Attractive Potential Energy Research Articles

Study on Colloidal Model of Petroleum Residues through the Attraction Potential between Colloids.

The samples of DaGang atmospheric residue (DG-AR), Middle East atmospheric residue (ME-AR), TaHe atmospheric residue (TH-AR), and their thermal reaction samples were chosen for study. All the samples were fractioned into six components separately, including saturates plus light aromatics, heavy aromatics, light resins, middle resins, heavy resins, and asphaltenes. The dielectric permittivity of the solutions of these components was measured, and the dielectric permittivity values of the components can be determined by extrapolation, which increased steadily from saturates plus light aromatics to asphaltenes. Moreover, the Hamaker constants of the components were calculated from their dielectric permittivity values. The Van der Waals attractive potential energy between colloids corresponding to various models could be calculated from the fractional composition and the Hamaker constants of every component. It was assumed that the cores of colloidal particles were formed by asphaltenes and heavy resins mainly; the other fractions acted as dispersion medium. For the three serials of thermal reaction samples, the Van der Waals attraction potential energy between colloids for this kind of model was calculated. For TH-AR thermal reaction samples, the Van der Waals attraction potential energy presented the maximum as thermal reaction is going on, which was near to the end of coke induction period.

Quarkonium binding and entropic force

A \(Q{\bar{Q}}\) bound state represents a balance between repulsive kinetic and attractive potential energy. In a hot quark–gluon plasma, the interaction potential experiences medium effects. Color screening modifies the attractive binding force between the quarks, while the increase of entropy with \(Q{\bar{Q}}\) separation gives rise to a growing repulsion. We study the role of these phenomena for in-medium \(Q{\bar{Q}}\) binding and dissociation. It is found that the relevant potential for \(Q{\bar{Q}}\) binding is the free energy \(F\); with increasing \(Q{\bar{Q}}\) separation, further binding through the internal energy \(U\) is compensated by repulsive entropic effects.

Arctic methane

What are the risks of a runaway greenhouse effect from methane release from hydrates in the Arctic? In January 2013, a dramatic increase of methane concentration up to 2000 ppb has been measured over the Arctic north of Norway in the Barents Sea. The global average being 1750 ppb. It has been suggested that methane normally trapped under the sea ice is now released more easily, especially in areas with reduced sea-ice extent. These high methane concentrations are possibly related to the destabilization of the methane gas hydrates in the Arctic shelf. Is this the start of a runaway greenhouse effect? Methane gas hydrates are solid, ice-like mixtures of water and methane. They are stable under relatively low temperatures and high pressures. These conditions can be found naturally in permafrost regions or under submarine continental slopes. Hydrates accumulate a large amount of gas. One cubic meter can include up to 160 m3 of methane. Their huge amount of stored energy and the wide geographical distribution make hydrates an attractive potential energy source. However, they are also suspected to be a source of global climate change and geological hazard, but the risk is still uncertain. Due to the quasi-stability of methane hydrates, any intervention in the temperature–pressure equilibrium could trigger hydrate dissociation followed by methane emissions, a runaway greenhouse effect, and potential underwater landslides. Here, some consequences for climate change of methane emissions to the atmosphere are analyzed in a scenario study.

APF-Based Car Following Behavior Considering Lateral Distance

Considering the influence of lateral distance on consecutive vehicles, this paper proposes a new car following model based on the artificial potential field theory (APF). Traditional car following behaviors all assume that the vehicles are driving along the middle of a lane. Different from the traditional car following principles, this incorporation of APF offers a potential breakthrough in the fields of car following theory. The individual vehicle can be represented as a unit point charge in electric field, and the interaction of the attractive potential energy and the repellent potential energy between vehicles simplifies the various influence factors on the target vehicle in actual following behavior. Consequently, it can make a better analysis of the following behavior under the lateral separation. Then, the proposed model has been demonstrated in simulation environment, through which the space-time trajectories and the potential energy change regulation are obtained. Simulations verify that the following vehicle's behavior is vulnerable to be affected by lateral distance, where the attractive potential energy tends to become repellent potential energy as the longitudinal distance decreases. The search results prove that the proposed model quantifies the relations between headway and potential energy and better reflects the following process in real-world situation.

