Differential Enthalpy Research Articles

Moisture adsorption isotherms of the borojó fruit (Borojoa patinoi. Cuatrecasas) and gum arabic powders

Sorption mechanisms of borojó pulp (BPP), three borojó phases (liquid phase (LP), medium phase (MP) and solid phase (SP)), gum arabic (GA), and a mixture of these (BP) produced by freeze-drying were interpreted through adsorption isotherms. The adsorption models adequately describe the experimental data over the entire measured range of aw; R2 was close to 1 for the studied conditions. For the thermodynamic properties, the differential and integral enthalpy of BP showed a synergic effect of the combined components. This shows that using a combination of polymers increases the interaction energy of water molecules with the surface of the material. The monolayers from the BET and GAB models showed similar moisture content values that corresponded to the minimum integral entropy. Finally, the behavior of the enthalpy–entropy compensation of water adsorption for the SP, MP and GA powders at low moisture contents demonstrated that the content is controlled by an entropic mechanism, for BP, BPP and LP powders, the water adsorption may be considered as mainly enthalpy-driven.

SORPTION ISOTHERMS AND THERMODYNAMIC ANALYSIS OF SEED FRUITS USED TO OBTAIN VEGETABLE OIL

Sorption isotherms of papaya, melon and grape seeds were determined and thermodynamic properties compared. The experiments were carried out at different temperature using the gravimetric method. The Henderson and GAB models were the best which represented the experimental data. Papaya and grape seeds are more stable with larger moisture content than melon seeds at 30°C. Grape seeds presented more resistance to lose water during the dehydration. The differential enthalpy and entropy decreased with increasing moisture content and satisfied the compensation theory. It was found that the sorption process investigated was enthalpy-driven.

Quasithermodynamic contributions to the fluctuations of a protein nanopore.

Proteins undergo thermally activated conformational fluctuations among two or more substates, but a quantitative inquiry on their kinetics is persistently challenged by numerous factors, including the complexity and dynamics of various interactions, along with the inability to detect functional substates within a resolvable time scale. Here, we analyzed in detail the current fluctuations of a monomeric β-barrel protein nanopore of known high-resolution X-ray crystal structure. We demonstrated that targeted perturbations of the protein nanopore system, in the form of loop-deletion mutagenesis, accompanying alterations of electrostatic interactions between long extracellular loops, produced modest changes of the differential activation free energies calculated at 25 °C, ΔΔG⧧, in the range near the thermal energy but substantial and correlated modifications of the differential activation enthalpies, ΔΔH⧧, and entropies, ΔΔS⧧. This finding indicates that the local conformational reorganizations of the packing and flexibility of the fluctuating loops lining the central constriction of this protein nanopore were supplemented by changes in the single-channel kinetics. These changes were reflected in the enthalpy–entropy reconversions of the interactions between the loop partners with a compensating temperature, TC, of ∼300 K, and an activation free energy constant of ∼41 kJ/mol. We also determined that temperature has a much greater effect on the energetics of the equilibrium gating fluctuations of a protein nanopore than other environmental parameters, such as the ionic strength of the aqueous phase as well as the applied transmembrane potential, likely due to ample changes in the solvation activation enthalpies. There is no fundamental limitation for applying this approach to other complex, multistate membrane protein systems. Therefore, this methodology has major implications in the area of membrane protein design and dynamics, primarily by revealing a better quantitative assessment on the equilibrium transitions among multiple well-defined and functionally distinct substates of protein channels and pores.

Energetics of CO2 Adsorption on Mg–Al Layered Double Hydroxides and Related Mixed Metal Oxides

Energetics of CO2 uptake by layered double hydroxides (LDH) of Mg and Al and their corresponding mixed metal oxides (MMO) of two different compositions (Mg/Al = 2:1 and 3:1) were investigated by gas adsorption calorimetry. The initial adsorption enthalpies for all the LDH and MMO are similar and in the range −90 to −120 kJ/mol, indicating strong chemisorption of CO2. However, the differential adsorption enthalpy varies substantially with increasing coverage for different samples. LDH prepared by coprecipitation (both Mg/Al = 2 and 3) and their corresponding MMO exhibit similar CO2 uptake, which are in the working sorption capacity range (0.6–1 mmol/g), but the one prepared by urea hydrolysis shows poor sorption capacity (0.02–0.35 mmol/g). An integral enthalpy of adsorption of −54 kJ/mol was observed for [Mg–Al–CO3] LDH prepared by urea hydrolysis and −59 kJ/mol and −57 kJ/mol for the ones prepared by coprecipitation with composition 3:1 and 2:1, respectively. Attenuated total reflectance spectroscopy measurements of samples after CO2 uptake aid in identifying the mode of binding of adsorbed carbonate, which gives information about strength of basic sites. The presence of monodentate carbonate in all samples suggests that strong basic sites are available in LDH and MMO, in agreement with the large exothermic initial adsorption enthalpies observed, indicating strong chemisorption.

