Microheterogeneity in CH3OH/CD3OH mixture

In Situ Species Analysis of a Lithium-Ion Battery Electrolyte Containing LiTFSI and Propylene Carbonate

In this study, we analyzed the species in a model electrolyte consisting of a lithium salt, lithium bis(trifluoromethane sulfone)imide (LiTFSI), and a widely used neutral solvent propylene carbonate (PC) with excess infrared (IR) spectroscopy, ab initio molecular dynamics simulations (AIMD), and quantum chemical calculations. Complexing species including the charged ones [Li+(PC)4, TFSI-, TFSI-(PC), TFSI-(PC)2, and Li(TFSI)2-] are identified in the electrolyte. Quantum chemical calculations show strong Li+···O(PC) interaction, which suggests that Li+ would transport in the mode of solvation-carriage. However, the interaction energy of each hydrogen bond in TFSI-(PC) is very weak, suggesting that TFSI- would transport in hopping mode. In addition, the concentration dependences of the relative population of the species were also derived, providing a scenario for the dissolving process of the salt in PC. These in-depth studies provide physical insights into the structural and interactive properties of the electrolyte of lithium-ion batteries.

Generalized Excess Spectroscopy

With a clear enhancement of the apparent resolution of experimentally determined spectra, excess spectroscopy has been developed as a powerful tool to study solution structures and molecular interactions. In the standard procedure of the method, excess spectra are calculated based on the ideal spectra constructed using two pure compounds. This limits the applications of the method when the pure compounds are unstable or their physical state is different from that of the mixtures. To overcome the problem or to extend the application, we propose generalized excess spectroscopy in this work, where the ideal spectrum is evaluated from the spectra of reference mixtures. After deducing the working equations, we performed digital simulation and then applied the novel approach to a binary system consisting of tert-butanol and carbon tetrachloride. Both results illustrated the feasibility and universality of the method.

Understanding the complexity of the structures in alcohol solutions by temperature-dependent near-infrared spectroscopy

For understanding the structures and the hydrogen bonding in alcohol solutions, the changes of the structures and hydrogen bonding with temperature were studied by temperature-dependent near-infrared (NIR) spectroscopy. The spectral features of eight alcohol species including the monomer, dimer and linear or cyclic aggregates (trimer, tetramer and polymer) were found from the resolution-enhanced spectra calculated by continuous wavelet transform. The changes of the eight species with concentration and temperature were analyzed using the intensity variation of the corresponding spectral features and two-dimensional correlation NIR spectroscopy. The aggregates were found to form at a very low concentration and the stability of the seven aggregates with temperature was found in an order of cyclic tetramer > linear polymer > linear tetramer > cyclic trimer > linear trimer > cyclic polymer > dimer. Furthermore, the formation of the aggregates was found to be affected by the chain length. The increase of the chain length is beneficial for the formation of cyclic tetramer and polymer due to the hydrophobic effect, but is an adverse effect for the formation of linear polymer.

Excess spectroscopy and its applications in the study of solution chemistry

Characterization of structural heterogeneity of liquid solutions and the pursuit of its nature have been challenging tasks to solution chemists. In the last decade, an emerging method called excess spectroscopy has found applications in this area. The method, combining the merits of molecular spectroscopy and excess thermodynamic functions, shows the ability to enhance the apparent resolution of spectra, provides abundant information concerning solution structures and intermolecular interactions. In this review, the thinking and mathematics of the method, as well as its developments, are presented first. Then, research progress related to the exploration of the method is thoroughly reviewed. The materials are classified into two parts, small-molecular solutions and ionic liquid solutions. Finally, potential challenges and the perspective for further development of the method are discussed.

Microheterogeneity in binary mixtures of water with CH3OH and CD3OH: ATR-IR spectroscopic, chemometric and DFT studies

Here we report ATR-IR spectroscopic study on the separation at a molecular level (microheterogeneity) and the degree of deviation of H₂O/CH₃OH and H₂O/CD₃OH mixtures from the ideal mixture. Of particular interest is the effect of isotopic substitution in methyl group on molecular structure and interactions in both mixtures. To obtain comprehensive information from the multivariate data we applied the excess molar absorptivity spectra together with two-dimensional correlation analysis (2DCOS) and chemometric methods. In addition, the experimental results were compared and discussed with the structures of various model clusters obtained from theoretical (DFT) calculations. Our results evidence the presence of separation at a molecular level and deviation from the ideal mixture for both mixtures. The experimental and theoretical results show that the maximum of these deviations appears at equimolar mixture. Both mixtures consist of three kinds of species: homoclusters of water and methanol and mixed clusters (heteroclusters). The heteroclusters exist in the whole range of mole fractions with the maximum close to the equimolar mixture. At this mixture composition near 55-60% of molecules are involved in heteroclusters. In contrast, the homoclusters of water occur in a limited range of mole fractions (X_ME < 0.85-0.9). Upon mixing the molecules of methanol form weaker hydrogen bonding as compared with the pure alcohol. In contrast, the molecules of water in the mixture are involved in stronger hydrogen bonding than those in bulk water. All these results indicate that both mixtures have similar degree of deviation from the ideal mixture.