Molecular dynamics simulation of triaxial compression of C60 and C80 solids

Present work investigates the triaxial compression behavior of face-centered cubic C₆₀ and C₈₀ solids using molecular dynamics simulation. Second-generation empirical bond-order potential governs the atomic interactions within a C₆₀ or C₈₀ molecule, whereas van der Waals potential dominates the interactions between C₆₀ or C₈₀ molecules. The equilibrium lattice spacings for C₆₀ and C₈₀ solids are obtained as 14.26 Å and 15.56 Å, respectively. Investigation focuses on the effects of: (i) van der Waals potential, (ii) temperature and (iii) loading rate, on the bulk moduli and hydrostatic stress vs. volumetric strain curves of C₆₀ and C₈₀ solids. Our results showed that these properties are dependent on loading rate and the choice of van der Waals potential, but insensitive to temperature change.     

The effect of van der Waals interactions on the properties of intrinsic defects in graphite

The formation energies and geometries of intrinsic interstitial and vacancy point defects in graphite are calculated using density functional theory and the generalized gradient approximation with a semi-empirical van der Waals (vdW) correction for dispersion interactions. Inclusion of the vdW terms in the simulations leads to energetically unfavorable out of plane distortions and inter-layer bonding for isolated vacancies (V) and interstitials (I), respectively. Conversely, despite changes in the calculated formation energies, inclusion of vdW terms only slightly affects the optimized inter-layer bonding in di-vacancies (VV), di-interstitials (I₂) and close interstitial-vacancy (I-V) pairs. On this basis, the interaction between defects is the origin of the inter-layer bonding in unsheared graphite. Partial shearing of graphite layers is shown to significantly lower (by 0.5 eV) the I-V recombination barrier with respect to the value calculated in perfectly AB-stacked hexagonal graphite (1 eV). We also find that the defect-defect interaction acts to cancel defect magnetic moments and excludes contributions of VV, I₂ and I-V pairs to the measured magnetic signal of irradiated graphite.     

Molecular Bonding Mechanism for Solid Adhesion

The purpose of this study is to understand why and how solids can be bonded together with and without an adhesive. Beside van der Waals interactions and chemical bonding, there are some intermediate interactions, such as the Coulombic and charge-transfer interactions. These interactions are also called molecular interactions. Thus, molecular bonding mechanism for solids deals mainly with the formation of an adhesive bond through molecular interactions. The driving forces for molecular interactions are discussed in terms of adhesive energy and separation distance. The functions of electrons are illustrated with molecular orbitals. Moreover, some unique interactions between a molecule and the surface of a solid are demonstrated with the results found by Hoffmann.     

Dispersive forces of particle–surface interactions: direct AFM measurements and modelling

Fundamentals of particle–particle interaction are of great interest in agglomeration processes. Particle adhesion depends on dispersive forces (van der Waals force), local chemical bindings, Coulomb force and capillary attractions. Additionally, surface properties like roughness, adsorption layers and surface chemistry strongly affect adhesion forces. van der Waals interactions are poorly understood because popular ab initio force calculations for molecules like density functional theory (DFT) often do not lead to proper results. van der Waals forces are difficult to measure directly. We present direct measurements of particle–particle and particle–surface interactions in the gas phase carried out with an atomic force microscope (AFM). Special emphasis is given to a proper statistical treatment of the data. For modelling of particle adhesion, we use computer-assisted empirical potential methods. Parameters like adsorbed water and surface roughness are considered. We extract parameters for weak interactions from the Lifshitz theory and gas adsorption data. Adsorbing molecules can be used as probes to measure dispersive forces. Studying surface and particle properties combined with computer-assisted modelling is a basic requisite to reach the aim of predicting particle–particle interactions in industrial processes.     

