Convergence of the Chemical Potential in Aqueous simulations

The chemical potential of a sphere in water is calculated using a molecular dynamics simulation. The convergence of a continuous sampling method is examined. Free energy calculations for forming a cavity or methane site in water with a thermodynamic integration are found to be strongly dependent on total simulation time length out to hundreds of picoseconds. The implications of insufficient sampling are given and related to relaxation time scales in the solvent about the solute in an Eisenberg-Kauzmann like analysis. Implications for more complex liquids are discussed.     

alpha-Amylase adsorption on starch crystallites

The goal of this work was to characterize the adsorption of Bacillus subtills alpha-amylase onto crystalline starchy materials of the B-type polymorph. Monodisperse spherulitic particles (R z6; 5.0 mum), essentially resistant to alpha-amylolysis at 25 degrees C were prepared from short amylose chains (DP(n) approximately 15). The alpha-amylase adsorbed specifically onto the spherulites, and adsorption was found to be a prerequisite step for hydrolysis. Adsorption was inhibited by the presence of maltose and maltotriose in the reaction mixture. Adsorption isotherm of the enzyme on the particles showed a well developed plateau of 1.62 mug/cm(2) at 25 degrees C corresponding to a monolayer adsorption process. The binding free energy calculated from the initial slope of the isotherm was DeltaG approximately -20.7 kJ/mol. This is smaller than published values for the binding of alpha-amylase to soluble amylosic chains (DeltaG < -30 kJ/mol).     

Density functional theory for the selective adsorption of small molecules on a surface modified with polymer brushes

A density functional method based on weighted density approximation is extended to study the selective adsorption of small molecules on a surface modified with end-grafted square-well chains. The excess part of the Helmholtz free energy functional is divided into two components: the hard sphere repulsion and the square-well attraction. The equation of state for hard sphere chain fluids developed by Liu et al. is used to calculate the repulsive part of the excess Helmholtz free energy functional, and the equation of state for square-well chain fluid with variable range developed by Li et al. is employed to calculate the attractive part. With this theoretical model, we examine the physical properties of the grafted polymer and the selective adsorption of small molecules on the modified surface.     

多微孔胺基树脂的制备及其对胆红素的吸附性能

采用悬浮聚合法制备了一定尺寸的多微孔聚苯乙烯(PS)微球,然后通过Friedel-Crafts反应(用氯乙酰氯替代了有致癌性的氯甲醚)和胺化反应得到新型的胺基树脂,并对反应条件进行了优化。结果表明,利用最优化条件制备的胺基树脂,其离子交换容量为4.1587mmol/g。考察了新型树脂对胆红素的吸附性能,其吸附量最大可达30.85mg/g,吸附率可达80%。     

胡萝卜吸附式低温干燥特性的研究

对胡萝卜吸附式低温干燥过程的干燥特性进行了试验研究。考察了干燥气体 (空气 )的湿度和风量以及物料粒度对胡萝卜干燥特性和复水比的影响 ,得到形状相似的干燥曲线 ,用自定义最小二乘法拟合均能获得较好的结果。结果表明 :吸附式低温干燥过程可使被干燥物料达到超干水平 (含水率 <5 % ) ;增加干燥气体流量对干燥过程进行有利 ,当气体流量从 2 0 0L/h增加到 4 0 0L/h时 ,胡萝卜含水率从 4 1 13%降低到 34 93% ;被干燥物料颗粒大小和形状对干燥过程有显著影响 ;经过吸附式干燥后的胡萝卜色泽鲜艳 ,无褐变 ,外表品质优于热风干燥。吸附式低温干燥后胡萝卜的复水比 (6 5 2 )显著高于热风干燥 (1 78)。     

A study of the adsorption of bacterial cells on porous materials

The paper presents experimental data on the adsorption of bacterial cells on porous materials.Translated from Mikrobiologiya, Vol. 73, No. 6, 2004, pp. 810–816.Original Russian Text Copyright © 2004 by Samonin, Elikova.     

