Giuseppe Della Gatta

Standards, calibration, and guidelines in microcalorimetry. Part 2. Calibration standards for differential scanning calorimetry* (IUPAC Technical Report)

Differential scanning calorimeters (DSCs) are widely used for temperature, heat capacity, and enthalpy measurements in the range from subambient to high temperatures. The present recommendations describe procedures and reference materials (RMs) for the calibration of DSCs. The recommendations focus on the calibration of the response of the instrument and on the estimation of the measurement uncertainty. The procedures for temperature, enthalpy, and heat-flow rate calibration are given in detail. Calibration on cooling has also been considered. Recommended RMs are listed, and the relevant properties of these materials are discussed.

The chemisorption of carbon monoxide on various transition aluminas

Carbon monoxide chemisorption was investigated at 36 °C on four transition aluminas (η, γ, θ, Alon C) by means of microcalorimetry and infrared spectroscopy. On all the aluminas coverages are fairly low, and the interaction only involves a coordination to surface cations. There is evidence for two families of admolecules, characterized by different stretching frequencies (Δ¯ v ∼ 30 cm −1 ) and adsorption heats (14 and 2 kcal/mole, respectively). The same species are found on all the aluminas although in different amounts. In particular the remarkable differences observed between η-alumina and γ-alumina seem to parallel their different catalytic activity.

Enthalpies and entropies of sublimation, vaporization and fusion of nine polyhydric alcohols

The thermodynamics of phase transitions of nine polyhydric alcohols have been studied by different experimental approaches. The enthalpy and entropy changes associated with sublimation of pentaerythritol and myo-inositol, and the vaporization of meso-erythritol, adonitol, D-arabinitol, xylitol, dulcitol, D-mannitol, D-sorbitol and myo-inositol were determined from the temperature dependence of their vapour pressures, as measured by the torsion–effusion method. The enthalpies and temperatures of fusion were measured by differential scanning calorimetry and relevant entropies were also obtained.Enthalpies and entropies of sublimation at 298 K have been derived from the experimental data by considering the appropriate Cp values partly estimated through the group additivity schemes. Results are fairly constant for each class of polyols and in particular ΔsubX0298(tetritols)≈ΔsubX0298(pentitols) < ΔsubX0298(hexitols), but D-sorbitol and myo-inositol show lower values. Sublimation is discussed on the basis of the crystal structure data in the literature.

Surface rehydration of variously dehydrated eta-alumina

Abstract The surface dehydration and rehydration of well-characterized eta-alumina have been investigated by means of gas adsorption microcalorimetry and infrared spectroscopy. Dehydration shows a multiplicity of surface hydroxyl species, some of which are not reformed on water adsorption at room temperature. The creation of an undissociated water phase takes place from the earliest stages of rehydration. Differential heats steeply decrease with surface coverage, while the adsorption energy distribution does not seem to depend on the dehydration degree.

Energetics of adsorption in the alumina—water system microcalorimetric study on the influence of adsorption temperature on surface processes

Abstract The interaction of water vapor with highly dehydroxylated η-alumina has been investigated at four adsorption temperatures (25, 100, 150, 200°C) by means of a microcalorimetric-volumetric technique. A part of the water molecules irreversibly chemisorbed at room temperature became reversibly adsorbed at the higher temperatures. Three types of adsorption mechanisms were noted: physisorption, only at the lowest temperature, coordination of molecular water on the dehydroxylated surface, and dissociative adsorption, within a wide energy spectrum. The molecular process appeared to be instantaneous, while thermokinetic data showed that dissociative adsorption was always activated. Increasing temperature resulted in greater filling up of the more energetic sites in the first stages of adsorption. In particular, from 150°C on the molecules appear to possess sufficient energy to obtain the most ordered surface occupation. The equilibrium between the two chemisorbed species was displaced toward dissociation by a temperature increase: Sites available for reversible adsorption were reduced, and the total heat released at high coverage decreased. These findings are interpreted on the basis of classical thermodynamic considerations.