Experimental and MD simulation study on the physical and mechanical properties of organically modified montmorillonite clay and compatibilized linear low density polyethylene nanocomposites

Understanding the interfacial interactions plays a key role in controlling mechanical and physical properties of polymer/clay nanocomposites (PCNs). In this work, the surface interactions between constituents of experimentally prepared PCNs which are the pristine linear low density polyethylene (PE) chains, PE compatibilizers, montmorillonite clay surface layer, and surfactants were studied quantitatively by employing molecular dynamics simulation technique. The interaction energy between the polymer and the clay was found to be inversely proportional with the volume of the surfactant which decreases the electrostatic interactions between the compatibilizer and the hydrophilic clay surface. However, the van der Waals (vdW) interactions between alkyl tails of surfactants and the PE chains increase with the tail length of the surfactants. The most attractive interaction was between the surfactant's head group and the clay surface. We showed that there existed fine balance between the electrostatic and vdW type forces on the stability and the enhanced properties of the PE–organoclay nanocomposites. Calculated interaction energies were then correlated to the experimentally measured mechanical properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45817.     

Measurement and correlation of solid drugs solubility in supercritical systems

A dynamic experimental set-up was utilized to measure ibuprofen solubility in supercritical CO2 at the pressure range of 8-13 MPa and the temperatures of 308, 313 and 318 K. Mole fraction values varied from 0.015＆#215;10^-3 to 3.261＆#215;10^-3 and correlated by using seven different semi empirical equations of state （Bartle, Modi-fied Bartle, Mendez-Teja, Modified Mendez-Teja, Kumar-Johnson, Sung-shim and Gordillo） as well as seven cubic equations of state （van der Waals, Redlich-Kwong, Soave-Redlich-Kwong, Peng-Robinson, Stryjek-Vera, Patel-Teja-Valderana and Pazuki）. Single and twin-parametric van der Walls mixing rules （vdW1, vdW2） were ap-plied in order to estimate the supercritical solution properties. The physicochemical properties were also obtained using Joback, Lydersen and Ambrose methods. Absolute average relatives deviation （AARD） were calculated and compared for all the correlating systems. Results showed that among the cubic equations of state （EOSs） the Pazuki equation （AARD=19.85% using vdW1 and AARD=8.79% using vdW2） and SRK equation （AARD=19.20%using vdW1 and AARD=10.03%using vdW2） predicted the ibuprofen solubility in supercritical CO2 with the least error in comparison to the others. Among the semi-empirical EOSs the most desirable deviation （AARD〈10%） was obtained by using Modified Bartle and Modified Mendez-Teja equations in all the studied temperatures.     

Integrated simulation of active carbon nanotube forest growth and mechanical compression

Carbon nanotube (CNT) forests are CNT populations that self-assemble into vertically oriented cellular arrays during growth. The anisotropic and inhomogeneous morphology of forests arises from complex mechanical interactions between CNTs during their collective growth and influences many forest properties. A time-resolved simulation is developed to model actively growing CNT populations having distributed properties and growth characteristics. The model considers van der Waals (vdW) attraction between neighboring CNTs and allows the growing and deforming CNTs to interact and react based on a balance of forces. Parametric variations of growth rate distribution and CNT occupation density generate variable CNT forest morphology in manners consistent with experimental observations. The forces opposing vdW bonding between contacting CNTs during forest growth are found to diminish with distance from the growth substrate and are proportional to CNT bending stiffness. Axial and transverse compression of simulated forests capture experimentally observed phenomena of coordinated axial buckling, transverse densification, and the foam-like force–displacement response that is typical of CNT forests. This new paradigm in CNT forest modeling may be used as an analytical tool to examine CNT forest growth kinetics, multi-physics CNT forest performance, and the post-synthesis processing and forming of CNT forest microstructures.     

Estimation of solubility of solids in supercritical carbon dioxide

Peng-Robinson equation of state(PR EOS)was chosen for modeling the thermodynamic be-havior of supercritical(SC)-CO_2/Solid systems.The necessary critical constants and acentric factorof the solute were obtained by the Sigmund and Trebble(1992)method based on the molecular weightand boiling temperature,and the vapor pressure of the solute was calculated by its meltingtemperature and heat of fusion.This approach compared very favorably with the conventional corres-ponding state theory,but without using critical constants and vapor pressure of solutes.Four mixingrules were tested for the calculation of solid solubility in SC-CO_2.van der Waals(vdW)mixing rulewith one parameter was considered to be most suitable for the estimation of solubility.A simplecorrelation was developed for the SC-CO_2/solid binary interaction coefficient k_(ij) with the meltingtemperature of pure solutes.The solubilities of solids in SC-CO_2 were estimated for eleven binarysystems at various temperatures,the total absolute average     

液体混合物密度的关联和预测

液体混合物密度的关联和预测陈新志，侯虞钧（浙江大学化工热力学研究室，杭州３１００２７）关键词密度，双液理论，液体混合物１引言液体的密度数据不仅在许多工程设计中不可缺少，而且是流体理论研究的重要参数。本文就是以双液理论和ｖａｎｄｅｒＷａａｌｓ混合法则建...     

