Repulsive Van Der Waals Interactions Research Articles

Performance and mechanism of atomizing dust removal from APG12/Active Water: Experimental and molecular simulation study

Coal mining processes produce large amounts of dust particles that seriously pollute the environment and threaten health. Spraying technology is a primary technique for dust removal. To date, experimental methods are mainly applied to study the atomization and dust removal properties of water droplets, but the intermolecular mechanism involved is unclear. First, a test platform was assembled to evaluate the atomization and dust removal performance of APG12/active water. Then, APG12 droplet and APG12 droplet/anthracite molecular models were constructed, and the underlying mechanism was elucidated through molecular simulation. The experimental results showed that the droplet diameter of the APG12/active water spray decreased by 14.15%, and the dust removal efficiencies of total dust and respirable dust increased by 39.55% and 61.51%, respectively. The simulation results indicated that APG12 molecules and H2O molecules exhibited attractive interactions. This interaction weakened the electrostatic attraction between the H2O molecules in the APG12/active water droplets. Moreover, the interaction enhanced the repulsive van der Waals interactions, reduced the number of hydrogen bonds, and increased the diffusion coefficient of H2O molecules. As a result, the droplets were relatively easy to break. In addition, APG12 molecules covered the surfaces of anthracite molecules, thus increasing the surface oxygen content and the electrostatic attraction, van der Waals forces and hydrogen bonding between H2O and coal molecules. This enhancement, in turn, improved the wettability of coal dust. Herein, the molecular mechanism of dust removal from APG12/active water was analysed, providing guidance for the development of new dust suppressants.

Molecular dynamics study on the effect of surface ionization on the interfacial heat transfer between silica and water

Understanding the thermal transport across the solid–liquid interface is crucial for the advancement of modern technology, with particular emphasis on the silica–water interface due to its universality and remarkable applications in energy conversion and medical treatment. Surface ionization is common for silica surfaces immersed in an aqueous environment, providing an adaptable way to modify silica surface properties. In this study, molecular dynamics simulation was employed to investigate the effect of surface ionization on the interfacial heat transfer between silica and water. It was demonstrated that increasing surface ionization significantly improves interfacial thermal conductance. Interactions between silica and water are strengthened so that more water molecules gather at the interface with higher ionization degree. In addition, increasing ionization results in more hydrogen bonds between ionized silica and water, producing more efficient interfacial heat paths. Somewhat counter-intuitively, although greater surface ionization does result in larger Coulombic forces, the primary contributor to enhanced interfacial thermal conductance is stronger repulsive van der Waals interactions, facilitated by decreased silica–water distances. Overall, this work successfully illustrated the mechanism of interfacial heat transfer enhancement due to surface ionization.

Influence of Steric and Dispersion Interactions on the Thermochemistry of Crowded (Fluoro)alkyl Compounds.

ConspectusAlkanes play a pivotal role in industrial, environmental, and biological processes. They are characterized by their carbon-carbon single-bond structure, remarkable stability, and conformational diversity. Fluorination of such compounds imparts unique physicochemical properties that often enhance pharmacokinetic profiles, metabolic stability, and receptor interactions while keeping beneficial properties. However, such per- and polyfluoroalkyl substances (PFAS) show a persistent presence in the environment and potential adverse health effects, which propelled them to the forefront of global environmental and health discussions. Alkyl compounds are also prototypical for stereoelectronic (SE) effects that are widely applied in chemistry. Substituents are typically described as electron-density-donating/withdrawing and/or responsible for sterically interacting with reagents or strategic groups in the molecule. That alkane branching can result in higher stability compared to less-branched isomers has been investigated in detail also by testing quantum chemical methods, in particular density functional theory (DFT). Alkane branching results in spatially compact structures with close intramolecular contacts so that at a specific size the detailed balance of attractive London dispersion and covalent versus repulsive Pauli exchange interactions shifts to new, chemically unfragile situations. This may lead to dissociation at room temperature and opens the central question: what is the smallest crowed alkane that cannot be made synthetically? In this Account, we try to shed light on the interplay among the various (free) energy components for crowded (fluoro)alkane dissociation. In this context, homolytic cleavage of the central C-C bond in a series of model alkanes of increasing size with tert-butyl (tBu), adamantyl (Ad), and [1.1.1]propellanyl (Prop) substituents is investigated. Reference energies are calculated at the PNO-LCCSD(T)-F12b level and used to benchmark the performance of contemporary DFT functionals. In line with previous conclusions, the application of dispersion corrections to density functionals is mandatory. For crowed structures, the accurate description of the midrange correlation effects, specifically repulsive van der Waals interactions, is crucial, and we observed that the density-dependent VV10 correction is superior to D4 in this context, although the asymptotic region is better described by the latter. The best available dispersion-inclusive functionals show systematic and reasonably small residual errors and can be safely applied to large systems (>100 atoms), for which coupled cluster methods with large basis sets are not computationally feasible anymore. For qualitatively correct predictions of synthetic accessibility under equilibrium conditions (free energy), the inclusion of thermostatistical (entropy) contributions is also essential. According to our results, tetra-tert-butylmethane (C17tBu) is the largest and most crowded system with a positive dissociation free energy and should be synthesizable. The difference between hydrogenated and perfluorinated systems originates from the increase in the steric repulsion of spatially close substituents, which is not compensated to the same extent by attractive orbital and dispersion interactions. A sometimes-assumed similar steric demand for fluorine and hydrogen atoms is not corroborated by our investigations on crowded systems. Perfluorination is found to substantially decrease thermal stability, rendering perfluorinated hexamethylethane (C8tBuF) the last potentially stable representative.

