Enthalpy Effects Research Articles

Single Event Kinetic Modeling of Complex Catalytic Processes

AbstractFor Abstract see ChemInform Abstract in Full Text.

Thermodynamics of Aqueous Solutions of 18-Crown-6 at 298.15 K: Enthalpy and Entropy Effects

We reported previously activity and activity coefficient data for aqueous solutions of 18-crown-6 (18C6) in the concentration range of 0.1–2.0 mol-kg−1 at 298.15 K. The results were interpreted in terms of the binding of four water molecules (two bridged and two singly H-bonded) inside the 18C6 cavity having a D3d conformation. In this work, we report our thermodynamic analysis of the Gibbs energy and enthalpy data (obtained using enthalpy virial data from literature) in aqueous solutions of 18C6 at 298.15 K. The excess enthalpy and Gibbs energy parameters are computed and further used to obtain excess entropies of solutions as a function of 18C6 concentration. The same data are utilized to compute the partial molar entropies of solvent and solute at finite, as well as at infinite, dilution of 18C6 in water. It is observed that ΔGmix, ΔHmix and TΔSmix values are all negative, whereas ΔGE values show a slightly positive variation as a function of the 18C6 concentration. The partial molar excess entropy of water, (\((\overline{S}_{1}-S_{1}^{\circ})^{\rm E}\), decreases (becomes negative) whereas that of 18C6, (\((\overline{S}_2-S_{2}^{\circ})^{\rm E}\), increases with a increase in the 18C6 concentration. These results are explained in terms of various effects, which include water structure making, incorporation of water molecules in the crown cavities and crown–crown hydrophobic interactions, which persist even at the lowest concentration studied.

Single Event Kinetic Modeling of Complex Catalytic Processes

Kinetic models for complex processes may contain an excessive number of rate parameters. Rather than simplifying the reaction scheme the approach illustrated here develops it in terms of the elementary steps of cation chemistry, using a computer algorithm. The resulting scheme, although gigantic, thus consists of a limited number of types of steps, generally involving series of homologous species. The rate coefficients of these steps are modeled based upon transition state theory and statistical thermodynamics. The single event concept explicits the effect of structure on the entropy contribution to the rate coefficient of the transformation, while the enthalpy effect is calculated using the Evans‐Polanyi relation. Together with thermodynamic constraints this approach drastically reduces the number of independent parameters to be determined from the experimental data. The approach is applied to the methanol‐to‐olefins process on ZSM5 and to a process with complex feedstock like the catalytic cracking of vacuum gas oil. In the latter case an additional problem is the requirement of an adequate feedstock definition. Other processes catalyzed by acid catalysts, eventually loaded with metals, like hydrocracking, catalytic reforming, alkylation and, more generall, processes involving series of homologous components can be dealt with along the same lines.

Application of PSRK for Process Design

A survey about the current status and potential of the predictive Soave-Redlich-Kwong (PSRK) group contribution equation of state (EoS) is given. The PSRK method combines the SRK EoS with the UNIFAC group contribution model by the PSRK mixing rule. The availability of a large number of interaction parameters between UNIFAC and new PSRK main groups allows calculating the phase equilibrium behavior and other thermodynamic quantities for various kinds of systems even with strong electrolytes. Different fields of application are discussed: the prediction of gas solubilities in asymmetric systems, the air solubility calculated for mixed solvents, the removal of H2S from natural gas by predicting the physical absorption, and the calculation of the enthalpy effects of absorption processes with pressure and temperature.

Addition of Carbon-Centered Radicals to Double Bonds: Influence of the Alkene Structure

The radical addition reaction to the double bond is well-recognized in organic chemistry as a powerful tool for C-C bond formation. The reactivity of three selected carbon centered radicals (aminoalkyl, methyl, and cyanomethyl) toward five double bonds, also representative of widespread monomers (vinyl ether, vinyl acetate, acrylonitrile, methyl acrylate, and ethylene), was examined in detail by using molecular orbital calculations. The observed reactivity is strongly influenced by the reaction exothermicity demonstrating that the energy barrier is governed in large part by the enthalpy term. The polar effect, as computed by molecular orbital calculations from the transition state structures, can drastically enhance the reactivity. A clear separation and quantification of the relative role of the polar and enthalpy effects in the different radical/double bond systems are obtained and the observed trend of reactivity is discussed. In addition to the effect of the charge-transfer configurations on the barrier, a large influence on the transition state geometry was evidenced.

