Cooperative binding of drugs on human serum albumin

In order to explain the adsorption isotherms of the amphiphilic penicillins nafcillin and cloxacillin onto human serum albumin (HSA), a cooperative multilayer adsorption model is introduced, combining the Brunauer–Emmet–Teller (BET) adsorption isotherm with an amphiphilic ionic adsorbate, whose chemical potential is derived from Guggenheim's theory. The non-cooperative model has been previously proved to qualitatively predict the measured adsorption maxima of these drugs [Varela, L. M., García, M., Pérez-Rodríguez, M., Taboada, P., Ruso, J. M., and Mosquera, V., 2001, J. chem. Phys., 114, 7682]. The surface interactions among adsorbed drug molecules are modelled in a mean-field fashion, so the chemical potential of the adsorbate is assumed to include a term proportional to the surface coverage, the constant of proportionality being the lateral interaction energy between bound molecules. The interaction energies obtained from the empirical binding isotherms are of the order of tenths of the thermal energy, therefore suggesting the principal role of van der Waals forces in the binding process.     

Adsorption of hydrogen in defective carbon nanotube: modelling and consequent investigations using statistical physics formalism

ABSTRACT

Experimental adsorption isotherms of hydrogen in CNT samples (CNT-A, activated CNT-A CNT-B and activated CNT-B) at T?=?77, 87 and 90?K have been fitted using some theoretical model expressions treated by statistical physics through the grand canonical ensemble. The monolayer model with single energy is selected to fit and interpret the experimental data obtained with CNTs. The physico-chemical parameters, interfering in the adsorption process and implicated in the model expressions, could be directly determined from the experimental data through numerical simulation. Three parameters of the model are fitted, namely the number of hydrogen molecule per site n, the interstitial site density N_m and the energetic parameter P_1/2. The evolution of these parameters as function of temperature is plotted and interpreted in relation to adsorption process. Finally, the thermo-dynamic potential functions, which involve in the adsorption mechanism like free enthalpy of Gibbs G_a, internal energy E_int and entropy S_a, are derived by statistical physics calculations from the selected model.     

The adsorption of argon on ZnO at 77 K

We have studied the adsorption of argon onto ZnO surfaces at 77 K by means of quasi-equilibrium adsorption volumetry coupled with high resolution microcalorimetry and Grand Canonical Monte-Carlo (GCMC) simulations. The adsorbate/surface adsorption potential function (PN type) used in the simulations, was determined on the basis of ab initio calculations (corrected for dispersion interactions). The first aspect of this work was to test the ability of a standard solid-state Hartree—Fock technique coupled with a perturbative semi-empirical approach in deriving a reliable adsorption potential function. The dispersion part of the adsorbate/surface interatomic potential was derived by using perturbation theory-based equations while the repulsive and induction interactions were derived from periodic Hartree—Fock (CRYSTAL92) calculations. GCMC simulations based on this adsorption potential allow one to calculate adsorption isotherms and isosteric heat versus loading curves as well as singlet distribution functions at 77 K for each type of ZnO (neutral and polar) faces. The combined analysis of the simulation data for all surfaces gives a good insight of the adsorption mechanism of argon onto ZnO surfaces at 77 K in agreement with experiment. As far as neutral surfaces are concerned, it is shown that adsorption first takes place within the ‘troughs’ which cover ZnO neutral surfaces. At low chemical potentials, these semi-channels are preferential adsorption sites in which we could detect a nearly one-dimensional adsorbate freezing in a commensurate phase at 77 K. The polar O faces are the most favourable surfaces for adsorption at higher chemical potentials.     

Mean field theory and monte carlo simulation of multisite adsorption

Multisite or $\text{1}\text{n}$ adsorption on single crystal surfaces takes place when the adatom diameter is greater than the distance between two neighbouring adsorption sites. The adsorption isotherm has been deduced using a mean field approximation. The same problem was studied by Monte Carlo simulation. The results show asymmetric adsorption isotherms with respect to a degree of coverage of 0.5. The critical temperature of a first order phase transition is increased as compared with the case of $\text{1}\text{1}$ adsorption. The theoretical and the simulated isotherms are in a good agreement for low adatom-adatom attraction. The theoretical and simulated isotherms obtained were used for interpretation of experimental results on electrolytic underpotential adsorption of Tl and Pb on Ag(100) faces.     

Dynamic effects of ion-lattice coupling for rare-earth ions

The adsorption isotherms for Kr monolayers on graphite measured by A. Thomy and X. Duval, J. chim. phys., 67, 1101 (1970) are transformed into the ‘spreading pressure’ as a function of the number density of adsorbed molecules and compared with the pressure of a two-dimensional Lennard-Jones fluid determined by the molecular dynamics technique. The comparison shows that the mobile-layer model for adsorption fails at low temperatures. The liquid-solid phase transitions in the adsorption monolayer appear at lower densities than in a two dimensional Lennard-Jones fluid and the adsorbed liquid layer is compressible in contrast to the two-dimensional Lennard-Jones liquid.     

