Mixtures of LiTFSI and urea: ideal thermodynamic behavior as key to the formation of deep eutectic solvents?

Efficient Molecular Dynamics Simulations of Deep Eutectic Solvents with First-Principles Accuracy Using Machine Learning Interatomic Potentials

In recent years, deep eutectic solvents emerged as highly tunable and ecofriendly alternatives to common organic solvents and liquid electrolytes. In the present work, the ability of machine learning (ML) interatomic potentials for molecular dynamics (MD) simulations of these liquids is explored, showcasing a trained neural network potential for a 1:2 ratio mixture of choline chloride and urea (reline). Using the ML potentials trained on density functional theory data, MD simulations for large systems of thousands of atoms and nanosecond-long time scales are feasible at a fraction of the computational cost of the target first-principles simulations. The obtained structural and dynamical properties of reline from MD simulations using our machine learning models are in good agreement with the first-principles MD simulations and experimental results. Running a single MD simulation is highlighted as a general shortcoming of typical first-principles studies if the dynamic properties are investigated. Furthermore, velocity cross-correlation functions are employed to study the collective dynamics of the molecular components in reline.

Quantitative Solvation Energies from Gas-Phase Calculations: First-Principles Charge Transfer and Perturbation Approaches

We present a first-principles approach for the calculation of solvation energies and enthalpies with respect to different ion pair combinations in various solvents. The method relies on the conceptual density functional theory (DFT) of solvation, from which detailed expressions for the solvation energies can be derived. In addition to fast and straightforward gas phase calculations, we also study the influence of modified chemical reactivity descriptors in terms of electronic perturbations. The corresponding phenomenological changes in molecular energy levels can be interpreted as the influence of continuum solvents. Our approach shows that the introduction of these modified expressions is essential for a quantitative agreement between the calculated and the experimental results.

Insights into the Effect of Lithium Doping on the Deep Eutectic Solvent Choline Chloride:Urea

Choline-based deep eutectic solvents (DESs) are potential candidates to replace flammable organic solvent electrolytes in lithium-ion batteries (LIBs). The effect of the addition of a lithium salt on the structure and dynamics of the material needs to be clarified before it enters the battery. Here, the archetypical DES choline chloride:urea at 1:2 mole fraction has been added with lithium chloride at two different concentrations and the effect of the additional cation has been evaluated with respect to the non-doped system via multinuclear NMR techniques. ¹H and ⁷Li spin-lattice relaxation times and diffusion coefficients have been measured between 298 K and 373 K and revealed a decrease in both rotational and translational mobility of the species after LiCl doping at a given temperature. Temperature dependent ³⁵Cl linewidths reflect the viscosity increase upon LiCl addition, yet keep track of the lithium complexation. Quantitative indicators such as correlation times and activation energies give indirect insights into the intermolecular interactions of the mixtures, while lithium single-jump distance and transference number shed light into the lithium transport, being then of help in the design of future DES electrolytes.

Ionic Conductivity Enhancement in UHMW PEO Gel Electrolytes Based on Room-Temperature Ionic Liquids and Deep Eutectic Solvents

Physical gels made of poly(ethylene oxide) (PEO) and deep eutectic solvents urea-Li bis(trifluoromethanesulfonyl)imide (TFSI) and ethylene glycol/LiTFSI, or pyrrolidinium ionic liquid solutions PYR13TFSI-LiTFSI and PYR14TFSI-LiTFSI, are prepared by a fast, single-step process, which involves no auxiliary solvents or intermediates and is reproducible and scalable. The properties of these gels are studied as a function of the PEO content and its molecular weight and the nature of the liquid electrolyte. The gels prepared with a low concentration (1-5 wt %) of ultrahigh molecular weight (UHMW) PEO are tough, stretchable materials which resemble soft elastomers and are also self-healing and transparent. Their rheology shows the conventional behavior of physical polymer gels, so that the higher the molecular weight of PEO, the lower the polymer concentration needed to produce the gel. However, the ion conductivities and diffusivities of the gels are striking, in many cases being equal to or significantly higher than those of pure liquid electrolytes. This ion conductivity enhancement is the highest for the lowest PEO concentration with the highest molecular weight. This unprecedented molecular weight dependence of conductivity and diffusivity is the result of two combined effects: the liquid electrolyte chemical structure modification as a consequence of the addition of PEO and the development of elastic networks, where ion mobility and rheology are uncoupled when the PEO added is of UHMW.

