Small Mole Research Articles

Ionizable Cationic Lipids and Helper Lipids Synergistically Contribute to RNA Packing and Protection in Lipid-Based Nanomaterials.

Lipid-based nanomaterials are used as a common delivery vehicle for RNA therapeutics. They typically include a formulation containing ionizable cationic lipids, cholesterol, phospholipids, and a small molar fraction of PEGylated lipids. The ionizable cationic lipids are considered a crucial element of the formulation for the way they mediate interactions with the anionic RNA as a function of pH. Here, we show, by means of molecular dynamics simulation of lipid formulations containing two different ionizable cationic lipids (DLinDMA and DLinDAP), that the direct interactions of those lipids with RNA, taken alone, may not be sufficient to determine the level of protection and packaging of mRNA. Our simulations help and highlight how the collective behavior of the lipids in the formulation, which determines the ability to envelop the RNA, and the level of hydration of the lipid-RNA interface may also play a significant role. This allows the drawing of a hypothesis about the experimentally observed differences in the transfection efficiency of the two ionizable cationic lipids.

Evaluation of the shadowgraph method for the determination of mutual and thermal diffusivities.

The present work provides a systematic study on the influence of sample properties and experimental conditions on the reliable accessibility of Fick or mutual diffusion coefficients D11 and thermal diffusivities a in binary liquid mixtures using the shadowgraph method. For this, mixtures with varying magnitudes of the Soret coefficient ST and their optical contrast factors were studied at a temperature of 298.15K and pressures between (0.1 and 0.65) MPa with varying magnitudes and orientations of the applied temperature and concentration gradients ∇T and ∇c. Experimental signals obtained in these investigations were analyzed with respect to the intensities of the signal contributions from non-equilibrium fluctuations (NEFs) in concentration and temperature, and the reliability of the determined D11 and a data was assessed by comparison to literature data. Larger signal intensities from NEFs and, therefore, a more reliable determination of diffusivities were given for sufficiently large magnitudes of ST, the optical contrast factors, and the applied ∇T and ∇c. At very small fluid layer thicknesses L ≤ 0.30mm, the associated reduction of signal statistics outweighing the expected increase of signal intensities at larger magnitudes of ∇T and ∇c as well as the influence of confinement imposed limitations for the determination of diffusivities in some cases. Furthermore, an influence of the mixture composition on signal intensities from concentration-NEFs was identified, where too small mole fractions of one component can hinder the determination of D11 in mixtures with small magnitudes of the optical contrast factor (∂n/∂c)T,p.

Anti-EGFR Antibody-Drug Conjugate Carrying an Inhibitor Targeting CDK Restricts Triple-Negative Breast Cancer Growth.

Anti-EGFR antibodies show limited response in breast cancer, partly due to activation of compensatory pathways. Furthermore, despite the clinical success of cyclin-dependent kinase (CDK) 4/6 inhibitors in hormone receptor-positive tumors, aggressive triple-negative breast cancers (TNBC) are largely resistant due to CDK2/cyclin E expression, whereas free CDK2 inhibitors display normal tissue toxicity, limiting their therapeutic application. A cetuximab-based antibody drug conjugate (ADC) carrying a CDK inhibitor selected based on oncogene dysregulation, alongside patient subgroup stratification, may provide EGFR-targeted delivery. Expressions of G1/S-phase cell cycle regulators were evaluated alongside EGFR in breast cancer. We conjugated cetuximab with CDK inhibitor SNS-032, for specific delivery to EGFR-expressing cells. We assessed ADC internalization and its antitumor functions in vitro and in orthotopically grown basal-like/TNBC xenografts. Transcriptomic (6,173 primary, 27 baseline, and matched post-chemotherapy residual tumors), single-cell RNA sequencing (150,290 cells, 27 treatment-naïve tumors), and spatial transcriptomic (43 tumor sections, 22 TNBCs) analyses confirmed expression of CDK2 and its cyclin partners in basal-like/TNBCs, associated with EGFR. Spatiotemporal live-cell imaging and super-resolution confocal microscopy demonstrated ADC colocalization with late lysosomal clusters. The ADC inhibited cell cycle progression, induced cytotoxicity against high EGFR-expressing tumor cells, and bystander killing of neighboring EGFR-low tumor cells, but minimal effects on immune cells. Despite carrying a small molar fraction (1.65%) of the SNS-032 inhibitor, the ADC restricted EGFR-expressing spheroid and cell line/patient-derived xenograft tumor growth. Exploiting EGFR overexpression, and dysregulated cell cycle in aggressive and treatment-refractory tumors, a cetuximab-CDK inhibitor ADC may provide selective and efficacious delivery of cell cycle-targeted agents to basal-like/TNBCs, including chemotherapy-resistant residual disease.