Exploring the aggregation of amyloid-β 42 through Monte Carlo simulations

Coarse-grained Monte Carlo simulations are performed for a disordered protein, amyloid-β 42 to identify the interactions and understand the mechanism of its aggregation. A statistical potential is developed from a selected dataset of intrinsically disordered proteins, which accounts for the respective contributions of the bonded and non-bonded potentials. While, the bonded potential comprises the bond, bend, and dihedral constraints, the nonbonded interactions include van der Waals interactions, hydrogen bonds, and the two-body potential. The two-body potential captures the features of both hydrophobic and electrostatic interactions that brings the chains at a contact distance, while the repulsive van der Waals interactions prevent them from a collapse. Increased two-body hydrophobic interactions facilitate the formation of amorphous aggregates rather than the fibrillar ones. The formation of aggregates is validated from the interchain distances, and the total energy of the system. The aggregate is structurally characterized by the root-mean-square deviation, root-mean-square fluctuation and the radius of gyration. The aggregates are characterized by a decrease in SASA, an increase in the non-local interactions and a distinct free energy minimum relative to that of the monomeric state of amyloid-β 42. The hydrophobic residues help in nucleation, while the charged residues help in oligomerization and aggregation.

Quantum order-by-disorder induced phase transition in Rydberg ladders with staggered detuning

^{87} Rb87Rb atoms are known to have long-lived Rydberg excited states with controllable excitation amplitude (detuning) and strong repulsive van der Waals interaction V_{{r} {r'}}Vrr′ between excited atoms at sites {r}r and {r'}r′. Here we study such atoms in a two-leg ladder geometry in the presence of both staggered and uniform detuning with amplitudes \DeltaΔ and \lambdaλ respectively. We show that when V_{{r r'}} \gg(\ll) \Delta, \lambdaVrr′≫(≪)Δ,λ for |{r}-{r'}|=1(>1)|r−r′|=1(>1), these ladders host a plateau for a wide range of \lambda/\Deltaλ/Δ where the ground states are selected by a quantum order-by-disorder mechanism from a macroscopically degenerate manifold of Fock states with fixed Rydberg excitation density 1/41/4. Our study further unravels the presence of an emergent Ising transition stabilized via the order-by-disorder mechanism inside the plateau. We identify the competing terms responsible for the transition and estimate a critical detuning \lambda_c/\Delta=1/3λc/Δ=1/3 which agrees well with exact-diagonalization based numerical studies. We also study the fate of this transition for a realistic interaction potential V_{{r} {r'}} = V_0 /|{r}-{r'}|^6Vrr′=V0/|r−r′|6, demonstrate that it survives for a wide range of V_0V0, and provide analytic estimate of \lambda_cλc as a function of V_0V0. This allows for the possibility of a direct verification of this transition in standard experiments which we discuss.

Conserved number fluctuations in interacting hadron resonance gas model

Fluctuations of net-baryon, net-charge and net-strangeness numbers are studied within the framework of ideal hadron resonance gas (IHRG), including attractive and repulsive van der Waals interactions. The binding energy and thermodynamic parameters are calculated for different extended versions of the HRG (E-HRG) model. The susceptibility ratios of conserved numbers are estimated in full phase-space using various HRG models that show a strong dependence on collision energies. The effect of kinematic cuts (transverse momentum (pT) and pseudo-rapidity (η)) on those ratios of conserved numbers are also discussed. It is observed that, the normalized susceptibility ratios as a function of collision energies strongly depend on lower pT cut-off and Δη window. The E-HRG models are very sensitive to kinematic acceptance even for net-proton fluctuations. The susceptibility ratios calculated from different HRG models with proper kinematic acceptances are compared with the experimental measurements, which will help to better understand the phase transition and existence of critical point in the QCD phase diagram.

