Bulk Liquid Properties Research Articles

Bursting of Underwater Oil Drops.

For decades, two main facets of underwater oil spills have been explored extensively-the rise of oil drops and resulting evolution of the oil slick at the air-water interface. We report on the bursting of rising oil drops at an air-liquid interface which precedes slick formation and reveal a counterintuitive bulge reversal that releases a daughter oil droplet inside the bulk as opposed to upward-shooting jets observed in bursting air bubbles. By unraveling the underlying physics we show that daughter droplet size and bulk liquid properties are correlated and their formation can be suppressed by an increase in the bulk viscosity.

Daughter oil droplet entrainment by oil-coated bubble bursting

Compound bubbles with a liquid coating in another continuous immiscible bulk phase are ubiquitous in a wide range of natural and industrial processes. Their formation, rise and ultimate bursting at the air–liquid interface play crucial roles in the transport and fate of natural organic matter and contaminants. However, the dynamics of compound bubbles has not received considerable attention until recently. Here, inspired by our previous work (Yang et al., Nat. Phys., vol. 19, 2023, pp. 884–890), we investigate the entrainment of daughter oil droplets in bulk water produced by a bursting oil-coated bubble. We document that the size of the entrained daughter oil droplet is affected by the oil coating fraction and the bulk liquid properties. We rationalize this observation by balancing the viscous force exerted by the extensional flow produced by bubble bursting with the capillary force resisting the deformation of the oil coating, and considering the subsequent end-pinching process which finally entrains the daughter oil droplets. We propose a scaling analysis for the daughter oil droplet size that well captures the experimental results for a wide range of oil coating fractions and Ohnesorge numbers of the bulk liquid. In addition, we discuss the non-monotonic variation of daughter droplet size with the Ohnesorge number, and show the eventual absence of daughter droplets because of the strong viscous effect in the high-Ohnesorge-number regime. Our findings may advance the fundamental understanding of compound bubble bursting and provide guidance and modelling constraints for bubble-mediated contaminant transport in liquids.

A Second Glass Transition Observed in Single-Component Homogeneous Liquids Due to Intramolecular Vitrification.

On supercooling a liquid, the viscosity rises rapidly until at the glass transition it vitrifies into an amorphous solid accompanied by a steep drop in the heat capacity. Therefore, a pure homogeneous liquid is not expected to display more than one glass transition. Here we show that a family of single-component homogeneous molecular liquids, titanium tetraalkoxides, exhibit two calorimetric glass transitions of comparable magnitude, one of which is the conventional glass transition associated with dynamic arrest of the bulk liquid properties, while the other is associated with the freezing out of intramolecular degrees of freedom. Such intramolecular vitrification is likely to be found in molecules in which low-frequency terahertz intramolecular motion is coupled to the surrounding liquid. These results imply that intramolecular barrier-crossing processes, typically associated with chemical reactivity, do not necessarily follow the Arrhenius law but may freeze out at a finite temperature.

Persistent Free Radicals in Petroleum

The persistent free radical content in petroleum is of the order 1018 spins/g (1 μmol/g), with higher and lower values found depending on origin and in different distillation fractions. The field of persistent free radicals in petroleum was reviewed with the aim of addressing and explaining apparent inconsistencies between free radical persistence and reactivity. The macroscopic average free radical concentration in petroleum is persistent over geological time, but individual free radical species in petroleum are short-lived and reactive. The persistent free radical concentration in petroleum can be explained in terms of a dynamic reaction equilibrium of free radical dissociation and association that causes a finite number of species at any given time to be present as free radicals. Evidence to support this description are observed changes in free radical concentration related to change in Gibbs free energy when the bulk liquid properties are changed and responsiveness of free radical concentration to dynamic changes in temperature. Cage effects, solvent effects, steric protection, and radical stabilization affect free radical reaction rate but do not explain the persistent free radical concentration in petroleum. The difference between persistent free radicals in straight-run petroleum and converted petroleum is that straight-run petroleum is an equilibrated mixture, but converted petroleum is not at equilibrium and the free radical concentration can change over time. Based on the limited data available, free radicals in straight-run petroleum appear to be part of the compositional continuum proposed by Altgelt and Boduszynski. Persistent free radical species are partitioned during solvent classification of whole oil, with the asphaltenes (n-alkane insoluble) fraction having a higher concentration of persistent free radicals than maltenes (n-alkane soluble) fraction. Attempts to relate persistent free radical concentration to petroleum composition were inconclusive.