Radial Corrugations of Multi-Walled Carbon Nanotubes Driven by Inter-Wall Nonbonding Interactions

We perform large-scale quasi-continuum simulations to determine the stable cross-sectional configurations of free-standing multi-walled carbon nanotubes (MWCNTs). We show that at an inter-wall spacing larger than the equilibrium distance set by the inter-wall van der Waals (vdW) interactions, the initial circular cross-sections of the MWCNTs are transformed into symmetric polygonal shapes or asymmetric water-drop-like shapes. Our simulations also show that removing several innermost walls causes even more drastic cross-sectional polygonization of the MWCNTs. The predicted cross-sectional configurations agree with prior experimental observations. We attribute the radial corrugations to the compressive stresses induced by the excessive inter-wall vdW energy release of the MWCNTs. The stable cross-sectional configurations provide fundamental guidance to the design of single MWCNT-based devices and shed lights on the mechanical control of electrical properties.     

Multiplicity of thermodynamic states of van der Waals gas in nanobubbles

The gas-containing nanobubbles have attracted extensive attention due to their remarkable properties and extensive application potential. However, a number of fundamental aspects of nanobubbles, including thermodynamic states for the confined gas, remain still unclear. Here we theoretically demonstrate that the van der Waals(vd W) gases confined in nanobubbles exhibit a unique thermodynamic state of remarkably deviating from the bulk gas phase, and the state transition behavior due to the sizedependent Laplace pressure. In general, the vd W gas inside nanobubbles present multiple stable or transient states, where 0–2 states are for supercritical gas and 0–4 for subcritical gas. Our further analysis based on Rayleigh–Plesset equation and free energy determination indicates that the gas states in nanobubbles exhibits different levels of stability, from which the coexistence of multiple bubble states and microphase equilibrium between droplets and bubbles are predicted. This work provides insight to understand the thermodynamic states appeared for gas in nanobubbles.     

A density dependent dispersion correction

Density functional approximations fail to provide an accurate treatment of weak interactions. More recent, but not readily available functionals can lead to significant improvements. A simple alternative to correct for the missing weak interactions is to add, a posteriori, an atom pair-wise dispersion correction. We here present a density dependent dispersion correction, dDXDM, which dramatically improves the performance of popular functionals (e.g., PBE-dDXDM or B3LYP-dDXDM) for a set of 145 systems featuring both inter- and intramolecular interactions. Whereas the highly parameterized M06-2X functional, the long-range corrected LC-BLYP and the fully non-local van der Waals density functional rPW86-W09 also lead to improved results as compared to standard DFT methods, the enhanced performance of dDXDM remains the most impressive.     

Measurement and calculation of solubility of quinine in supercritical carbon dioxide

Solubility of quinine in supercritical carbon dioxide(SCCO_2) was experimentally measured in the pressure range of 8 to 24 MPa, at three constant temperatures: 308.15 K, 318.15 K and 328.15 K. Measurement was carried out in a semi-dynamic system. Experimental data were correlated by iso-fugacity model(based on cubic equations of state, CEOS), Modified Mendez–Santiago–Teja(MST) and Modified Bartle semi-empirical models. Two cubic equations of state: Peng–Robinson(PR) and Dashtizadeh–Pazuki–Ghotbi–Taghikhani(DPTG) were adopted for calculation of equilibrium parameters in CEOS modeling. Interaction coefficients(k_(ij) l_(ij)) of van der Waals(vdW) mixing rules were considered as the correlation parameters in CEOS-based modeling and their contribution to the accuracy of model was investigated. Average Absolute Relative Deviation(AARD) between correlated and experimental data was calculated and compared as the index of validity and accuracy for different modeling systems. In this basis it was realized that the semi-empirical equations especially Modified MST can accurately support the theoretical studies on phase equilibrium behavior of quinine–SCCO_2 media. Among the cubic equations of state DPGT within two-parametric vd W mixing rules provided the best data fitting and PR within one-parametric vd W mixing rules demonstrated the highest deviation respecting to the experimental data. Overall, in each individual modeling system the best fitting was observed on the data points attained at 318 K, which could be perhaps due to the moderate thermodynamic state of supercritical phase.     

