Differential Enthalpy Research Articles

Enthalpies of absorption of CO2 in aqueous alkanolamine solutions from e-NRTL model

Abstract Differential enthalpies of absorption of CO2 with aqueous solutions of 2-aminoethanol (MEA) and N-methyldiethanolamine (MDEA) were calculated from reaction equilibrium constants of each of the key reactions taking place in the MEA + H2O + CO2 and MDEA + H2O + CO2 systems. A set of equilibrium constants was selected for the modeling, based on comparison of existing correlations for the reaction equilibrium constants, K e q , and corresponding heats of reaction, Δ H r , with experimentally measured K e q and Δ H r found in literature. Activity coefficients of all species in the solutions were calculated using the electrolyte NRTL model. Enthalpies of absorption from this work are compared with experimental data from literature and with predictions from the Aspen and Deshmukh–Mather models. The importance of good correlations for the temperature dependencies of the equilibrium constants is shown.

Effect of Temperature on the Solubility of Short-Chain Carboxylic Acids in Water

The aqueous solubilities of pentanoic and hexanoic acids were measured between 298.15 and 333.15 K at intervals of 5.00 K. Nonlinear dependences were found for the variation of solubility with temperature. The temperature dependences of the solubility of the acids in water are described by nonlinear van’t Hoff plots. Experimental data were fitted by the method of least-squares to a second-order polynomial equation, which was then used to determine the differential solution enthalpies at selected temperatures. The values for the apparent Gibbs energy and the apparent entropy of solution are calculated.

Enthalpy of absorption of CO 2 with alkanolamine solutions predicted from reaction equilibrium constants

Differential enthalpies of absorption of CO 2 in aqueous solutions of 2-aminoethanol (MEA) and N-methyldiethanolamine were predicted from reaction equilibrium constants using the Gibbs–Helmholtz equation. Correlations for the reaction equilibrium constants and enthalpies of reaction found in the literature for each of the individual reactions taking place at CO 2 absorption were compared to experimental data, and from this, a set of equations was selected for the Deshmukh–Mather model in this work. The carbamate dissociation constant for MEA was fitted to experimental P CO 2 and Δ H abs data. Heat contributions from each of the individual reactions taking place in the systems MEA+H 2O+CO 2 and MDEA+H 2O+CO 2 were calculated and presented as functions of loading and temperature. Predicted enthalpies of absorption agree well with the data from literature and with experimental data from this work. The calculation procedure described in this work may be used for the adjustment of reaction equilibrium constants by fitting the equilibrium model also to experimentally measured heats of absorption data in addition to P CO 2 data.

Molecular Simulation of Excess Isotherm and Excess Enthalpy Change in Gas-Phase Adsorption

We present a new approach to calculating excess isotherm and differential enthalpy of adsorption on surfaces or in confined spaces by the Monte Carlo molecular simulation method. The approach is very general and, most importantly, is unambiguous in its application to any configuration of solid structure (crystalline, graphite layer or disordered porous glass), to any type of fluid (simple or complex molecule), and to any operating conditions (subcritical or supercritical). The behavior of the adsorbed phase is studied using the partial molar energy of the simulation box. However, to characterize adsorption for comparison with experimental data, the isotherm is best described by the excess amount, and the enthalpy of adsorption is defined as the change in the total enthalpy of the simulation box with the change in the excess amount, keeping the total number (gas + adsorbed phases) constant. The excess quantities (capacity and energy) require a choice of a reference gaseous phase, which is defined as the adsorptive gas phase occupying the accessible volume and having a density equal to the bulk gas density. The accessible volume is defined as the mean volume space accessible to the center of mass of the adsorbate under consideration. With this choice, the excess isotherm passes through a maximum but always remains positive. This is in stark contrast to the literature where helium void volume is used (which is always greater than the accessible volume) and the resulting excess can be negative. Our definition of enthalpy change is equivalent to the difference between the partial molar enthalpy of the gas phase and the partial molar enthalpy of the adsorbed phase. There is no need to assume ideal gas or negligible molar volume of the adsorbed phase as is traditionally done in the literature. We illustrate this new approach with adsorption of argon, nitrogen, and carbon dioxide under subcritical and supercritical conditions.

