Solvent effect on heats of protonation of some amines

The heats of protonation of n-pentylamine, dipentylamine, tributylamine and heats for the first protonation step of 1,8-diamino-3,6-dioxaoctan, diazacrownether 22 and cryptand 222 in pure acetonitril and propylene carbonate and of diazacrownether 22 and dibenzosubstituted ligand 22BB in water + acetonitrile mixtures have been measured at 298.15 K using calorimetric titrations. The values of the reaction enthalpies in the solvents as well as the data in aqueous and methanol solutions reported in literature are analysed in terms of the simple electrostatic model and thermodynamic parameters of transfer (solvation) of the reactants. Estimation of the electrostatic and covalent contributions to standard enthalpy of transfer of the reactions from water to non-aqueous and mixed solvents has been made.     

The low-temperature heat capacity and saturation vapor pressure of ethyl ester of butanoic acid

The heat capacity, thermodynamic properties of fusion, and purity of the ethyl ester of butanoic acid were determined by adiabatic calorimetry in the temperature range from 8 to 372 K. The pT-parameters of the ester for the equilibrium liquid-vapor were measured by comparative ebulliometry in the “atmospheric” range of pressure from 10.8 to 101.7 kPa. The obtained data were used to derive the normal boiling temperature (T _n.b), the enthalpies of vaporization at T = 298.15 K and T _n.b, and the main thermodynamic functions (changes of S, H, G) in the crystal and liquid states of the temperature interval studied and in the ideal gas state at T = 298.15 K. The experimental vapor pressures of the narrow temperature interval, ΔT = 62 K were extended to the entire range of the liquid, T _cr − T _tp⁰ = 394.3 K, from the triple, T _tp⁰, to the critical, T _cr, temperatures.     

Thermodynamics of complex formation in mixed solvents <Emphasis Type="Italic">K</Emphasis> and Δ<Emphasis Type="Italic">H</Emphasis> for the formation reaction of [Gly18C6] at 298.15 K

The influence of H₂O–EtOH and H₂O–Acetone mixed solvents at various compositions on the thermodynamics of complex formation reaction between crown ether 18-crown-6 (18C6) and glycine (Gly) was studied. The standard thermodynamic parameters of the complex [Gly18C6] (log K°, Δ_r H°, Δ_r S°) were calculated from thermochemical data at 298.15 K obtained by titration calorimetry. The complex stability and its formation enthalpy increase with increasing the non aqueous component concentration in both mixed solvents. The thermodynamic data were discussed on the basis of the solvation thermodynamic approach and the solvation contributions of the reagents and of the complex to the complex stability were analyzed.     

A thermodynamic study on the complexation between riboflavin and a diaminotriazine derivative mediated by triple hydrogen bonds at water/oil interfaces

The changes in Gibbs free energy (ΔG _int), enthalpy (ΔH _int) and entropy (TΔS _int) upon complexation between riboflavin (RF) and N,N-dioctadecyl-[1,3,5]triazine-2,4,6-triamine (DTT), mediated by triple hydrogen bonds at water/carbon tetrachloride, trichloroethylene and chloroform interfaces, were determined via temperature-controlled interfacial tension measurements. It was shown that hydrogen bonding interactions between RF and DTT were best characterized by large and negative ΔH _int values, unlike those predicted from either the polarity in each phase or the arithmetic average of the polarities in the two phases. Furthermore, the ΔH _int values became more positive as the dielectric constant of the oil phase was increased. These results strongly indicate that ΔH _int is governed by the dielectric properties of the oil phase. Adsorption of RF, DTT and the RF-DTT complex at the water/oil interface gave rise to restrictions on the translational and rotational motions of these species, as demonstrated by the ΔS _int values observed, which is another characteristic of interfacial complexation. The thermodynamic parameters evaluated in the present study revealed the characteristic complexation behavior that occurs at a water/oil interface, as mediated by hydrogen bonding.     

