cation in order to simulate the individual vibrational spectra of these species in solution. This study shows that the stretching modes of carboxylate are sensitive to both solvent-shared and contact ion pair formation. FTIR spectra of solutions of increasing concentrations indeed reveal several spectral changes consistent with the presence of specific types of solvent-shared and contact ion pairs. By providing relevant guidelines for the interpretation of solution phase IR spectra, this work illustrates the potential of the approach for the elucidation of supramolecular structures in electrolyte solutions.

Dimers of acetic acid in helium nanodroplets

Two metastable dimers are created inside superfluid helium and studied using infrared spectroscopy to provide insight into condensed phase structures.

Formation of small clusters of NaCl dihydrate in the gas phase

Sodium chloride dihydrates form cuboid structures in initial stages of nucleation, preserving the NaCl interaction.

Emergence of Solvent-Separated Na+-Cl- Ion Pair in Salt Water: Photoelectron Spectroscopy and Theoretical Calculations

Solvation of salts in water is a fundamental physical chemical process, but the underlying mechanism remains unclear. We investigated the contact ion pair (CIP) to solvent-separated ion pair (SSIP) transition in NaCl(H₂O)_n clusters with anion photoelectron spectroscopy and ab initio calculations. It is found that the SSIP type of structures show up at n = 2 for NaCl^-(H₂O)_n anions. For neutral NaCl(H₂O)_n, the CIP structures are dominant at n < 9. At n = 9-12, the CIP structures and SSIP structures of NaCl(H₂O)_n are nearly degenerate in energy, coincident to the H₂O:NaCl molar ratio of NaCl saturated solution and implying that the CIP and SSIP structures can coexist in concentrated solutions. These results are useful for understanding the solvation of salts at the molecular level.