Molecular Liquids Research Articles

Universal Evolution of Fickian Non-Gaussian Diffusion in Two- and Three-Dimensional Glass-Forming Liquids.

Recent works show that glass-forming liquids display Fickian non-Gaussian Diffusion, with non-Gaussian displacement distributions persisting even at very long times, when linearity in the mean square displacement (Fickianity) has already been attained. Such non-Gaussian deviations temporarily exhibit distinctive exponential tails, with a decay length λ growing in time as a power-law. We herein carefully examine data from four different glass-forming systems with isotropic interactions, both in two and three dimensions, namely, three numerical models of molecular liquids and one experimentally investigated colloidal suspension. Drawing on the identification of a proper time range for reliable exponential fits, we find that a scaling law λ(t)∝tα, with α≃1/3, holds for all considered systems, independently from dimensionality. We further show that, for each system, data at different temperatures/concentration can be collapsed onto a master-curve, identifying a characteristic time for the disappearance of exponential tails and the recovery of Gaussianity. We find that such characteristic time is always related through a power-law to the onset time of Fickianity. The present findings suggest that FnGD in glass-formers may be characterized by a "universal" evolution of the distribution tails, independent from system dimensionality, at least for liquids with isotropic potential.

Molecular mobility of liquid crystalline polyurethanes modified by polyhedral oligomeric silsesquioxanes

Molecular mobility and liquid crystalline order formation of liquid crystalline polyurethanes (LCPU) modified by trisilanolphenyl-POSS was investigated by using dielectric spectroscopy (DRS), differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Hybrid materials with 2, 4 and 6 wt% of POSS load were studied. Small-angle X-ray scattering (SAXS) results suggested that the nanoscale morphology was not significantly altered by the introduction of POSS into the LCPU matrix. The presence of liquid crystal phase transitions was confirmed by both DRS and DSC measurements. Dielectric study showed changes in relaxation steps and conductivity in the LC transitions region due to a shift in phase order. Using partially cross-linked POSS particles, an enhancement of the material's rubber modulus was observed in the DMA measurement. Deterioration in charge transfer was observed for the composites, which was attributed to the crosslinking nature of the silsesquioxane used. Further, dynamic calorimetric studies in different cooling rates were performed for detailed study of liquid crystalline transitions. In summary, it was found that the addition of POSS causes a nucleation effect and does not impair LC phase formation.

Liquid Structures and Diffusion Dynamics of Alkyl-Pyrene Liquids Studied by Molecular Dynamics Simulations.

Functional molecular liquids (FMLs) based on alkylated π-conjugated molecules have attracted attention as solvent-free and nonvolatile liquid materials with prominent optoelectronic features. Recently, novel FML compounds containing pyrene as the functional core were synthesized, and their rheological and photochemical properties were investigated. Although the molecules differ only in the number of alkyl chain substituents and their substitution positions, their viscosity coefficients are largely different beyond the Stokes-Einstein relation on the assumption of identical microscopic friction, indicating that local microscopic molecular interactions are crucial for the macroscopic rheological properties. Here, we report a theoretical study on the rheological properties of the alkyl-pyrene liquids by means of atomistic molecular dynamics (MD) simulations. We performed long-time MD simulations for tens of microseconds to obtain ample statistical samples of the alkyl-pyrene liquids and analyzed their liquid structures and diffusion dynamics based on spatiotemporal correlation functions. We found the formation of characteristic local liquid structures of π-π stacking of the pyrene moieties and locally anisotropic and anomalous diffusion dynamics, which remarkably vary depending on the alkyl substituent patterns. The present results provide an atomistic insight into the macroscopic rheological properties of alkyl-π FMLs and molecular design strategy for them.

Exploring the Microscopic Aspects of 1-Methyl-3-octylimidazolium Tetrafluoroborate Mixtures with Formamide, N-Methylformamide, and N,N-Dimethylformamide by Multiple Spectroscopic Techniques and Computations.