A novel spectral model for prediction of nongray radiative properties in gaseous combustion

Radiative heat transfer during gaseous combustion displays very strong spectral or “nongray” behavior, which is both very difficult to characterize and to evaluate. A novel spectral model based on the full-spectrum correlated k-distribution (FSCK) method combined with the multilayer perceptron (MLP), i.e., NFM model, is developed for the prediction of nongray radiative properties in gaseous combustion. Followed by the nature of FSCK method, the NFM model is designed as a simple MLP structure that has only two outputs to directly give both correlated k-value and corresponding ka-value. This not only avoids on-the-fly calculations of a-values but also saves storage by the simple structure. Moreover, by constructing training data with a number of small mole fractions, the errors caused by nonlinear effects are found to be mitigated during the use of NFM model. Several test cases have been carried out to compare the developed NFM model and other spectral tools including FSCK look-up tables and traditional FSCK MLP (TFM) model. Results show that the NFM model can achieve excellent accuracy that is even better than FSCK look-up tables at a tiny computational cost that is far less than that of TFM model.

Influence of the H2 proportion on NH3/H2/air combustion in hot and low-oxygen coflows

This study investigated the effect of the H2 fraction on reaction dynamics and NOx formation in a premixed NH3/H2/air jet flame within a hot coflow (JHC) through numerical simulations. We found that even a small mole fraction of H2 (e.g., XH2, F = 5%) significantly enhances the ignition of the premixed NH3/air flame. However, the presence of H2 markedly intensifies the main combustion reactions only when XH2, F ≥ 30%, resulting in a sharp increase in temperature and heat release rate. Moreover, as XH2, F increases, the NOx emission first increases until XH2, F = 75% and then decreases. Without any H2 addition, a high emission of unburned NH3 occurs in the pure NH3 JHC flame. On the other hand, when XH2, F ≥ 50%, the emissions of unburned H2 and OH increase rapidly due to extremely high temperatures. Notably, maintaining XH2, F between 5% and 20% minimizes the emissions of NOx, NH3, H2, and OH. Furthermore, the preferential diffusion of H2 plays a key role in enhancing the production of active radicals (OH, O, and H), thereby significantly boosting the initiation and combustion reactions of NH3/H2 blended fuel.

Colossal barocaloric effect of the spin-crossover compound {Fe(pz)2(BH3CN)2} near room temperature

As one of the most likely alternatives to traditional vapor compression refrigeration technology, solid refrigeration technology based on the barocaloric effect (BCE) has attracted extensive attention in recent years. Spin-crossover (SCO) compounds are considered suitable for working at low driving pressures due to high-pressure sensitivity and small hysteresis width. However, the entropy change (ΔSSCO) of the SCO compound is smaller than that of other excellent barocaloric materials (plastic crystals and two-dimensional perovskites). Here, we report the BCE of the SCO compound {Fe(pz)2(BH3CN)2} (pz = pyrazine) with a smaller molar mass and a third source of entropy change besides electron and vibrational entropy changes. Compound {Fe(pz)2(BH3CN)2} exhibits high pressure sensitivity (dT1/2dP= 20.2 K kbar−1) as well as entropy change (ΔSSCO= 202 J kg−1 K−1). The maximum values of reversible isothermal entropy change (ΔSit,rev,max) and adiabatic temperature change (ΔTad,rev,max) at 1 kbar are only 103 J kg−1 K−1 and ∼0 K, respectively, due to the hysteresis behavior. However, at sufficiently high driving pressures, ΔSit,rev,max exceeds 200 J kg−1 K−1, and ΔTad,rev,max can reach ∼47 K, which exceeds all SCO compounds reported in BCE studies and is comparable to some plastic-crystalline and two-dimensional perovskite barocaloric materials. The excellent BCE of the SCO compound {Fe(pz)2(BH3CN)2} is mainly due to its small molar mass, which makes the unit mass compound exhibit higher ΔSSCO, while the introduction of the third source of entropy change—the reorientation entropy change (ΔSreo), only plays a small role. This is expected to promote the practical application of SCO compounds as barocaloric refrigerants.

