Enthalpy Components Research Articles

Specific heat and thermodynamic characteristics of the system Bi-Sr-Ca-Mg-Cu-O in the temperature range 4.2?300 K

Results are presented from an experimental study of specific heat of the superconductive metal oxide system Bi -Sr-Ca-Mg-Cu-O over the temperature range 4.2–300 K. Temperature-dependent components of entropy and enthalpy are calculated. A correlation is made between Tc and standard entropy and enthalpy values for high temperature superconductors of various classes.

Enthalpy and entropy contributions to solvent effects on adduct formation

Thermodynamic parameters for adducts of organometallic acids with tributylphosphine and triphenylphosphine oxide in various solvents have been determined by NMR and calorimetric analysis. These data combined with previously reported data on hydrogen-bonded and other adduct systems have been used to correlate and interpret solvent effects on the thermodynamics of adduct formation in organic media. Linear free energy (LFE) relationships are analyzed in terms of solvent effects on the enthalpy and entropy components of adduct formation. Empirical parameters related to electron pair donating and accepting abilities along with bulk solvent properties give LFE correlations which may be interpreted in terms of solute-solvent interactions

Thermodynamic and structural properties of acrylonitrile–butadiene rubber/polyethylene blends

AbstractThermodynamics of interaction, adhesive properties, and the structure of acrylonitrile‐butadine rubber/low density polyethylene blends have been investigated. Enthalpy and entropy components of interaction parameters supporting an incompatibility of studied systems and a possibility of highly organized structure formation on the phase interface were determined by inversion gas chromatography. The formation of highly organized structures on the interface is made possible by a local diffusion of polybutadiene fragments into polyethylene phase. That is why acrylonitrile microblocks are oriented. By this manner adhesion between rubber and polyethylene phase is enforced and simultaneously a number of morphological changes take place.

Thermodynamics of the solvation of the anions of cyclic and acyclic phosphoric acids

1. The enthalpies of solution were measured by calorimetric method for the tetramethylammonium salts of 1,3-propylene-, 1,3-butylene-, diethyl-, and 1,4-butylenephosphoric acids in water, ethanol, and nitromethane at 298.15 K. 2. The enthalpies of transfer of all the investigated solvents from ethanol to nitromethane were similar, whereas in the transition from water to the anhydrous solvents the enthalpies of transfer of the salts of the six-membered cyclic phosphoric acids differed by 1.5 kcal/mole from those for the salts of diethyl-and 1,4-butylenephosphoric acids. 3. The change of the enthalpy component for the resolvation of the anions is determining in the variation of the free energies of resolvation of the phosphoric acids.

Stereochemistry of seven-membered heterocycles. Communication 18. Graphic processing of dipole moments of cis-2-ARYL-4-R-1,3-Dioxa-5,6-benzocycloheptenes and conformational characteristics of the 4-methyl group

1. By graphic processing of the dipole moments of cis-2-aryl-4-R-1,3-dioxa-5,6-benzo-cycloheptenes replacement of the H atom by Me(Ph) at C-4 was shown not to change the position of chair-twist equilibrium. 2. The chair-chair conformational energy and the enthalpy component of the Ce~Te conformation energy for the 4-methyl group were determined.

A b i n i t i o calculation of the thermochemical data on the H3O++H2O=H5O+2 gas-phase clustering

Thermochemical data for the clustering H3O++H2O=H3O+⋅⋅⋅H2O have been estimated theoretically. Basis set dependence of the entropy change as well as of the enthalpy change has been examined. The double-zeta plus polarization quality is required to reproduce them satisfactorily. The stabilizing dispersion energy which corresponds to an intermolecular correlation correction is calculated to be about 3 kcal/mol. The role of components of enthalpy and entropy changes has been discussed.

Fundamental difference between the molecular interactions of agonists and antagonists with the beta-adrenergic receptor.

Antagonist binding to the beta-adrenergic receptor is largely entropy driven, with only a small enthalpy component. The binding of agonists, on the other hand, is associated with a large decrease in enthalpy which permits a highly unfavourable decrease in entropy. The thermodynamic differences between the binding of agonists and antagonists may provide new insights into the molecular basis for hormone stimulation of adenylate cyclase activity.