Phase behavior of mixture of supercritical CO2 + ionic liquid: Thermodynamic consistency test of experimental data

Various models have been applied composed of the Peng‐Robinson equation of state (PR‐EoS) and the Soave‐Redlich‐Kwong equation of state (SRK‐EoS) associated with three mixing rules including the following: Wong‐Sandler (WS), van der Waals one (vdW1), and van der Waals two (vdW2) for phase behavior modeling of mixtures of supercritical CO₂ + different ionic liquids in vapor–liquid equilibrium (VLE) region. It has been found that the PR EoS implying the WS mixing rule can be used as a reliable thermodynamic model to perform a thermodynamic consistency test on the experimental data of phase behaviors of the supercritical CO₂ + ionic liquid systems (19 commonly‐used ionic liquids have been studied). The results show that 40% of the experimental data seem to be thermodynamically consistent, 55.5% seem to be thermodynamically inconsistent, and 4.5% seem to be not fully consistent. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3892–3913, 2013     

Direct measurement of grafting strength between an individual carbon nanotube and a carbon fiber

Hierarchical structures consisting of carbon nanotubes (CNTs) grafted onto a carbon fiber (CF) have the potential to improve the performance of fiber/polymer composites. The strength between a CNT and a CF is a key factor that influences the load-transfer behavior and inter-laminar properties. Here, we directly measured the grafting strength of a chemically bonded CNT–CF hierarchical structure by detaching individual CNT from the CF substrate and simultaneously recording the force–displacement characteristics in a scanning electron microscopy equipped with a nano-manipulator. We observed a relatively wide distribution of the maximum forces at complete detachment for different grafted CNTs, which ranges from below the van der Waals (vdW) force existing at the CNT–CF interface up to 7 times higher than that. For a typical configuration where a CNT is partially anchored on a CF, we obtained grafting strengths in the range of 5–90 MPa, which are dominated by the vdW force as well as other factors such as chemical bonding. Our results, based on the measurements at individual nanostructure level, might be useful for designing and fabrication of high performance hierarchical composites.     

Molecular modeling and atomistic simulation strategies to determine surface properties of perfluorinated homopolymers and their random copolymers

Molecular mechanics (MM) and molecular dynamics (MD) simulations on ten perfluoroalkyl methacrylates and four copolymers derived from methyl methacrylate (MMA) and 1,1-dihydroperfluorohendecyl methacrylate (F10MA) in different ratios have been performed to predict surface properties. 1,1-Dihydroperfluorohendecyl methacrylate, which contained highest number of fluorine atoms, exhibited lowest surface energy, a trend that is in accordance with experimental observations. Density calculations on selected perfluoroalkyl methacrylates have been performed using NPT dynamics, for which no experimental data are available. Computations were performed to obtain bulk properties like cohesive energy density and solubility parameter through MM and MD simulations in the NVT ensemble under periodic boundary conditions. From the equilibrated structures, surface energies were computed, which compared well with the experimental data reported in the literature. Surface energies of copolymers decreased with increasing number of perfluoroalkyl groups. Various components of energetic interactions have been examined in detail in order to gain a better insight into interactions between bulk structure and the film. The dominant contributions are from van der Waals and Coulombic energy terms. The computed mass density profile for thin films gave an indication whether the film is of sufficient thickness so that the interior of the film is indistinguishable from the bulk structure. The total pair correlation and bond correlation functions have been analyzed to confirm the amorphous nature of the simulated structures.     