Understanding hydrogen sorption in a polar metal-organic framework with constricted channels

A high fidelity molecular model is developed for a metal-organic framework (MOF) with narrow (approximately 7.3 Å) nearly square channels. MOF potential models, both with and neglecting explicit polarization, are constructed. Atomic partial point charges for simulation are derived from both fragment-based and fully periodic electronic structure calculations. The molecular models are designed to accurately predict and retrodict material gas sorption properties while assessing the role of induction for molecular packing in highly restricted spaces. Thus, the MOF is assayed via grand canonical Monte Carlo (GCMC) for its potential in hydrogen storage. The confining channels are found to typically accommodate between two to three hydrogen molecules in close proximity to the MOF framework at or near saturation pressures. Further, the net attractive potential energy interactions are dominated by van der Waals interactions in the highly polar MOF - induction changes the structure of the sorbed hydrogen but not the MOF storage capacity. Thus, narrow channels, while providing reasonably promising isosteric heat values, are not the best choice of topology for gas sorption applications from both a molecular and gravimetric perspective.

Electrostatic contribution to DNA condensation—application of ‘energy minimization’ in asimple model in the strong Coulomb coupling regime

The process of bending of straight DNA to a circular form in the presence of any of themono-, di-, tri- or tetravalent counterions has been simulated in a strong Coulomb couplingenvironment, employing a previously developed energy minimization simulation technique.The inherent characteristics of the simulation technique allow the monitoring of therequired electrostatic contribution to the bending. The curvature of the bending has beenfound to play a crucial role in facilitating the electrostatic attractive potential energy. Thetotal electrostatic potential energy has been found to decrease with bending, whichindicates that bending straight DNA to a circular form or to a toroidal form in thepresence of neutralizing counterions is energetically favourable and is practically aspontaneous phenomenon.

Attractive energy contribution to nanoconfined fluids behavior: the normal pressure tensor

The aim of our research is to demonstrate the role of attractive intermolecular potential energy on normal pressure tensor of confined molecular fluids inside nanoslit pores of two structureless purely repulsive parallel walls in xy plane at z = 0 and z = H, in equilibrium with a bulk homogeneous fluid at the same temperature and at a uniform density. To achieve this we have derived the perturbation theory version of the normal pressure tensor of confined inhomogeneous fluids in nanoslit pores: $$ P_{ZZ} = kT\rho \left( {Z_{1} } \right) + \pi kT\rho \left( {Z_{1} } \right)\int\limits_{ - d}^{0} {\rho \left( {Z_{2} } \right)} Z_{2}^{2} g_{Z,H} (d){\text{d}}Z_{2} - \frac{1}{2}\iint {\int\limits_{0}^{2\pi } {\phi^{\prime } \left( {\vec{r}_{2} } \right)\rho \left( {Z_{1} } \right)\rho \left( {Z_{2} } \right)g_{Z,H} (r_{2} )} }{\frac{{Z_{2}^{2} }}{{(R_{2}^{2} + Z_{2}^{2} )^{{\frac{1}{2}}} }}}R_{2} {\text{d}}R_{2} {\text{d}}Z_{2} {\text{d}}\Uptheta ;\quad \left| {\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {r}_{2} } \right| > d $$ In which R, Θ, Z are the cylindrical coordinate’s notations and \( \left| {\vec{r}_{2} } \right| = \sqrt {R_{2}^{2} + Z_{2}^{2} }\). Assuming the truncated-Lennard-Jones potential, the third term represents the attractive intermolecular potential energy contribution to the normal pressure tensor. We report solution of this equation for the truncated-Lennard-Jones confined fluid in nanoslit pores, and we demonstrate the role of attractive potential energy by comparing the results of Lennard-Jones and hard-sphere fluids. Our numerical calculations show that the normal pressure tensor has an oscillatory form versus distance from the walls for all confined fluids. The oscillations increase with reduced bulk density and decrease with fluid–fluid attraction. It also becomes broad and smooth with pore width at constant temperature and density. In comparison with hard-sphere confined fluids, the values of the normal pressure for LJ fluids at all distances from the walls are less than the hard-sphere fluids. This analytic pressure tensor equation is a useful tool to understand the role of attractive and repulsive forces in the normal pressure tensor and to predict phase behavior of nanoconfined fluids.

