Effects of Surface Oxygen of Activated Carbon on Alkaloid Adsorption: A Molecular Dynamics Simulation Study

The influence of the density and the type of surface oxygen on the adsorption of berberine alkaloid onto activated carbon was investigated using the molecular dynamics simulation method in vacuum. The carbon surface consisted of a basal plane of graphite and surface oxygen groups which were bonded on the graphite plane in a regular square array with various densities. Two types of surface oxygen groups, =O and —OH, were employed. The simulation results showed that the berberine alkaloids were favorable to be adsorbed on the negative charged carbon surfaces. It was indicated that the vdw attraction of the carbon surface to the alkaloid molecule dominates the adsorption only at the lower surface density of oxygen. It is also indicated that a good adsorptive selectivity for a certain berberine alkaloid can be obtained by controlling the density of surface oxygen.The adsorption simulation of berberine alkaloids onto activated carbon in the presence of water was also carried out by using a dome-shape molecular model for presenting the alkaloid/water/carbon system. It was found that the adsorption of berberine alkaloids on the activated carbon which has a higher density of surface oxygen was strongly inhibited by the presence of water.     

A Monte Carlo study of spherical electrical double layer of macroions-polyelectrolytes systems in salt free solutions

A canonical Monte Carlo simulation is performed to investigate the microstructure and the electrical double layer (EDL) of polyelectrolytes around macroions in the bulk systems based on the primitive model. We explore the influences of particles size, chain length, and charge density of polyelectrolytes on the microscopic behavior of the macroions-polyelectrolytes systems. The simulation results show that the surface charge density and the chain length of the polyelectrolytes are two key factors that affect the microstructure of polyelectrolytes around the macroions and potential of mean force between the macroions as well as the zeta potential of the spherical EDL constructed by polyelectrolytes. The high surface charge density of a polyelectrolyte leads to the polyelectrolyte acting as a bridge for the aggregation of macroions, causing the presence of the attraction between macroions. The polyelectrolytes with a long chain length present a cooperativity effect for the adsorption of the polyelectrolytes on the surface of the macroions. Furthermore, the two key factors both induce the overcharge of the macroions. The longer the chain length and the higher surface charge density of the polyelectrolytes, the stronger is the overcharge.     

Molecular dynamics simulation of the density and surface tension of water by particle-particle particle-mesh method

In this work, molecular dynamics simulation is performed to study the density and surface tension of water for a range of temperatures from 300 to 600 K. The extended simple point charge interaction potential for water is used. The particle-particle particle-mesh method, which automatically includes untruncated long-range terms, is used for the Lennard-Jones and the Coulombic terms. The results show that the long-range correction for the Lennard-Jones term is very important for the calculation of surface tension. It is found that the calculated density and surface tension of water fit well with experimental data for temperatures less than 500 K. Near the critical temperature, the simulation results are off from the experimental data.     

Further investigation about Lagrangian theorem-based density functional approximation: test by non-uniform polymer melt

A Lagrangian theorem-based density functional approximation [S. Zhou, New J. Phys. 4 (2002) 36] for hard sphere fluid is employed to describe non-uniform polymer melt in the framework of density functional theory. A required bulk second order direct correlation function (DCF) within the whole density range is obtained by solving the polymer-RISM integral equation, the associated adjustable parameter is specified by a hard wall sum rule, and is found to be a negative value when the bulk density is low and the number of chain segment is large. However, the mathematically meaningless value can be physically meaningful by the observation that the present recipe can produce out density profile in very good agreement with simulation data not only at the contact region, but also at the region far away from the surface, and that the predicted global quantities such as surface excess and surface tension are also in good agreement with the simulation data. It is considered that the LTDFA has a property of self correction, which enables the LTDFA-based DFA for non-uniform polymer melt performs quite well even with a not very accurate second order DCF as input. Potential applications of the self correction peculiarity are discussed.     

Wetting of Crystalline Solids by Associating Fluids

The wetting behavior of Lennard-Jones associating fluid on a crystalline surface is studied using Monte Carlo simulation in the isobaric-isothermal ensemble. We explore the structure of the adsorbed film, characterized by the distribution of all particles and the distributions of centers of mass of dimers, as well as by orientational distributions of the dimers. The simulations performed indicate the existence of a prewetting-like behavior of the adsorption isotherm. The results for a crystalline surface are compared with simular results obtained for a structureless surface. Finally, we present some density functional theory evaluations and compare them with the simulation results at a high temperature. Copyright 2000 Academic Press.     