Two-Dimensional Six-Body van der Waals Interactions

Van der Waals interactions, primarily attractive van der Waals interactions, have been studied over one and half centuries. However, repulsive van der Waals interactions are less widely studied than attractive van der Waals interactions. In this article, we focus on repulsive van der Waals interactions. Van der Waals interactions are dipole–dipole interactions. In this article, we study the van der Waals interactions between multiple dipoles. Specifically, we focus on two-dimensional six-body van der Waals interactions. This study has many potential applications. For example, the result may be applied to physics, chemistry, chemical engineering, and other fields of sciences and engineering, such as breaking molecules.

Macroscopic Behavior and Microscopic Structure Evolution of Marine Clay in One-Dimensional Compression Revealed by Discrete Element Simulation

Marine clay has been attracting in-depth research on its mechanical behavior and internal structure evolution, which are crucial to marine infrastructure safety. In the formation process of marine clay, including the sedimentation and consolidation stages, the compression behavior and internal structure evolution are highly dependent on the pore water salinity. Discrete element method (DEM) simulation is a powerful tool to study the microscopic mechanics behind the complicated macroscopic mechanical behavior of marine clay. In this study, a DEM simulation scheme is systematically proposed to numerically study the macroscopic beahvior and microscopic structure evolution of marine clay in one-dimensional compression that mimics the marine clay formation process. First, the proposed calculation scheme for double layer repulsive interaction and van der Waals interaction is introduced. Then, the developed DEM simulation scheme is validated by satisfactorily reproducing the experimentally observed one-dimensional compression curves and internal structure transition from an edge-to-edge/edge-to-face flocculated structure to a face-to-face dispersed structure. Finally, evolutions of coordinate number and fabric anisotropy are quantitatively evaluated in the microscopic view. The noticeable effects of ion concentration on the internal structure evlotion and mechanical behavior of marine clay have been examined and discussed.

Temperature and Pressure Dependence of the Viscosity of the Ionic Liquids 1-Hexyl-3-methylimidazolium Tetrafluoroborate and 1-Ethyl- and 1-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amides

The viscosities of the ionic liquids 1-hexyl-3-methylimidazolium tetrafluoroborate, [HMIM]BF4, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide, [EMIM][Tf2N], and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide, [HMIM][Tf2N], have been measured at pressures up to 400 MPa and between 278 and 363 K for [HMIM]BF4, 960 mPa·s maximum viscosity, between 273 and 363 K for [HMIM][Tf2N], 731 mPa·s maximum viscosity, and between 268 and 363 K for [EMIM][Tf2N], 478 mPa·s maximum viscosity, with a falling-body viscometer. The expanded relative uncertainty is estimated at 100·Ur(η) = 2.5. Modified Litovitz and Vogel–Fulcher–Tammann (VFT) equations are used to represent the temperature and pressure dependence. Comparison is made with other 1-alkyl-3-methylimidazolium ionic liquids measured in this laboratory using density scaling. The values of the scaling factor, γ < 3, are consistent with an interplay between the repulsive van der Waals interactions between the ions, which yield high γ, and the Coulombic attractions of oppositely charged ions that act to decrease it. The Angell equation relating the strength factor D, the VFT parameter T0, and the glass temperature Tg is confirmed and extended to estimate the pressure dependence of Tg for 1-alkyl-3-methylimidazolium ionic liquids. Densities between 273 and 363 K at atmospheric pressure are also reported for [HMIM]BF4.

Orientation analysis of sum frequency generation spectra of di-chain phospholipids: Effect of the second acyl chain

Sum Frequency Generation (SFG) spectroscopy is widely used for studying the di-chain phospholipid monolayers incorporated in model cell membranes. In this context, it is frequently assumed, without justification, that the chains are identical, so their individual contributions to the SFG spectra are indistinguishable. However, the combination of both attractive and repulsive Van der Waals interactions between the chains results in a finite angle between their two terminal methyl groups, resulting in non-equivalent contributions to the non-linear susceptibility. This work describes the application of the underlying non-linear theory required to produce the accurate SFG spectral simulations needed to test this assertion and therefore provides the necessary quantitative validation. For phospholipids comprising two identical saturated chains, which typically have small angles of divergence, these simulations predict only small deviations in the SFG intensities from those calculated assuming a single methyl orientation. Non-identical tails, however, with differences in the degree or type of chain unsaturation, or in the parity of the chain lengths, show much larger discrepancies than the assumption of a single chain. In these cases, the two tails must be treated as separate entities, and their structural relationship must be incorporated into the interpretation of their SFG spectra. A second important result from the simulations arises from the systematic nature of the deviations, which shows that even small intensity changes should not be quickly dismissed on the basis of being subsumed by the uncertainties associated with spectral noise.