Thermodynamic Properties of Liquids Including Solutions. XV. Intramolecular Contacts and Their Effects on Excess Enthalpies of Solutions of Chain Molecules, Especially Benzene Solutions of Normal Alkanes and Polymethylenes

The theory of excess enthalpy (HE) and enthalpic component of the interaction parameter (χh), dealt with in previous papers of this series, is extended to allow for the effects of intramolecular contacts in chain molecule solutes. The number of these contacts, as a function of the number of mers in the chain molecules and their concentration in the solution, is estimated from enumerations over a class of self-avoiding walks (paths) in a lattice-graph. Applied to solutions of polymethylenes in benzene, for which the equilibrium constant parameter (K) has been previously estimated to be unity, we find intramolecular contacts to have little effect on the excess enthalpy at 25°C even for long chains. The enthalpic part of the interaction function is predicted to depend strongly on the extent of intramolecular contacting. Extensions of the theory to solutions for which K≠1 are briefly considered.

The linear free-energy relation in the thermodynamics of complex formation. Part 2. Analysis of the formation constants of complexes of the large metal ions silver(I), mercury(II), and cadmium(II) with ligands having ‘soft’ and nitrogen-donor atoms

A potentiometric and calorimetric study has been made of the complex formation of AgI, HgII, and CdII with ligands containing ‘soft’ and N-donor atoms. It has been found that linear enthalpy relations exist, and that they mirror quite closely the division of the free-energy data into separate groups for the ‘soft’ and N-donor atom ligands, for the pair of metal ions AgI–HgII and for AgI–CdII. It seems that a linear entropy relation also exists, to within experimental error, for the pair AgI–HgII which has a gradient and intercept that can be rationalised in terms of the Powell–Latimer–Cobble semi-empirical theory of ionic entropy. The entropy contributions are small compared with those of the enthalpy in these predominantly enthalpy-stabilised complexes. The significance of the relatively simple behaviour of the free energies compared with their enthalpy and entropy components is discussed.

Thermodynamic aspects of some radioassays.

In a number of radioimmunoassays and radiotransin assays, effective equilibrium constants have been measured at different temperatures in order to define the relative contribution of changes of entropy and enthalpy to the change in free binding energy. In systems with a large enthalpy component, the lowest possible incubation temperature maximizes sensitivity, and control of temperature throughout the assay is important. Conversely, when enthalpy change is small, a high temperature allows rapid attainment of equilibrium without loss of sensitivity. At a theoretical level, the thermodynamic characteristics of binding may allow some insight into the nature of the binding process.

On the Temperature Coefficient of the Solubility of Some Glycyl Peptides in Water-Ethanol Mixtures

Abstract The solubility of glycine, diglycine, and triglycine was investigated in water-ethanol mixtures (20% and 50% ethanol) as a function of the temperature. The transfer free energy, ΔFtrconh, for the peptide group was evaluated and the separate enthalpy and entropy components of the transfer process were determined. In 20% ethanol solutions, the hypothesis of additivity of the contribution of various molecular groups to the transfer parameters was verified. Values obtained as the difference triglycine-diglycine in 20% ethanol at 25° were ΔFtrconh = +120 cal per mole, ΔStrconh = -1.7 e.u., ΔHtrconh = -390 cal per mole. These values appear to be in excellent agreement with previous results obtained for the coil → helix transformation of poly-l-ornithine in 20% ethanol solutions. It has been confirmed that helical stabilization in mixed solvents is an entropic effect, arising from the negative value of the transfer entropy, ΔStrconh (from water to water-organic solvent mixtures).

Thermodynamic properties of liquids, including solutions. VIII. Comparison of properties of several polymer and solvent systems

AbstractThe writer's new theory of solutions has, so far, been applied in detail to experimental data from four types of polymer solutions: rubber plus benzene, polyisobutylene plus benzene, polyisobutylene plus cyclohexane, and poly(propylene oxide) plus carbon tetrachloride. The parameters deduced for these systems are here compared and interpreted in terms of molecular and intermolecular properties. The effects of these parameters on the magnitudes and concentration dependence of the related thermodynamic properties (especially the interaction coefficient χ and its enthalpy and entropy components) are discussed and illustrated.

