Thermodynamics and kinetics of adsorption of atoms and molecules by carbon nanotubes

Thermodynamics and kinetics of adsorption of atoms and molecules by carbon nanotubes are investigated. Expressions are derived that can be used to perform calculations taking into account various external factors and interactions and, therefore, analyze both physical and chemical adsorption processes and determine its thermodynamic parameters. A mathematical model is developed that makes it possible to find parameters of desorption kinetics separately for molecules adsorbed at different locations and reduce possible systematic errors in analysis of experimental results. Theoretical models are validated by analyzing physical and chemical adsorption of hydrogen and physical adsorption of oxygen.     

Effect of pore sizes in silicate glass on the degree of association of dye molecules

We have studied the pore size dependences of association processes for dye molecules incorporated into the matrix of a porous silicate glass. We have determined the structures of the associates of dye molecules as a function of the type of dye, the dye concentration, and the matrix pore sizes. We have established a nonuniform distribution of the complex dye molecules in the porous glass matrix, which is responsible for the higher efficiency of the association processes for the adsorbed molecules compared with alcoholic solutions of these dyes. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 824–827, November–December, 2006.     

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.     

The kinetics of field adsorption

In an applied field of ～ 1 V/Å or higher, gas atoms and molecules near the electrode surface can be attracted to the surface and be field adsorbed there. Field adsorption can occur at a temperature much higher than that in ordinary physical adsorption. For example, field adsorption of He and Ne on metal surfaces can occur at a temperature above 100 K. The kinetic of field adsorption on spherical, cylindrical and field ion emitter surfaces are discussed. Methods for measuring the field adsorption energy, the gas supply constant, and other physical parameters using the pulsed-laser time-of-flight atom-probe are described. These parameters are essential for describing the kinetics of field adsorption. We consider field adsorption and desorption of both atomic and molecular gases. In molecular gases, field dissociation of field desorbed species can occur. A proper account of this effect has to be included in the data analysis. A case with two adsorption states at different adsorption sites is also considered. Field adsorption must play an important role in the early stages of cloud formation, and in the adsorption-desorption and catalytical reactions on the surface of interstellar particles, etc.     

Influence of Solvent on Dye-Sensitized Solar Cell Efficiency: What is so Special About Acetonitrile? (Part. Part. Syst. Charact. 5/2021)

The selection of solvent for preparing a working electrode (and to act as the electrolyte) is known to influence the efficiency of dye-sensitized solar cells. In this topical review, results taken from a systematic study are presented from the authors’ own lab examining how protic and aprotic solvents, as well as solvent polarity, affect adsorption of carboxylic dyes on the titanium dioxide nanoparticle surface and electron injection from the dye to the semiconductor. Adsorption of dye molecules on nanoparticle surfaces is measured through second harmonic light scattering and electron injection through ultrafast transient mid-infrared absorption. It is revealed that protic solvents do not allow direct adsorption of the dye onto the semiconductor surface, due to hydrogen bonding with the dye and competitive binding to the semiconductor surface. Aprotic solvents, on the other hand, support solvation of the dye molecules but also facilitate dye adsorption on the semiconductor nanoparticle. Among aprotic solvents, it is found that solvents with higher polarity result in larger adsorption free energy for the dye and faster electron injection. Overall, these studies reveal that aprotic solvents with high solvent polarity (such as acetonitrile) yield more efficient solar cell devices.     

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In this second paper we consider three possible models for the adsorption of CO/Mo(110). Taking into account the size of molecules we establish simple criteria for the adsorption in each of the two states we have determined experimentally (Part I). On this basis we simulate the adsorption kinetics corresponding to the three models. Comparison with experimental results shows that only one model is adequate. In this model the CO molecules of state No. 1 are bridged ones and their adsorption is a random processus. In the state No. 2 the CO molecules are perpendicular to the Mo(110) plane and only the C atoms are bounded to Mo atoms. The simulated structure of the adsorbed film is completely disordered, which corresponds to the LEED results (Part I).     

Adsorption of a tripeptide on platinum electrodes: II. Kinetics and isotherms

Changes in the capacitance of a platinum electrode due to adsorption of the tripeptide Bz-Phe-Val-Arg-pNA have been measured and analyzed. The experimental isotherm can be described with a Langmuir isotherm if changes in molecular conformation are taken into account. The desorption and adsorption kinetics both exhibit two time constants which are proposed to reflect that the molecules are adsorbed in at least two different conformations. Measurements of the initial rate of adsorption are consistent with the isotherm and the desorption kinetics. The difference between the electrically determined isotherm and one determined by ellipsometry is also explained by a change in conformation of the molecules on the surface.     