A Neural-Network-Optimized Hydrogen Peroxide Pairwise Additive Model for Classical Simulations.

We have developed an all-atom pairwise additive model for hydrogen peroxide using an optimization procedure based on artificial neural networks (ANNs). The model is based on experimental molecular geometry and includes a dihedral potential that hinders the cis-type configuration and allows for crossing the trans one, defined between the planes that have the two oxygen atoms and each hydrogen. The model's parametrization is achieved by training simple ANNs to minimize a target function that measures the differences between various thermodynamic and transport properties and the corresponding experimental values. Finally, we evaluated a range of properties for the optimized model and its mixtures with SPC/E water, including bulk-liquid properties (density, thermal expansion coefficient, adiabatic compressibility, etc.) and properties of systems at equilibrium (vapor and liquid density, vapor pressure and composition, surface tension, etc.). Overall, we obtained good agreement with experimental data.

Nanoscopic study on carvone-terpene based natural deep eutectic solvents.

Terpene-based natural deep eutectic solvents (NADES) formed by using carvone as the hydrogen bond acceptor and a series of organic acids including tartaric, succinic, malic, and lactic acids as hydrogen bond donors are studied using a combination of molecular simulation methods. Density functional theory was used to study small molecular clusters and the topological characterization of the intermolecular forces using the atoms-in-a-molecule approach. Close-range interactions between the optimized carvone bases eutectic solvents between carbon dioxide have been studied for potential utilization of these solvents for gas capture purposes. Furthermore, COSMO-RS calculations have been carried out for the carbon dioxide solubilization performance of NADES compounds and to obtain s-profiles to infer the polarity and H-bond forming ability of the studied solvents. On the other hand, molecular dynamics simulations were carried out to analyze the bulk liquid properties and their relationship with relevant macroscopic properties (e.g., density or thermal expansion). Last but not least, relevant toxicity properties of the studied systems were predicted and reported in this work. The reported results provide the characterization of environmentally friendly NADES and show the suitability of carvone for advanced applications as carbon dioxide solubilizers.

Theoretical insights into the cineole-based deep eutectic solvents.

Deep eutectic solvents based on cineole as hydrogen bond acceptors and organic acids (succinic, malic, and lactic) as hydrogen bond donors are studied using a theoretical approach. The nature, strength, and extension of hydrogen bonding are analyzed, thus quantifying this prevailing interaction and its role in the fluid properties. Density functional theory was used to study small molecular clusters, and the topological characterization of the intermolecular forces was carried out using atoms in a molecule theory. Classical molecular dynamics simulations were considered to study nanoscopic bulk liquid properties and their relationship with relevant macroscopic properties such as density or thermal expansion. The reported results provide the characterization of environmentally friendly deep eutectic solvents and show the suitability of cineole for developing these sustainable materials.

Ultrafast Dynamics in Nonaromatic Cation Based Ionic Liquids: A Femtosecond Raman-Induced Kerr Effect Spectroscopic Study

Abstract Herein, the data of the intermolecular vibrations of forty nonaromatic cation based ionic liquids (ILs) at 293 K measured by femtosecond Raman-induced Kerr effect spectroscopy are reported. The low-frequency spectra in the frequency range of 0.3–700 cm−1 were obtained by Fourier transform deconvolution analysis. The line shapes of the low-frequency spectra below ∼200 cm−1 were discussed on the basis of the ion species. The spectral intensity in nonaromatic cation based ILs was much lower than that in aromatic cation based ILs owing to the absence of the aromatic ring, i.e., the libration of the aromatic species had a strong spectral intensity in the low-frequency region. However, nonaromatic cation based ILs with a flat anion, such as dicyanamide and tricyanomethanide, showed stronger spectral intensity because of the libration of the anion. Other unique spectral features were also discussed in the context of the structure of the ion species. Liquid properties, such as density, viscosity, electrical conductivity, and surface tension, were also estimated. On comparing the low-frequency spectra with the bulk liquid properties of the nonaromatic cation based ILs, a mild linear relationship between the first moment of the low-frequency spectrum and a bulk parameter comprised of surface tension and density was observed.