PbTe/CdTe量子点的光学增益

PbTe/CdTe量子点是一类新型异系低维结构材料,实验发现具有强的室温中红外光致发光现象.为研究这一材料体系的发光特性,建立了理论模型,计算了PbTe/CdTe量子点的光学跃迁和增益.模型基于k·p包络波函数方法并考虑了PbTe能带结构的各向异性.分析了量子点光学增益与量子点尺寸、注入载流子浓度的关系.结果表明,当注入载流子浓度在(0.3—3)×10¹⁸cm^-3范围时,尺寸为15—20nm的量子点可以产生 关键词： PbTe/CdTe量子点 光学增益 铅盐矿半导体     

Statistical thermodynamics of isolated rigid rod-like molecules near a surface

The configurational behaviour and thermodynamic properties of a dilute gas of rigid rod-like molecules in the vicinity of a macroscopic planar adsorption surface are investigated using statistical mechanics. The interaction energy between the surface and a rod-like molecule is determined as a function of both its molecular centre of mass separation R and its orientation relative to the surface. In calculating this interaction energy, each rod segment and molecule comprising the surface is assumed to interact through a Lennard-Jones pair potential. The average molecular order parameter is then determined as a function of R. We find that an isolated rod-like molecule tends to align nearly parallel to the surface for small separations. However, as R increases the order parameter first passes through a maximum then decays to zero as R ^-5 for large R. The configurational behaviour of an isolated rod-like molecule located between two parallel adsorption surfaces is also considered briefly. The surface spreading pressure, excess surface energy and entropy are also obtained for a dilute gas of rod-like molecules near a surface. We find that the extent of surface binding increases nearly exponentially with molecular length at constant temperature and surface density, and that the excess surface energy and entropy are essentially proportional to the molecular length.     

Theoretical studies of argon adsorption in MCM-41 mesoporous systems

Adsorption isotherms are predicted for spherical adsorbates in cylindrical channels of MCM-41 mesoporous materials over a wide range of temperatures by using the “fragment method”. This prediction shows that an equilibrium capillary condensation is impossible for pores with diameters smaller than 2.5 nm. The adsorbate distribution in relatively large pore channels was described by the quasi-chemical approximation (QCA) that takes into account direct pair correlations between interacting molecules. In order to improve the lattice-gas model in the vicinity of the critical point, a calibration function that takes into account information from the fragment method, was introduced into the QCA equations. The influence of the size factor of pores on argon adsorption isotherms was demonstrated.     

An analytical theory for diffusion of fluids in crystalline nanoporous materials

An analytical theory for diffusion of fluids in zeolites and other nanoporous materials has been developed. The theory incorporates molecular level information about the nanoporous material, which is obtainable from an energy minimization and does not require molecular dynamics computer simulations. The theory is statistical mechanical in nature and assumes a lattice composed of adsorption sites. The theory yields a self-diffusion coefficient, which is a function of (i) temperature, (ii) adsorbate density, (iii) adsorbate size, (iv) adsorbate-adsorbate energetic interaction and (v) adsorbate-pore energetic interaction. The theory is generalized and is applicable to nanoporous materials with three-dimensional porous networks (e.g. faujusite) and one-dimensional porous networks (e.g. A1P0₄-5).

The theory is self-contained and incorporates no fitting parameters. The theory does not require computational effort beyond a few seconds on a standard personal computer.     

Studies of adsorption equilibria and kinetics in the systems: Aqueous solution of dyes-mesoporous carbons

Two carbonaceous materials were synthesized by using the method of impregnation of mesoporous silicas obtained by applying the Pluronic copolymers as pore-creating agents. The isotherms of adsorption of methylene blue and methyl orange from aqueous solutions were measured by the static method. The profiles of adsorbate concentration change in time were obtained from the UV-vis spectra. The adsorption isotherms and kinetic dependence were discussed in the terms of theory of adsorption on heterogeneous surfaces.     

Tuning of the periodicity of stable self-organized metallic templates

The atomic and electronic structures of Pb bilayer/Pt(111) are investigated with two theoretical calculations.We find that the stable (2 × 2)/(3 × 3) Pb/Pt(111) structure is a promising candidate for being used as a template with self-organized ordered Pb semi-cluster array on the first Pb monolayer.This stable structure can realize the ordered Au single-atom array around the Pb semi-clusters that can cause selective adsorption of noble atoms.The size of Pb magic number semi-cluster plays a more important role in determining the periodicity of the template than the lattice constant misfit between the substrate and the overlayer.This leads to quite a different periodicity between the two stable templates,which are (2 × 2)/(3 × 3) Pb/Pt(111) and Pb/Cu(111).Therefore,by considering the size of the stable semi-clusters and carefully selecting different substrate materials,we can tune the density of Pb semi-clusters as the nucleation points and then tune the periodicity of the stable template.     