Dissolution Properties of 1,3,3-Trinitroazetidine in Ethyl Acetate and <em>N</em>,<em>N</em>-Dimethylformamide

The enthalpies of dissolution for 1,3,3-trinitroazetidine (TNAZ) in ethyl acetate (EA) and N, N-dimethylformamide (DMF) were measured using a RD496-2000 Calvet Microcalorimeter at 298.15 K under atmospheric pressure. Differential enthalpies (△(subscript dif)H) and molar enthalpies (△(subscript sol)H) were determined for TNAZ in different solvents. The corresponding kinetic equations that describe the two dissolution processes are dα/dt=10^(-7.26)(1-α)^0.88 for the dissolution of TNAZ in ethyl acetate and dα/dt=10^(-7.21)(1-α)^0.66 for the dissolution of TNAZ in N, N-dimethylformamide.

A new microcalorimeter of adsorption for the determination of differential enthalpies

An adsorption microcalorimeter for the determination of the differential heat of adsorption is designed and built, and some of the results are presented here. For this purpose, a Calvet heat-conducting microcalorimeter is developed and is connected to a gas volumetric unit built in stainless steel to record adsorption isotherms. The microcalorimeter is electrically calibrated to establish its sensitivity and reproducibility, obtaining K = 154.34 ± 0.23 W V −1 and, chemically, it is examined with a THAM–HCL system, obtaining Δ f = −30.49 ± 0.05 kJ mol −1.The adsorption microcalorimeter is used here to obtain adsorption isotherms and the corresponding differentials heats, using as reference solid, a zeolite ZSM-5 type.

Adsorption properties and surface chemistry of MgO

The adsorption properties of MgO, which is used as a sorbent and catalyst support, were studied using gas chromatography. The test absorbents used were n-alkanes (which show only nonspecific dispersion interactions when physisorbed on any adsorbent) and adsorbates whose molecules are capable of specific interactions with the surface reactive sites of MgO. Adsorption isotherms were measured for CHCl3, CH3NO2, CH3CN, (CH3)2CO, CH3COOC2H5, and (C2H5)2O on MgO at 50–100°C. Differential molar enthalpy changes (−ΔH), equal to molar heats of adsorption, were determined. For polar adsorbates, contributions from dispersive and specific interactions into −ΔH were determined. The electron-acceptor and electron-donor abilities of the MgO surface were estimated.

Amine functionalised metal organic frameworks (MOFs) as adsorbents for carbon dioxide

Three different porous metal organic framework (MOF) materials have been prepared with and without uncoordinated amine functionalities inside the pores. The materials have been characterized and tested as adsorbents for carbon dioxide. At 298 K the materials adsorb significant amount of carbon dioxide, the amine functionalised adsorbents having the highest CO2 adsorption capacities, the best adsorbing around 14 wt% CO2 at 1.0 atm CO2 pressure. At 25 atm CO2 pressure, up to 60 wt% CO2 can be adsorbed. At high pressures the CO2 uptake is mostly dependent on the available surface area and pore volume of the material in question. For one of the iso-structural MOF pairs the introduction of amine functionality increases the differential adsorption enthalpy (from isosteric method) from 30 to around 50 kJ/mole at low CO2 pressures, while the adsorption enthalpies reach the same level at increase pressures. The high pressure experimental results indicate that MOF based solid adsorbents can have a potential for use in pressure swing adsorption of carbon dioxide at elevated pressures.

Determination of Differential Enthalpy and Isotherm by Adsorption Calorimetry

An adsorption microcalorimeter for the simultaneous determination of the differential heat of adsorption and the adsorption isotherm for gas-solid systems are designed, built, and tested. For this purpose, a Calvet heat-conducting microcalorimeter is developed and is connected to a gas volumetric unit built in stainless steel to record adsorption isotherms. The microcalorimeter is electrically calibrated to establish its sensitivity and reproducibility, obtaining K=154.34±0.23 WV−1. The adsorption microcalorimeter is used to obtain adsorption isotherms and the corresponding differential heats for the adsorption of CO2 on a reference solid, such as a NaZSM-5 type zeolite. Results for the behavior of this system are compared with those obtained with commercial equipment and with other studies in the literature.