Update of the CHARMM36 United Atom Chain Model for Hydrocarbons and Phospholipids.

Accurate lipid force field (FF) parameters used in molecular dynamics (MD) simulations are crucial for understanding the properties of lipid-containing systems and biological processes related to lipids. The last update of the CHARMM36 united atom chain model (C36UA) was in 2013 [Lee, S. J. Phys. Chem. B 2014, 118, 547 556]; it utilized CHARMM36 (C36) lipid FF parameters for headgroups and OPLS-UA Lennard-Jones (LJ) parameters for tails. Simulations with the FF were able to reproduce many experimental observables of lipid bilayers accurately, but to be more applicable for a wide range of lipids, additional FF parameter optimization was needed. In this work, we present an update of the model, named C36UAr. The parameterization included the LJ parameters for hydrocarbons and related dihedrals. Bulk liquid properties (density, heat of vaporization, isothermal compressibility, and diffusion constant) of model compounds were used as targets for the LJ parameter fitting, and dihedrals were fit to either quantum mechanical (QM) or potential of mean force (PMF) calculations using C36. Thermodynamic reweighting was used to further improve the parameters. Bilayer simulations of various lipid headgroups (phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol) and tails (saturated, monounsaturated, and polyunsaturated) were performed to validate the model, and significant improvements were seen in bilayer properties, including surface area, membrane thicknesses, NMR deuterium order parameters, and density profiles. C36UAr was also compared to the hydrogen mass repartitioning (HMR) method. The high accuracy and competitive efficiency shown in this study make C36UAr one of the best choices for studies of membrane structure and membrane-associated proteins.

The structure of turbulence and mixed-phase cloud microphysics in a highly supercooled altocumulus cloud

Abstract. Observations of vertically resolved turbulence and cloud microphysics in a mixed-phase altocumulus cloud are presented using in situ measurements from an instrumented aircraft. The turbulence spectrum is observed to have an increasingly negative skewness with distance below cloud top, suggesting that long-wave radiative cooling from the liquid cloud layer is an important source of turbulence kinetic energy. Turbulence measurements are presented from both the liquid cloud layer and ice virga below. Vertical profiles of both bulk and microphysical liquid and ice cloud properties indicate that ice is produced within the liquid layer cloud at a temperature of −30 ∘C. These high-resolution in situ measurements support previous remotely sensed observations from both ground-based and space-borne instruments and could be used to evaluate numerical model simulations of altocumulus clouds at spatial scales from eddy-resolving models to global numerical weather prediction models and climate simulations.

Bubble growth inside an evaporating liquid droplet introduced in an immiscible superheated liquid

A numerical bubble growth model for two phase droplets in an immiscible superheated liquid is developed by solving the mass and energy conservation equations. The model is applicable for concentric two-phase droplets (where the bubble is completely surrounded by the volatile liquid) like those seen during evaporation of refrigerants suspended in water. The effect of pressure driven growth in the initial phases of bubble growth is shown to be minimal and is therefore ignored to simplify the model. While previous works assumed that the droplet is initially uniformly superheated, the current model assumes a more accurate temperature profile obtained by solving the radial heat diffusion equation in the droplet and surrounding liquid. The model is validated by measuring bubble growth rate within an FC-72 droplet introduced in a water bath and it is seen that the results are in close agreement with the model predictions. Experimental results from various cases in literature are also found to be well represented by the model. The effect of the liquid superheat and initial bubble diameter on bubble growth rate is also studied. Finally, a parametric study on the effect of droplet and bulk liquid properties on bubble growth rate is conducted to highlight the relative importance of the thermal conductivity and thermal mass ratios of the two liquids.