Computer Simulations of Deep Eutectic Solvents: Challenges, Solutions, and Perspectives

Deep eutectic solvents (DESs) are one of the most rapidly evolving types of solvents, appearing in a broad range of applications, such as nanotechnology, electrochemistry, biomass transformation, pharmaceuticals, membrane technology, biocomposite development, modern 3D-printing, and many others. The range of their applicability continues to expand, which demands the development of new DESs with improved properties. To do so requires an understanding of the fundamental relationship between the structure and properties of DESs. Computer simulation and machine learning techniques provide a fruitful approach as they can predict and reveal physical mechanisms and readily be linked to experiments. This review is devoted to the computational research of DESs and describes technical features of DES simulations and the corresponding perspectives on various DES applications. The aim is to demonstrate the current frontiers of computational research of DESs and discuss future perspectives.

Specific Ion Effects in Different Media: Current Status and Future Challenges

We discuss the current state of research as well as the future challenges for a deeper understanding of specific ion effects in protic and aprotic solvents as well as various additional media. Despite recent interest in solute or interfacial effects, we focus exclusively on the specific properties of ions in bulk electrolyte solutions. Corresponding results show that many mechanisms remain unknown for these simple media, although theoretical, computational, and experimental studies have provided some insights into explaining individual observations. In particular, the importance of local interactions and electronic properties is emphasized, which enabled a more consistent interpretation of specific ion effects over the past years. Despite current insufficient knowledge, we also discuss future challenges in relation to dynamic properties as well as the influence of different concentrations, different solvents, and solute contributions to gain a deeper understanding of specific ion effects for technological applications.

Long-Range Electrostatic Colloidal Interactions and Specific Ion Effects in Deep Eutectic Solvents

While the traditional consensus dictates that high ion concentrations lead to negligible long-range electrostatic interactions, we demonstrate that electrostatic correlations prevail in deep eutectic solvents where intrinsic ion concentrations often surpass 2.5 M. Here we present an investigation of intermicellar interactions in 1:2 choline chloride:glycerol and 1:2 choline bromide:glycerol using small-angle neutron scattering. Our results show that long-range electrostatic repulsions between charged colloidal particles occur in these solvents. Interestingly, micelle morphology and electrostatic interactions are modulated by specific counterion condensation at the micelle interface despite the exceedingly high concentration of the native halide from the solvent. This modulation follows the trends described by the Hofmeister series for specific ion effects. The results are rationalized in terms of predominant ion-ion correlations, which explain the reduction in the effective ionic strength of the continuum and the observed specific ion effects.

Lithium-salt-based deep eutectic solvents: Importance of glass formation and rotation-translation coupling for the ionic charge transport

Lithium-salt-based deep eutectic solvents, where the only cation is Li⁺, are promising candidates as electrolytes in electrochemical energy-storage devices, such as batteries. We have performed broadband dielectric spectroscopy on three such systems, covering a broad temperature and dynamic range that extends from the low-viscosity liquid around room temperature down to the glassy state approaching the glass-transition temperature. We detect a relaxational process that can be ascribed to dipolar reorientational dynamics and exhibits the clear signatures of glassy freezing. We find that the temperature dependence of the ionic dc conductivity and its room-temperature value also are governed by the glassy dynamics of these systems, depending, e.g., on the glass-transition temperature and fragility. Compared to the previously investigated corresponding systems, containing choline chloride instead of a lithium salt, both the reorientational and ionic dynamics are significantly reduced due to variations in the glass-transition temperature and the higher ionic potential of the lithium ions. These lithium-based deep eutectic solvents partly exhibit significant decoupling of the dipolar reorientational and the ionic translational dynamics and approximately follow a fractional Debye-Stokes-Einstein relation, leading to an enhancement of the dc conductivity, especially at low temperatures. The presented results clearly reveal the importance of decoupling effects and of the typical glass-forming properties of these systems for the technically relevant room-temperature conductivity.