Model system for concentrated electrolyte solutions: Thermodynamic and transport properties of ethylammonium nitrate in acetonitrile and in water

Ethylammonium nitrate (ETAN), a low melting fused salt which is completely miscile in water and in many non-aqueous solvents, was used as a model system for the study of concentrated non-aqueous electrolyte solutions. Acetonitrile (AN) was chosen as a representative aprotic solvent. Some data were also obtained for water as solvent. The properties investigated over the whole mole fraction range, many as a function of temperature, were solid-liquid phase diagram, volume, heat capacity, conductivity and viscosity. Most properties in both solvents vary in a regular fashion over the whole mole fraction range and the properties at high concentration rapidly tend to those of the molten salt. The apparent volumes and heat capacities vary linearly with lnX₂ over a large mole fraction range. There is evidence of significant association in AN (K _A =1094 l-mol⁻) but not in water. The low concentration thermodynamic data were fitted with an association model using the above K _A to obtain the partial molar quantities of ETAN at infinite dilution and in the associated state. These latter values are of the same magnitude as the molar quantities of the molten electrolyte.     

Thermodynamic studies of hydriodic acid in ethylene glycol-water mixtures from electromotive force measurements

The standard potentials of the Ag—AgI electrode in twenty ethylene glycol—water mixtures covering the whole range of solvent composition have been determined from the e.m.f. measurements of the cell Pt¦H₂(g, 1 atm)¦HOAc (m ₁), NaOAc (m ₂), KI(m ₃), solvent¦AgI¦Ag at nine different temperatures ranging from 15 to 55°C. The temperature variation of the standard e.m.f. has been utilized to compute the standard thermodynamic functions for the cell reaction, the primary medium effects of various solvents upon HI, and the standard thermodynamic quantities for the transfer of HI from the standard state in water to the standard states in the respective solvent media. The chemical effects of solvents on the transfer process have been obtained by subtracting the electrostatic contributions from the total transfer quantities. The results have been discussed in the light of ion—solvent interactions as well as the structural changes of the solvents.     

Thermodynamics of Cadmium Chloride in 2-Butanone + Water Mixtures (5, 10, and 15 Mass%) from Electromotive Force Measurements

The emf (electromotive force) of the cell: CdHg_x (two phase) | CdCl₂ (m) | AgCl | Ag in 2-butanone + water mixtures (containing 5, 10, and 15 mass% 2-butanone) was measured at varying temperature (293.15, 298.15, 303.15, 308.15, and 313.15 K) and in the CdCl₂ molality range from 0.002 to 0.02 mol-kg^–1. At each temperature the standard emf of the cell (E_m^o) was determined using potentiometric data and literature values for the stability constants of chlorocadmium complexes. The E_m^o values were used to calculate the standard thermodynamic quantities for the cell reaction, the stoichiometric mean molal activity coefficients of CdCl₂, and the thermodynamic functions for CdCl₂ transfer from water to 2-butanone + water mixtures. The transfer process is a forced one and results in an entropy decrease. The transfer functions were compared to those obtained for the same electrolyte in acetone + water mixtures, as well as to those for HBr in ketone + water mixtures. Medium effects upon CdCl₂ were calculated and discussed for the examined mixtures (2-butanone + water).     

Dependence of the thermodynamic characteristics of the complexation of alanine-18-crown-6 on the composition of water-ethanol solvent

Standard thermodynamic parameters (Δ_r G○, Δ_r H○, TΔ_r S○) for the complexation reaction of 18-crown-6 ether (18C6) with D,L-alanine (Ala) in mixed water-ethanol (H₂O-EtOH) solvents are calculated from the data of calorimetric titrations performed at T = 298.15 K. It is established that an increase in the concentration of EtOH in mixed solvent leads to a rise in stability and an increase in the exothermicity of [Ala18C6] molecular complex formation; changes in the energetics of reaction upon a change in the solvent composition are determined by changes in the solvation state of 18C6, which is typical of the reactions of molecular complex formation of 18C6 with D,L-alanine and glycine in water-organic solvents.     