Derivation of the equation of motion for resonantly excited molecular J aggregates taking into account multiparticle effects

We have analyzed a model of molecular J aggregates in the form of a chain of three-level molecules. Equations of motion for these chains with taking into account multiparticle contributions caused by different mechanisms by which molecules interact with each other (dipole-dipole interactions, exciton-exciton annihilation, etc.) are presented. These contributions describe not only the interaction of any pair of molecules with each other, but also the interaction of a given molecule simultaneously with two, three, or more number of molecules. In the general case, it is necessary to take into account effects related to correlations between such molecules. To take into account these correlation effects, it is necessary to derive equations of motion from the first principles. As a result, a hierarchy of mutually coupled equations for the expectation values of the products of operators that refer to different molecules of the chain. In this work, we take into account only two-particle correlations between molecules. This, in turn, has led us to the necessity of taking into account equations for the two-particle expectation values with subsequent factorization of higher-order averages to obtain a closed system of equations. Correct taking into account of the mechanism of exciton-exciton annihilation from first principles has made it necessary to introduce multiparticle contributions into equations of motion that describe the relaxation of the system due to this mechanism and that are absent in a purely phenomenological consideration. By this means, we have obtained a set of equations that makes it possible to successively and rigorously take into account two-particle correlations between molecules of J aggregates.     

The effect of dye photodegradation on the performance of dye lasers

Detailed experimental study of laser dye stability and the subsequent effects of photodegradation products on the operation of a Nd:YAG laser pumped Rhodamine-6G pulsed dye oscillator has been carried out. Deterioration of laser output on account of dye photodegradation resulting in the loss of active dye molecules and the generation and accumulation of reaction products that introduce a loss at the laser and pump wavelengths has been theoretically simulated in terms of a time dependent rate equation model. Our experimental observations have been found to be in good agreement with our theoretical estimates. We have also investigated the deterioration in the performance of a copper vapour laser pumped tunable dye laser with a high average power and a high repetition rate operating over an extended period of time. Our theoretical model, when suitably modified, provided useful estimates for the extent of deterioration in laser performance of such a dye laser with a high average power and a high repetition rate on account of photodegradation of the active dye molecules. PACS 42.55.Mv; 42.60.By; 42.70.Hj     

基于混合加宽的宽带激光脉冲放大的物理模型

建立了宽带激光脉冲放大的物理模型.模型引入交叉弛豫时间表征不同频率响应下反转粒子与激光作用后重新形成分布的快慢，模型考虑的整体非均匀加宽、局部均匀加宽的混合加宽机理弥补了现有放大模型仅考虑均匀加宽的不足.基于此物理模型，开发了宽带激光传输放大的模拟设计软件CPAP，并讨论了在一定条件下此宽带模型可回归到窄带模型.     

On the stability of parameters of molecular models in the parametric method of the theory of vibronic spectra

We have studied the stability of the system of parameters used in the parametric method of the theory of vibronic spectra of polyatomic molecules with respect to whether the initial structural-dynamic model of the ground state of a molecule is formed either by the fragmentary method or by direct quantum-chemical calculation. Modeling of excited states and spectra of the stilbene-h ₁₂ and stilbene-d ₁₂ molecules showed that, although the initial models of the ground states are significantly different, calculated changes in the geometry that occur upon excitation and electronic-vibrational spectra of the models are close and quantitatively agree with experiment. This indicates that the parameters of the method are stable and are applicable for performing predictive calculations of vibronic spectra based on models of the ground states created by different methods. We show that the fragmentary method has a considerable advantage, since models created by this method take into account the continuity of the structure in series of related compounds and calculations can be easily performed for molecules of arbitrarily high complexity. We show that direct quantum-chemical calculations are important in the case of molecules with unknown structural-dynamic models of fragments in the ground states.     

