Gas adsorption process in activated carbon over a wide temperature range above the critical point. Part 2: conservation of mass and energy

Simulations of the thermal effects during adsorption cycles are a valuable tool for the design of efficient adsorption-based systems such as gas storage, gas separation and adsorption-based heat pumps. In this paper, we present simulations of the thermal phenomena associated with hydrogen, nitrogen and methane adsorption on activated carbon for supercritical temperatures and high pressures. The analytical expressions of adsorption and of the internal energy of the adsorbed phase are calculated from a Dubinin-Astakhov adsorption model using solution thermodynamics. A constant adsorption volume is assumed. The isosteric heat is also calculated and discussed. Finally, the mass and energy rate balance equations for an adsorbate/adsorbent pair are presented and are shown to be in agreement with desorption experiments.     

Energy dissipation by pulsating air bubbles in a viscoelastic medium

A theoretical discussion is presented on the mechanisms by which an isolated pulsating air bubble in a viscoelastic solid dissipates its energy. The analysis is limited to the situation where the amplitude of motion is assumed to be sufficiently small that the stress strain relations may be described by linear equations with convected differentiation replaced by ?/?t. The theoretical thermal, radiation, and viscous damping constants are calculated for resonant air bubbles in unvulcanized natural rubber; however, the results are typical of elastomers in general.     

Relation between Energy Contribution to Elastic Force and Strain Dependence of Modulus for Polybutadiene Vulcanizates

It is well known that the modulus G = r/(λ - λ^?2) varies with deformation, thus deviating from the predictions of statistical theories of rubber elasticity which require it to be constant. It has also been found that there is a nonnegligible energy contribution to the elastic force. It is postulated that these two phenomena are related because both arise from energetic interaction between chains.

Based on the lateral order of chains indicated by x-ray fiber diagrams of elongated noncrystallizable elastomers, it is suggested that energetic interaction of chains is induced by strain orientation. Proportionality between these two is assumed. The orientation distribution functions of end-to-end vectors and of statistical chain segments are considered. The proportionality constants are determined from the energy contribution to the strain dependence of the coefficient of thermal expansion. With the aid of these constants the modulus, corrected for energy contribution, is calculated. The observed and calculated elongation dependence of G agree reasonably well.

It is concluded that an energy interaction between aligned chains can account for the deviation of the observed stress elongation relation from the predictions of entropy elasticity theories.     

First-principles studies on the efficient photoluminescent iridium(III) complexes with C^N═N ligands

The electronic structures and photophysical properties of several homoleptic iridium complexes IrL(3) with C^N═N ligands, including 1 (L = 3,6-diphenylpyridazine), 2 (L = 1,4-diphenylphthalazine), 3 (L = 3-phenyl-5H-indeno[1,2-c]pyridazine), and 4 (L = 3-phenylbenzo[h]cinnoline), are investigated using the density functional method. The comparison between the calculated results of the four complexes shows that the assumed complex 4 may possess higher photoluminescent quantum efficiency than complexes 1-3 and is the potential candidate to be an efficient green-emitting material. The photophysical properties of the assumed complex 3 can be comparable to that of experimentally found complex 1. For 1 and 3, the emission energies are nearly the same, consistent with their similar HOMO-LUMO energy gaps. Their emission characters are also similar and mainly dominated by one ligand. For 4 and the experimentally found complex 2, although they have similar HOMO-LUMO energy gaps, and their luminescent nature is nearly the same and dominated by the three ligands, the emission spectrum of 4 is blue-shifted as compared to that of 2.     

Cylindrical cell model for the electrostatic free energy of polyelectrolyte complexes

