Showing papers on "Enthalpy published in 2011"

Adsorption isotherms and thermochemical data of fd&c red n° 40 binding by chitosan

Abstract: Chitosan is a natural polymer that has been employed in dye adsorption. In this work, adsorption and thermodynamic data for the interaction of FD&C Red n° 40 food dye with chitosan in aqueous solutions were investigated. The equilibrium adsorption isotherms were determined by the batch method, from 298 to 338 K. Adsorption data were adjusted to five isotherm models: Langmuir, Freundlich, Redlich-Peterson, Temkin and Dubinin-Radushkevich, in order to determine which presented the best adjustment to the experimental data. Error analysis showed that the Langmuir isotherm model was the most appropriate for fitting the experimental data, with a maximum monolayer adsorption of 3065.8 µmol g-1 at 308 K. Negative enthalpy (-112.7 kJ mol-1), entropy (-0.338 kJ mol-1 K-1) and Gibbs free energy (-15.6 to 1.0 kJ mol-1) values demonstrated that the adsorption process is exothermic, spontaneous, favorable, and that randomness of the system decreases during the adsorption process.

Electrical conductivity and defect equilibria of Pr0.1Ce0.9O2−δ

Abstract: Praseodymium-cerium oxide (PCO) solid solutions exhibit mixed ionic electronic conductivity (MIEC) behavior in a relatively high and readily accessible oxygen partial pressure (PO2) regime and as such serve as a model system for investigating the correlation between thermodynamic and kinetic properties and performance figures of merit in the areas of high temperature energy conversion, automotive control, and gas sensing applications. In this paper, we present measurements on the non-stoichiometry of Pr0.1Ce0.9O2−δ and develop a defect equilibria model to predict the dependence of the concentration of all the dominant charge carriers on temperature, PO2, and Pr fraction. The predictive model is then employed to describe the measured electrical conductivity and oxygen nonstoichiometry whereby pre-exponentials and enthalpies of defect formation and migration are extracted.

Thermodynamics of Ligand Binding and Efficiency

Abstract: Analysis of the experimental binding thermodynamics for approximately 100 protein-ligand complexes provides important insights into the factors governing ligand affinity and efficiency. The commonly accepted correlation between enthalpy and -TΔS is clearly observed for this relatively diverse data set. It is also clear that affinity (i.e., ΔG) is not generally correlated to either enthalpy or -TΔS. This is a worrisome trend since the vast majority of computational structure-based design is carried out using interaction energies for one, or at most a few, ligand poses. As such, these energies are most closely comparable to enthalpies not free energies. Closer inspection of the data shows that in a few cases the enthalpy (or -TΔS) is correlated with free energy. It is tempting to speculate that this could be an important consideration as to why some targets are readily amenable to modeling and others are not. Additionally, analysis of the enthalpy and -TΔS efficiencies shows that the trends observed for ligand efficiencies with respect to molecular size are primarily a consequence of enthalpic, not entropic, effects.

Benzene under high pressure: a story of molecular crystals transforming to saturated networks, with a possible intermediate metallic phase.

Abstract: In a theoretical study, benzene is compressed up to 300 GPa. The transformations found between molecular phases generally match the experimental findings in the moderate pressure regime (<20 GPa): phase I (Pbca) is found to be stable up to 4 GPa, while phase II (P43212) is preferred in a narrow pressure range of 4–7 GPa. Phase III (P21/c) is at lowest enthalpy at higher pressures. Above 50 GPa, phase V (P21 at 0 GPa; P21/c at high pressure) comes into play, slightly more stable than phase III in the range of 50–80 GP, but unstable to rearrangement to a saturated, four-coordinate (at C), one-dimensional polymer. Actually, throughout the entire pressure range, crystals of graphane possess lower enthalpy than molecular benzene structures; a simple thermochemical argument is given for why this is so. In several of the benzene phases there nevertheless are substantial barriers to rearranging the molecules to a saturated polymer, especially at low temperatures. Even at room temperature these barriers should all...

Universal relation for size dependent thermodynamic properties of metallic nanoparticles

Abstract: The previous model on surface free energy has been extended to calculate size dependent thermodynamic properties (i.e., melting temperature, melting enthalpy, melting entropy, evaporation temperature, Curie temperature, Debye temperature and specific heat capacity) of nanoparticles. According to the quantitative calculation of size effects on the calculated thermodynamic properties, it is found that most thermodynamic properties of nanoparticles vary linearly with 1/D as a first approximation. In other words, the size dependent thermodynamic properties Pn have the form of Pn = Pb(1 − K/D), in which Pb is the corresponding bulk value and K is the material constant. This may be regarded as a scaling law for most of the size dependent thermodynamic properties for different materials. The present predictions are consistent literature values.