Depletion-Induced Shape and Size Selection of Gold Nanoparticles

For nanoparticle-based technologies, efficient and rapid approaches that yield particles of high purity with a specific shape and size are critical to optimize the nanostructure-dependent optical, electrical, and magnetic properties, and not bias conclusions due to the existence of impurities. Notwithstanding the continual improvement of chemical methods for shaped nanoparticle synthesis, byproducts are inevitable. Separation of these impurities may be achieved, albeit inefficiently, through repeated centrifugation steps only when the sedimentation coefficient of the species shows sufficient contrast. We demonstrate a robust and efficient procedure of shape and size selection of Au nanoparticles (NPs) through the formation of reversible flocculates by surfactant micelle induced depletion interaction. Au NP flocculates form at a critical surfactant micelle molar concentration, C(m)* where the number of surfactant micelles is sufficient to induce an attractive potential energy between the Au NPs. Since the magnitude of this potential depends on the interparticle contact area of Au NPs, separation is achieved even for the NPs of the same mass with different shape by tuning the surfactant concentration and extracting flocculates from the sediment by centrifugation or gravitational sedimentation. The refined NPs are redispersed by subsequently decreasing the surfactant concentration to reduce the effective attractive potential. These concepts provide a robust method to improve the quality of large scale synthetic approaches of a diverse array of NPs, as well as fine-tune interparticle interactions for directed assembly, both crucial challenges to the continual realization of the broad technological potential of monodispersed NPs.

State resolved measurements of a C1H2 removal confirm predictions of the gateway model for electronic quenching

Collisional quenching of electronically excited states by inert gases is a fundamental physical process. For reactive excited species such as singlet methylene, (1)CH(2), the competition between relaxation and reaction has important implications in practical systems such as combustion. The gateway model has previously been applied to the relaxation of (1)CH(2) by inert gases [U. Bley and F. Temps, J. Chem. Phys. 98, 1058 (1993)]. In this model, gateway states with mixed singlet and triplet character allow conversion between the two electronic states. The gateway model makes very specific predictions about the relative relaxation rates of ortho and para quantum states of methylene at low temperatures; relaxation from para gateway states leads to faster deactivation independent of the nature of the collision partner. Experimental data are reported here which for the first time confirm these predictions at low temperatures for helium. However, it was found that in contrast with the model predictions, the magnitude of the effect decreases with increasing size of the collision partner. It is proposed that the attractive potential energy surface for larger colliders allows alternative gateway states to contribute to relaxation removing the dominance of the para gateway states.

A new correlation on predicting self- and mutual-diffusion coefficient of Lennard–Jones chain fluid

Based on the Chapman–Enskog theory of diffusion and molecular dynamics simulation data for Lennard–Jones chain (LJC) fluid, a new semi-empirical correlation for calculating the self-diffusion coefficient of LJC fluid is proposed. The new correlation introduces in two correction functions with six fitting parameter to modify the impact of intermolecular repulsive and attractive potential energy on molecular friction coefficient. The new correlation represents the experimental self-diffusion coefficients with an average absolute deviation (AAD) of 3.46% for 23 polyatomic compounds (1102 experimental data points) over wide ranges of temperature and pressure. On this basis, the van der Waals mixing rule is adopted to calculate the mutual-diffusion coefficient of binary LJC fluid. By comparison of calculated results with the experimental data of 12 binary LJC systems over wide range of temperature and composition, the average absolute deviation is 6.98% which verifies the accuracy and the effectiveness of the new correlation.