Promotion effect of Ga3+ in ZnAlGa-x mixed oxides obtained from their layered double hydroxides on the enhancement in the transient photocurrent response, the inhibition in the recombination rate of photogenerated (e−, h+) charges and the photocatalytic activity in the photodegradation of 4-chlorophenol

ZnAlGa-x (x = 0, 0.5, 0.8, 1.0, 2.0 and 3.0 mol % of Ga3+) mixed oxides (MOs) obtained by calcination of their corresponding layered double hydroxides (LDHs) were characterized and evaluated in the photodegradation of 4-chlorophenol in aqueous media (4-CP) using UV light irradiation. Structural studies by X-ray diffraction (XRD) of ZnAlGa-x MOs indicated that the incorporation of a small molar concentration of Ga3+ (0.5–0.8 mol %) within the ZnAl LDH structure changed the micro-crystalline structure to an amorphous structure. A small molar content of Ga3+ in the ZnAlGa3+ MOx promotes the formation of a high amount of structural oxygen vacancies as evidenced by Raman and EPR spectroscopy studies. Transient photocurrent response (TPC) and fluorescence spectroscopy experiments reveal that the incorporation of a small molar concentration of Ga3+ in the structure of ZnAl MO promotes the enhancement in the transient photocurrent response, the separation of photogenerated charges and the inhibition of the recombination rate of (e−, h+) pairs. Furthermore, ZnAlGa-x MOs with a small molar concentration of Ga3+ reduced their band gap energies below of 3.0 eV. ZnAl MO photodegradated 69.0 % of 4-CP after 4 h of reaction. However, the photoconversion % of 4-CP increased to 80–90 % at 3 h of reaction using ZnAlGa-x MOs with a small molar concentration of Ga3+. ZnAlGa-x with small Ga3+ concentration mineralized approximately 70–75 % of total disolved organic carbon at 3 h of reaction. The photocatalytic degradation reaction of 4-CP fitted well to a pseudo-first order kinetics of Langmuir-Hinshelwood-type. The kinetic parameters of ZnAlGa-x MOs confirmed that ZnAlGa-0.8 MO was the best photocatalyst to photodegradate 4-CP in aqueous media. The highest photocatalytic activity and efficacy of ZnAlGa-0.8 MO to mineralize 4-CP in aqueous medium may be due to its higher amount of oxygen vacancies contained in its structure promoting a better separation of charges and the inhibition of the recombination rate of photogenerated (e−, h+) charges.

Typical Fine Structure and Seismogenic Mechanism Analysis of the Surface Rupture of the 2022 Menyuan Mw 6.7 Earthquake

On 8 January 2022, a seismic event of significant magnitude (Mw 6.7, Ms 6.9) occurred in the northeastern region of the Tibetan Plateau. This earthquake was characterized by left-lateral strike-slip motion, accompanied by a minor reverse movement. The Menyuan earthquake resulted in the formation of two main ruptures and one secondary rupture. These ruptures were marked by a left-lateral step zone that extended over a distance of 1 km between the main ruptures. The length of the rupture zones was approximately 37 km. The surface rupture zone exhibited various features, including left-lateral offset small gullies, riverbeds, wire fences, road subgrades, mole tracks, cracks, and scarps. Through a comprehensive field investigation and precise measurement using unmanned aerial vehicle (UAV) imagery, 111 coseismic horizontal offsets were determined, with the maximum offset recorded at 2.6 ± 0.3 m. The analysis of aftershocks and the findings from the field investigation led to the conclusion that the earthquake was triggered by the Lenglongling fault and the Tuolaishan fault. These faults intersected at a release double-curved structure, commonly referred to as a stepover. During this particular process, the Lenglongling fault was responsible for initiating the coseismic rupture of the Sunan–Qilian fault. It is important to note that the stress applied to the Tuolaishan fault has not been fully relieved, indicating the presence of potential future hazards.