The microscopic aspects of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF4]) mixtures with formamide (FA), N-methylformamide (NMF), and N,N-dimethylformamide (DMF) were investigated using spectroscopic techniques of femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES), FT-IR, and NMR. Molecular dynamics simulations and quantum chemistry calculations were also performed. According to fs-RIKES, the first moment of the low-frequency spectrum bands mainly originating from the intermolecular vibrations in the [MOIm][BF4]/FA and [MOIm][BF4]/DMF systems changed gradually with the molecular liquid mole fraction XML but that in the [MOIm][BF4]/NMF system was constant up to XNMF = 0.7 and then gradually increased in the range of XNMF ≥ 0.7. Excluding the contribution of the 2D hydrogen-bonding network due to the presence of FA in the low-frequency spectrum band, the XML dependence of the normalized first moment of the low-frequency band in the [MOIm][BF4]/FA and [MOIm][BF4]/NMF systems revealed that the normalized first moment did not remarkably change in the range of XML < 0.7 but drastically increased in XML ≥ 0.7. FT-IR results indicated that the amide C═O band shifted to the low-frequency side with increasing XML for the three mixtures due to the hydrogen bonds. The imidazolium ring C-H band also showed a similar tendency to the amide C═O band. 19F NMR probed the microenvironment of [BF4]- in the mixtures. The [MOIm][BF4]/NMF and [MOIm][BF4]/DMF systems showed an up-field shift of the F atoms of the anion with increasing XML, and the [MOIm][BF4]/FA system exhibited a down-field shift. Steep changes in the chemical shifts were confirmed in the region of XML > 0.8. On the basis of the quantum chemistry calculations, the observed chemical shifts with increasing XML were mainly attributed to the many-body interactions of ions and amides for the [MOIm][BF4]/FA and [MOIm][BF4]/DMF systems. Meanwhile, the long distance between the cation and the anion was due to the high dielectric medium for the [MOIm][BF4]/NMF system, which led to an up-field shift.

Overall picture and details of diffusion dynamics for liquid benzene by quasielastic neutron scattering and mode distribution analysis

We investigated the diffusion dynamics of liquid benzene as a basic molecular liquid. Quasielastic neutron scattering (QENS) was utilized to elucidate the overall behavior of molecular diffusion in liquid benzene. Mode distribution analysis, which does not require specific model assumptions and can describe molecular dynamics by QENS as a distribution of Lorentz functions, revealed that the diffusion of benzene molecules is represented by three dynamic modes. The Q-dependencies of the scattering intensity and the relaxation time were analyzed using diffusion models constructed to match each dynamic mode. (1) The slowest mode is translational diffusion, described as continuous diffusion rather than jump diffusion. (2) The intermediate-speed mode is rotational diffusion, which is due to the three-dimensional reorientation of benzene molecules with a jump angle of approximately 90°. (3) The fastest mode involves local fluctuations, which may originate from translational oscillations and librations of the benzene crystal. The overall picture of the diffusion dynamics and characteristics of the individual modes were clarified without using a model, which is a new approach in the field of diffusion dynamics of molecular liquids.

Development and potential use of MWCNT suspended in vegetable oil as a cutting fluid in machining of Monel 400

The heat dissipation problem that arises when machining difficult to cut materials can be mitigated by using nano-cutting fluids, which have a much higher thermal conductivity value than the basic lubricants. Since the heat carrying capacity of the Minimum Quantity Lubrication (MQL) system is significantly lower than that of other coolants, a number of nano-additives are used to increase its cooling effectiveness. Therefore, this study firstly focuses on the assessment of nano-cutting fluids and theirsignificant efficacy in the machining operation of nickel-based alloys. Multi-walled carbon nanotube (MWCNT) suspended in vegetable oil is compared with distinct cooling environmental conditions namely, dry, MQL and cryogenic carbon dioxide (CO2). The novelty of this work is that the different volume fractions of 0.2 – 1 wt% of MWCNT were dispersed to study the contact angle, viscosity and thermal conductivity of the prepared molecular liquid and then, the potential of MWCNT suspended in vegetable oil is used to evaluate the outcomes of surface roughness (Ra), cutting temperature (CT) and tool wear (Vb) while machining Monel 400 alloy. The outcomes affirmed that employing nano MQL in machining operations resulted in a mean reduction of about 65% in the temperature evolved concerning no coolant condition. Additionally, the use of nanoparticles leads to an improved surface finish and abridged tool wear.