Interactions of a DNA G-quadruplex with TMAO and urea: a molecular dynamics study on co-solute compensation mechanisms

We study the individual and combined influence of TMAO and urea on a basket-type DNA G-quadruplex by means of atomistic molecular dynamics (MD) simulations. In combination with the Kirkwood-Buff theory of solutions, we propose a simple mechanism to elucidate the impact of TMAO and urea on the G-quadruplex. Our results reveal the importance of the molecular accumulation around the DNA in terms of stabilizing or destabilizing effects. The results for mixtures show only a weak interaction between both co-solutes, which highlights the additivity of contributions. Despite the fact, that TMAO can to some extent compensate the adverse impact of urea on the G-quadruplex structure, the destabilizing influence is not completely eliminated. This observation opens the door for further research on selective stabilization of DNA G-quadruplexes by modulating the concentrations of TMAO and urea in solution.

Theoretical Insights into Specific Ion Effects and Strong-Weak Acid-Base Rules for Ions in Solution: Deriving the Law of Matching Solvent Affinities from First Principles

We present a detailed study of specific ion effects, volcano plots and the law of matching solvent affinities by means of a conceptual density functional theory (DFT) approach. Our results highlight that specific ion effects and the corresponding implications on the solvation energy are mainly due to differences in the electric chemical potentials and chemical hardnesses of the ions and the solvent. Our approach can be further used to identify reliable criteria for the validity of the law of matching solvent affinities. Basic expressions are derived, which allow us to study the limiting conditions for this empirical observation with regard to matching chemical reactivity indices. Moreover, we show that chaotropic and kosmotropic concepts and their implications for the stability of ion pairs are directly related to a generalized strong and weak acids and bases (SWAB) principle for ions in solution, which is also applicable to rationalize the shape of volcano plots for different solvents. In contrast to previous assumptions, all empirical findings can be explained by the properties of local solvent-ion complexes which dominate the specific global behavior of ion pairs in solution.

Excited State Dynamics of Isolated 6- and 8-Hydroxyquinoline Molecules

The photoinduced dynamics of isolated n-hydroxyquinoline (nHQ) molecules (n=6,8) was investigated in femtosecond pump-probe experiments. A qualitative difference was found between 8HQ and 6HQ. After an initial rapid decay corresponding to the departure of the initial wavepacket out of the Franck-Condon region of the excitation, the 8HQ probe signal decays to zero in 0.37 ps whereas a much longer time constant of 10.4 ps is observed in 6HQ. This interrogates on the role played by the intramolecular H-bond N · · · HO which is at play the 8HQ molecule. Ab-initio were performed at the MCSCF/aug-cc-pVDZ level on the 8HQ molecule to help the discussion. A complex energy landscape was found, which includes a conical intersection.

Theoretical and Computational Insight into Solvent and Specific Ion Effects for Polyelectrolytes: The Importance of Local Molecular Interactions

Polyelectrolytes in solution show a broad plethora of interesting effects. In this short review article, we focus on recent theoretical and computational findings regarding specific ion and solvent effects and their impact on the polyelectrolyte behavior. In contrast to standard mean field descriptions, the properties of polyelectrolytes are significantly influenced by crucial interactions with the solvent, co-solvent and ion species. The corresponding experimental and simulation results reveal a significant deviation from theoretical predictions, which also highlights the importance of charge transfer, dispersion and polarization interactions in combination with solvation mechanisms. We discuss recent theoretical and computational findings in addition to novel approaches which help broaden the applicability of simple mean field theories.

Specific Ion Effects and the Law of Matching Solvent Affinities: A Conceptual Density Functional Theory Approach

We study the principles behind specific ion effects of alkali and halide ions in various protic and aprotic solvents by means of a conceptual density functional theory (DFT) approach. The results of our calculations are in good agreement with experimental data and underline the crucial role of frontier molecular orbital energies. Further analysis reveals that the electronegativities and chemical hardness values of the considered ion and solvent species provide a molecular rationale for specific ion effects and the law of matching water affinities. Based on the analytical expressions and DFT calculations, we show that solvent affinities and the occurrence of specific ion effects, among other molecular mechanisms and interactions, are mainly due to electronegativity differences between the ions and the surrounding solvent molecules.