Critically shielded potential in a three-dimensional electron gas: The induced charge density at the origin

The induced electron density at the position of a critically shielded single point charge embedded in a three-dimensional degenerate electron gas is studied at low densities, in a comparative manner. The partial-wave expansion method for scattering states and averaging over the occupied Fermi sphere are used for an attractive potential energy of Hulthen form. The third-order differential equation of March and Murray [N.H. March, A.M. Murray, Phys. Rev. 120 (1960) 830] for the diagonal of the one-particle density matrix is also investigated, for the same continuous set of energy eigenvalues. The comparative study, performed for the s-type component of the total electron density, explicitly demonstrates the equivalence of the two methods.

QTAIM Analysis of Ligand Properties and Mechanisms of Tuning of 6-Membered Ring N-Heterocyclic Carbenes in Transition Metal Complexes through Ring-Substituent Variation

Tuning of the ligand properties of a series of N-heterocyclic carbenes has previously been achieved through the variation of substituents at the ring boron sites. Analysis of the topology of the electron density using the quantum theory of atoms in molecules (QTAIM) reveals sensitive changes in the integrated atomic properties and the curvature of the valence shell charge concentration of the carbene carbon (C2). Amino substitution induces greater ligand-to-metal sigma-donation in the transition metal complex, which in turn weakens the attractive potential energy density on the interatomic surface of the trans-carbonyl group leading to the decrease in the trans C-O stretching frequency observed experimentally and theoretically. The distribution of charge concentrations within the inner-valence shell of cobalt in the transition metal complexes recovers the QTAIM analogs of ligand-to-metal sigma-donation and metal-to-ligand pi-back-donation. Investigation of the virial of the Ehrenfest force acting on the interatomic surfaces shows the operative mechanism of ligand tuning to be the inductive withdrawal of charge from the ring by nitrogen and subsequent back-polarization of remaining charge toward C2. This mechanism is at odds with the orbital viewpoint of exo nitrogen-to-ring-boron pi-back-donation; however, it is the electronic forces that govern the bonding and charge distribution within a molecule in a stationary state.

Separation of nanocolloids driven by dielectrophoresis: A molecular dynamics simulation

The nonequilibrium molecular dynamics (MD) method was used to model the nanocolloids and the solvent particles. By introducing a non-uniform electric field, colloids were polarized to have opposite polarities. Separation of colloids driven by dielectrophoresis (DEP) could be seen clearly under a strong electric field at low temperatures. Analyzing the ratio of DEP velocities of colloids to thermal velocities of neutral solvent particles showed that when the ratio was correspondingly big, collision between colloids and solvent particles would be intense, making the DEP velocity of colloids fluctuate frequently. By changing the electric field strength, it was found that the enhancement of electric field strength would quicken the separation of colloids. But when the electric field strength increased to a certain degree, the separation motion would be slow because of the strong friction resistance of the solvent particles to the colloids. Moreover, studying the separation reason of colloids based on the potential energy showed that after colloids were polarized, the attractive potential energy among the colloids would be weaker than before, while the increase of temperature would reduce the attractive potential energy and increase the repulsive potential energy, which accorded with the DLVO theory.

Monte Carlo study on ultrasound backscattering by three-dimensional distributions of red blood cells

A Monte Carlo study on ultrasound backscattering by red blood cells (RBCs) is presented for three-dimensional (3D) distributions of particles. The cells were treated as classical spherical particles and accordingly, the Boltzmann distribution was considered to describe probability distribution of energy states of a system composed of such particles. The well-known Metropolis algorithm can generate configurations according to that probability distribution and therefore, was employed in this study to simulate some realizations of both nonaggregating and aggregating RBCs. The study of nonaggregating particles was motivated to compare simulations with existing experimental results and consequently, to validate the model. In the case of aggregating RBCs, the interaction potential between cells was modeled with the Morse potential and the frequency-dependent backscattering coefficient (BSC) was investigated at different hematocrits (H, particle volume fractions). The impact of aggregation potential on the spectral slope (SS) was also evaluated. It is shown that BSC increased as the magnitude of aggregating potential was raised and the effect was more pronounced at higher hematocrits. Moreover, spectral slopes at nonaggregating and low aggregating conditions were found to be around 4, which is consistent with the Rayleigh scattering theory. However, it had diminished significantly, particularly at higher hematocrits as the magnitude of the attractive potential energy was raised. For instance, at H=40% SS dropped from 4.04 for nonaggregating particles to 3.62 at the highest aggregating potential considered in this study. Our results suggest that this 3D model is capable of reflecting the effects of RBC aggregation on BSC and SS.