Adsorptive Separation of Isobutene and Isobutane on Cu3(BTC)2

The metal organic framework material Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate) has been synthesized using different routes: under solvothermal conditions in an autoclave, under atmospheric pressure and reflux, and by electrochemical reaction. Although the compounds display similar structural properties as evident from the powder X-ray diffraction (XRD) patterns, they differ largely in specific surface area and total pore volume. Thermogravimetric and chemical analysis support the assumption that pore blocking due to trimesic acid and/or methyltributylammoniummethylsulfate (MTBS) which has been captured in the pore system during reaction is a major problem for the electrochemically synthesized samples. Isobutane and isobutene adsorption has been studied for all samples at different temperatures in order to check the potential of Cu3(BTC)2 for the separation of small hydrocarbons. While the isobutene adsorption isotherms are of type I according to the IUPAC classification, the shape of the isobutane isotherm is markedly different and closer to type V. Adsorption experiments at different temperatures show that a somewhat higher amount of isobutene is adsorbed as compared to isobutane. Nevertheless, the differential enthalpies of adsorption are only different by about 5 kJ/mol, indicating that a strong interaction between the copper centers and isobutene does not drive the observed differences in adsorption capacity. The calculated breakthrough curves of isobutene and isobutane reveal that a low pressure separation is preferred due to the peculiar shape of the isobutane adsorption isotherms. This has been confirmed by preliminary breakthrough experiments using an equimolar mixture of isobutane and isobutene.

Supramolecular Enantiodifferentiating Photocyclodimerization of 2-Anthracenecarboxylate Mediated by Capped γ-Cyclodextrins: Critical Control of Enantioselectivity by Cap Rigidity

A series of gamma-cyclodextrins (CDs) modified with capping and noncapping aromatic group(s) were synthesized to mediate the enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylic acid (AC). The complexation behavior of these gamma-CDs with AC was studied by circular dichroism, UV-vis, and NMR spectroscopy to reveal the formation of stable 1:2 host-guest complexes in all cases. The capped gamma-CD with a biphenyl group bridging the A and D glucose units was shown to confine the included AC molecules most strictly among the capped and noncapped gamma-CDs examined. Photocyclodimerization of AC mediated by capped gamma-CDs considerably improved the yield and enantiomeric excess (ee) of the head-to-head photodimer 3. The ee and the absolute configuration of syn-head-to-tail photodimer 2 critically depended on the rigidity of capping. Thus, the flexibly capped and rim-substituted gamma-CDs afforded 2 in moderate ee's of around 40%, whereas gamma-CD with a rigid biphenyl cap gave the antipodal 2 in -58% ee. Interestingly, the ee of 2 mediated by flexibly capped gamma-CDs was highly sensitive to the temperature variation as a consequence of large differential entropy changes in the enantiodifferentiation process. In contrast, the entropy effect does not appear to play a significant role in the photocyclodimerization of AC with rigidly capped gamma-CDs. The differential enthalpy and entropy changes obtained for the enantiodifferentiating photocyclodimerization mediated by native and most of the modified gamma-CDs gave an excellent enthalpy-entropy compensation plot with an exception of the biphenyl-capped gamma-CD, indicating the operation of significantly different enantiodifferentiation mechanism within the rigidly capped cyclodextrin cavity.

Moisture sorption isotherms and thermodynamic properties of tow mints: Mentha pulegium and Mentha rotundifolia

Moisture sorption isotherms of Mentha pulegium and Mentha rotundifolia were determined at 30, 40 and 50 °C using the static gravimetric method. GAB, modified Halsey and Peleg models were fitted to the experimental data and the agreement between experimental and predicted values was satisfactory. Calculation of the thermodynamic properties was further used to provide an understanding of the properties of water and energy requirements associated with the sorption behaviour. Isosteric heats of sorption were calculated through direct use of moisture isotherms by applying the Clausius-Clapeyron equation. The differential enthalpy and entropy decreased with increasing moisture content and were adequately characterized by a power law equation. A plot of differential heat versus entropy satisfied the enthalpy-entropy compensation theory.