Chromosomal Evolution of the Talpinae.

In recent years, the number of mole species with species status confirmed by genetic methods has been continuously increasing. Unfortunately, cytogenetic data are not yet available for all species. Here, for the first time, a GTG-banded karyotype of the small-toothed mole from Vietnam, Euroscaptor parvidens, a representative of the Eastern clade of the genus Euroscaptor, has been described. Through comparative analysis of available Euroscaptor (Euroscaptor parvidens, Euroscaptor klossi, and Euroscaptor malayana) and Oreoscaptor (Oreoscaptor mizura) karyotypes, we found cytogenetic signatures for each of the studied species. Zoo-FISH with sorted chromosomes of the Siberian mole (Talpa altaica) on chromosome sets of the small-toothed mole (E. parvidens), the small Japanese mole (Mogera imaizumii) from the closely related genus, and the Japanese shrew mole (Urotrichus talpoides) from the tribe Urotrichini made it possible to identify syntenic regions between these species. We propose a possible ancestral karyotype of the tribe and, based on it, traced the features of chromosomal rearrangements accompanying the divergence of moles. The low rates of chromosomal evolution within the species of the genus Talpa-T. altaica and T. europaea-and the high rates of karyotypic reshuffling within the Asian genera of the tribe were confirmed. The karyotype of the Japanese mountain mole O. mizura seems to be the most conserved among the Asian moles. The most frequently occurring types of chromosomal rearrangements in moles are the pericentric inversions and amplification of heterochromatin. The pericentric inversions on four pairs of autosomes are shared between the closely related genera Euroscaptor, Oreoscaptor, and Mogera, while many more apomorphic rearrangements have occurred in each lineage additionally. The highest rate of chromosomal changes, with five rearrangements occurring over approximately 7 million years, was recorded in the lineage of the small-toothed mole.

First record of the genus Pseudaeginella Mayer, 1890 (Crustacea, Amphipoda, Caprellidae) with a new species from Korean waters.

A new species of the genus Pseudaeginella Mayer, 1890 belonging to the family Caprellidae Leach, 1814 was collected from the South Sea in Korea. Pseudaeginellacarinaspinosasp. nov. is morphologically similar to related congeners belonging to the genera Paradeutella Mayer, 1890 and Pseudaeginella, in having dorsal projections on pereonites, triarticulate mandibular palp, small or absent molar, and uniarticulate pereopods 3 and 4. However, this new species is distinguished from its congeners by the position and size of dorsal projection. This is the first record of Pseudaeginella from the Northwest Pacific region, including Korea, and a key to species of the genus Pseudaeginella is also provided.

Novel Chimeric Amino Acid-Fatty Alcohol Ester Amphiphiles Self-Assemble into Stable Primitive Membranes in Diverse Geological Settings.

Primitive cells are believed to have been self-assembled vesicular structures with minimal metabolic components, that were capable of self-maintenance and self-propagation in early Earth geological settings. The coevolution and self-assembly of biomolecules, such as amphiphiles, peptides, and nucleic acids, or their precursors, were essential for protocell emergence. Here, we present a novel class of amphiphiles-amino acid-fatty alcohol esters-that self-assemble into stable primitive membrane compartments under a wide range of geochemical conditions. Glycine n-octyl ester (GOE) and isoleucine n-octyl ester (IOE), the condensation ester products of glycine or isoleucine with octanol (OcOH), are expected to form at a mild temperature by wet-dry cycles. The GOE forms micelles in acidic aqueous solutions (pH 2-7) and vesicles at intermediate pH (pH 7.3-8.2). When mixed with cosurfactants (octanoic acid [OcA]; OcOH, or decanol) in different mole fractions [XCosurfactant = 0.1-0.5], the vesicle stability range expands significantly to span the extremely acidic to mildly alkaline (pH 2-8) and extremely alkaline (pH 10-11) regions. Only a small mole fraction of cosurfactant [XCosurfactant = 0.1] is needed to make stable vesicular structures. Notably, these GOE-based vesicles are also stable in the presence of high concentrations of divalent cations, even at low pHs and in simulated Hadean seawater composition (without sulfate). To better understand the self-assembly behavior of GOE-based systems, we devised complementary molecular dynamics computer simulations for a series of mixed GOE/OcA systems under simulated acidic pHs. The resulting calculated critical packing parameter values and self-assembly behavior were consistent with our experimental findings. The IOE is expected to show similar self-assembly behavior. Thus, amino acid-fatty alcohol esters, a novel chimeric amphiphile class composed of an amino acid head group and a fatty alcohol tail, may have aided in building protocell membranes, which were stable in a wide variety of geochemical circumstances and were conducive to supporting replication and self-maintenance. The present work contributes to our body of work supporting our hypothesis for synergism and coevolution of (proto)biomolecules on early Earth.