Effect of non-uniform reaction rates at solid–oxide interfaces on the electrochemical impedance

An impedance model is developed for a porous oxygen electrode on top of a solid oxide conductor, taking into account adsorption and surface transport along the pore walls, interfacial diffusion and reaction along the interface, as well as current distribution (2D) in the electrolyte. All parameters are in principal measurable. Simulated impedance spectra typically exhibit two semicircles, one related to the charge transfer reaction at the interface (high frequencies), and one related to mass transfer limitations (low frequencies). The resolution of these two semicircles, however, depend on the relative contributions of these two processes to the overall potential losses, and the magnitude of the interfacial capacitance relative to the other kinetic and transport parameters, as well as geometrical parameters.     

多孔硅／激光染料复合体的发光

将激光染料分子有效地嵌入多孔硅中，获得多孔硅／染料分子复合体，嵌入微量梁料分子时，复合体的发光兼有多孔硅和激光染料两者的发光特征，增加染料分子的嵌入量，复合体的发光增强，荧光的时间响应快、脉宽窄，主要表面为激光染料的发光特性。     

双包层掺染料聚合物光纤放大器的增益性能分析

双包层聚合物光纤放大器可以减小染料的热漂白,使用速率方程的方法,给出了一个能够全面描述双包层掺染料聚合物光纤放大器增益性能的模型,模型既考虑了染料三重态能级对速率方程的影响,又考虑了双包层结构对泵浦光的影响,可以在稳态的情况下计算放大器的各种增益性能指标.     

The role of surface tension in elastic wave scattering in an inhomogeneous poroelastic medium

It is well known that many porous media such as rocks have heterogeneities at nearly all scales. We applied Biot's poroelastic theory to study the propagation of elastic waves in isotropic porous matrix with spherical inclusions. It is assumed that the heterogeneity dimension exceeds significantly the pore size. Modified boundary conditions on poroelastic interface are used to take into account the surface tension effects. The effective wavenumber is calculated using the Waterman and Truell multiple scattering theory, which relates the effective wave number to the amplitude of the wave field scattered by a single inclusion. The calculations were performed for a medium containing fluid-filled cavities or porous inclusions contrasting in saturating fluid elastic properties. The results obtained show that when we consider elastic wave propagation in poroelastic medium containing soft inclusions, it is necessary to take into account the capillary pressure. The influence of the surface tension depends on the diffraction parameter and it is a maximum in the low frequency range.     

The role of surface tension in elastic wave scattering in an inhomogeneous poroelastic medium

It is well known that many porous media such as rocks have heterogeneities at nearly all scales. We applied Biot's poroelastic theory to study the propagation of elastic waves in isotropic porous matrix with spherical inclusions. It is assumed that the heterogeneity dimension exceeds significantly the pore size. Modified boundary conditions on poroelastic interface are used to take into account the surface tension effects. The effective wavenumber is calculated using the Waterman and Truell multiple scattering theory, which relates the effective wave number to the amplitude of the wave field scattered by a single inclusion. The calculations were performed for a medium containing fluid-filled cavities or porous inclusions contrasting in saturating fluid elastic properties. The results obtained show that when we consider elastic wave propagation in poroelastic medium containing soft inclusions, it is necessary to take into account the capillary pressure. The influence of the surface tension depends on the diffraction parameter and it is a maximum in the low frequency range.     

Two models of anisotropic propagation of a cardiac excitation wave

Propagation of the action potential in the real heart is direction-dependent (anisotropic). We propose two general physical models explaining this anisotropy on the cellular level. The first, “delay” model takes into account the frequency of the cell-cell transitions in different directions of propagation, assuming each transition requires some small time interval. The second model relies on the assumption that the action potential transmits to the next cell only from the area at the pole of the previous cell. We estimated parameters of both models by doing optical mapping and fluorescent staining of cardiac cell samples grown on polymer fiber substrate. Both models gave reasonable estimations, but predicted different behaviors of the anisotropy ratio (ratio of the highest and lowest wave velocities) after addition of the suppressor of sodium channels such as lidocaine. The results of the experiment on lidocaine effect on anisotropy ratio were in favor of the first, “delay” model. Estimated average cell-cell transition delay was 240 ± 80 μs, which is close to the characteristic values of synaptic delay.     

A kinetic analysis of Langmuir model for adsorption within the framework of Jovanovic theory; a generalization of the Jovanovic isotherm

This paper presents a comparative kinetic analysis of the Langmuir and Jovanovic models for adsorption of gases on solid surfaces. Taking into account of the settling times distribution of the adsorbed molecules on the surface, the authors suggest a generalization of the Jovanovic isotherm.