Effect of temperature on adsorption of mixtures in porous materials

The effect of temperature on the adsorption of a simple mixture (Ar/Kr) in disordered porous materials is investigated by means of molecular simulation. In the larger mesopores of porous silica glasses, capillary condensation occurs upon decreasing the temperature. At temperatures above the capillary condensation temperature, Kr is preferentially adsorbed at the pore surface and Ar adsorption occurs in regions of low Kr density. For temperatures below the capillary condensation temperature, Ar density surprisingly increases as temperature increases, the behaviour that is consistent with an over-solubility effect. In contrast, in the disordered sub-nanoporous carbon, filling of the pores occurs in a reversible and continuous way upon decreasing the temperature, owing to the small size and amorphous shape of the pores. These results show that the crossover between capillary condensation and continuous reversible filling observed for pure fluids in pores also exists for mixtures. We also show that the Kr selectivity exhibits a minimum in the disordered porous silica that is located at the capillary condensation temperature. In contrast, in the disordered porous carbon where no capillary condensation occurs, the selectivity decreases monotonically with increasing the temperature. These results shed light on low-temperature adsorption of mixtures confined in porous materials and provide a guide to design efficient phase separation processes.     

Predicting CO2 adsorption and reactivity on transition metal surfaces using popular density functional theory methods

ABSTRACT

In this work, with Ni (110) as a model catalyst surface and CO₂ as an adsorbate, a performance study of Density Functional Theory methods (functionals) is performed. CO being a possible intermediate in CO₂ conversion reactions, binding energies of both, CO₂ and CO, are calculated on the Ni surface and are compared with experimental data. OptPBE-vdW functional correctly predicts CO₂ binding energy on Ni (?62?kJ/mol), whereas CO binding energy is correctly predicted by the rPBE-vdW functional (?138?kJ/mol). The difference in computed adsorption energies by different functionals is attributed to the calculation of gas phase CO₂. Three alternate reaction systems based on a different number of C=O double bonds present in the gas phase molecule are considered to replace CO₂. The error in computed adsorption energy is directly proportional to the number of C=O double bonds present in the gas phase molecule. Additionally, both functionals predict similar carbon–oxygen activation barrier (40?kJ/mol) and equivalent C1s shifts for probe species (?2.6?eV for CCH₃ and +1.5?eV CO₃^?), with respect to adsorbed CO₂. Thus, by including a correction factor of 28?kJ/mol for the computed CO₂ gas phase energy, we suggest using rPBE-vdW functional to investigate CO₂ conversion reactions on different metals.     

羟基磷灰石Zeta电位对其蛋白吸附性能的影响

目的:为了寻找羟基磷灰石(HA)颗粒的蛋白吸附性能与其Zeta电位之间的关系,为研发具有优秀骨修复能力的HA材料提供理论依据。方法:本文采用水热反应法,合成了形状规则的带状HA颗粒,并采用SEM、BET、XRD对其进行表征。检测了HA颗粒在不同p H、PO_4~(3-)浓度中的Zeta电位;选择小牛血清白蛋白(BSA)、溶菌酶(LYS)两种蛋白作为模型蛋白,测试了HA颗粒在不同p H、PO_4~(3-)浓度环境中对BSA或LYS的吸附量。结果:随着吸附体系的p H值逐渐增大,HA颗粒表面负电性迅速增加,HA颗粒对BSA的吸附能力逐渐减弱,对LYS的吸附能力逐渐增强。随着吸附体系中PO_4~(3-)浓度逐渐增大,HA颗粒表面负电性呈先增大后减小的变化趋势,对BSA、LYS的吸附性能均逐渐减弱。结论:p H和PO_4~(3-)浓度的变化,都能引起HA颗粒Zeta电位的变化,前者引起静电作用类型、强度的变化而改变HA蛋白吸附性能;后者引入竞争吸附,蛋白吸附量随竞争离子浓度增多而逐渐减少。     

Computer Simulation Study of the Chemical Potential of Argon Adsorbed on Graphite

Abstract

A multilayer film of argon adsorbed on the basal plane of graphite at 103 K was simulated using isokinetic molecular dynamics. The local chemical potentials in the film were evaluated using three algorithms suggested in the literature: test particle insertion, real particle calculation and the ratio method, which relies on calculations of the local energy distribution functions for test and real particles. Although none of these was suitable for calculations involving the partially solidified first adsorbed layer, the test particle and the ratio method produced useful results for regions in the film corresponding to second and higher layers. The ratio method is shown to be the most realistic, giving constant local chemical potentials of reasonably high precision for all points other than in the first layer.     