Theoretical description of aggregation of cationic gemini surfactants in the bulk solution and on the silica surface

A theory of cationic dimeric (gemini) surfactant adsorption onto negatively charged surface is presented. In the proposed model it is assumed that the adsorbed phase is a mixture of singly dispersed molecules of surfactant and spherical, globular and cylindrical aggregates of different dimensions. Only the “excluded area” interactions between the adsorbed species are considered and the effects of surface heterogeneity on monomer adsorption are taken into account. The aggregation behavior of gemini surfactants is based on the additive free energy model proposed by Camesano and Nagarajan (2000). The calculated surfactant adsorption isotherms and the differential molar enthalpies of micellisation and adsorption are compared with the experimental results obtained for a series of gemini surfactants depending on the length of a spacer, temperature or the presence of electrolyte. On the basis of theoretical results the evolution of adsorbed phase of gemini surfactants with the increasing adsorption is discussed. It is shown that the evaluated cmc values and the dimensions of surfactant aggregates are in a good agreement with experiment. Unfortunately, the theoretical model does not describe properly the temperature dependence of micellisation process.

Chromatographic determination of the differential isosteric adsorption enthalpies and differential entropies on ordered silica materials

The adsorption properties of the ordered mesoporous siliceous materials: MCM-41C16 (denoted as C16), MCM-41C16-SH and MCM-41C16-NH 2 (known as MCMs) having different surface functionalities were studied by inverse gas chromatography to assess their suitability for adsorption of analytes from gas and liquid phases. Polar and non-polar adsorbates were employed. The differential isosteric enthalpies, −Δ H ads, and differential entropies, −Δ S ads, of adsorption of different ‘molecular probes’ were determined chromatographically. A mathematical link between the −Δ H ads, and −Δ S ads magnitudes and experimental data was derived through an Antoine-type equation. The present studies have been entirely restricted to the region of low adsorbate concentration. The problem of the interrelationship between the −Δ H ads, and −Δ S ads values, known as the ‘thermodynamic compensation effect’, and interpretation of chromatographic data for the adsorption of different adsorbates on the MCMs have been considered in the light of both experimental data obtained in the present studies and the data available in the literature for siliceous adsorbents with randomly ordered structures. It was shown chromatographically that there is substantial parallelism between the magnitudes of the differential isosteric enthalpy and differential entropy for some ‘molecular probes’ chromatographed on C16 and its derivatives. Complementary information was obtained by atomic-force microscopy (AFM), X-ray photoelectron spectroscopy and X-ray diffraction (XRD) spectroscopy.

Thermodynamic analysis of experimental sorption isotherms of loquat and quince fruits

Sorption isotherms of loquat and quince fruits were determined by static gravimetric method at different temperatures, in the range from 20 to 65°C. The curves obtained can be considered as type II at 20°C and type III at higher temperatures according to the Brunauer–Emmett–Teller (BET) classification. Equilibrium moisture content data were correlated by different mathematical models usually applied to foodstuffs (GAB, Peleg and Halsey). The best fit of the experimental data was obtained with Peleg and GAB models. Experimental data were analysed by a thermodynamic approach to obtain such properties as net isosteric heat, net equilibrium heat, differential and integral entropy that provide a deeper understanding of the properties of water and energy requirements associated with sorption process. Particularly, for loquat and quince fruits, the differential enthalpy and entropy decreased with increasing moisture content and satisfied the compensation theory. The net integral enthalpy show maximum values (32, 26, 24kJ/mol for loquat pulp, loquat seeds and quince, respectively) and the net integral entropy has the opposite behaviour with minimum values (−90.8, −70.8, −68.1J/molK for loquat pulp, loquat seeds and quince, respectively) at specified moisture contents.

Thermodynamic properties of moisture desorption of raw pinhão (Araucaria angustifolia seeds)

SummaryThe seeds of Araucaria angustifolia, named pinhão, are consumed in the south and southeast of Brazil. They are big in size and have high nutritious value. The literature about technological aspects of pinhão is very scarce and there are no reports about moisture sorption models. In this work, moisture desorption isotherms of raw pinhão were determined at 15, 25, 30 and 40 °C. Results show that temperature has little effect on the sorption behaviour and the Chirife model was found to best represent the experimental data. The isosteric heat of sorption (differential enthalpy) was calculated through direct use of moisture desorption isotherm by applying the Clausius‐Clapeyron equation. The differential enthalpy of desorption decreased with increasing moisture content. The enthalpy–entropy compensation theory was applied to desorption isotherms and plots of differential enthalpy vs. differential entropy for pinhão provided the isokinetic temperature, indicating an enthalpy‐controlled desorption process.