Associative phase separation (complex coacervation) in a mixture of oppositely charged polyelectrolytes can lead to different types of (inter-)polyelectrolyte complexes (soluble micelles, macroscopic precipitation). In a previous report [Langmuir 2004, 20, 2785-2791], we presented a model for the electrostatic free energy change when (weakly charged) polyelectrolyte forms a homogeneous complex phase. The influence of ionization of the polymer on the electrostatic free energy of the complex was incorporated but the influence of complex density neglected. In the present effort, cylindrical cells are assumed around each polyelectrolyte chain in the complex, and on the basis of the Poisson-Boltzmann equation, the electrostatic free energy is calculated as a function of the complex density. After combination with Flory-Huggins mixing free energy terms and minimization of the total free energy, the equilibrium complex density is obtained, for a given ratio of polycations to polyanions in the complex. The analysis is used in an example calculation ofpolyelectrolyte film formation by alternatingly applying a polycation and a polyanion solution. The calculation suggests that the often observed exponential growth of a polyelectrolyte film when the polymer is weakly charged has a thermodynamic origin: the polyelectrolyte complex shifts repeatedly between two equilibrium states of different densities and compositions. However, when the polyelectrolytes are strongly charged the difference in the compositions between the two equilibrium states is very small, and exponential growth by an absorption mechanism is no longer possible.     

Application of the Monte Carlo method to modeling of kinetic processes with the participation of polyatomic molecules and clusters: 1. Kinetics of energy transfer during collisions of polyatomic molecules

The Monte Carlo method was used to model the collisional energy transfer for polyatomic molecules within the framework of the statistical theory of reactions. A model describing energy transfer through the formation of a statistical collisional complex was suggested. It was assumed that the total energy of the complex was randomized in the course of collisions and statistically distributed among the internal and translational degrees of freedom. The method was verified by comparing the equilibrium distribution functions for the vibrational, rotational, and total energies of the molecule. The mean energy portion and the root-mean-square energy portion transferred per collision, as functions of the total molecular energy, were determined. The relaxation parameters of the population distribution over energy after a sharp increase in the bath-gas temperature were calculated.     

A model study of the laser-induced stabilization of a collision complex

The possibility of stabilizing a collision complex by stimulated emmission with coherent radiation is explored for a model. three-atom system. The lifetimes of several resonance states of this system were obtained previously and are used in a simple two-level model to describe the interaction of radiation with a resonance state and a true bound state. For the case where the energy difference between the two levels is time independent, simple, analytical expressions are used to obtain the time-dependent transition probability. Numerical solutions are obtained for the more realistic case where this energy difference is time dependent. For both cases, a substantial transition probability for stabilization is calculated for moderate laser intensities (e.g. 8.1 kW/cm² for an assumed dipole matrix element of 0.01 D).     

Effect of free volume fluctuations on polymer relaxation in the glassy state. II. Calculated results

The relaxation following a change in temperature of amorphous polymers near the glass transition has been calculated. The calculation uses a chain model consisting of cis and trans backbone rotational states. The relaxation is assumed to proceed by localized conformational changes whose rates are controlled by the fractional free volume in small enough regions of the polymer that thermal fluctuations need to be considered. The relaxation is treated as a stochastic process, and an approximate solution is obtained for a finite set of relaxation environments. Using what is believed to be the most plausible set of parameters for polystyrene, relaxation curves are computed for the internal energy that are very similar to the curves obtained by Kovacs and others for the volumetric relaxation of poly(vinyl acetate) and polystyrene.     

A Modified statistical theory of collisional energy transfer in thermal reactions

A fully statistical kernel describing the probability of energy transfer in collisions between polyatomic reactant (A) and heat bath (M) molecules in a thermal system is developed, proceeding through the formation of an intermediate collision complex (AM) whose internal degrees of freedom are assumed to exchange energy. After pointing out that this kernel does not give a quantitatively useful answer, the kernel is modified by introducing the concept that the collision complex lifetime is due to orbiting collisions, and that the (AM) lifetime must equal collision duration. This puts two constraints on the internal degrees of freedom of (AM): (1) those that correlate with relative translation and intrinsic rotation of separated A and M (= transitional modes) can contain only an amount of energy not exceeding E_*, which is the maximum energy for which orbiting can occur; (2) those that correlate with internal degrees of freedom of M must have a density of states such that, subject to constraint (1), the lifetime of (AM) is equal to collision duration. It turns out, quite unambiguously, that the appropriate density of states is equivalent to just one oscillator of M participating in energy exchange. Calculations of average amount of energy transferred (Δ E>) in the system CH₃NC + M show good quantitative agreement with experiment for both polar and non-polar M. The modified theory does not give any appreciable dependence of Δ E> on the size of M because collision duration is assumed to depend only on the long-range part of the potential.     