DIRECT DETERMINATION OF ADSORPTION HEATS

Energy changes occurring during adsorption in gas-solid and solution-solid systems are considered in connection with measurement of the adsorption enthalpies. Some experimental apparatuses allowing determination both of heat evolved and adsorbed amounts are described, with special regard to those using heat-flow microcalorimeters. Calibration techniques and experimental data reliability are also discussed.

Energies of different surface rehydration processes on “eta”, “theta” and “alpha” aluminas

The adsorption of water vapour on completely dehydroxylated samples of η-, θ- and α-alumina has been investigated at 150°C by a microcalorimetric technique.The same rehydration mechanisms have been found on all three aluminas, essentially dissociative and coordinative chemisorption. The hydroxylation process appears to be inhibited on passing from η- to θ- and α-alumina, the coordination of molecular water in different forms being favoured. However decreases in adsorption capacity and energies are evidenced, which may be correlated both with different alumina structures and with the modification of surface reactivity induced by the increasing preparation temperatures.

Enthalpies of solution at infinite dilution for linear and branched aliphatic aldehydes in heptane. Calorimetric measurements compared with disquac predictions

Abstract The molar enthalpies of solution at infinite dilution of 16 linear and branched alkanals in heptane were determined calorimetrically at 298.15 K. These data and the literature data for the molar excess Gibbs energies, molar excess enthalpies, and activity coefficients at infinite dilution of alkanal + n-alkane mixtures were examined on the basis of the DISQUAC group contribution model. The quasi-chemical and dispersive interaction parameters were compared with those of alkanones. There is clear evidence for both a steric and an inductive effect exerted by the alkyl groups adjacent to the carbonyl group. The steric effect results in a regular decrease of the quasi-chemical parameters; the inductive effect increases the dispersive parameters. DISQUAC offers a consistent description of the thermodynamic properties of alkanal + n-alkane mixtures, even at infinite dilution.

Heat capacities of urea, N-methylurea, N-ethylurea, N-(n)propylurea, and N-(n)butylurea in the range 200 to 360 K

Abstract The heat capacities at constant pressure of urea, N -methylurea, N -ethylurea, N -( n )propylurea and N -( n )butylurea were measured by differential scanning calorimetry over the range 200–360 K, in gas-flow conditions. A He + Ne mixture and N 2 only were selected as the purge gases below and above room temperature, respectively. Benzoic acid was used as the reference material and the accuracy of the measurements was estimated to be within 2% across the temperature range. The heat capacities at 298.15 K were obtained by interpolating the smoothed fitting equations. No anomalies were detected for urea or N -methylurea. One fully reversible solid-solid transition was observed for N -ethylurea at T = (294.5 ± 2.0) K, with Δ trs H m = (1.0 ± 0.3) kJ mol −1 , and for N -( n )propylurea at T = (289.6 ± 1.0) K, with Δ trs H m = (3.0 ± 0.2) kJ mol −1 . Two solid-solid transitions were found for N -( n )butylurea at T = (315.0 ± 2.0) K, with Δ trs H m = (7.0 ± 2.0) kJ mol −1 and at T = (346.0 ± 1.2) K, with Δ trs H m = (1.0 ± 0.1) kJ mol −1 , the first one being recoverable at temperatures lower than 240 K only.

Use of isothermal microcalorimetry data for the determination of integral molar entropies of adsorption at the gas—solid interface by a quasi-equilibrium procedure

Abstract The aim of this paper is to describe the present position in the experimental determination of entropies of adsorption. After pointing out the limitations of the “isosteric” approach and after stressing the fact that the most interesting entropy to know is the integral molar entropy of adsorption, the authors show how the use of isothermal adsorption microcalorimetry, with a special quasi-reversible procedure, allows this entropy to be determined satisfactorily from the thermodynamic point of view.