Adsorption of carbon dioxide in SAPO STA-7 and AlPO-18: Grand Canonical Monte Carlo simulations and microcalorimetry measurements

The adsorption properties of carbon dioxide in SAPO STA-7 has been investigated by combining Grand Canonical Monte Carlo simulation and microcalorimetry. The modeling approach, based on both newly derived interatomic potentials for describing the interaction between CO2 and the Bronsted acid sites, and a realistic description of the silicon distribution within the framework, provides isotherms and evolutions of the differential enthalpy of adsorption as a function of coverage for the STA-7 material characterised by a silicon fraction of 0.19. The simulated results are revealed to be in good agreement with the experimental data that allowed us to provide a possible microscopic mechanism of CO2 adsorption in this material. These whole results are thus compared to those obtained for the purely aluminophosphate form of the AlPO-18. It is clearly emphasized that distinct adsorption behaviours are observed in these two systems, depending on the energetic characteristics of their surfaces.

Thermodynamic Properties and Moisture Sorption Isotherms of Argania spinosa and Zygophyllum gaetulum

Almond of Argania spinosa and Zygophyllum gaetulum moisture sorption isotherms were determined within the range of 0.1-0.9 water activity at three temperatures (30, 40 and 50°C), using saturated salt solutions dynamic method. Sorption isotherms of Argania spinosa and Zygophyllum gaetulum have, respectively a sigmoid shape (Type 2) and (Type 5). The GAB, modified Oswin, modified Henderson, Peleg, modified BET and modified Halsey models were tested to fit the experimental data. In relation to the isotherm models the best models were, GAB, modified Oswin and Peleg obtaining minor values of % mean relative errors and higher correlation coefficient. Isosteric heats of sorption were calculated through direct use of moisture isotherms by applying the Clausius-Clapeyron equation. The differential enthalpy and entropy decreased with increasing moisture content and were adequately characterized by polynomial function. A plot of differential heat versus entropy satisfied the enthalpy-entropy compensation theory. The comparison between the sorption isotherms and thermodynamic properties of A. spinosa almond and Z. gaetulum was studied.

The temperature dependence of the exchange enthalpy of calcium and sodium ions on polymethacrylic and polyacrylic cationites

The influence of temperature on the exchange of calcium and sodium ions from solutions of 2.3–2.8 g-equiv/l concentrations on KB-2e4 gel polyacrylic cationite, KB-4P2 and KB-4 gel polymethacrylic cationites, and Purolite C104 polyacrylic cationite was studied over the temperature range 273–400 K. It was shown that, simultaneously with a substantial increase in selectivity with respect to calcium ions, the differential enthalpy of the ion exchange reaction increased linearly on all polyacrylic and polymethacrylic cationites as the temperature grew.

Synthesis, Characterization and Thermodynamic Parameters of Anionic Gemini Surfactants with Different Spacer Groups

Abstract The aim of this work is to investigate the micellization and adsorption thermodynamics (ΔmicH°, ΔmicG°, ΔmicS°, ΔadsH°, ΔadsG°, ΔadsS°) for three novel anionic Gemini surfactants with the same structure except the spacer length of the polymethylene chain have been synthesized based on nonylphenol (NP). The surface tension at cmc (γcmc) and critical micelle concentration (cmc) of the surfactants in aqueous solutions have been investigated. C20 values (surfactant concentration in the solution phase that will reduce the surface tension of the solvent by 20 mNm−1) were also obtained. The critical micellar concentration (cmc) and the minimum average area per surfactant molecule (Amin) at the air-water interface of Gemini surfactant decrease with increasing number of methylene moieties in the spacer and, consequently, with the enhanced hydrophilic character of the molecule. The results were discussed with particular emphasis on the effect of the length spacer on the hydrophilic character of the spacer group. The surface studies of the molecules confirm that the exothermic values of the differential enthalpy of micellization (ΔmicH°) decrease as the methylene moieties are lengthened. Moreover, the values of the free energy (ΔmicG°) and entropy (ΔmicS°) of micellization indicate an entropy driven phenomenon for all surfactants. The effect of length spacer on surfactant adsorption for Gemini surfactants was utilized to calculate such thermodynamic parameters as the free energy of adsorption (ΔadsG°) and the heat of adsorption (ΔadsH°). The low value of ΔadsH° is an evidence for the physical adsorption of anionic surfactants. The relatively large value of entropic contribution (TΔadsS°) indicates that the adsorption of anionic surfactants is entropically governed.