Monte Carlo simulation and molecular theory of tethered polyelectrolytes

We investigate the structure of end-tethered polyelectrolytes using Monte Carlo simulations and molecular theory. In the Monte Carlo calculations we explicitly take into account counterions and polymer configurations and calculate electrostatic interaction using Ewald summation. Rosenbluth biasing, distance biasing, and the use of a lattice are all used to speed up Monte Carlo calculation, enabling the efficient simulation of the polyelectrolyte layer. The molecular theory explicitly incorporates the chain conformations and the possibility of counterion condensation. Using both Monte Carlo simulation and theory, we examine the effect of grafting density, surface charge density, charge strength, and polymer chain length on the distribution of the polyelectrolyte monomers and counterions. For all grafting densities examined, a sharp decrease in brush height is observed in the strongly charged regime using both Monte Carlo simulation and theory. The decrease in layer thickness is due to counterion condensation within the layer. The height of the polymer layer increases slightly upon charging the grafting surface. The molecular theory describes the structure of the polyelectrolyte layer well in all the different regimes that we have studied.     

Sr掺杂对CaO(100)表面吸附甲醇影响的分子模拟

借助分子模拟手段,研究了锶掺杂对氧化钙表面甲醇吸附行为的影响。构建了甲醇在CaO(100)和CaO(100)-Sr表面吸附的模型,计算了甲醇在氧化钙表面的吸附能和解离活化能,分析了甲醇在氧化钙表面成键的态密度以及锶掺杂前后甲醇在氧化钙表面电荷布局和差分电荷密度,评估了锶掺杂量对氧化钙表面甲醇吸附性能的影响。结果表明,锶掺杂能够显著强化氧化钙对甲醇的吸附性能,降低甲醇的解离活化能,且吸附性能随锶掺杂量的增加而增强;甲醇在氧化钙表面吸附时活化,锶掺杂后活化程度增加。     

The colloidal force of bead-spring chains in a good solvent

A recently developed density functional theory (DFT) for tethered bead-spring chains is used to investigate colloidal forces for the good solvent case. A planar surface of tethered chains is opposed to a bare, hard wall and the force exerted on the bare wall is calculated by way of the contact density. Previously, the case of large wall separation was investigated. The density profiles of the unperturbed chains, in that case, were found to be neither stepfunctions nor parabolas and were shown to accurately predict computer simulation results. In the present paper, the surface forces that result from the distortion of these density profiles at finite wall separation is studied. The resulting force function is analyzed for varying surface coverages, wall separations, and chain lengths. The results are found to be in near quantitative agreement with the scaling predictions of Alexander [S. Alexander, J. Phys. (Paris) 38, 983 (1977)] when the layer thickness is "correctly" defined. Finally, a hybrid Alexander-DFT theory is suggested for the analysis of experimental results.     

Assembly of copolymer blend on nanopatterned surfaces: a molecular simulation study

We report a molecular simulation study on the assembly of an (A7B5)5/A7B5 copolymer blend on nanopatterned surfaces. The density distributions, anisotropic radii of gyration, and conformations of both copolymers are quantitatively characterized. As the width of stripes on the surface decreases, the shape and thickness of the assembled film are found to be in qualitative agreement with those from experiments. The simulation results indicate that the shape and conformation of ordered film can be modulated by tuning the adsorption energy between the surface and the polymer or by adjusting the width of the stripes on the surface. We can regulate the width of the stripes to obtain a desired polymer conformation without altering the assembled film. In remarkable contrast to the pure copolymer, the radii of gyration of the blend in three directions are consistently smaller. The simulation reveals that the addition of a short chain during assembly is of central importance in restructuring the conformations of the long chain.     

Solvation forces between molecularly rough surfaces

Surface heterogeneity affects significantly wetting and adhesion properties. However, most of the theories and simulation methods of calculating solid-fluid interactions assume a standard thermodynamic model of the Gibbs' dividing solid-fluid interface, which is molecularly smooth. This assumption gives rise to a layering of the fluid phase near the surface that is displayed in oscillating density profiles in any theories and simulation models, which account for the hard core intermolecular repulsion. This layering brings about oscillations of the solvation (or disjoining) pressure as a function of the gap distance, which are rarely observed in experiments, except for ideal monocrystal surfaces. We present a detailed study of the effects of surface roughness on the solvation pressure of Lennard-Jones (LJ) fluids confined by LJ walls based on the quenched solid density functional theory (QSDFT). In QSDFT, the surface roughness is quantified by the roughness parameter, which represents the thickness of the surface "corona" - the region of varying solid density. We show that the surface roughness of the amplitude comparable with the fluid molecular diameter effectively damps the oscillations of solvation pressure that would be observed for molecularly smooth surfaces. The calculations were done for the LJ model of nitrogen sorption at 74.4 K in slit-shaped carbon nanopores to provide an opportunity of comparing with standard adsorption experiments. In addition to a better understanding of the fundamentals of fluid adsorption on heterogeneous surfaces and inter-particle interactions, an important practical outcome is envisioned in modeling of adsorption-induced deformation of compliant porous substrates.     