The role of cholesterol in curvature and stripe width evolution in model lung surfactant monolayers.

Lung surfactant (LS) is a lipid-protein monomolecular layer coating the alveolar air-liquid interface. A primary role of LS is to reduce the interfacial tension to facilitate breathing. Simple two and three component systems are used to model LS in studies to test the relationship between composition, morphology, rheological, and phase behavior. Failure of LS is associated with acute respiratory distress syndrome (ARDS). The primary lipid in LS is dipalmitoylphosphatidylcholine (DPPC) which accounts for 40-80 percent of native and replacement LS therapies. The addition of fatty acids, fatty alcohols, and cholesterol to replacement LS therapies is still an area of research. We utilize mixtures of DPPC and hexadecanol (HD) or palmitic acid (PA) with small mole fractions of dihydrocholesterol (DChol). The morphology of 2-dimensional self-assembled monolayers with coexisting crystalline-like domains (liquid-condensed phase) within a liquid-like matrix (liquid-expanded phase) is investigated. With the addition of DChol to the DPPC/HD or DPPC/PA system, a finger instability is seen during monolayer compression followed by a time dependent circle-to-stripe transition at a fixed interfacial tension. In this poster we will discuss recently published results on the equilibrium nature of these films (REF: Valtierrez et al., Sci. Adv. v:8:14, 2022) and extend this work to explore the role of cholesterol in determining morphology and curvature.

Dielectric Breakdown Properties of He-H2 Mixtures for High Temperature Superconducting Power Devices

In this study, the electron energy distribution function (EEDF), the electron swarm parameters , the effective ionization coefficients, and the critical field strength (dielectric strength) in binary He-H2 gas mixture which is used as cryogenic for high-temperature superconducting power applications, are evaluated using two-term solution of the Boltzmann equation over the range of E/N ( the electric field to gas density) from 1 to 100 Td ( 1 Td=10-17 Vcm2) at temperature 77 K and pressure 2MPa, taking into account elastic ( momentum transfer) and inelastic cross-sections. Using the electron energy distribution function (EEDF) electron swarm parameters (electron drift velocity, mean electron energy, diffusion coefficient, electron mobility, ionization and attachment coefficient) are calculated. At low reduced electric field strength E/N, the EEDF is close to Maxwellian distribution, at high E/N, due to vibrational excitation of H2, the calculated distribution function is non-Maxwellian. Besides, the Boltzmann equation analysis showed as the small mole fraction of H2 in the He-H2 mixture is increased, the electron energy distribution function EEDF shifts to lower energy region, the density-reduced ionization coefficient α/N and density-reduced effective ionization coefficient (α-η)/N decreases, whereas density-reduced attachment coefficient η/N, density-reduced critical electric field strength increases, (E/N)crt and critical electric field Ecrt increases. It is found that dielectric field strength depends on pressure and temperature. To confirm the validity of the two term solution of Boltzmann equation analysis, a set of elastic and inelastic cross-sections for each gas He and H2 are used to calculate the electron swarm parameters and dielectric field strength. Compared with previous experimental and theoretical literatures, the values obtained are generally in good agreement.

