Physics-Informed Multifidelity Gaussian Process: Modeling the Effect of Water and Temperature on the Viscosity of a Deep Eutectic Solvent

Knowledge of shear viscosity as function of temperature and composition of an aqueous deep eutectic solvent mixture is essential for process design but can be highly challenging and costly to measure. The present work proposes to combine a small set of experimentally determined viscosities with a small set of simulated values within a linear multifidelity approach to predict the dependency of shear viscosity on temperature and composition. This method provides a simple approach that requires a physics-based transformation of viscosity data prior to training, without the need for additional data such as densities. This allows reduction in cost with experiments and reduces the number of experiments and simulations required to characterize a specific system. The data-driven component of the model does not concern the viscosity itself but rather the excess free energy term within the framework of a mixture viscosity model according to Eyring's absolute rate theory. Moreover, we illustrate the application of kernel-based machine learning approaches to daily research questions where data availability is limited compared to the data set size typically required for neural networks.

Effects of Hydrogen Bond Networks on Viscosity in Aqueous Solutions

In aqueous solutions, the impact of ions on hydrogen bond networks plays a crucial role in transport properties. We used molecular dynamics simulations to explain how ions affect viscosity through structural changes. We developed a quantitative model to describe the effect of ions on viscosity. The model comprises two parts: the addition of ions alters hydrogen bond networks, and changes in hydrogen bond networks exponentially lead to changes in viscosity. The influence of ions on hydrogen bond networks involves the following mechanisms: first, ions can disrupt the tetrahedral structures within the first solvation shell into three-coordinated structures through substitution; second, structural changes within the first shells affect the global hydrogen bond network through electrostatic forces and the hindrance of ionic volumes. By analyzing the mechanisms of how hydrogen bond networks determine viscosity through the decomposition of viscosity, we found that the proportion of potential viscosity in aqueous solutions primarily increases due to the enhancement of non-hydrogen bonding interactions, and the proportion of hydrogen bonding viscosity decreases accordingly. Our results demonstrate that hydrogen bond networks are crucial for describing the changes in transport phenomena affected by external factors.

Viscosity of Ionic Liquids: Theories and Models

Ionic liquids (ILs) offer a wide range of promising applications due to their unique and designable properties compared to conventional solvents. Further development and application of ILs require correlating/predicting their pressure-viscosity-temperature behavior. In this review, we firstly introduce methods for calculation of thermodynamic inputs of viscosity models. Next, we introduce theories, theoretical and semi-empirical models coupling various theories with EoSs or activity coefficient models, and empirical and phenomenological models for viscosity of pure ILs and IL-related mixtures. Our modelling description is followed immediately by model application and performance. Then, we propose simple predictive equations for viscosity of IL-related mixtures and systematically compare performances of the above-mentioned theories and models. In concluding remarks, we recommend robust predictive models for viscosity at atmospheric pressure as well as proper and consistent theories and models for P-η-T behavior. The work that still remains to be done to obtain the desired theories and models for viscosity of ILs and IL-related mixtures is also presented. The present review is structured from pure ILs to IL-related mixtures and aims to summarize and quantitatively discuss the recent advances in theoretical and empirical modelling of viscosity of ILs and IL-related mixtures.

Ionic liquid mixtures as energy storage materials: a preliminary and comparative study based on thermal storage density

Fifteen equimolar binary mixtures are synthesized and thermophysically evaluated in this study. These mixtures are derived from six ionic liquids (ILs) based on methylimidazolium and 2,3-dimethylimidazolium cations with butyl chains. The objective is to compare and elucidate the impact of small structural changes on the thermal properties. The preliminary results are compared to previously obtained results with mixtures containing longer eight-carbon chains. The study demonstrates that certain mixtures exhibit an increase in their heat capacity. Additionally, due to their higher densities, these mixtures achieve a thermal storage density equivalent to that of mixtures with longer chains. Moreover, their thermal storage density surpasses that of some conventional materials commonly used for energy storage.

Collectivity in ionic liquids: a temperature dependent, polarizable molecular dynamics study

We use polarizable molecular dynamics simulations to study the thermal dependence of both structural and dynamic properties of two ionic liquids sharing the same cation (1-ethyl-3-methylimidazolium). The linear temperature trend in the structure is accompanied by an exponential Arrhenius-like behavior of the dynamics. Our parameter-free Voronoi tessellation analysis directly casts doubt on common concepts such as the alternating shells of cations and anions and the ionicity. The latter tries to explain the physico-chemical properties of the ionic liquids based on the association and dissociation of an ion pair. However, cations are in the majority of both ion cages, around cations and around anions. There is no preference of a cation for a single anion. Collectivity is a key factor in the dynamic properties of ionic liquids. Consequently, collective rotation relaxes faster than single-particle rotations, and the activation energies for collective translation and rotation are lower than those of the single molecules.

Pressing matter: why are ionic liquids so viscous?