Synthesis and characterization of fluorinated benzoxazole polymers with high Tg and low dielectric constant

Next generation microelectronic packaging requirements are driving the need to produce increasingly lower dielectric constant materials while maintaining high thermal stability and ease of processing. Efforts have focused on the synthesis and analysis of new polymers with the goals of high thermal stability [degradation temperature (T_d) > 400 °C, low glass‐transition temperature (T_g) > 350 °C], low water uptake (<1%), solubility in selected organic solvents, dielectric constant less than 2.5, and low thermal expansion coefficient. These stringent combined goals have been largely achieved with flexible aromatic benzoxazole polymers. Intramolecular hydrogen bonding between pendant hydroxyl groups and the double‐bond nitrogen of the benzoxazole has been exploited to increase the polymer T_g, whereas the incorporation of perfluoroisopropyl units effectively decreases the dielectric constant. Out‐of‐plane impedance measurements on films of materials in this family (38–134 μm thick) have resulted in typical dielectric values of 2.1–2.5 at 1 MHz, depending on copolymer ratios and functionalizations. Results have been correlated with optical waveguide measurements of films 4‐μm thick to determine film anisotropy and the high‐frequency dielectric constant, and have been corroborated by in‐plane interdigitated electrode dielectric measurements on samples 0.75 μm thick. Candidate materials exhibited extremely low water uptake (0.2%) even after submersion in boiling water for several days. Dynamic mechanical analysis of the polymers enabled the determination of the influence of intermolecular hydrogen bonding on the T_g and loss tangent magnitude. Finally, the coefficient of thermal expansion has been examined and correlated with copolymer constitution. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1991–2003, 2000     

Synthesize,crystal structure,heat capacities and thermodynamic properties of a potential enantioselective catalyst

A new potential enantioselective catalyst derived from ferrocene, 1-{(R)-1-[(S)-2-(diphenylphosphino)ferrocenyl]ethyl}-benzimidazole (DPFEB), was prepared and its absolute structure was characterized by means of single crystal X-ray diffraction. The molar heat capacity of DPFEB was measured by means of temperature modulated differential scanning calorimetry over the temperature range of 200–530 K, and the thermodynamic functions of [H _T  − H _298.15] and [S _T  − S _298.15] were calculated. Further more, thermogravimetry experiment revealed that DPFEB exhibited a three step thermal decomposition process with the final residual of 28.7%.     

Thermodynamics of cross-linked and branched (co)polymers based on <Emphasis Type="Italic">tris</Emphasis>- and <Emphasis Type="Italic">bis</Emphasis>-(pentafluorophenyl)germanes

The temperature dependence of the heat capacity of cross-linked and branched (co)polymers based on tris- and bis-(pentafluorophenyl)germanes is studied in the temperature range of 6–7 to 535–570 K, using adiabatic vacuum and differential scanning calorimeters. In the indicated temperature range, physical transformations are revealed and their thermodynamic characteristics are determined. The obtained experimental data are used to calculate the thermodynamic functions of (co)polymers: C _p /°, H°(T) - H°(0), S°(T) - S°(0), and G°(T) - H°(0) in the range of T → 0 to 535 K for the branched (co)polymer and from T → 0 to 500 K for the cross-linked polymer. Their standard entropies of formation are determined at 298.15 K. The obtained results are compared with analogous data for hyperbranched perfluorinated polyphenylenegermane studied earlier. The effect of the structure of polyphenylenegermanes on their thermodynamic properties is analyzed.     

Thermodynamic data of the NZP compounds family

Summary The thermodynamic data for NZP compounds MZr₂(PO₄)₃ (M=Na, K, Rb, Cs, Zr_0.25) and Na₅D(PO₄)₃ (D=Ti, Zr) are reported. The heat capacities of the phosphates were measured between T=7 and T=640 K. The standard enthalpies entropies, and Gibbs functions of formation at T=298.15 K were derived. The obtained thermodynamic characteristics of phosphates of the NZP type structure and literature data are summarized. Thermodynamic functions of reactions of solid-state synthesis were calculated and the usability of ceramic technology for obtaining NZP compounds was proved.     