Two-dimensional infrared-Raman spectroscopy as a probe of water’s tetrahedrality

Two-dimensional spectroscopic techniques combining terahertz (THz), infrared (IR), and visible pulses offer a wealth of information about coupling among vibrational modes in molecular liquids, thus providing a promising probe of their local structure. However, the capabilities of these spectroscopies are still largely unexplored due to experimental limitations and inherently weak nonlinear signals. Here, through a combination of equilibrium-nonequilibrium molecular dynamics (MD) and a tailored spectrum decomposition scheme, we identify a relationship between the tetrahedral order of liquid water and its two-dimensional IR-IR-Raman (IIR) spectrum. The structure-spectrum relationship can explain the temperature dependence of the spectral features corresponding to the anharmonic coupling between low-frequency intermolecular and high-frequency intramolecular vibrational modes of water. In light of these results, we propose new experiments and discuss the implications for the study of tetrahedrality of liquid water.

Bridgman formula for the thermal conductivity of atomic and molecular liquids

A simple and popular Bridgman’s model predicts a linear correlation between the thermal conductivity coefficient and the sound velocity of dense liquids. A proportionality coefficient proposed originally is fixed and independent of the liquid molecular structure. This work reports a systematic analysis of correlations between thermal conductivity and sound velocity in simple model systems (hard sphere and Lennard-Jones fluids), monoatomic liquids (argon and krypton), diatomic liquids (nitrogen and oxygen), and several polyatomic liquids (water, carbon dioxide, methane, and ethane). It is demonstrated that linear correlations are well reproduced for model fluids as well as real monoatomic and diatomic liquids, but seem less convincing in polyatomic molecular liquids. The coefficient of proportionality is not fixed; it is about unity for monoatomic liquids and generally increases with molecular complexity. Some implications for the possibility to predict the thermal conductivity coefficients are discussed.

A bibliometric analysis for estimating the global research trends related to applications of ionic liquids in drug delivery

Many scientists are interested in ionic liquids (ILs) as they are efficient solvents, especially for drug delivery systems. A bibliometric analysis was performed in this article to assess the applicability of ILs in drug delivery. 519 publications from the Web of Science (WoS) database were analysed using VOSviewer and MS Excel. An analysis was conducted to identify publication trends, well-known authors, institutions, countries, sources, and the most cited articles in this field. This research comprised 1984 authors from 574 institutions and 52 countries.Regarding the total number of publications produced, Masahiro Goto, Muhammad Moniruzzaman, and Rie Wakabayashi were the most prolific authors. Based on the number of publications, the top three institutions were Kyushu University, Universiti Teknologi PETRONAS, and Shandong University. For the period from 2007 and 2021, the most productive countries were China, India, and the United States. In both the number of publications produced and the number of citations received, the Journal of Molecular Liquids has maintained its dominant position. This study demonstrated the continued use of ILs in drug delivery applications, indicating that more investigation might aid in advancing drug delivery systems.

Metallic glacial glass

A novel glassy substance known as metallic glacial glass has been recently discovered by liquid-to-liquid transition or glaciation of some metallic-glass forming liquids. Without changing the original composition and amorphous nature of the material, glaciation gives the glass intriguing properties, including high strength, high hardness, and improved thermal stability. The metallic glacial glass can be preserved at ambient temperature, above which sits the glass transition temperature, making it suitable for material applications that have not been possible with other glacial phases in other liquid systems. A brief history of the glacial phase in the triphenyl phosphite molecular liquid with similar thermodynamics and kinetics to metallic glacial glass is introduced, emphasizing the common questions faced. Different phase-transition pathways for supercooled liquids of principal crystallization, primary crystallization, quasicrystallization, short-range ordering, phase separation, mesophase formation, and glaciation are compared, highlighting the large enthalpy change of glaciation enabling a new landscape of the glassy state. Requirements for identifying glaciation out of other possibilities are specified. Future research directions regarding both scientific and practical needs are proposed. The review concludes with a roadmap that may lead to more compositions of metallic glacial glasses.