Multiple Subunit Fitting into a Low-Resolution Density Map of a Macromolecular Complex Using a Gaussian Mixture Model

Recently, electron microscopy measurement of single particles has enabled us to reconstruct a low-resolution 3D density map of large biomolecular complexes. If structures of the complex subunits can be solved by x-ray crystallography at atomic resolution, fitting these models into the 3D density map can generate an atomic resolution model of the entire large complex. The fitting of multiple subunits, however, generally requires large computational costs; therefore, development of an efficient algorithm is required. We developed a fast fitting program, “ gmfit”, which employs a Gaussian mixture model (GMM) to represent approximated shapes of the 3D density map and the atomic models. A GMM is a distribution function composed by adding together several 3D Gaussian density functions. Because our model analytically provides an integral of a product of two distribution functions, it enables us to quickly calculate the fitness of the density map and the atomic models. Using the integral, two types of potential energy function are introduced: the attraction potential energy between a 3D density map and each subunit, and the repulsion potential energy between subunits. The restraint energy for symmetry is also employed to build symmetrical origomeric complexes. To find the optimal configuration of subunits, we randomly generated initial configurations of subunit models, and performed a steepest-descent method using forces and torques of the three potential energies. Comparison between an original density map and its GMM showed that the required number of Gaussian distribution functions for a given accuracy depended on both resolution and molecular size. We then performed test fitting calculations for simulated low-resolution density maps of atomic models of homodimer, trimer, and hexamer, using different search parameters. The results indicated that our method was able to rebuild atomic models of a complex even for maps of 30 Å resolution if sufficient numbers (eight or more) of Gaussian distribution functions were employed for each subunit, and the symmetric restraints were assigned for complexes with more than three subunits. As a more realistic test, we tried to build an atomic model of the GroEL/ES complex by fitting 21-subunit atomic models into the 3D density map obtained by cryoelectron microscopy using the C7 symmetric restraints. A model with low root mean-square deviations (14.7 Å) was obtained as the lowest-energy model, showing that our fitting method was reasonably accurate. Inclusion of other restraints from biological and biochemical experiments could further enhance the accuracy.

On the Mechanism of Hydrogen Storage in a Metal−Organic Framework Material

Monte Carlo simulations were performed modeling hydrogen sorption in a recently synthesized metal-organic framework material (MOF) that exhibits large molecular hydrogen uptake capacity. The MOF is remarkable because at 78 K and 1.0 atm it sorbs hydrogen at a density near that of liquid hydrogen (at 20 K and 1.0 atm) when considering H2 density in the pores. Unlike most other MOFs that have been investigated for hydrogen storage, it has a highly ionic framework and many relatively small channels. The simulations demonstrate that it is both of these physical characteristics that lead to relatively strong hydrogen interactions in the MOF and ultimately large hydrogen uptake. Microscopically, hydrogen interacts with the MOF via three principle attractive potential energy contributions: Van der Waals, charge-quadrupole, and induction. Previous simulations of hydrogen storage in MOFs and other materials have not focused on the role of polarization effects, but they are demonstrated here to be the dominant contribution to hydrogen physisorption. Indeed, polarization interactions in the MOF lead to two distinct populations of dipolar hydrogen that are identified from the simulations that should be experimentally discernible using, for example, Raman spectroscopy. Since polarization interactions are significantly enhanced by the presence of a charged framework with narrow pores, MOFs are excellent hydrogen storage candidates.