335弱碱性阴离子交换树脂对甘草酸的吸附过程

研究了335弱碱性阴离子交换树脂对甘草酸的吸附过程。拟合得到的吸附等温线方程为:c1/[q×(329-c1)]=0.035 8 1.872(c1/329),符合BET方程,计算得出335树脂的饱和吸附量是524.2 mg.g-1。通过吸附动力学曲线的研究,表明该树脂属于慢型吸附类型,得到树脂对甘草酸的吸附穿透曲线,穿透容量为42.00 mg.g-1,饱和容量近似为203.0 mg.g-1,交换柱的利用率小于0.206 9。用碱性洗脱液不易将树脂上吸附的甘草酸洗脱下来,利于甘草浸膏溶液中甘草酸和其它组分的分离。     

Understanding the effect of adsorption on activated processes using molecular theory and simulation

Most chemical reactions of practical interest are catalysed by porous materials, which can improve reaction rates and equilibrium yields through various interactions with the reacting mixture. Nucleation, another kind of activated process, can also be substantially affected by the presence of a surface or by the confinement within a porous material. There are several different effects that can influence such activated processes, such as the reduced dimensionality of the porous space or the adsorbed layer, physical or chemical interactions with the adsorbate and transport limitations. This paper presents a unifying perspective on the influence of each of these effects on chemical reactions and crystallisation processes through the discussion of selected examples from the literature. The common aspects and the differences between these two different kinds of activated processes are considered in the context of each effect. Finally, some of the important issues that could benefit from the development of new molecular simulations methods are discussed.     

Optimized Effective Potential for Atoms and Molecules

We describe the optimized effective potential method of density functional theory and the semi-analytical approximation due to Krieger, Li and Iafrate. Results for atomic and molecular systems including correlation contributions are presented and compared with conventional Kohn–Sham methods. The combination of the exact exchange energy functional with the correlation energy functional of Colle and Salvetti works extremely well for atomic systems, while further improvement is required for molecular systems.     

A hybrid-type approach with MD and DFT calculations for evaluation of redox potential of molecules

A simple calculation method to evaluate the redox potential of molecules by using a hybrid-type calculation with molecular dynamics (MD) and density functional theory calculations is presented with discussions of the difference of the redox potential. In our hybrid method, the standard Gibbs free energy of the molecules, acetone and 3-pentanone, in the redox reaction, is estimated from the average of ionisation free energy and the excess chemical potentials of the reduced and oxidised molecules according to the Born–Haber cycle by sampled configurations from the MD simulation. The difference of the redox potentials between the two molecules is in agreement with the experimental data within the standard deviation.     

Calibration of a Linear Free Energy Estimation Approach for Estimating Stability Constants for Metal-Bacterial Surface Complexes

In this study, we use the measured extent of metal adsorption onto bacterial cells to constrain a linear free energy relationship that allows estimation of unknown stability constants for metal-bacterial surface complexes based on the value of corresponding aqueous metal-acetate stability constants. A previous study (Fein et al., 2001 Fein, J B, Martin, A M and Wightman, P G. 2001. Metal adsorption onto bacterial surface: Development of a predictive approach. Geochim Cosmochim Acta, 65: 4267–4273. [Crossref], [Web of Science ®] , [Google Scholar]) used metal adsorption experiments to constrain a similar relationship, but the experiments were conducted using acid-washed bacteria, and subsequent evidence (Borrok et al., 2004a Borrok, D, Fein, J B, Tischler, M, O'Loughlin, E, Meyer, H, Liss, M and Kemner, K M. 2004a. The effect of acidic solutions and growth conditions on the adsorptive properties of bacterial surfaces. Chem Geol, 209: 107–119. [Crossref], [Web of Science ®] , [Google Scholar]) shows that the acid-washing step affects the extent of adsorption of a number of metals onto bacterial surfaces. We measured the adsorption of Zn, Ni, Co, Sr, and Nd onto Bacillus subtilis in 0.1 M NaClO₄ as a function of pH and metal:bacterial site ratio, using a non-electrostatic discrete four-site model of the bacterial protonation reactions as a basis for the metal adsorption modeling. The adsorption of the divalent cations (Zn, Ni, Co, and Sr) could best be modeled by considering adsorption reactions involving three sites on the bacterial surface; we used a one-site model to account for the Nd data that covered a more restricted pH range. The calculated stability constants for metal-Site 2 bacterial surface complexes are used to re-calibrate the linear free energy relationship previously defined by Fein et al. (2001) Fein, J B, Martin, A M and Wightman, P G. 2001. Metal adsorption onto bacterial surface: Development of a predictive approach. Geochim Cosmochim Acta, 65: 4267–4273. [Crossref], [Web of Science ®] , [Google Scholar]. There is a significant difference between the original and the re-calibrated lines for weakly binding cations such as Sr^{2 +}, but the difference becomes negligible for the stronger-binding cations. Because the linear free energy relationship defined in this study was calibrated from experiments that involved bacteria that were not exposed to acidic conditions, the estimated stability constant values that result from using this relationship are likely to reasonably reflect bacterial adsorption behaviors that occur in realistic geologic settings.     