Application of the reference interaction site model self-consistent field method based on the Dirac-Hartree–Fock wave function to a chemical reaction

The reference interaction site model self-consistent field (RISM-SCF) method is a combined method of the electronic structure theory of molecules and the integral equation theory of molecular liquids. The RISM-SCF method based on the Dirac-Hartree-Fock wave function, recently proposed, is applied to a chemical reaction, specifically, a Menshutkin reaction in aqueous solution. The Helmholtz energy profile along the reaction coordinate is calculated and the characteristics of the reaction are discussed based on energy component analysis.

Johari–Goldstein β relaxation in glassy dynamics originates from two-scale energy landscape

Supercooled liquids undergo complicated structural relaxation processes, which have been a long-standing problem in both experimental and theoretical aspects of condensed matter physics. In particular, past experiments widely observed for many types of molecular liquids that relaxation dynamics separated into two distinct processes at low temperatures. One of the possible interpretations is that this separation originates from the two-scale hierarchical topography of the potential energy landscape; however, it has never been verified. Molecular dynamics simulations are a promising approach to tackle this issue, but we must overcome laborious difficulties. First, we must handle a model of molecular liquids that is computationally demanding compared to simple spherical models, which have been intensively studied but show only a slower process: α relaxation. Second, we must reach a sufficiently low-temperature regime where the two processes become well-separated. Here, we handle an asymmetric dimer system that exhibits a faster process: Johari-Goldstein β relaxation. Then, we employ the parallel tempering method to access the low-temperature regime. These laborious efforts enable us to investigate the potential energy landscape in detail and unveil the first direct evidence of the topographic hierarchy that induces the β relaxation. We also successfully characterize the microscopic motions of particles during each relaxation process. Finally, we study the correlation between low-frequency modes and two relaxation processes. Our results establish a fundamental and comprehensive understanding of experimentally observed relaxation dynamics in supercooled liquids.

Using Machine Learning To Predict Partition Coefficient (Log P) and Distribution Coefficient (Log D) with Molecular Descriptors and Liquid Chromatography Retention Time.

During preclinical evaluations of drug candidates, several physicochemical (p-chem) properties are measured and employed as metrics to estimate drug efficacy in vivo. Two such p-chem properties are the octanol-water partition coefficient, Log P, and distribution coefficient, Log D, which are useful in estimating the distribution of drugs within the body. Log P and Log D are traditionally measured using the shake-flask method and high-performance liquid chromatography. However, it is challenging to measure these properties for species that are very hydrophobic (or hydrophilic) owing to the very low equilibrium concentrations partitioned into octanol (or aqueous) phases. Moreover, the shake-flask method is relatively time-consuming and can require multistep dilutions as the range of analyte concentrations can differ by several orders of magnitude. Here, we circumvent these limitations by using machine learning (ML) to correlate Log P and Log D with liquid chromatography (LC) retention time (RT). Predictive models based on four ML algorithms, which used molecular descriptors and LC RTs as features, were extensively tested and compared. The inclusion of RT as an additional descriptor improves model performance (MAE = 0.366 and R2 = 0.89), and Shapley additive explanations analysis indicates that RT has the highest impact on model accuracy.

Quest for a Rational Molecular Design of Alkyl-Distyrylbenzene Liquid by Substitution Pattern Modulation.