Monte Carlo simulation of the electrical differential capacitance of a double electrical layer formed at the heterogeneous metal oxide/electrolyte interface

A semiempirical simulation study of differential capacitance c(d) is presented for the case in which the formula for the macroscopic surface potential is known. Both the differential capacity and the surface potential are treated as unique functions of the potential determining ions (H+ ions). The effects of surface heterogeneity on the surface charge density curves sigma0 = f(pH), and capacity curves c(d) = f(pH), as well as on the position of maximum capacitance are discussed. The "model" effects (influence of the model parameter on the results) are presented.     

Nucleation kinetics of heterogeneous catalyst: Pd/thin layer of MgO (100)

The kinetics describing the formation of the system Pd/thin layer of MgO (100) surface was investigated by applying programs developed on the basis of Fortran Software. The simulation is based on studies related to nucleation, crystallite growth, coalescence and diffusion of clusters on the surface of thin films. The density of Pd islands is obtained by simulating the deposition of 1 × 10¹³ atoms cm^–2 s^–1 in 150 s. The density of clusters first increases to reach a plateau. Then, it either remains some time at a nearly constant value or decreases slowly after the saturation regime gives way to the coalescence stage. This phenomenon is explained via island migration process at the surface. The coalescence time strongly depends on the deposition temperature, the coalescence occurs when the substrate temperature is high. Also, the surface coverage decreases when the substrate temperature increases. The coalescence is pronounced at a low surface coverage. It involves the migration on the surface, defined as the dynamic coalescence and precedes the process of the formation of immobile islands that predominates at high extents of surface coverage.     

Effect of SDBS solution on the surface potential properties of the Zhaozhuang coal

It is of great significance to study the effect of surfactants on the coal surface potential for effective dust suppression in mining faces. The effect of different concentrations of sodium dodecyl benzene sulfonate (SDBS) solution on the surface potential of the Zhaozhuang coal was measured by atomic force microscopy (AFM). The experimental results show that the SDBS solution has significant influence on the surface potential of the Zhaozhuang coal. The electrical characteristics of the coal surface at the nanometer scale are different from those of macroscopic or the mesoscopic level. The surface potential of coal is basically a normal distribution, showing certain electrical characteristics. The mean value of the surface potential of the Zhaozhuang coal is increased with the increase in concentration of the SDBS solution; when the concentration of the SDBS solution is 0.3%, the mean value of surface potential is 5.59 mv, which is about two times of the mean value of the surface potential without SDBS solution added. With the increase of the concentration of the SDBS solution, the maximum value of the surface potential of the Zhaozhuang coal increases, and the minimum value decreases. It shows that the SDBS solution has a significant effect on the potential distribution law and the magnitude of the coal surface. Subsequently, on the basis of the constructed Zhaozhuang coal macromolecule model, xtb and Multiwfn simulation software were used to calculate the molecular surface electrostatic potential value and electron density value of the Zhaozhuang coal molecules after adding water molecules. The variation law for the electrostatic potential surface of the molecule was obtained after adding numbers of water molecules to the surface of the coal molecule. The simulation results show that the area proportion of absolute molecular surface electrostatic potential greater than 10 kcal/mol is increased with the growth in the number of water molecules, while the area proportion of absolute molecular surface electrostatic potential less than 10 kcal/mol is decreased. Because of the free state O─H bond polarity of water molecules, the charges on the molecular surface are rearranged in order to change the electron density on the surface of coal molecules, which affects the overall electrostatic potential of the configuration.     

Mercedes-Benz water molecules near hydrophobic wall: integral equation theories vs Monte Carlo simulations

Associative version of Henderson-Abraham-Barker theory is applied for the study of Mercedes-Benz model of water near hydrophobic surface. We calculated density profiles and adsorption coefficients using Percus-Yevick and soft mean spherical associative approximations. The results are compared with Monte Carlo simulation data. It is shown that at higher temperatures both approximations satisfactory reproduce the simulation data. For lower temperatures, soft mean spherical approximation gives good agreement at low and at high densities while in at mid range densities, the prediction is only qualitative. The formation of a depletion layer between water and hydrophobic surface was also demonstrated and studied.     

Solvation structure and dynamics for passivated Au nanoparticle in supercritical CO2: a molecular dynamic simulation

All-atomic molecular dynamics simulations have been performed to study the interfacial structural and dynamical properties of passivated gold nanoparticles in supercritical carbon dioxide (scCO(2)). Simulations were conducted for a 55-atom gold nanocore with thiolated perfluoropolyether as the packing ligands. The effect of solvent density and surface coverage on the structural and dynamical properties of the self-assembly monolayer (SAM) has been discussed. The simulation results demonstrate that the interface between nanoparticle and scCO(2) solvent shows a depletion region due to the preclusion of SAM. The presence of scCO(2) solvent around the passivated Au nanoparticle can lead to an enhanced extension of the surface SAM. Under full coverage, the structure and conformation of SAM are insensitive to the density change of scCO(2) fluid. This simulation results clarify the microscopic solvation mechanism of passivated nanoparticles in supercritical fluid medium and is expected to be helpful in understanding the scCO(2)-based nanoparticle dispersion behavior.     