Structure and magnetic properties of off-stoichiometric Y 2Co 17 and Gd 2Co 17 compounds

Off-stoichiometric single-phase Gd 2+ x Co 17−2 x and Y 2+ x Co 17−2 x alloys were successfully prepared. For the Y 2+ x Co 17−2 x ( x = 0–0.25) samples, the room temperature easy magnetization direction (EMD) is in the basal plane. For the Gd 2Co 17 compounds, the EMD at room temperature lies in the basal plane, whereas for the Gd 2+ x Co 17−2 x ( x = 0.11) compound, the EMD lies on an easy-cone. With increasing x, the anisotropy fields of the Y 2+ x Co 17−2 x samples first increase quickly from 1.2 T for x = 0 to 1.4 T for x = 0.08, remain unchanged until x = 0.19, then decrease again to 1.1 T at x = 0.25. A spin-reorientation transition was found in the Gd 2+ x Co 17−2 x compounds, while there is no spin-reorientation for the Y 2+ x Co 17−2 x samples. The difference in anisotropy between the Gd 2+ x Co 17−2 x and Y 2+ x Co 17−2 x compounds was explained in terms of size effects, enthalpy effects and dipole–dipole interactions.

Deep Quenching: A Special Method to Study Stress‐Induced Crystallization and Control the Lamellar Growth Direction

AbstractSummary: Liquid‐nitrogen quenching was applied to study the enthalpy effect on the stress‐induced crystallization of microbial polyesters. Crystallization bands of poly(3‐hydroxybutyrate) exhibited the potential to reveal the stress distribution in the melt; while crystallization of poly[(3‐hydroxybutyrate)‐co‐(3‐hydroxyhexanoate)] gave shish‐kebab structures. Polarized‐light micrographs confirmed that the enhanced nucleation was attributed to the tensile stress. Furthermore, control of the quenching direction provides a method to direct the lamellar growth.Polarized‐light micrographs of PHB film crystallized at 90 °C after quenching in liquid nitrogen from the melt. The normal of the bands, namely the lamellar growth direction, runs predominantly parallel to the stress direction.imagePolarized‐light micrographs of PHB film crystallized at 90 °C after quenching in liquid nitrogen from the melt. The normal of the bands, namely the lamellar growth direction, runs predominantly parallel to the stress direction.

Reactivity of Carbon-Centered Radicals toward Acrylate Double Bonds: Relative Contribution of Polar vs Enthalpy Effects

The different factors controlling the reactivity of a large series of carbon-centered radicals toward the methyl acrylate monomer unit were examined in detail by using molecular orbital calculations. In agreement with the state correlation diagram, the energy barrier is governed for a large part by the enthalpy term, as supported by an increase of reactivity with increasing exothermicity of the reaction. However, important polar effects, as evidenced by molecular calculations on the transition states, were also highlighted: they dramatically enhance the reactivity of the nucleophilic radicals (aminoalkyl or dialkylketyl radicals) as well as the electrophilic radicals (malonyl radical). Their contribution to the decrease of the barrier was evaluated by using a model based on chemical descriptors. This allows a clear separation of the relative role of the polar and enthalpy effects for 22 radicals.