A Review of Adhesion Mechanisms Using the Peel Test in Air and Liquid Media

This paper deals with the analysis of peel energy of assemblies measured in different environments, i.e. in air and in the presence of liquids, and constitutes a brief review of the work of Professor Schultz' team in this domain. It is shown how such measurements can lead to a better knowledge of the nature as well as of the magnitude of fundamental interactions established at the interface between two solids. Earlier experiments have shown that peel energy can be expressed as a product of three terms corresponding, respectively, to the reversible energy of interfacial adhesion, the hysteretic losses of the bulk materials and the molecular dissipation near the crack front during peeling. This approach is well-verified when only physical interactions (van der Waals) are involved at the interface. However, more complex cases correspond to systems where specific interactions are also established between both materials, in particular acid-base interactions and creation of chemical bonds. In both cases, peel measurements in liquid media can lead to the determination of fundamental parameters, such as the interfacial density of specific interactions at the interface and the acid-base or chemical components of the work of adhesion. Finally, the effect of interdiffusion phenomena on peel energies can also be investigated in the case of elastomer/elastomer assemblies.     

Density functionals with broad applicability in chemistry

Although density functional theory is widely used in the computational chemistry community, the most popular density functional, B3LYP, has some serious shortcomings: (i) it is better for main-group chemistry than for transition metals; (ii) it systematically underestimates reaction barrier heights; (iii) it is inaccurate for interactions dominated by medium-range correlation energy, such as van der Waals attraction, aromatic-aromatic stacking, and alkane isomerization energies. We have developed a variety of databases for testing and designing new density functionals. We used these data to design new density functionals, called M06-class (and, earlier, M05-class) functionals, for which we enforced some fundamental exact constraints such as the uniform-electron-gas limit and the absence of self-correlation energy. Our M06-class functionals depend on spin-up and spin-down electron densities (i.e., spin densities), spin density gradients, spin kinetic energy densities, and, for nonlocal (also called hybrid) functionals, Hartree-Fock exchange. We have developed four new functionals that overcome the above-mentioned difficulties: (a) M06, a hybrid meta functional, is a functional with good accuracy "across-the-board" for transition metals, main group thermochemistry, medium-range correlation energy, and barrier heights; (b) M06-2X, another hybrid meta functional, is not good for transition metals but has excellent performance for main group chemistry, predicts accurate valence and Rydberg electronic excitation energies, and is an excellent functional for aromatic-aromatic stacking interactions; (c) M06-L is not as accurate as M06 for barrier heights but is the most accurate functional for transition metals and is the only local functional (no Hartree-Fock exchange) with better across-the-board average performance than B3LYP; this is very important because only local functionals are affordable for many demanding applications on very large systems; (d) M06-HF has good performance for valence, Rydberg, and charge transfer excited states with minimal sacrifice of ground-state accuracy. In this Account, we compared the performance of the M06-class functionals and one M05-class functional (M05-2X) to that of some popular functionals for diverse databases and their performance on several difficult cases. The tests include barrier heights, conformational energy, and the trend in bond dissociation energies of Grubbs' ruthenium catalysts for olefin metathesis. Based on these tests, we recommend (1) the M06-2X, BMK, and M05-2X functionals for main-group thermochemistry and kinetics, (2) M06-2X and M06 for systems where main-group thermochemistry, kinetics, and noncovalent interactions are all important, (3) M06-L and M06 for transition metal thermochemistry, (4) M06 for problems involving multireference rearrangements or reactions where both organic and transition-metal bonds are formed or broken, (5) M06-2X, M05-2X, M06-HF, M06, and M06-L for the study of noncovalent interactions, (6) M06-HF when the use of full Hartree-Fock exchange is important, for example, to avoid the error of self-interaction at long-range, (7) M06-L when a local functional is required, because a local functional has much lower cost for large systems.     

Simple Views on Metal/Oxide Interfaces: Contributions of the Long-Range Interactions to the Adhesion Energy

The image and van der Waals contributions to the metal/oxide work of adhesion are compared through the extent to which they follow the known prevalent trends, i.e. the increase in work of adhesion (a) with narrowing oxide band gap and (b) with increasing conduction electron density of the metal. The van der Waals interaction is shown to follow both trends, while the image term is suggested to be significant only for dense metals in contact with very ionic oxides. The relative contribution of these long-range interactions to the overall metal/oxide work of adhesion is found to be maximized for systems involving metals with low electronic densities and oxides with wide band gaps. At variance, high metallic electronic densities and narrow oxide gaps likely favour short-range interactions arising from charge transfer.