表面电荷及外电场对液固界面热阻的影响

采用非平衡分子动力学方法模拟了外电场及固体表面电荷对水与固体间界面热阻的影响. 结果表明,外加电场平行于界面时, 其对界面热阻几乎没有影响, 而垂直于界面时, 界面热阻将随着电场强度的增大而减小. 壁面带正电荷或负电荷都将使得界面热阻减小. 界面热阻与表面电荷密度及电场强度均满足二次函数关系. 模拟结果表明施加外电场和表面电荷是控制液固界面热阻的有效方法.     

Relation between packing density and thermal transitions of alkyl chains on layered silicate and metal surfaces

Self-assembled layers of alkyl chains grafted onto the surfaces of layered silicates, metal, and oxidic nanoparticles are utilized to control interactions with external media by tuning the packing density of the chains on the surface, head group functionality, and chain length. Characterization through experiment and simulation shows that the orientation of the alkyl layers and reversible phase transitions on heating are a function of the cross-sectional area of the alkyl chains in relation to the available surface area per alkyl chain. On even surfaces, a packing density less than 0.2 leads to nearly parallel orientation of the alkyl chains on the surface, a high degree of conformational disorder, and no reversible melting transitions. A packing density between 0.2 and 0.75 leads to intermediate inclination angles, semicrystalline order, and reversible melting transitions on heating. A packing density above 0.75 results in nearly vertical alignment of the surfactants on the surface, a high degree of crystalline character, and absence of reversible melting transitions. Curved surfaces can be understood by the same principle, taking into account a local radius of curvature and a distance-dependent packing density on the surface. In good approximation, this simple model is independent from the length of the alkyl chains (a minimum length of C10 is required to form sufficiently distinctive patterns), the chemical nature of the surface, and of the surfactant head group. These structural details primarily determine the functionality of alkyl modified surfaces and the temperature of thermal transitions.     

Density functional theory for the recognition of polymer at nanopatterned surface

The recognition of homopolymer at nanopatterned surface has been investigated by density functional theory (DFT). Chain conformation and pattern transfer parameter predicted from the DFT are in good agreement with Monte Carlo simulation results. The theory describes satisfactorily the transition from depletion at low packing fractions to adsorption and double-layer adsorption at high packing fractions and also accounts for the crucial effect of the segment-wall interaction. It is found that homopolymer is better recognized at a low bulk density and a stronger interaction with the surface. The polymer can not only recognize the surface but also invert the surface at high bulk densities. The chain in the solution-wall interface exhibits a typical "brush" conformation with a length approximated by half the length of polymer chain.     

Calculating a Characteristic Bulk Current Density in the Cathode of a Hydrogen-Oxygen Fuel Cell with a Solid Polymer Electrolyte

The magnitude of currents of electrodes in hydrogen-oxygen fuel cells of all types is shown to be fully determined by values of the effective coefficient of gas diffusion, the effective coefficient of ionic conduction, and the characteristic bulk current density. The characteristic bulk current density is estimated in two versions for cathodes with Nafion: the catalyst is distributed in the bulk of substrate grains or at their external surface. The currents commensurate with those observed in experiments are given only by the second version. Means of computer-aided simulation are used to imitate the formation of fractal films composed of the catalyst particles on the surface substrate grains. The simulation means made it possible to link the magnitude of the specific surface area of platinum particles with its weight content in substrate grains. Electrochemical characteristics of the cathode with Nafion-the potential dependence of the optimum magnitude of the overall current and the thicknesses of the active layer and the weight of platinum in it, as well as the magnitudes of the optimum current generated by a unit weight of platinum—are calculated. A notion of “ norm” is introduced for the characteristic bulk current density of the cathode. 1 × 10^?3 A cm^?3 is the electrochemical-process intensity, which the technology of preparation of active layers of cathodes can provide at this stage in the development of fuel cells with a solid polymer electrolyte.     

尿素在ZnO(1010)表面的吸附

运用VASP(Vienna ab-initio simulation package),采用基于密度泛函理论(DFT)的第一原理计算,研究了尿素在ZnO(101軈0)表面的吸附行为.计算结果表明:尿素分子在ZnO(101軈0)表面主要发生分子吸附,稳定的吸附产物通过尿素分子中的氮原子或氧原子与表面锌原子之间的键合作用而形成,吸附能分别为-1.48和-1.41eV;表面吸附的尿素分子也可以发生解离,生成表面吸附的异氰酸根、氨气和一个表面羟基,吸附能为-1.66eV.