An attempt at quantum thermal physics

Summary This paper outlines an alternative exposition of the structure of quantum thermodynamics which is essentially based on Carnot's theory where fluxes of caloric are identified with negative information fluxes. It is further assumed that the thermal energy evolved by thermal processes is identical with the electromagnetic zero-point background energy evolved by the destruction of information inscribed in a structural unit (qubit). Theoretical arguments on an elementary level are accompanied by illustrative examples.     

Calculations of fine-structure resolved collisional rate coefficients for the NH(X3Σ-)-He system

We present fine-structure-resolved collisional rate coefficients for the NH(X(3)Σ(-))-He van der Waals complex. The calculations are based on the state-of-the-art potential energy surface [Cybulski et al., J. Chem. Phys. 122, 094307 (2005)]. Close-coupling calculations of the collisional excitation cross sections of the fine-structure levels of NH by He are calculated for total energies up to 3500 cm(-1), which yield, after thermal average, rate coefficients up to 350 K. The fine-structure splitting of rotational levels is taken into account rigorously. The propensity rules between fine-structure levels are reported, and it is found that F-conserving cross sections are much larger than F-changing cross sections, as expected from theoretical considerations. The calculated rate coefficients are compared with available experimental measurements at room temperature and a fairly good agreement is found between experimental and theoretical data. The agreement confirms the relatively good quality of the scattering calculations and also the accuracy of the potential energy surface used in this work. The new set of thermal rate coefficients for this system may be used for improvements in astrophysical and atmospherical modeling.     

Determination of the thermal energy and its distribution in peptides

The mean thermal energy and its distribution has been calculated for various peptides. The table, figures, and analytical expressions provided in the paper can be used to determine the mean thermal energy and its distribution in peptides in a very simple way, without having to use complex mathematics. Accuracy is ∼±6%. The same expressions can be used for most organic compounds with an estimated accuracy of ∼±10%. Data for W(CO)₆, the “thermometer molecule,” are also given.     

Effect of substituents on electron energy redistribution in uracil derivatives and their ionization in polar solvents

It is assumed that the chemical reactivity may be described in terms of the appropriate energy density calculated for each atom. Indices of energy density for deprotonization of uracil, 5-nitro- and 5-aminouracil are presented. The results of a quantum chemical calculation of the deprotonization energy are in qualitative agreement with experimental pK and ΔH values.     

Nonclassical nucleation kinetics in the crystallization of a supercooled melt

A molecular dynamics simulation of homogenous nucleation of a crystal in supercooled aluminum melt is performed. Nucleation rates at a temperature of 900 K in the range of pressures from 12 to 15 GPa are calculated. Analysis of the mean first-passage times of crystalline cluster sizes is performed. A stepwise dependence of the mean first-passage time on crystal nucleus size is observed, in contrast to the sigmoidal dependence that follows from classical nucleation theory. Based on the data from atomistic simulations, it is established that the form of the free energy barrier during nucleation differs significantly from the one assumed in classical nucleation theory for a spherically symmetric nucleus. It is supposed that the observed differences are apparently due to the complex structure of the crystalline nucleus.     

Investigation of the thermal properties of [Cd(hydet-en)2Pd(CN)4] AND [Zn(hydet-en)2Pd(CN)4] single crystals

Thermal properties of the single crystals have been investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC) techniques. The thermodynamic parameters such as activation energy and enthalpy and thermal stability temperature of the samples were calculated from the differential thermal analysis (DTA) and TG data. The activation energies for first peak of DTA curves were found as 496.65 (for Cd–Pd) and 419.37 kJ mol^–1 (for Zn–Pd). For second peak, activation energies were calculated 116.56 (for Cd–Pd) and 173.96 kJ mol^–1 (for Zn–Pd). The thermal stability temperature values of the Cd–Pd and Zn–Pd compounds at 10°C min^–1 heating rate are determined as approximately 220.7 and 203°C, respectively. The TG results suggest that thermal stability of the Cd–Pd complex is higher than that of the Zn–Pd complex.     