Alkyl-π functional molecular liquids (FMLs) are of interest for fabricating soft electronic devices due to their fluidic nature and innate optoelectronic functions from the π-conjugated moiety. However, predictable development of alkyl-π FMLs with the desired liquid and optoelectronic properties is challenging. A series of alkyl-distyrylbenzene (DSB) liquids was studied in terms of the substituent position effect by attaching 2-octyldodecyl chains at (2,4-), (2,5-), (2,6-), and (3,5-). The effect of the alkyl chain length was investigated by attaching 2-hexyldecyl, 2-decyltetradecyl (C10 C14 ), and 2-dodecylhexadecyl at the (2,5-) substituent position. The 2,5-C10 C14 substituent pattern constructed a superior alkyl-DSB liquid with a lower viscosity, intrinsic optical properties, and high thermal- and photo-stabilities. The discovered 2,5-C10 C14 was applied to dicyanostyrylbenzene and comparable liquid physical and optical superiorities were confirmed. This molecular design is useful for creating alkyl-π FMLs with the aforementioned advantages, which are applicable for deformable and flowable optoelectronic devices.

Quantification and Distribution of Three Types of Hydrogen Bonds in Mixtures of an Ionic Liquid with the Hydrogen-Bond-Accepting Molecular Solvent DMSO Explored by Neutron Diffraction and Molecular Dynamics Simulations.

The concept of hydrogen bonding is celebrating its 100th birthday. Hydrogen bonds (H-bonds) play a key role in the structure and function of biological molecules, the strength of materials, and molecular binding. Herein, we study H-bonding in mixtures of a hydroxyl-functionalized ionic liquid with the neutral, H-bond-accepting molecular liquid dimethylsulfoxide (DMSO) using neutron diffraction experiments and molecular dynamics simulations. We report the geometry, strength, and distribution of three different types of H-bond OH···O, formed between the hydroxyl group of the cation and either the oxygen atom of another cation, the counteranion, or the neutral molecule. Such a variety of different strengths and distributions of H-bonds in one single mixture could hold the promise of providing solvents with potential applications in H-bond-related chemistry, for example, to alter the natural selectivity patterns of catalytic reactions or the conformation of catalysts.

Combining the Tait equation with the phonon theory allows predicting the density of liquids up to the Gigapascal range

Predicting the density of liquids at ultrahigh pressures in the case when only the data measured at ambient pressure are available is a long-standing challenge for thermodynamic research. In this work, we archived this goal for molecular liquids by applying the half-sum of the Tait equation and the Murnagnan equation in the form coordinated with Tait’s at low pressure for predicting the density of molecular liquids up to the pressures more than 1 GPa with uncertainty comparable with the experimental one. It is shown that the control parameter, which is needed in addition to the initial density and the isothermal compressibility can be found using the speed of sound and the density at ambient pressure and has a clear physical interpretation in terms of the characteristic frequency of intermolecular oscillation mimicking the limiting frequency of Debye’s theory of heat conductivity of solids. This fact is discussed as arguing in favour of the modern phonon theory of liquid thermodynamics and expands it range of applicability to the volumetric properties of liquids at temperatures far below the critical one. The validity of the model is illustrated with the case study of classic Bridgman’s dataset as well as with some examples of ultrahigh-pressure data obtained by the diamond anvil cell and shock wave compression methods.

Lubricity of chelated orthoborate-phosphonium ionic liquids on tetrahedral amorphous carbon and steel surfaces

In boundary lubrication, formation of tribofilms and subsequent friction and wear properties are strongly influenced by lubricant affinity to the lubricating surfaces. Ionic liquids (ILs), which are composed of ions, are expected to provide enhanced surface affinity compared to molecular liquids. This is also of interest for non-ferrous surfaces such as diamond-like carbon (DLC) coatings. DLC technology is widely utilised for sliding components, but DLC performance with conventional lubricants does not always fulfil the requirements. In this study, we compare lubricating performance of two non–halogenated ILs with a common cation, trihexyl(tetradecyl)phosphonium, but different anions, bis(oxalato)borate and bis(mandelato)borate, in the steel/steel and steel/DLC contacts. The tribofilms formed and their lubrication mechanism are investigated using a wide range of surface analysis techniques: Atomic Force Microscopy (AFM), nano-indentation, SEM-EDX and X-ray Photoelectron Spectroscopy (XPS). It is demonstrated that both ILs exhibited better lubricating performance than PAO oils of similar viscosity. It is also shown that the lubricating performance is controlled by the chemical stability of ILs, surface hardness and topography. The phosphonium bis(oxalato)borate IL exhibited excellent lubricity but its performance was adversely affected by the rough and very hard DLC surface that disrupted a delicate balance between the rates of tribofilm formation and removal. A higher chemical stability of the phosphonium bis(mandelato)borate IL facilitated formation of a non-sacrificial ionic tribofilm that reduced friction to a lower extent than the phosphonium bis(oxalato)borate IL but without accelerated tribochemical wear. The results demonstrate the potential for controlling lubrication properties by tuning functional groups of anion structure and subsequent chemical reactivity of ILs with lubricated surfaces.