Low-temperature heat capacity and standard entropy of PdO(cr.)

The temperature dependence of the heat capacity C _p ^o = f(T) of palladium oxide PdO(cr.) was studied for the first time in an adiabatic vacuum calorimeter in the range of 6.48–328.86 K. Standard thermodynamic functions C _p ^o(T), H ^o(T) — H ^o(0), S ^o(T), and G ^o(T) — H ^o(0) in the range of T → 0 to 330 K (key quantities in different thermodynamic calculations with the participation of palladium compounds) were calculated on the basis of the experimental data. Based on an analysis of studies on determining the thermodynamic properties of PdO(cr.), the following values of absolute entropy, standard enthalpy, and Gibbs function of the formation of palladium oxide are recommended: S ^o(298.15) = 39.58 ± 0.15 J/(K mol), Δ_f H ^o(298.15) = −112.69 ± 0.32 kJ/mol, Δ_f G ^o(298.15) = −82.68 ± 0.35 kJ/mol. The stability of Pd(OH)₂ (amorph.) with respect to PdO(cr.) was estimated.     

A segment concept in the dissolution of stereoregular isotactic polyacrylonitrile into three dinitrile solvents, CN-(CH2) n -CN (n = 1, 2, and 3), at elevated temperatures

The dependence of the dissolution temperature (T _sol) of isotactic polyacrylonitrile (PAN) on tacticity was studied for three dinitrile solvents. A linear relationship was obtained in the inverse plots of the tacticity dependence of the T _sol of PAN. A phenomenological analogy between the tacticity dependence of the T _sol of isotactic PAN and the molecular-weight dependence of the glass-transition temperature of amorphous polystyrene is discussed from a thermodynamic point of view. The linear relationships in both phenomena are explained in terms of a common mechanism: a breakdown of thermodynamic competition, enthalpy, and entropy through the segment mobility. The significance of segment concept and molecular mobility at elevated temperatures are discussed. Received: 7 July 1999/Accepted in revised form: 21 October 1999     

Physicochemical properties of {1-methyl piperazine (1) + water (2)} system at T = (298.15 to 343.15) K and atmospheric pressure

Densities (ρ) and viscosities (η) of aqueous 1-methylpiperazine (1-MPZ) solutions are reported at T = (298.15 to 343.15) K. Refractive indices (n_D) are reported at T = (293.15 to 333.15) K, and surface tensions (γ) are reported at T = (298.15 to 333.15) K. Derived excess properties, except excess viscosities (Δη), are found to be negative over the entire composition range. The addition of 1-MPZ reduces drastically the surface tension of water. The temperature dependence of surface tensions is explained in terms of surface entropy (S^S) and enthalpy (H^S). The measured and derived properties are used to probe the microscopic liquid structure of the bulk and surface of the aqueous amine solutions.     

Depolarization thermocurrent studies in poly(hydroxyethyl acrylate)/water hydrogels

Detailed investigations on the dielectric relaxation mechanisms in poly(hydroxyethyl acrylate) (PHEA), by means of the thermally stimulated depolarization currents (TSDC) method in the temperature range 77-300 K are reported. There is particular interest in the dependence of the dielectric relaxation mechanisms on the water content h, h = 0 ? 0.5 w/w, in an attempt to contribute to a better understanding of the physical structure of water in the PHEA hydrogels. We employ thermal sampling (TS) and partial heating (PH) techniques to experimentally analyze the observed complex relaxation processes, due to the secondary (β_sw) and the main (α) relaxation, into approximately single responses and to determine the spectra of activation energies E(T) at different h values. Measurements with different electrode configurations reveal different aspects of the dynamics of the relaxation mechanisms and allow the distinction between dipolar and conductivity relaxation contributions. It is shown that by means of these techniques we can determine certain temperature characteristics for the α relaxation and investigate their dependence on water content. We discuss the relation of these characteristic temperatures to the calorimetric glass transition temperature T_g. © 1994 John Wiley & Sons, Inc.     