Room temperature ionic liquids are considered to have huge potential for practical applications such as batteries. However, their high viscosity presents a significant challenge to their use changing from niche to ubiquitous. The modelling and prediction of viscosity in ionic liquids is the subject of an ongoing debate involving two competing hypotheses: molecular and local mechanisms versus collective and long-range mechanisms. To distinguish between these two theories, we compared an ionic liquid with its uncharged, isoelectronic, isostructural molecular mimic. We measured the viscosity of the molecular mimic at high pressure to emulate the high densities in ionic liquids, which result from the Coulomb interactions in the latter. We were thus able to reveal that the relative contributions of coulombic compaction and the charge network interactions are of similar magnitude. We therefore suggest that the optimisation of the viscosity in room temperature ionic liquids must follow a dual approach.

Perspectives in the Computational Modeling of New Generation, Biocompatible Ionic Liquids

In this Perspective, I review the current state of computational simulations on ionic liquids with an emphasis on the recent biocompatible variants. These materials are used here as an example of relatively complex systems that highlights the limits of some of the approaches commonly used to study their structure and dynamics. The source of these limits consists of the coexistence of nontrivial electrostatic, many-body quantum effects, strong hydrogen bonds, and chemical processes affecting the mutual protonation state of the constituent molecular ions. I also provide examples on how it is possible to overcome these problems using suitable simulation paradigms and recently improved techniques that, I expect, will be gradually introduced in the state-of-the-art of computational simulations of ionic liquids.

Textural control of ionosilicas by Ionic Liquid templating

Due to their unique self-assembly properties, ionic liquids (ILs) are versatile soft templates for the formation of mesoporous materials. Here, we report the use of ionic liquids as soft templates...

A global transform for the general formulation of liquid viscosities with significant linearizing benefits: a case study on ionic liquid mixtures

Among the different thermophysical properties of significant importance in the study of transport phenomena, viscosity has long defied attempts at presenting a unified model applicable to both pure compounds and binary mixtures within extended temperature and composition ranges. Similar to the role that ln(P^sat.)-T^-1 profiles play in linearizing vapor pressure data, here it is contended for the first time that it is highly beneficial to study viscosity in the space, as opposed to the commonly used logarithmic space. Within this framework, the groundbreaking Lewis-Squires model is modified and extended to mixtures by studying 184 binary systems with 89 distinct ILs and 17 polar and nonpolar solutes. The dataset covers a total of 1104 isotherms with 10 909 data points, having quite an extensive range of viscosities, from 0.447 to 113 733 (mPa s). Breaking a highly regarded convention upheld for decades, the proposed model does not stipulate the availability of pure state viscosity and/or volumetric data over the whole temperature range, which acts as a bottleneck for the current literature models that have all been cast in the same mold inspired by thermodynamic mixing rules. In practice, without utilizing any volumetric properties, a single pure solvent viscosity datum at any reference temperature is the only requirement in the interpolative mode, which is complemented by the binary viscosity data at the reference isotherm of choice in the extrapolative mode of the proposed model that is concurrently applicable to both pure components and binary mixtures. An AARD of 2.82% was obtained in estimating the binary viscosity data using the new algorithm, while deviations of 13.58%, 6.79%, 6.17%, and 3.41% were obtained for the Grunberg and Nissan, Fang and He, Jouyban-Acree, and Eyring-MTSM models, respectively. Moreover, for extrapolation of the binary data measured at room temperature, a capability exclusive to the proposed model, an overall AARD of 5.66% was obtained for the mixtures of interest. With significant inherent flexibility, the new methodology could also be employed to represent the uncommon composition dependence of certain IL systems exhibiting multiple local extrema, with AARDs close to half of those of the current models.

Thermodynamic, transport, and structural properties of hydrophobic deep eutectic solvents composed of tetraalkylammonium chloride and decanoic acid

With the emergence of hydrophobic deep eutectic solvents (DESs), the scope of applications of DESs has been expanded to include situations in which miscibility with water is undesirable. Whereas most studies have focused on the applications of hydrophobic DESs from a practical standpoint, few theoretical works exist that investigate the structural and thermodynamic properties at the nanoscale. In this study, Molecular Dynamics (MD) simulations have been performed to model DESs composed of tetraalkylammonium chloride hydrogen bond acceptor and decanoic acid hydrogen bond donor (HBD) at a molar ratio of 1:2, with three different cation chain lengths (4, 7, and 8). After fine-tuning force field parameters, densities, viscosities, self-diffusivities, and ionic conductivities of the DESs were computed over a wide temperature range. The liquid structure was examined using radial distribution functions (RDFs) and hydrogen bond analysis. The MD simulations reproduced the experimental density and viscosity data from the literature reasonably well and were used to predict diffusivities and ionic conductivities, for which experimental data are scarce or unavailable. It was found that although an increase in the cation chain length considerably affected the density and transport properties of the DESs (i.e., yielding smaller densities and slower dynamics), no significant influence was observed on the RDFs and the hydrogen bonds. The self-diffusivities showed the following order for the mobility of the various components: HBD > anion > cation. Strong hydrogen bonds between the hydroxyl and carbonyl groups of decanoic acid and between the hydroxyl group of decanoic acid and chloride were observed to dominate the intermolecular interactions.