Thermodynamic and Transport Properties of Two-Temperature Nitrogen-Oxygen Plasma

Thermodynamic and transport properties are computed for a 17 species model of nitrogen-oxygen plasma under different degrees of thermal non-equilibrium, pressures and volume ratios of component gases. In the computation electron temperatures range from 300 to 45,000 K, mole fractions range from 0.8 to 0.2, pressures range from 0.1 atmosphere to 5 atmospheres, and thermal nonequilibrium parameters (T_e/T_h) range from 1 to 20. It is assumed that all the electrons follow a temperature T_e and the rest of the species in the plasma follow a temperature T_h. Compositions are calculated using the two temperature Saha equation derived by van de Sanden et al. Updated energy level data from National Institute of Standards and Technology (NIST) and recently compiled collision integrals by Capitelli et al., have been used to obtain thermodynamic and transport properties. In the local thermodynamic equilibrium (LTE) regime, the results are compared with published data and an overall good agreement is observed.     

Kinetic parameters from thermogravimetric study of used rubber granulates–polyurethane composites

From the TG data of rubber granulates, different polyurethane and composites it can be seen that the thermal decomposition for the rubber granulate and all of the composites start above 520 K. Two major mass losses for the rubber granulates and majority of the composites were observed and thermal decomposition is essentially complete by ~820 K. The changes of activation energies of lower and higher temperature decomposition, calculated according to the different equations were observed for a priori assumed first-order reaction for devolatilisation. Differences between determined and calculated results could suggest a possible reaction between polyurethane agents and rubber granulate during the composites formations.     

Molecular dynamics study of thermal phenomena in an ultrathin liquid film sheared between solid surfaces: the influence of the crystal plane on energy and momentum transfer at solid-liquid interfaces

A molecular dynamics study has been performed on a liquid film sheared between moving solid walls. Thermal phenomena that occur in the Couette-like flow were examined, including energy conversion from macroscopic flow energy to thermal energy, i.e., viscous heating in the macroscopic sense, and heat conduction from the liquid film to the solid wall via liquid-solid interfaces. Four types of crystal planes of fcc lattice were assumed for the surface of the solid wall. The jumps in velocity and temperature at the interface resulting from deteriorated transfer characteristics of thermal energy and momentum at the interface were observed. It was found that the transfer characteristics of thermal energy and momentum at the interfaces are greatly influenced by the types of crystal plane of the solid wall surface which contacts the liquid film. The mechanism by which such a molecular scale structure influences the energy transfer at the interface was examined by analyzing the molecular motion and its contribution to energy transfer at the solid-liquid interface.     

Complexing properties of nucleic-acid constituents adenine and guanine complexes

Cobalt, nickel and copper complexes of adenine and guanine, as nucleic-acid constituents, were prepared. The adenine and guanine complexes are of tetrahedral and octahedral geometries, respectively. All are of high spin nature. The nickel complexes are of 2:1 metal:ligand ratio with Ni...Ni direct interaction in the guanine complex. The coordination bonds of adenine metal complexes are calculated and follow the order: Cu(II)-adenine < Ni(II)-adenine < Co(I)-adenine. The Cu(II)-adenine complex is the stronger following the softness of the copper, while that of guanine is less covalent. The copper complexes are with stronger axial field. The differential thermal analysis (DTA) and TGA of the complexes pointed to their stability. The mechanism of the thermal decomposition is detected. The thermodynamic parameters of the dissociation steps are evaluated. The complexes are of semi-conducting behaviour for their technical applications. Empirical equations are deduced between the electrical conducting and the energy of activation of the complexes.     

Studies on transition metal murexide complexes

Iron, cobalt, nickel, copper, zinc, cadmium and mercury murexide complexes have been prepared and characterized. The thermal properties of these complexes are studied deeply by DTA technique where their thermal peaks are explained. The multi-stages thermal decomposition mechanism is proposed. The thermodynamic parameters of the decomposition steps are calculated. The entropy change values for all complexes are of the same magnitude and all transition states of the thermal reactions are more ordered than the reactants. The thermal reactions proceed in complicated mechanisms. The fractions appeared in the calculated orders of the thermal reactions confirmed that these reactions proceeded via complex mechanisms.