Kinetics and thermodynamic study of aniline adsorption by multi-walled carbon nanotubes from aqueous solution.

Abstract: Multi-walled carbon nanotubes (MWCNTs) were used in the adsorptive removal of aniline, an organic pollutant, from an aqueous solution. It was found that carbon nanotubes with a higher specific surface area adsorbed and removed more aniline from an aqueous solution. The adsorption was dependent on factors, such as MWCNTs dosage, contact time, aniline concentration, solution pH and temperature. The adsorption study was analyzed kinetically, and the results revealed that the adsorption followed pseudo-second order kinetics with good correlation coefficients. In addition, it was found that the adsorption of aniline occurred in two consecutive steps, including the slow intra-particle diffusion of aniline molecules through the nanotubes. Various thermodynamic parameters, including the Gibbs free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°), were calculated. The results indicated that the spontaneity of the adsorption, exothermic nature of the adsorption and the decrease in the randomness reported as ΔG°, ΔH° and ΔS°, respectively, were all negative.

Aggregation behavior of amino acid ionic liquid surfactants in aqueous media.

Abstract: Self-aggregation of amino acid ionic liquid surfactants (AAILSs) in aqueous solution has been investigated through surface tension, conductivity, steady-state fluorescence, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The critical aggregation concentration (cac) of AAILSs obtained from different techniques showed fairly good agreement. Surface tension measurements have been used to derive surface adsorption properties such as adsorption efficiency (pC20), effectiveness of surface tension reduction (Πcac), and minimum surface area per molecule (Amin) at the air–water interface. Temperature-dependent conductivity measurements have been used to obtain the degree of counterion binding (β), and the thermodynamic parameters such as standard free energy (ΔGagg0), enthalpy (ΔHagg0), and entropy (ΔSagg0) of aggregation. The aggregation number (Nagg) for various AAILSs has been derived by using the fluorescence quenching technique. Size of the aggregates has been obtained from DLS and...

Kinetics and mechanistic analysis of an extremely rapid carbon dioxide fixation reaction

Abstract: Carbon dioxide may react with free or metal-bound hydroxide to afford products containing bicarbonate or carbonate, often captured as ligands bridging two or three metal sites. We report the kinetics and probable mechanism of an extremely rapid fixation reaction mediated by a planar nickel complex [NiII(NNN)(OH)]1- containing a tridentate 2,6-pyridinedicarboxamidate pincer ligand and a terminal hydroxide ligand. The minimal generalized reaction is M-OH + CO2 → M-OCO2H; with variant M, previous rate constants are ≲103 M-1 s-1 in aqueous solution. For the present bimolecular reaction, the (extrapolated) rate constant is 9.5 × 105 M-1 s-1 in N,N′-dimethylformamide at 298 K, a value within the range of kcat/KM≈105–108 M-1 s-1 for carbonic anhydrase, the most efficient catalyst of CO2 fixation reactions. The enthalpy profile of the fixation reaction was calculated by density functional theory. The initial event is the formation of a weak precursor complex between the Ni-OH group and CO2, followed by insertion of a CO2 oxygen atom into the Ni-OH bond to generate a four center Ni(η2-OCO2H) transition state similar to that at the zinc site in carbonic anhydrase. Thereafter, the Ni-OH bond detaches to afford the Ni(η1-OCO2H) fragment, after which the molecule passes through a second, lower energy transition state as the bicarbonate ligand rearranges to a conformation very similar to that in the crystalline product. Theoretical values of metric parameters and activation enthalpy are in good agreement with experimental values [ΔH‡ = 3.2(5) kcal/mol].

Solution Thermodynamics and Preferential Solvation of Meloxicam in Propylene Glycol + Water Mixtures

Abstract: The equilibrium solubilities of the analgesic drug meloxicam (MEL) in propylene glycol + water mixtures were determined at several temperatures from 293.15 to 313.15 K. The Gibbs energy, enthalpy, and entropy of solution and of mixing were obtained from these solubility data. The solubility was maximal in neat propylene glycol and very low in pure water at all temperatures studied. A nonlinear plot of ΔsolnH° versus ΔsolnG° gave a negative slope from pure water up to 0.80 mass fraction of propylene glycol and a positive slope above this composition up to neat propylene glycol, at the mean temperature 303.15 K. Accordingly, the driving mechanism for MEL solubility in the water-rich mixtures was the entropy, probably due to water-structure loss around nonpolar moieties of the drug, while for the propylene glycol-rich mixtures it was the enthalpy, probably due to better solvation of the drug. The preferential solvation of MEL by the components of the solvent was estimated by means of the inverse Kirkwood-Buff integral method, showing rather small preferential solvation of MEL by propylene glycol at all compositions.