Analysis of solution nonideality of a pseudomorphic drug system through a comprehensive thermodynamic framework for the design of a crystallization process

Solutions of a semipolar drug belonging to the alphaV betaiii integrin antagonist class of compounds were studied in a comprehensive thermodynamic framework. The solubility of two pseudomorphic forms (an anhydrate and a monohydrate) was measured at several temperatures and various solvent mixtures of acetonitrile and water. Both forms displayed a “bell”-shaped solubility behavior as a function of cosolvent composition. Thermodynamic framework used to analyze the data comprised van't Hoff and enthalpy–entropy compensation analyses. The two pseudomorphs exhibited linear temperature dependence from 25 to 65°C at all solvent compositions (i.e., ideal behavior with temperature for fixed solvent composition). Plots of enthalpy of solublization and Gibbs free energy showed two distinct regions with contrasting thermodynamic, and consequently, underlying structural properties (indicating non-deal behavior with solvent composition for a fixed temperature). Solubility increased due to entropy effects in the acetonitrile rich region, whereas enthalpy effects dominated solublization in the water-rich region. Quantification of this phenomenon by plotting ΔH versus ΔG showed considerable nonlinearity, and that the two regions were separated by a significant discontinuity—a trend rarely seen before in the literature. The reason behind this behavior is believed to be due to the complex interactions in the solution of the drug in water acetonitrile solvent system. A very significant aspect of the comprehensive thermodynamic analysis is that it helped explain the puzzling feature of the data, which showed that the free energy of phase transformation between the two pseudomorphic forms for a given temperature was not independent of the solvent composition. The resulting explanation has major consequences for crystallization process development.

A novel modification on melt intercalation via exothermal MMT

This article presented a novel modification on the melt intercalation. Montmorillonite (MMT) with exothermal enthalpy effect was prepared by the compounding of MMT with AIBN in solution. An exfoliated polystyrene (PS)/MMT nanocomposites could be obtained by the introduction of exothermal MMT via melt mixing, and the comparison with the counterpart indicated the exfoliation of MMT was accelerated by the in situ exothermal enthalpy and nitrogen released during melt intercalation. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2230–2236, 2004

Formation, Structure and Magnetic Properties of SmCo9.8V2.2.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structural relaxation and configurational statistics of the orientational glass CuCN

Time- and temperature-dependent enthalpy effects due to structural relaxation of an orientational glass of CuCN have been studied. The relaxation is found to be nonexponential, with a structure-independent characteristic time, the time- and temperature-dependent fictive temperature indicating only enthalpy change. Also, configurational statistics in the lattice hole model, which had been used for determining the entropy of polymers [J. Chem. Phys. 116, 2310 (2002)], has now been used to calculate the configurational entropy and heat capacity of the apparently one-dimensional -C-N-C- chains in the disordered structure of CuCN crystal. The calculation yields a heat capacity change of 1.2 J/mol K, as observed by experiments, when a plausible value for the energy of orientation of the CN groups in the CuCN chain is used.

Thermodynamics of the Hydrophobicity in Crystallization of Insulin

For insight into the solvent structure around protein molecules and its role in phase transformations, we investigate the thermodynamics of crystallization of the rhombohedral form of porcine insulin crystals. We determine the temperature dependence of the solubility at varying concentration of the co-solvent acetone, C ac = 0%, 5%, 10%, 15%, and 20%, and find that, as a rule, the solubility of insulin increases as temperature increases. The enthalpy of crystallization, Δ H cryst o , undergoes a stepwise shift from ∼−20 kJ mol −1 at C ac = 0%, 5%, and 10% to ∼−55 kJ mol −1 at C ac = 15% and 20%. The entropy change upon crystallization Δ S cryst o > 0 is ∼35 J mol −1 K −1 for the first three acetone concentrations, and drops to ∼−110 J mol −1 K −1 at C ac = 15% and 20%. Δ G cryst o indicates release of solvent, mostly water, molecules structured around the hydrophobic patches on the insulin molecules’ surface in the solution. As C ac increases to 15% and above, unstructured acetone molecules apparently displace the waters and their contribution to Δ S cryst o > 0 is minimal. This shifts Δ S cryst o > 0 to a negative value close to the value expected for tying up of one insulin molecule from the solution. The accompanying increase in Δ H cryst o suggests that the water structured around the hydrophobic surface moieties has a minimal enthalpy effect, likely due to the small size of these moieties. These findings provide values of the parameters needed to better control insulin crystallization, elucidate the role of organic additives in the crystallization of proteins, and help us to understand the thermodynamics of the hydrophobicity of protein molecules and other large molecules.