Repulsive potential peaks

In this article, we study four-body repulsive van der Waals interactions. Specifically, we study the repulsive potential peaks as a function of the principal quantum number n. It is discovered that the repulsive potential peak’s height increases as the energy difference between the two coupling states increases. This study is beneficial for achieving tunable atom–atom interactions, quantum tunneling, precision measurement, ultracold plasma, as well as quantum computing. For example, if we would like to study quantum tunneling, it is imperative to know the potential height or the potential peak’s height. This article provides a hint on how to achieve a potential peak with a particular height.

Critical point of nuclear matter and beam-energy dependence of net-proton number fluctuations

The beam energy dependence of net baryon number susceptibilities is studied in the framework of the hadron resonance gas model with the attractive and repulsive van der Waals interactions between baryons. The collision energy dependences for the skewness $S\sigma$ and kurtosis $\kappa\sigma^2$ deviate significantly from the Poisson baseline and demonstrate the existence of rich structures at moderate collision energies. This behavior may result from the critical end point of the nuclear liquid-gas first order phase transition. In particular, $\kappa\sigma^2$ shows a non-monotonic energy dependence, and, in contrast to the standard scenario for the QCD critical point, it does not decrease at low collision energies. It is also found that the measurable net proton fluctuations differ significantly from the net baryon fluctuations when interactions between baryons cannot be neglected. The results are compared with the experimental net proton number fluctuations measured by the STAR collaboration.

Repulsive van der Waals interaction between a quantum particle and a conducting toroid

We calculate the non-retarded dispersion force exerted on an electrically polarizable quantum particle by a perfectly conducting toroid, which is one of the most common objects exhibiting a non-trivial topology. We employ a convenient method developed by Eberlein and Zietal that essentially relates the quantum problem of calculating dispersion forces between a quantum particle and a perfectly conducting surface of arbitrary shape to a corresponding classical problem of electrostatics. Considering the quantum particle in the symmetry axis of the toroid, we use this method to find an exact analytical result for the van der Waals interaction between the quantum particle and the conducting toroid. Surprisingly, we show that for appropriate values of the two radii of the toroid the dispersive force on the quantum particle is repulsive. This is a remarkable result since repulsion in dispersive interactions involving only electric objects (and particles) in vacuum is rarely reported in the literature. Final comments are made about particular limiting cases as, for instance, the quantum particle-nanoring system.

Porous silicon film formation from silicon-nanoparticle inks: The possibility of effects of van der Waals interactions on uniform film formation

Porous Si films were formed on electrically insulative, semiconductive, and conductive substrates by depositing aqueous and nonaqueous Si nanoparticle inks. In this study, we focused on whether the Si ink deposition resulted in the formation of uniform porous Si films on various substrates. As a result of the experiments, we found that the inks showing better substrate wettabilities did not necessarily result in more uniform film formation on the substrates. This implies that the ink-solvent wettability and the nanoparticle–substrate interactions play important roles in the uniform film formation. As one of the interactions, we discussed the influence of van der Waals interactions by calculating the Hamaker constants. The calculation results indicated that the uniform film formation was hampered when the nanoparticle surface had a repulsive van der Waals interaction with the substrate.

The Effect of Network Solvation on the Viscoelastic Response of Polymer Hydrogels.

The majority of investigations consider the deformation response of hydrogels, fully controlled by the deformation behavior of their polymer network, neglecting the contribution caused by the presence of water. Here, we use molecular dynamics simulation in an attempt to include the effect of physically bound water via polymer chain solvation on the viscoelastic response of hydrogels. Our model allows us to control the solvation of chains as an independent variable. The solvation of the chain is independent of other factors, mainly the effect (pH) which interferes significantly in experiments. The solvation of hydrophilic chains was controlled by setting a partial charge on the chains and quantified by the Bjerrum length (BL). The BL was calculated from the partial electric charge of the solvent and macromolecular network. When the BL is short, the repulsive Van der Waals interactions are predominant in the vicinity of macromolecules and solvation is not observed. For a long BL, the water molecules in the solvation zone of network are in the same range as attractive intermolecular forces and the solvation occurs. The model also allows the consideration of molecules of water attached to two chains simultaneously, forming a temporary bridging. By elucidating the relations between solvation of the network and structural changes during the network deformation, one may predict the viscoelastic properties of hydrogels knowing the molecular structure of its polymer chains.