Precise correlation of propylene-propane system and its analysis of relative volatility

In this research, a prediction method for the vapor-liquid equilibrium compositions of the propylene-propane mixture by the combined use of the COSMO-RS method and the Peng-Robinson equation of state is examined. Employing the van der Waals (VdW) mixing rule, the average deviations from this method for both the pressures and vapor phase compositions are no more than 0.62% and 0.75%, respectively. The average deviations for the relative volatility and tray efficiency are 0.93% and 5.52%, respectively. This method successfully describes the relative volatility of this system is not only related to the temperature but also dependent on the propylene mole fraction at low temperatures and high propylene concentrations. The density, excess molar volume and surface tension data are listed. The calculations explain the irregular phenomenon of the relative volatility. The results show that the non-ideality of this system comes from the dispersions between the components dominates the bulk liquid properties at low temperatures and high propylene concentrations.

Structural analysis of zwitterionic liquids vs. homologous ionic liquids.

Zwitterionic liquids (Zw-ILs) have been developed that are homologous to monovalent ionic liquids (ILs) and show great promise for controlled dissolution of cellulosic biomass. Using both high energy X-ray scattering and atomistic molecular simulations, this article compares the bulk liquid structural properties for novel Zw-ILs with their homologous ILs. It is shown that the significant localization of the charges on Zw-ILs leads to charge ordering similar to that observed for conventional ionic liquids with monovalent anions and cations. A low-intensity first sharp diffraction peak in the liquid structure factor S(q) is observed for both the Zw-IL and the IL. This is unexpected since both the Zw-IL and IL have a 2-(2-methoxyethoxy)ethyl (diether) functional group on the cationic imidazolium ring and ether functional groups are known to suppress this peak. Detailed analyses show that this intermediate range order in the liquid structure arises for slightly different reasons in the Zw-IL vs. the IL. For the Zw-IL, the ether tails in the liquid are shown to aggregate into nanoscale domains.

Martini straight: Boosting performance using a shorter cutoff and GPUs

In molecular dynamics simulations, sufficient sampling is of key importance and a continuous challenge in the field. The coarse grain Martini force field has been widely used to enhance sampling. In its original implementation, this force field applied a shifted Lennard-Jones potential for the non-bonded van der Waals interactions, to avoid problems related to a relatively short cutoff. Here we investigate the use of a straight cutoff Lennard-Jones potential with potential modifiers. Together with a Verlet neighbor search algorithm, the modified potential allows the use of GPUs to accelerate the computations in Gromacs. We find that this alternative potential has little influence on most of the properties studied, including partitioning free energies, bulk liquid properties and bilayer properties. At the same time, energy conservation is kept within reasonable bounds. We conclude that the newly proposed straight cutoff approach is a viable alternative to the standard shifted potentials used in Martini, offering significant speedup even in the absence of GPUs.