On the mathematics of associated solutions

Many “non-ideal” or “excess” mixing properties in solid or liquid solutions arise from homogeneous order-disorder or speciation reactions. Simple Taylor expansions, such as the commonly used Margules expansion can never exactly match the enthalpy and entropy effects associated with such homogeneous reactions. Solution modeling based on associated solution theory produces what can be a more phenomenologically sound representation of the energetics of mixing in such solutions, as well as allowing incorporation of constraints from x-ray crystallography and a wide range of spectroscopic methods. Additional non-ideal interactions between species can be incorporated by combining regular- and associated-solution theory. We present a detailed description of robust algorithms allowing calculation of homogeneous equilibrium, estimation of model properties and calculation of thermodynamic properties. Matrix algorithms are presented to allow calculation of species standard state and mixing properties in non-ideal associated solutions. Species stability and phase equilibrium calculations in associated solutions require the second derivative of Gibbs free energy with respect to the component vector (the Gibbs Hessian). A general algorithm is presented to calculate the Gibbs Hessian in ideal and non-ideal associated solutions. These algorithms can be applied in any number of components and species. Species can be of arbitrary stoichiometry.

Experimental Investigation of High-Enthalpy Effects on Attachment-Line Boundary-Layer Transition

A series of experiments was performed on high-enthalpy effects on hypersonic boundary-layer transition using nitrogen, air, and carbon dioxideas test gases in the T5 hypervelocity shock tunnel. Previous experiments on a sharp cone showed significant high enthalpy effects. This series concerns the attachment line boundary layer on swept cylinders with sweep angles of 60 and 45 deg. The observed trend of transition Reynolds number with enthalpy, which is found to be similar to that in the cone results, shows strong transition delay at the larger sweep angle for carbon dioxide, whereas no significant effect is observed in nitrogen and air. The acoustic wave absorption rate due to relaxation shows a quite similar trend in enthalpy dependence to transition Reynolds number for the carbon dioxide case. This suggests that the dominant effect in delaying transition in the case is vibrational relaxation. The comparisons between the magnitude of the strongest amplification rate from an inviscid linear stability analysis and the absorption rate due to relaxation show that they are of the same order of magnitude for carbon dioxide and that absorption is not significant for nitrogen or air, which supports the stated understanding of the effect of relaxation on transition.

The effect of gas pressure on the melting behavior of compounds

The melting and crystallization behavior of pure compounds under gas pressure was experimentally investigated using a scanning transitiometer, which is a sensitive twin calorimeter combined with a computer driven PVT control. This technique allows to determine different thermodynamic effects occurring with changes of state. While one of the three state variables pressure P, volume V, or temperature T is kept constant, another one is changed with time, both the enthalpy effect and the change of the remaining third state variable can simultaneously be measured with high precision. Thermodynamic equilibrium can be achieved during the entire process because the state is changed very slowly. The solid–liquid phase change of dimethylterephthalate (DMT) and biphenyl was investigated under He, CO 2, and N 2O pressure up to 30 MPa, and the SLG three phase lines were determined. The solid–liquid phase change appeared unexpectedly as a pair of two different heat signals.

Formation, structure and magnetic properties of SmCo 9.8V 2.2

The structure and magnetic properties of the SmCo 9.8V 2.2 compound were investigated by means of X-ray diffraction analysis and magnetic measurement. SmCo 9.8V 2.2 crystallizes in a tetragonal structure with space group I4/mmm. The lattice parameters are a=8.3802(1) Å and c=4.7047(1) Å. Both the atomic size effect and enthalpy effect favor a preferential occupation of V on the 8 i site. Magnetic measurements indicate that the SmCo 9.8V 2.2 compound is ferromagnetic with a Curie temperature of 360 K. The easy magnetization direction of SmCo 9.8V 2.2 is perpendicular to the c-axis. The saturation magnetization and the magnetocrystalline anisotropy field of the SmCo 9.8V 2.2 compound at 5 K are 35.4 emu/g and 77 kOe, respectively.