Thermodynamic study of the interaction of an organic ion, hydroxytetracycline, with linear and cross-linked polyelectrolytes

1. The thermodynamic functions characterizing the intermolecular interaction were determined according to the temperature dependence of the selectivity of the binding in the system of organic ion of hydroxytetracycline (HTC) and polyelectrolyte. 2. The HTC-polyelectrolyte interaction is intensified by the transition from soluble to cross-linked polyelectrolytes. 3. The entropy and enthalpy components of the free energy change for the HTC-polyelectrolyte system are correlated with the increase in the amount of the cross-linking agent and pass through a maximum at 0.5% DVB. 4. An interpretation is given for the thermodynamic data on the basis of the assumption of supplementary (in addition to coulombic) interaction of HTC with polyelectrolytes.

An inverse solution procedure for turbulent swirling boundary layer combustion flow

A detailed description is presented of the mathematical and computational aspects of the general inverse solution procedure developed for turbulent swirling boundary layer combustion flow. The situation is that a nonrecirculating swirling turbulent flame in which experimental time-mean measurements of the axial and swirl velocities ν z and ν θ , temperature T and chemical species concentrations m j have been made. The problem is to calculate the distributions of the significant turbulent momentum, enthalpy and chemical species' flux components ( τ rz , τ rθ , ( J h ) r and ( J i ) r ) and associated exchange coefficients, Prandtl and Schmidt numbers. A solution is provided by the inverse solution TEXCO code; it provides a link between mean measurements and certain correlations of turbulent fluctuation components and throws light on the appropriateness or otherwise of any given turbulence model for the flow under consideration. The method is applied to both isothermal and chemically reacting flows, and calculations show that previous assumptions of isotropy of the turbulent stress tensor and constancy of Prandtl-Schmidt numbers are not generally valid. The exchange coefficients are shown to be functions of the degree of swirl and position in the flowfield. For the isothermal case, it is shown that the assumption of an isotropic uniform mixing length parameter distribution is quite feasible for weak swirl but is progressively less valid as the degree of swirl increases. For the flame case, similar results are obtained and the turbulent viscosity is found to be highly nonisotropic.

Hydrolysis of amides. IV. Dilute acid hydrolysis of amides of the type R2CH-CONH2 and R3C-CONH2

Enthalpies and entropies of activation have been derived from rate constants measured over a range of temperature for the dilute acid hydrolysis of isobutyramide, diethylacetamide, α-methylbutyramide, cyclohexaneoarboxamide, cyclopentane-carboxamide, and trimethylacetamide. ��� These data, in combination with data for other amides produced earlier, show the hydrolysis rate constants to be dominated by steric influences but with a perturbing effect which is proportional to the number of α-hydrogens present in the substituent. The "α-hydrogen bonding" concept of hyperconjugation appears to offer a reasonable explanation of these perturbations. ��� Controlled steric changes are shown to be non-additive for this reaction and to exert their influence primarily through the enthalpy component of the observed free energy changes.

Interaction between proteins and salt solutions. I. Thermodynamic parameters of dilution for gelatin and collagen

AbstractIntrinsic viscosity–temperature studies for isoelectric gelatin in KCl and KSCN aqueous solutions and equilibrium degree of swelling, ν−1, measurements for amorphous crosslinked rat tail tendons in the same solutions were carried out. On increasing salt concentration Cs, both [η] and ν−1 increase, go through a maximum, and then decrease at high Cs, KCl being more effective than KSCN in bringing about this decrease. The trend observed is similar to the variation of solubility of polypeptides and soluble proteins with Cs. By regarding a water–salt solution of a given Cs as a single diluent interacting with the protein modified by solvation and binding of ions, usual polymer solution theories were applied to the experimental results. Thus, quantities related to the entropy and enthalpy components of the excess chemical potential of the diluents were obtained. The data indicate that the initial increase of [η] and ν−1 with Cs results from the balance of an enthalpy component which, on increasing Cs, becomes less favorable to dilution and an entropy component which, conversely, becomes more favorable. The subsequent decrease of [η] and ν−1 with Cs is due to the prevailing of the enthalpy component. The maximum is reached at higher Cs for KSCN than for KCl, primarily because of the large increase in the entropy component operated by the former salt. The increase of the entropy parameter with Cs is associated to a breaking down of the coordinated water structure and to an alteration of the conformation of the macro molecules due to ion–dipole interaction and to ion absorption. The decrease in exothermicity of dilution with Cs indicates a reduced thermodynamic affinity of the diluent toward the polymer which probably results from alteration of the nature of both polymer and diluent.