Hydrogen Storage in Magnesium Hydride : The Molecular Approach

Abstract: Safe and convenient storage of hydrogen is one of the nearfuture challenges. For mobile applications there are strict volume and weight limitations, and these limitations have steered investigations in the direction of compact, solid, lightweight main-group hydrides. Whereas ammonia–borane (NH3BH3) is a nontoxic, nonflammable, H2-releasing solid with a record hydrogen density of 19.8 wt%, it releases hydrogen in an irreversible process. Metal hydrides such as MgH2 are less rich in hydrogen (7.7 wt%) but advantageously display reversible hydrogen release and uptake: MgH2QMg+ H2. [3] Although bulk MgH2 seems an ideal candidate for reversible hydrogen storage, it is plagued by high thermodynamic stability, which translates into relatively high hydrogen desorption temperatures and slow release and uptake kinetics. The kinetics can be improved drastically by doping the magnesium hydride with transition metals and by ball milling or surface modifications. The high hydrogen release temperature (over 300 8C), however, is due to unfavorable thermodynamic parameters (DH= 74.4(3) kJmol ; DS= 135.1(2) Jmol K ), which originate from the enormous lattice energy for [MgH 2]1 (DH= 2718 kJmol ) relative to that of bulk Mg (DH= 147 kJmol ). Although thermodynamic values are intrinsic to the system, recent theoretical calculations demonstrate that for very small (MgH2)n clusters (n< 19), the enthalpy of decomposition sharply reduces with cluster size. Downsizing the particles has a dramatic effect on the stability of saltlike (MgH2)n but much less on that of the metal clusters Mgn. For a Mg9H18 cluster of approximately 0.9 nm diameter a desorption enthalpy of 63 kJmol 1 was calculated, from which a decomposition temperature of about 200 8C can be estimated. At the extreme limit, molecular MgH2 is calculated to be unstable even towards decomposition into its elements (DH= 5.5 kJmol ). The sharp decrease of stability for (MgH2)n clusters with n< 19 can be understood by the rapid increase in surface/volume ratios: surface atoms have a lower coordination number and are loosely bound. It is of interest to note that only clusters with n 19 ( 1.3 nm) have a core with the typical a-MgH2 rutile geometry (six-coordinate Mg and three-coordinate H). Apparently this is the critical size from which clusters start to show bulk behavior. These insights led to increased research activity on the syntheses of MgH2 nanoparticles, either by special ballmilling techniques or by incorporation into confined spaces. Thus nanoparticles in the range of 1–10 nm have been reported. Hydrogen elimination studies indeed show a small reduction of DH and H2 desorption temperatures, but dramatic effects can only be expected for particles smaller than 1 nm. Production of magnesium hydride particles in the subnanometer range would benefit from a molecular “bottomup” approach. We recently reported a simple synthesis protocol for the first soluble calcium hydride complex 1 by the silane route [Eq. (1)], and Jones et al. reported the

Calculating thermodynamic properties from fluctuations at small scales.

Abstract: We show how density and energy fluctuations of small nonperiodic systems embedded in a reservoir can be used to determine macroscopic thermodynamic properties like the enthalpy density and the ther...

Modified α Function for the Peng—Robinson Equation of State To Improve the Vapor Pressure Prediction of Non-hydrocarbon and Hydrocarbon Compounds

Abstract: On the basis of the available vapor pressures for 59 non-hydrocarbon and hydrocarbon compounds, including heavy alkanes up to n-tritetracontane (n-C43H88), a modified α function for the Peng−Robinson equation of state (PR-EOS) has been developed to more accurately determine the vapor pressure for pure non-hydrocarbon and hydrocarbon compounds, especially heavy components. To balance the characterization of both light and heavy compounds, the Pitzer acentric factor is first redefined in terms of reduced vapor pressure at a reduced temperature of 0.6. In comparison to the evaluated α functions used for the PR-EOS, it is found that the newly developed α function with the redefined acentric factor provides a more accurate prediction of vapor pressures with a percentage average absolute deviation of 1.90% and a percentage maximum absolute deviation of 21.22% for the 59 chemical species. In addition, the newly developed α function results in the best prediction of the vaporization enthalpy data with an average ...