Uranium extraction from partially neutralised and diluted phosphoric acid using a synergistic organophosphorous solvent mixture

A process is described for the separation of uranium from partially neutralised and diluted phosphoric acid (PNDA) which is generated in fertiliser plants when phosphoric acid is used to scrub ammoniacal vapours from the neutralisation reactors. The separation process is based on solvent extraction using a synergistic mixture of di-2-ethyl hexyl phosphoric acid (DEHPA) with tri- n-alkyl phosphine oxide (TAPO). The effects of process parameters including concentration of DEHPA and TAPO, temperature and degree of neutralisation of acid have been investigated. Laboratory scale and pilot plant scale tests have been carried out. The extraction reaction is found to be exothermic with enthalpy effect of 30 kJ/mol. Tests on stripping of extracted uranium and recovery of uranium have been carried out and results are reported. Analysis of data indicates significant differences in the mechanism of uranium extraction from PNDA and the mechanism reported in literature for extraction of uranium from weak phosphoric acid using a similar solvent mixture.

Effect of temperature on Cs+ sorption and desorption in subsurface sediments at the Hanford Site, U.S.A.

The effects of temperature on Cs+ sorption and desorption were investigated in subsurface sediments from the U.S. Department of Energy Hanford Site. The site has been contaminated at several locations by the accidental leakage of high-level nuclear waste (HLW) containing 137Cs+. The high temperature of the self-boiling, leaked HLW fluid and the continuous decay of various radionuclides carried by the waste supernatant have resulted in elevated vadose temperatures (currently up to 72 degrees C) below the Hanford S-SX tank farm that have dissipated slowly from the time of leakage (1970). The effect of temperature on Cs+ sorption was evaluated through batch binary Cs(+)-Na+ exchange experiments on pristine sediments, while Cs+ desorption was studied in column experiments using 137Cs(+)-contaminated sediments. Cs+ adsorption generally decreased with increasing temperature, with a more apparent decrease at low aqueous Cs+ concentration (10(-10)-10(-6) mol/L). Cs+ desorption from the contaminated sediments increased with increasing temperature. The results indicated that the free energy of Na(+)-Cs+ exchange on the Hanford sediment had a significant enthalpy component that was estimated to be -17.87 (+/- 2.01) and -4.82 (+/- 0.44) kJ/mol (at 298 degrees C) for the high- and low-affinity exchange sites, respectively. Both Cs+ adsorption and desorption at elevated temperature could be well simulated by a two-site ion exchange model, with the conditional exchange constants corrected by the exchange enthalpy effect. The effect of temperature on Cs+ desorption kinetics was also evaluated using a stop-flow technique. The kinetics of desorption of the exchangeable pool (which was less than the total adsorbed concentration) were found to be rapid under the conditions studied.

The effect of MgNi 2 intermetallic compound on nanostructurization and amorphization of Mg–Ni alloys processed by controlled mechanical milling

Two Mg–Ni alloys with 27.9±11.1 and 57.5±0.8 at% Ni, fabricated by ingot metallurgy (IM) and containing ∼9 and ∼79 vol% of the MgNi 2 phase, respectively, were ball (mechanically) milled in a magnetic Uni-Ball-Mill 5 under controlled shearing mode for 10, 30, 70 and 100 h. The evolution of the microstructure of milled powders is presented. It is observed that the Mg 2Ni phase undergoes a partial amorphization in the Mg–Ni alloys containing ∼79 vol% of the MgNi 2 phase while no amorphization of Mg 2Ni is observed in the alloy containing only ∼9 vol% of MgNi 2. The results are rationalized in terms of the enthalpy effects based on the application of Miedema’s semi-empirical model to the phase changes in ball-milled intermetallics and the critical nanograin size required to be formed in the Mg 2Ni phase before triggering its amorphization, which is enhanced by the presence of hard MgNi 2 phase during ball milling. The milled powders of 27.9±11.1 at% Ni alloy, after long-term milling for 100 h, did not absorb hydrogen.