The Role of Adsorbed Ions during Electrocatalysis in Protic Ionic Liquids

Due to their high proton conductivities and thermal and chemical stabilities, the applications of protic ionic liquids (PILs) have increased significantly in recent years. A particularly promising application of PILs is in the development of protic ionic liquid electrolyte fuel cells (PILFCs). Unlike the polymer electrolytes used in conventional proton exchange membrane fuel cells (PEMFCs), PILs can conduct protons at elevated temperatures and without deliberate humidification, meaning that water management in PILFCs is simpler than in PEMFCs. In addition, by operating PILFCs at elevated temperatures, efficiencies can exceed those of PEMFCs due to a decrease in the overpotential of the cathodic oxygen reduction reaction (ORR).1 Despite the increasing use of PILs (and ionic liquids in general) in devices such as fuel cells, electrochemical studies of PILs usually focus on the relationships between bulk liquid properties, such as the conductivity and viscosity, and electrochemical behavior. The effects of ion adsorption during electrocatalysis in ionic liquids have not been studied thus far and no fuel cell electrocatalysts have been designed specifically for use in ionic liquids. In contrast, electrocatalysis in aqueous electrolytes is well understood and the field of “surface electrocatalysis” has yielded major advances in PEMFC electrocatalyst design.2 If intermediate temperature (> 100 °C) PILFCs are to compete with conventional PEMFCs, it is likely that “task-specific” electrocatalysts and ionic liquids will be required and this depends on us understanding surface electrocatalysis in ionic liquids. In this contribution, the role of electrode-adsorbate interactions on electrocatalysis of the ORR and CO oxidation in a series of ammonium-based PILs will be discussed. The PILs (see, for example, the insets of Figures 1A and 1B) were prepared by adding the appropriate Brønsted acid to Brønsted base. The resulting PILs were dried at 6 × 10-2 mbar and 70 °C for 48 hr and the trace H2O contents of the PILs were < 0.05 %. Figure 1A shows typical CVs recorded during oxidation of CO at Pt in dimethylethylammonium triflate, [dmea][TfO]. Reduction of protons and surface oxides (points a and b, respectively, in the background CV), were suppressed in the presence of CO and CO oxidation was observed above 1.0 V. “Pre-ignition” of CO oxidation was observed as a shoulder on the main CO oxidation wave (point c) and can be attributed to the co-deposition of underpotential deposited hydrogen (Hupd) with CO onto the Pt surface. As the Hupd was oxidized, Pt sites were freed up allowing OH nucleation and early oxidation of CO. In contrast, Hupd deposition was completely suppressed in diethylmethylammonium bis(trifluoromethanesulfonyl)imide, [dema][Tf2N], due to strong adsorption of the [Tf2N]- anions on the Pt surface. Consequently, pre-ignition of CO oxidation in [dema][Tf2N] was also suppressed (Figure 1B).The presence of adsorbed [Tf2N]- anions had a significant impact on the rate of the ORR in PILs and increased the ORR overpotential drastically. Adsorption of oxides at positive potentials also had a significant effect on ORR electrocatalysis in PILs. For example, the Tafel slope for the ORR at Pt in diethylmethylammonium triflate, [dema][TfO], changed from 70 mV decade-1 when surface oxides were present to 117 mV decade-1 when the oxides were reduced.3 Extension of the “surface electrocatalysis” approach described here to well-defined surfaces will reveal whether ion adsorption from PILs exhibits structural sensitivity and will yield new insights into electrocatalysis in PILs. Ultimately, this approach may lead to the rational design of electrocatalysts for use in PIL electrolytes.

Adsorption of benzyldimethyldodecylammonium chloride onto stainless steel using the quartz crystal microbalance and the depletion methods: an optimisation study

The adsorption behaviour of benzyldimethyldodecylammonium chloride (C12BDMAC) corrosion inhibitor from aqueous solutions onto a stainless steel surface has been investigated using the quartz crystal microbalance (QCM) technique and the depletion-HPLC method. Three different experimental procedures were used to carry out the adsorption measurements using the QCM. The results revealed that the employed experimental procedure had a marked influence on both of the measured frequency and dissipation energy changes. Additionally, the measured changes in the normalized frequency and dissipation changes were found to be overtone-dependent. The adsorbed amounts determined by QCM were higher than those determined by depletion due to strong contributions from the bulk liquid properties. The direct application of the measured frequency shifts corresponding to the lowest dissipation changes, ΔflowestD, in the Sauerbrey equation was effective in minimising these contributions within an acceptable experimental tolerance. A correction procedure, based on the Kanazawa equation, has been proposed and applied successfully to separate the contribution from the bulk effects to the calculated adsorbed mass. The adsorption isotherms obtained from the corrected frequency shifts and the depletion method were perfectly matched over the whole range of concentrations investigated.

Evanescent Wave-Based Particle Tracking Velocimetry for Nanochannel Flows

Understanding fluid flows in 10-1000 nm space, which we call extended nanospace, is important for novel nanofluidic devices in analytical chemistry. This study therefore developed a particle tracking velocimetry for measuring velocity distribution in nanochannel flows, by using the evanescent wave illumination. 64 nm fluorescent nanoparticles were used as flow tracer. The particle position was determined from fluorescent intensity by the evanescent wave field, with a spatial resolution smaller than light wavelengths. The time resolution of 260 μs was achieved to make error by the Brownian diffusion of the tracer small to be neglected. An image processing by multitime particle tracking was established to detect the tracer nanoparticles of weak fluorescent intensity. Though the measurement region was affected by nonuniform particle distribution with the electrostatic interactions, pressure-driven flows of water in a nanochannel of 50 μm width and 410 nm depth were successfully measured. The results of the velocity distribution in the depth-wise direction approximately showed agreement with the fluid dynamics with the bulk liquid properties from the macroscopic view, however, suggested slip velocities even in the hydrophilic channel. We suggest a possibility of appearance of molecular behavior in the fluid near the wall within 10 nm-order scale.