SANS Study of Third Phase Formation in the HCl‐TBP‐n‐Octane System

Third phase formation in the extraction of HCl by tri‐n‐butyl phosphate (TBP) in n‐octane has been investigated. The extraction of the acid is accompanied by the simultaneous extraction of large amounts of water and brings about organic phase splitting when the equilibrium HCl concentration in the aqueous phase is higher than 7.6 M. For a description of mechanism and energetics of these phenomena, a combination of coordination chemistry and concepts and techniques that are typical of colloid chemistry is needed. SANS data for organic phases loaded with progressive amounts of HCl and water have been interpreted using the Baxter model for hard spheres with surface adhesion. According to this model, the increase in scattering intensity in the low Q range observed when increasing amounts of solutes are extracted into the organic phase, arises from van der Waals interactions between reverse micelles containing up to seven TBP molecules. At the onset of third phase formation, the potential energy of attraction between micelles is about twice the average thermal energy. The results obtained for HCl extraction are compared with those reported previously for the extraction of metal nitrates under the same conditions. Work performed under the auspices of the U.S. Department of Energy, Office of Basic Energy Science, Division of Chemical Sciences, Biosciences and Geosciences (for the part performed at the Chemistry Division of ANL) and Division of Material Science (for the part performed at INPS), under contract No. W‐31‐109‐ENG‐38.

On the Extensivity of the EntropySq, theq-Generalized Central Limit Theorem and theq-Triplet

First, we briefly review the conditions under which the entropy $S_q$ can be extensive (Tsallis, Gell-Mann and Sato, Proc. Natl. Acad. Sc. USA (2005), in press; cond-mat/0502274), as well as the possible $q$-generalization of the central limit theorem (Moyano, Tsallis and Gell-Mann, cond-mat/0509229). Then, we address the $q$-triplet recently determined in the solar wind (Burlaga and Vinas, Physica A {\bf 356}, 375 (2005)) and its possible relation with the space-dimension $d$ and with the range of the interactions (characterized by $\alpha$, the attractive potential energy being assumed to decay as $r^{-\alpha}$ at long distances $r$).

Interactions between amphiphilic graft copolymer-intercalated liposomes

Amphiphilic graft copolymers containing both the mPEG and cholesterol side chains were synthesized and successfully incorporated into liposomes via the cholesterol side chains. Excellent colloidal stability of the copolymer-intercalated liposomes was achieved. By contrast, the native liposome lost its colloidal stability rapidly upon aging. Isothermal titration microcalorimetry experiments along with the theoretical calculation of the net potential energy between two vesicles were carried out to study the inter-vesicle interactions. The electrostatic repulsion potential energy was comparable to van der Waals attraction potential energy for the native liposome and the net repulsive potential energy barrier was not high enough to prevent vesicles from coalescence. On the other hand, the steric repulsion force provided by the surface mPEG was the predominant parameter that controlled the colloidal stability of the copolymer-modified liposomes.

Trajectory Studies of Methyl Radical Reaction with Iodine Molecule

The reaction of methyl radical with iodine molecule on an attractive potential energy surface is studied by classical trajectory procedures. The reaction occurs over a wide range of impact parameters with the majority of reactive events occurring in the backward rebound region on a subpicosecond scale. A small fraction of reactive events take place in the forward hemisphere on a longer time scale. The ensemble average of reaction times is 0.36 ps. The occurrence of reactive events is strongly favored when the incident radical and the target molecule align in the neighborhood of collinear geometry. Since the rotational velocity of I2 is slow, the preferential occurrence of reactive events at the collinear configuration of <TEX>$CH_3{\ldots}I{\ldots}$</TEX>I leads to the reaction exhibiting an anisotropic dependence on the orientation of <TEX>$I_2$</TEX>. During the collision, there is a rapid flow of energy from the <TEX>$H_3C{\ldots}$</TEX>I interaction to the I-I bond. The <TEX>$CH_3I$</TEX> translation and <TEX>$H_3C$</TEX>-I vibration share nearly all the energy released in the reaction, and the distribution of the vibrational energy is statistical. The reaction probability is <TEX>$\cong$</TEX>0.4 at the <TEX>$CH_3$</TEX> and I2 temperatures maintained at 1000 K and 300 K, respectively. The probability is weakly dependent on the <TEX>$CH_3\;and\;I_2$</TEX> temperatures between 300 K and 1500 K.