Five-body van der Waals interactions

We report on the five-body repulsive and attractive van der Waals interactions between the strongly dipole-dipole coupled Rydberg states. Compared to four-body van der Waals interactions, five-body van der Waals interactions show more energy levels and more potential wells caused by avoided crossings. This research bridges the few-body physics and many-body physics. Other disciplines, such as chemistry, biology, and medical fields, will also benefit from better understanding van der Waals interactions.

Van der Waals Interactions in Hadron Resonance Gas: From Nuclear Matter to Lattice QCD.

An extension of the ideal hadron resonance gas (HRG) model is constructed which includes the attractive and repulsive van der Waals (VDW) interactions between baryons. This VDW-HRG model yields the nuclear liquid-gas transition at low temperatures and high baryon densities. The VDW parameters a and b are fixed by the ground state properties of nuclear matter, and the temperature dependence of various thermodynamic observables at zero chemical potential are calculated within the VDW-HRG model. Compared to the ideal HRG model, the inclusion of VDW interactions between baryons leads to a qualitatively different behavior of second and higher moments of fluctuations of conserved charges, in particular in the so-called crossover region T∼140-190 MeV. For many observables this behavior resembles closely the results obtained from lattice QCD simulations. This hadronic model also predicts nontrivial behavior of net-baryon fluctuations in the region of phase diagram probed by heavy-ion collision experiments. These results imply that VDW interactions play a crucial role in the thermodynamics of hadron gas. Thus, the commonly performed comparisons of the ideal HRG model with the lattice and heavy-ion data may lead to misconceptions and misleading conclusions.

Solvation of asphaltenes in supercritical water: A molecular dynamics study

To improve the understanding of the condensation of asphaltenes in supercritical water (SCW), the solvation of asphaltenes under SCW environments was investigated with molecular dynamics simulation. The simulation results indicate that the repulsive van der Waals interaction has a primary influence on the nature and the magnitude of the interaction between asphaltenes and SCW. A cavity short-range solvent structure surrounding asphaltenes thus is formed, and the solvation free energy (Gsol) of asphaltenes in SCW has a positive sign in most cases. The scale of cavities and the value of Gsol both are determined mainly by the bulk density of water. By the repulsive asphaltenes/SCW interaction and the attractive π–π aromatic interaction, the aggregation of asphaltenes in SCW occurs spontaneously. The asphaltene clusters formed, containing asphaltene monomers and nanoaggregates, present a coke-like supramolecular structure. Benefiting from excellent diffusivity in SCW, the aggregation of asphaltenes in SCW can be accomplished rapidly.

Possible existence of van der Waals macrodimers

ABSTRACTFew-body interactions offer the opportunity to study the isolated atom to few-body coupled molecules, and to condensed matter transitions. Atoms in molecules and in condensed matters are coupled by different orders of multipole–multipole interactions, which all stem from different orders of approximations from coulomb interactions between multiple charges. The lowest order multipole–multipole interaction is the dipole–dipole interaction, which is proportional to the size of the dipole. In this article, we use Rydberg atoms, which have more than 1000 times greater electric dipoles than the ground state atoms, to study the van der Waals interaction between few bodies. In addition to the large dipoles, the kinetic energy of the atoms is significantly reduced by reducing the temperature, which makes these interactions stable and observable. Here, we report on the 2D and 3D few-body interaction potentials and possible ways of creating semistable molecules in such an ultracold Rydberg gas with a temperature of ≈100 nK. Although we use Rydberg atoms in this article, this calculation can be applied to other states too. The results reported here are useful for studying repulsive van der Waals interactions and creating ultracold molecules.

Measurement of the van der Waals interaction by atom trajectory imaging

We study the repulsive van der Waals interaction of cold rubidium $70S_{1/2}$ Rydberg atoms by analysis of time-delayed pair correlation functions. After excitation, Rydberg atoms are allowed to accelerate under the influence of the van der Waals force. Their positions are then measured using a single-atom imaging technique. From the average pair correlation function of the atom positions we obtain the initial atom-pair separation and the terminal velocity, which yield the van der Waals interaction coefficient $C_{6}$. The measured $C_{6}$ value agrees well with calculations. The experimental method has been validated by simulations. The data hint at anisotropy in the overall expansion, caused by the shape of the excitation volume. Our measurement implies that the interacting entities are individual Rydberg atoms, not groups of atoms that coherently share a Rydberg excitation.