Sulfur Dioxide and Sulfolane as Additives in Organic Electrolytes to Develop Room-Temperature Sodium Batteries

Sodium metal anodes have attracted great attention for the development of a next generation of high-energy batteries because of their high theoretical capacity (1166 mAh·g−1), low redox potential (−2.71 V vs. SHE), and abundance. However, sodium reacts with most of the liquid electrolytes described to date and it has the shortcoming of dendrite formation during sodium deposition. Several strategies have been proposed to overcome these issues, including the incorporation of electrolyte additives. This work reports on the use of SO2 and sulfolane as additives in organic electrolytes to modify the sodium–electrolyte interphase, making the sodium plating/stripping process more robust. Not only is the process more stable in the case of sodium metal anodes, but also the use of copper substrates is enabled. In fact, high-quality sodium films on copper have been attained by adding small mole fractions of the additives, which paves the way for the development of anode-free batteries. In a general vein, this work stresses the importance of researching on compatible and cost-effective additives that can be easily implemented in practice.

Local Structure of DMF-Water Mixtures, as Seen from Computer Simulations and Voronoi Analysis.

Molecular dynamics simulations of mixtures of N,N-dimethylformamide (DMF) with water of various compositions, covering the entire composition range, are performed on the canonical (N,V,T) ensemble. The local structure of the mixtures is analyzed in terms of radial distribution functions and the contributions of the first five neighbors to them, various order parameters of the water molecules around each other, and properties of the Voronoi polyhedra of the molecules. The analyses lead to the following main conclusions. The two molecules are mixing with each other even on the molecular scale; however, small self-aggregates of both components persist even at their small mole fraction values. In particular, water-water H-bonds exist in the entire composition range, while water clusters larger than 3 and 2 molecules disappear above the DMF mole fraction values of about 0.7 and 0.9, respectively. The O atoms of the DMF molecules can well replace water O atoms in the hydrogen-bonding network. Further, the H-bonding structure is enhanced by the presence of the hydrophobic CH3 groups of the DMF molecules. On the other hand, the H-bonding network of the molecules gradually breaks down upon the addition of DMF to the system due to the lack of H-donating groups of the DMF molecules. Finally, in neat DMF, the molecules form weak, CH-donated H-bonds with each other; however, these H-bonds disappear upon the addition of water due to the increasing competition with the considerably stronger OH-donated H-bonds DMF can form with the water molecules.

Structural and thermodynamics properties of pure phase alkanes, monoamides and alkane/monoamide mixtures with an ab initio based force-field model

A polarizable force-field (FF) model for short- and long-alkane chains and amide derivatives was constructed based solely on accurate quantum chemical (QC) calculations. First, the FF model accuracy was accessed by performing molecular dynamics (MD) simulations to calculate liquid-phase thermodynamic and structural properties for alkanes, for which experimental data are available. Second, The FF was then used to perform molecular dynamics simulations to calculate thermodynamic, structural and excess properties of monoamide/dodecane mixtures, namely DEHiBA/dodecane and DEHBA/dodecane. Aggregation phenomena appear for both types of mixtures and monoamide pure phases. A detailed structural analysis revealed, at small monoamide mole fraction the formation of dimers, while trimerization at larger monoamide concentrations and in their pure phases. Analysis of the relative orientation of the dimers have also been performed and showed a small difference for both phases.

Ammonium ion pre‐intercalated manganese dioxide with hydrogen bond for high‐rate and stable zinc‐ion batteries

AbstractThe low‐cost and high‐safety zinc‐ion batteries (ZIBs) are expected to be promising alternatives of lithium‐ion batteries (LIBs). Nevertheless, sluggish kinetics and unstable structure of cathode lead to the low‐rate capability and poor cycling stability of ZIBs, which severely hinder their commercial application. Herein, a (NH4)xMnO2cathode with ammonium ions (NH4+) pre‐intercalation is reported. The NH4+ions with small molar mass expand the layer spacing, accelerating ion diffusion and act as “structural pillars”, preventing structural collapse. Moreover, hydrogen bond between NH4+and MnO2promotes the charge transfer and further stabilize the layered structure. Therefore, the (NH4)xMnO2cathode delivers a high specific capacity (228.7 mAh g−1at 0.3 A g−1), a superior high‐rate performance (122.5 mAh g−1at 3.0 A g−1), and an outstanding long‐term cycling stability without obvious capacity fading after 1500 cycles at 2.0 A g−1. This work provides a new perspective for future development of high‐performance ZIBs.image