Composite surface for blocking bacterial adsorption on protein biochips

The design and fabrication of protein biochips requires characterization of blocking agents that minimize nonspecific binding of proteins or organisms. Nonspecific adsorption of Escherichia coli, Listeria innocua, and Listeria monocytogenes is prevented by bovine serum albumin (BSA) or biotinylated BSA adsorbed on SiO(2) surfaces of a biochip that had been modified with a C(18) coating. Biotinylated BSA forms a protein-based surface that in turn binds streptavidin. Because streptavidin has multiple binding sites for biotin, it in turn anchors other biotinylated proteins, including antibodies. Hence, biotinylated BSA simultaneously serves as a blocking agent and a foundation for binding an interfacing protein, avidin or streptavidin, which in turns anchors biotinylated antibody. In our case, the antibody is C11E9, an IgG-type antibody that binds Listeria spp. Nonspecific adsorption of another bacterium, Escherichia coli, is also minimized due to the blocking action of the BSA. The blocking characteristics of BSA adsorbed on C(18)-derivatized SiO(2) surfaces for construction of a protein biochip for electronic detection of pathogenic organisms is investigated.     

Effect of pH and aluminum on surface properties of barley roots as determined from water vapor adsorption

Water vapor adsorption isotherms were used for the estimation of surface areas and adsorption energy distribution functions of roots of barley grown at different pH levels and at a toxic Al level (10 mg·dm⁻³), induced at tillering and shooting stages of plants growth. Values of surface area as well as energy distributions were the same for the roots grown at all pH values studied: 2, 4 and 7 and not dependent on the age of the plants indicating that the protons do not alter the physicochemical build-up of the surface of roots. However, significant changes of the root surface properties under the influence of aluminum: increase of surface area, average adsorption energy and amount of highly energetic adsorption sites together with a decrease of low energetic sites were observed.     

Root surface area measurements based on adsorption and desorption of nitrite

A method, based on the negative adsorption of NO₂ ^-, has been developed to determine surface area of roots. Young roots of 3–4 year old plants ofAcacia catechu, Eucalyptus camaldulensis andLeucaena leucocephala were up-rooted and cut into 18 cylindrical pieces. Each root piece was immersed individually for 10 sec in 0.05M, 0.10M and 0.15M aqueous NaNO₂ solution and the excess solution on the root surface allowed to drain off. It was then transferred to conical flask containing distilled water and shaken for 15 min for desorption of nitrite. A known quantity of this aliquot was reacted with 1% acidic sulphanilamide and 0.02% NED HCl. A pink colour developed, and its optical density was read at 540 nm. A positive linear correlation was noted between colour density and root surface area. The respective correlation coefficientvalues for 0.05M, 0.10M and 0.15M NaNO₂ solutions were 0.973, 0.963 and 0.964 forAcacia catechu, 0.933, 0.903 and 0.898 forEucalyptus camaldulensis and 0.968, 0.976 and 0.972 forLeucaena leucocephala (significant atp<0.001). The method was successfully adopted to determine the root surface area of seedlings ofAlbizia lebbek, A. procera, Acacia auriculiformis, A. nilotica, Dalbergia latifolia andD. sissoo.     

Marginal Magnesium Doping for High‐Performance Lithium Metal Batteries

Due to unparalleled theoretical capacity and operation voltage, metallic Li is considered as the most attractive candidate for lithium‐ion battery anodes. However, Li metal electrodes suffer from uncontrolled dendrite growth and consequent interfacial instability, which result in an unacceptable level of performance in cycling stability and safety. Herein, it is reported that a marginal amount (1.5 at%) of magnesium (Mg) doping alters the surface properties of Li metal foil drastically in such a way that upon Li plating, a highly dense Li whisker layer is induced, instead of sharp dendrites, with enhanced interfacial stability and cycling performance. The effect of Mg doping is explained in terms of increased surface energy, which facilitates plating of Li onto the main surface over the existing whiskers. The present study offers a useful guideline for Li metal batteries, as it largely resolves the longstanding shortcoming of Li metal electrodes without significantly sacrificing their main advantages.