Control of Phase Separation and Crystallization for High‐Efficiency and Mechanically Deformable Organic Solar Cells

Stretchable organic solar cells (OSCs) have great potential as power sources for the next‐generation wearable electronics. Although blending rigid photovoltaic components with soft insulating materials can easily endow the mechanical ductility of active layers, the photovoltaic efficiencies usually drops in the resulting OSCs. Herein, a high photovoltaic efficiency of 15.03% and a large crack‐onset strain of 15.70% is simultaneously achieved based on a ternary blend consisting of polymer donor poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐fluoro)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c']dithiophene‐4,8‐dione)] (PM6), non‐fullerene accepter 2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐ethylhexyl)‐3,9‐diundecyl‐12,13‐dihydro‐[1,2,5]thiadiazolo[3,4‐e]thieno[2′′,3′′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2‐g]thieno[2′,3′:4,5]thieno[3,2‐b]indole‐2,10‐diyl)bis (methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (Y6), and soft elastomer polystyrene‐block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene (SEBS) through the control of phase separation and crystallization. By employing a high‐boiling point solvent additive 1‐chloronaphthalene (CN) with different solubilities for PM6 and Y6, the aggregation dynamics of PM6 and Y6 as well as the film solidification process are dramatically altered, allowing for the different molecular rearrangement and liquid–liquid phase separation evolution. Consequently, the ternary film with optimal CN content presents decreased SEBS domains and moderately improved molecular ordering of PM6 and Y6, enabling effective mechanical deformation and charge generation/transport. The revealed corrections between the film‐formation process, film microstructure, and photovoltaic/mechanical characteristics in the ternary blend provide deep understanding of the morphology control toward high‐performance stretchable OSCs.

Analyzing ionic liquid systems using real-time electron microscopy and a computational framework combining deep learning and classic computer vision techniques

Electron microscopy (EM) is one of the most important methods for characterizing various systems, and it is traditionally applied to static solid structures. Remarkable recent developments have opened multiple possibilities for in situ observation of different phenomena, including liquid phase processes. In contrast to routine solid-state EM measurements with static images, electron microscopy in liquids often deals with ubiquitous dynamics, which can be recorded as video streams. Providing much information about the sample, real-time EM increases the complexity of data analysis, challenging researchers to develop new, highly efficient systems for data processing. The present work proposes a framework for data analysis in real-time electron microscopy. Multiple algorithm choices are compared, and efficient solutions are described. Using the best algorithm, combining classical computer vision methods and deep learning-based denoising, the unique anisotropic effect of the electron beam in microstructured ionic liquid-based systems was discovered. The developed method provides an efficient approach for studying the structure and transformation of soft micro-scale domains in molecular liquids. The corresponding software was made publicly available, and detailed instructions to reapply it to other problems were provided.

Medulloblastoma: From TP53 Mutations to Molecular Classification and Liquid Biopsy.

A recent paradigm shift in the diagnostics of medulloblastoma allowed the distinction of four major groups defined by genetic data rather than histology. This new molecular classification correlates better with prognosis and will allow for the better clinical management of therapies targeting druggable mutations, but also offer a new combination of monitoring tumor development in real-time and treatment response by sequential liquid biopsy. This review highlights recent developments after a century of milestones in neurosurgery and radio- and chemotherapy, but also controversial theories on the cell of origin, animal models, and the use of liquid biopsy.