Successive defects asymmetric simple exclusion processes with particles of arbitrary size

This paper uses various mean-field approaches and the Monte Carlo simulation to calculate asymmetric simple exclusion processes with particles of arbitrary size in the successive defects system. In this system, the hopping probability p (p<1) and the size d of particles are not constant. Through theoretical calculation and computer simulation, it obtains the exact theoretical results and finds that the theoretical results are in agreement with the computer simulation. These results are helpful in analysing the effect of traffic with different hopping probabilities p and sizes d of particle.     

First-principles study of structural,mechanical, lattice dynamical and thermal properties of nodal-line semimetals ZrXY (X=Si,Ge; Y=S,Se)

Abstract

The nodal-line semimetals are new and very promising materials for technological applications. To understand their structural, mechanical, lattice dynamical and thermal properties in detail, we have investigated theoretical study of ZrXY (X = Si,Ge; Y = S,Se) using Density Functional Theory for the first time. Obtained lattice parameters are in excellent agreement with previous experimental data. These nodal-line semimetals obey the mechanical stability conditions for tetragonal structure. We obtain Bulk modulus, Shear modulus, Poisson’s ratio, Pugh ratio, sound velocities and thermal conductivity using elastic constant. All the materials behave in brittle manner. Poisson’s ratio data and Bader charge analysis results indicate that the ionic bonding characters are dominant. Next, the lattice dynamical properties are calculated. Phonon density of states shows that nodal-line semimetals ZrXY are also dynamically stable in the tetragonal structure. Raman and IR active phonon modes are determined. Highest optical mode at gamma point corresponds to A_2u (IR active) and E_g (Raman active) modes for ZrXSe and ZrXS, respectively. Based on phonon density of states, thermal properties such as Helmholtz free energy, entropy, heat capacity at constant volume and Debye temperature are also presented and discussed.     

X-ray diffraction studies on dilute solid solution of NaCl with Ag impurities

Abstract

X-ray investigations of Bragg scattering measurements have been carried out on the dilute solid solutions of Ag _x Na_1?x Cl for x = 0.03 and 0.10. The integrated intensities are measured on spherical single crystals, using a home-built manual X-ray diffractometer. The analysis of experimental data indicates an enhancement of Debye-Waller factor in this system. The size effect distortion factors evaluated are compared with the calculated values on the basis of different theoretical models and the results are discussed. The measured cell constant and density data indicate the existence of vacancies in these crystals.     

Monte Carlo simulation of carbon monoxide,carbon dioxide and methane adsorption on activated carbon

In this study, the adsorption capacity of pure and activated carbon with regard to carbon monoxide (CO), carbon dioxide (CO₂) and methane (CH₄) gases at 298?K and pressure from 0.01 up to 2.0?MPa has been investigated computationally. Computational work refers to Monte Carlo (MC) simulation of each adsorbed gas on a graphite model with varying density of activation sites. The Grand Canonical Monte Carlo (GCMC) simulation technique was employed to obtain the uptake of each adsorbed gas by considering a graphite model of parallel sheets activated by carboxyl and hydroxyl groups, as observed experimentally. The simulation adsorption data for these gases within the examined carbon pore material are presented and discussed in terms of the adsorbate fluid molecular characteristics and corresponding interactions between adsorbate species and adsorbent material. We found that the simulated adsorption uptake of the examined graphite model under these conditions with regard to the aforementioned fluids increases in the order CO?<?CH₄?<?CO₂.     

Theory and simulation of coupled grain boundary migration and grain rotation in low angle grain boundaries

Abstract

Microstructure evolution due to coupled grain boundary migration and grain rotation in low angle grain boundaries is studied through a combination of molecular dynamics and phase field modeling. We have performed two dimensional molecular dynamics simulations on a bicrystal with a circular grain embedded in a larger grain. Both size and orientation of the embedded grain are observed to evolve with time. The shrinking embedded grain is observed to have two regimes: constant dislocation density on the grain boundary followed by constant rate of increase in dislocation density. Based on these observations from the molecular dynamics simulations, a theoretical formulation of the kinetics of coupled grain rotation is developed. The grain rotation rate is derived for the two regimes of constant dislocation density and constant rate of change of dislocation density on the grain boundary during evolution. The theoretical calculation of the grain rotation rate shows strong dependence on the grain size and compares very well with the molecular dynamics simulations. A multi-order parameter based phase field model with coupled grain rotation is developed using the theoretical formulation to model polycrystalline microstructure evolution.     

Effect of oxygen surface groups on adsorption of benzene derivatives from aqueous solutions onto active carbon samples

The process of adsorption of selected benzene derivatives from aqueous solution is investigated on two carbonaceous materials of differentiated surface properties - quantity of oxygen functional groups. Carbon samples were prepared by removal of external layers from granules of unmodified and oxidized active carbon. The surface and structure characteristics of carbon samples were estimated by various methods. The experimental isotherms of organics adsorption from liquid phase were measured and interpreted in terms of the theory of adsorption on heterogeneous solid surfaces. The influence of differences in adsorbate and adsorbent properties on adsorption uptake was analyzed. The adsorption effectiveness was regarded as a result of the differences in adsorbate hydrophobicities and the effect of specific interactions of its functional groups with active sites on carbon surface.