Rarefied gas mixtures with large species mass ratio: Outflow into vacuum

The theoretical study of the processes of the outflow of a binary gas mixture from a source into a vacuum through an orifice in an infinitely thin wall is presented. Two mixtures with a large species mass ratio K are considered: Au–Ne (K = 9.76) and Au–He (K = 49.21). The work continues the study of the flow of Ag–He mixture (K = 26.95) started in Bykov and Zakharov [“Binary gas mixture outflow into vacuum through an orifice,” Phys. Fluids 32, 067109 (2020)]. The results of the direct simulation Monte Carlo made it possible to propose approximations of the mass flow rates of the species and the mixture depending on the species mass ratio, the flow rarefaction degree, and the mole fraction of light species in the source. It is shown that with an increase in the parameter K, an increase in the dimensionless mass flow rate of the mixture referred to the corresponding free molecular value is observed. The maximum dimensionless flow rate corresponds to the near-continuum regime and exceeds the value obtained using the hydrodynamic approximation and the equivalent single gas approach. A variation of K also leads to changes in the spatial distributions of the dimensionless density and velocity of the mixture and some axial focusing of the flow. An increase in the species mass ratio for the case of a small initial mole fraction of the heavy species in the source for a flow regime close to the hydrodynamic one leads to an increase in acceleration and axial focusing of the heavy species.

Concentration dependence of dynamics and hydrogen bonding in aqueous solutions of urea, methyl-substituted ureas, and trimethylamine N-oxide

Organic cosolutes can have profound effects on protein stability, and their behavior in aqueous solutions can provide a molecular understanding of these effects. Urea denatures proteins while methyl-substituted ureas are stronger denaturants and the similar cosolute trimethylamine N-oxide (TMAO) is a protein stabilizer. Here, differences between aqueous solutions of urea, dimethylurea (DMU), tetramethylurea (TMU), and TMAO are explored via molecular dynamics simulations. A consistent force field for urea and the methylureas is first developed that reproduces their experimental structure and dynamics in aqueous solution. Simulations show that the cosolutes generally diffuse more slowly with increasing concentration up to ∼ 4 M and that since the cosolutes are hydrogen bonded to water, bulk water also diffuses slower. However, the diffusion of TMU shows a dramatic increase at higher molar concentrations because due to the very small mole fraction of water in the TMU solutions so that it behaves more like pure liquid TMU. The cosolutes also generally diffuse more slowly with increasing size, although the diffusion of TMAO is even slower than DMU because of greater hydration of its oxygen. The slower diffusion of TMAO leads to larger viscosities, which further increases hydrogen bond lifetimes and leads to even slower diffusion in a feedback loop. Ultimately, this strengthens hydrogen bonds within TMAO solutions relative to urea and methylurea solutions.

Salt-in-Ionic-Liquid Electrolytes: Ion Network Formation and Negative Effective Charges of Alkali Metal Cations.

Salt-in-ionic liquid electrolytes have attracted significant attention as potential electrolytes for next generation batteries largely due to their safety enhancements over typical organic electrolytes. However, recent experimental and computational studies have shown that under certain conditions alkali cations can migrate in electric fields as if they carried a net negative effective charge. In particular, alkali cations were observed to have negative transference numbers at small mole fractions of alkali-metal salt that revert to the expected net positive transference numbers at large mole fractions. Simulations have provided some insights into these observations, where the formation of asymmetric ionic clusters, as well as a percolating ion network, could largely explain the anomalous transport of alkali cations. However, a thermodynamic theory that captures such phenomena has not been developed, as ionic associations were typically treated via the formation of ion pairs. The theory presented herein, based on the classical polymer theories, describes thermoreversible associations between alkali cations and anions, where the formation of large, asymmetric ionic clusters and a percolating ionic network are a natural result of the theory. Furthermore, we present several general methods to calculate the effective charge of alkali cations in ionic liquids. We note that the negative effective charge is a robust prediction with respect to the parameters of the theory and that the formation of a percolating ionic network leads to the restoration of net positive charges of the cations at large mole fractions of alkali metal salt. Overall, we find excellent qualitative agreement between our theory and molecular simulations in terms of ionic cluster statistics and the effective charges of the alkali cations.