Structural and Rheological Properties of Meibomian Lipid

We explore the unique rheological and structural properties of human and bovine meibomian lipids to provide insight into the physical behavior of the human tear-film lipid layer (TFLL). Bulk rheological properties of pooled meibomian lipids were measured by a commercial stress-controlled rheometer; a home-built interfacial stress rheometer (ISR) probed the interfacial viscoelasticity of spread layers of meibomian lipids. Small- and wide-angle x-ray scattering detected the presence and melting of dispersed crystal structures. Microscope examination under cross polarizers provided confirmation of ordered crystals. A differential scanning calorimeter (DSC) analyzed phase transitions in bulk samples of bovine meibum. Bulk and interfacial rheology measurements show that meibum is extremely viscous and highly elastic. It is also a non-Newtonian, shear-thinning fluid. Small- and wide-angle x-ray diffraction (SAXS and WAXS), as well as differential scanning calorimetry (DSC) and polarizing microscopy, confirm the presence of suspended lamellar-crystal structures at physiologic temperature. We studied meibum architecture and its relation to bulk and interfacial rheology. Bovine and human meibomian lipids exhibit similar physical properties. From all structural probes utilized, we find a melt transition near eye temperature at which lamellar crystals liquefy. Our proposed structure for the tear-film lipid layer at physiologic temperature is a highly viscoelastic, shear-thinning liquid suspension consisting of lipid lamellar-crystallite particulates immersed in a continuous liquid phase with no long-range order. When spread over on-eye tear, the TFLL is a duplex film that exhibits bulk liquid properties and two separate interfaces, air/lipid and water/lipid, with aqueous protein and surfactantlike lipids adsorbed at the water/lipid surface.

Characterising highly active nuclear waste simulants

Reprocessing of spent nuclear fuel produces a highly active liquor (HAL) waste stream, which is typically stored over extended periods of many years in waste tanks equipped with extensive heat exchange capability. Over time, particulates are known to precipitate from the HAL within these tanks. Particle simulants provide a route for understanding the physical behaviour of these HAL solids under different agitation and transfer conditions. Particle and dispersion characterisation techniques are used here to understand the behaviour of two types of simulant HAL solids, viz. caesium phosphomolybdate (CPM) and zirconium molybdate (ZM), in dispersion. Distinct properties are established for CPM and ZM and compared to a common oxide particle material titanium dioxide (TiO2). The results of this study highlight the influence of key aspects of the HAL particulates, such as size and shape, on relevant solid–liquid properties such as sedimentation and rheology. The influence of bulk liquid properties such as electrolyte concentration and pH is also investigated. The results indicate various possible behaviours within the tanks which may impact the storage, remobilisation and pipeline transport of this class of nuclear waste.

Molecular dynamics simulation of liquid methanol. I. Molecular modeling including C–H vibration and fermi resonance

A flexible and polarizable methanol model has been developed on the basis of charge response kernel (CRK) theory. The present CRK methanol model well reproduces bulk liquid and interfacial properties, including density, enthalpy of vaporization, diffusion coefficient, surface tension, and radial distribution functions. The modeling of intramolecular potential incorporates the anharmonic coupling effects pertinent to the Fermi resonance of stretching and bending overtones, with its effective quantum correction. Therefore, the present methanol model can describe the vibrational spectroscopic features of infrared, Raman, and sum frequency generation spectra of C-H or C-D stretching region of methanol or deuterated methanol on the same footing. This model allows for further detailed analysis of C-H vibrations of alkyl moieties by molecular dynamics simulation.