Dissociation constant of protonated tris(hydroxymethyl)aminomethane in N-methylpropionamide and related thermodynamic quantities from 10 to 55°C

The dissociation constant of protonated tris(hydroxymethyl)aminomethane (tris·H⁺) in the solvent N-methylpropionamide (NMP) has been determined at intervals of 5°C from 10 to 55°C by measurement of the emf of cells without liquid junction using hydrogen and silver-silver chloride electrodes. At 25°C, pK _a was found to be 8.831, as compared with 8.075 in water. The standard changes in Gibbs energy, enthalpy, and entropy for the dissociation process have been evaluated from the dissociation constant and its change with temperature. By comparison with similar data for the dissociation of tris·H⁺ in water, thermodynamic functions for the transfer from water to NMP have been derived. The dissociation process is isoelectric, and the solvent dielectric constant is high (=176 at 25°C). Consequently, electrostatic charging effects are expected to be minimal, and the change in dissociation constant depends primarily on solute-solvent interactions. The results, combined with transfer energies for HCl, tris, and tris·HCl from emf and solubility measurements, demonstrate that the decreased acidic strength of tri·H⁺ in NMP is attributable in large part to the fact that NMP is less effective than water in stabilizing tris and its salts.     

Dependence of the enthalpies of formation of Ag<Superscript>+</Superscript> complexes with glycinate ion and the protonation of glycinate ion on the content of aqueous ethanol solvent

Enthalpy changes are determined by calorimetry for the reactions of glycinate ion (Gly⁻) proto- nation and its complexation with Ag⁺ ion at a temperature of 298 K and ionic strength 0.1 (NaClO₄) in an aqueous ethanol solvent containing 0.0–0.4 and 0.0–0.3 mole fraction of alcohol, respectively. An abnormal relationship of enthalpy changes is found for the processes of stepwise formation of mono- and bis-glycinates of silver(I) in water. It is shown that varying the ethanol content has virtually no effect on the exothermicity of Ag⁺ complexation reactions with glycinate ions at either coordination step and does not change the relationship of the step enthalpies. An analogy is observed in the relationship of solvation contributions from the reagents to the value of Δ_tr H° for the reactions of glycinate ion protonation and its complexation with silver(I) in aqueous ethanol solvents.     

Molecular complex formation between l-phenylalanine and 18-crown-6 in H2O–DMSO solvents studied by titration calorimetry at T = 298.15 K

The standard thermodynamic parameters (Δ_r G°, Δ_r H°, TΔ_r S°) of the reaction of molecular complex formation between 18-crown-6 ether (18C6) and l-phenylalanine (Phe), [Phe18C6], have been obtained from calorimetric titration experiments carried out by means of the microcalorimetric system TAM III (TA Instruments, USA) at T = 298.15 K in water–dimethylsulfoxide (H₂O–DMSO) solvents. Results show that the increase of the DMSO concentration in the mixed solvents brings about an increase of the [Phe18C6] complex stability and of the exothermicity of the reaction of complex formation.     

Thermochemical properties of free radicals from G3MP2B3 calculations

For a set of 32 selected free radicals, energy minimum structures, harmonic vibrational wave numbers ω_e, principal moments of inertia I_A, I_B, and I_C, heat capacities C°_p(T), entropies S°(T), thermal energy contents H°(T) ? H°(0), and standard enthalpies of formation Δ_fH°(T) were calculated at the G3MP2B3 level of theory in the temperature range 200–3000 K. In this article, thermodynamic functions at T = 298.15 K are presented and compared with recent experimental values. The mean absolute deviation between calculated and experimental Δ_fH°(298.15) values resulted in 3.91 kJ mol^?1, which is close to the average experimental uncertainty of ± 3.55 kJ mol^?1. The influence of hindered rotation on thermodynamic functions is studied for isopropyl and tert‐butyl radicals. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 550–560, 2002