The effect of chicken skin gelatin and whey protein interactions on rheological and thermal properties

Physical, thermal and microstructural properties of whey protein isolate (WPI) and unutilized chicken skin gelatin mixtures were investigated for the first time. Small deformation rheology indicated that combinations of gelatin (3, 5 and 10%) and 10% whey protein (WPI) in distilled water produced high elastic modulus (G′) values of 1860, 23,914 and 20,145 Pa, respectively, compared with 120 Pa for 10% WPI alone, due to synergistic interaction. Frequency sweeps showed increased strength of networks in gels containing higher gelatin concentrations in WPI/gelatin mixtures. Gelatin gels were more stable and stronger than 10% (w/w) whey protein gels and did not exhibit frequency dependence for G′ and G″. Large deformation gel strength values of all samples increased significantly (p < 0.05) with increasing gelatin concentration and were greater at each concentration compared to gelatin alone. Differential scanning calorimetry transition temperature (Tm) and enthalpy change (H) of gelatin and whey protein mixed in the ratios 3:10, 5:10 and 10:10 (w/w) confirmed gelatin reversibility on heating to 90 °C and cooling to 10 °C and irreversible denaturation of WPI on heating. The addition of 3, 5 or 10% gelatin to whey protein increased whey protein Tm and decreased gelatin Tm. However, the presence of 10% (w/w) WPI significantly increased the ΔH values to 0.62, 1.34 and 2.20 J/g for 3, 5 and 10% (w/w) gelatin solutions respectively, indicating whey–gelatin interaction. Chicken skin gelatin gels exhibited a fine network whereas whey protein gels comprised aggregates and mixtures led to phase separation. This unutilized non-mammalian gelatin may lead to novel industrial applications.

Thermochemical properties of 2,4-dinitroanisole in N-methyl pyrrolidone and dimethyl sulfoxide

The thermochemical properties of 2,4-dinitroanisole (DNAN) in N-methy pyrrolidone (NMP) and dimethyl sulfoxide(DMSO) were studied using a RD496-2000 Calvet microcalorimeter at four different temperatures. The heat effects were measured for DNAN dissolved in NMP and DMSO and the relationships between the heat effects and the amounts of the substance were determined. The molar enthalpies and the differential molar enthalpies of dissolution processes were also obtained from the experimental data. The corresponding kinetic equations describing the two dissolution processes at different temperatures were discussed.

Energetics of CO2 and H2O adsorption on zinc oxide.

Adsorption of H2O and CO2 on zinc oxide surfaces was studied by gas adsorption calorimetry on nanocrystalline samples prepared by laser evaporation in oxygen to minimize surface impurities and degassed at 450 °C. Differential enthalpies of H2O and CO2 chemisorption are in the range -150 ±10 kJ/mol and -110 ±10 kJ/mol up to a coverage of 2 molecules per nm(2). Integral enthalpy of chemisorption for H2O is -96.8 ±2.5 kJ/mol at 5.6 H2O/nm(2) when enthalpy of water condensation is reached, and for CO2 is -96.6 ±2.5 kJ/mol at 2.6 CO2/nm(2) when adsorption ceases. These values are consistent with those reported for ZnO prepared by other methods after similar degas conditions. The similar energetics suggests possible competition of CO2 and H2O for binding to ZnO surfaces. Exposure of bulk and nanocrystalline ZnO with preadsorbed CO2 to water vapor results in partial displacement of CO2 by H2O. In contrast, temperature-programmed desorption (TPD) indicates that a small fraction of CO2 is retained on ZnO surfaces up to 800 °C, under conditions where all H2O is desorbed, with adsorption energies near -200 kJ/mol. Although molecular mechanisms of adsorption were not studied, the thermodynamic data are consistent with dissociative adsorption of H2O at low coverage and with several different modes of CO2 binding.

Investigation of hydrogen desorption from CaSiH by means of calorimetric method

The present work deals with the study of the reaction of hydrogen desorption from the CaSiHX hydride by means of the calorimetric method. The dehydrogenation of the CaSiHX hydride was carried out at 548 K. For a calorimetric study, the installation composed of the differential heat-conducting Tian–Calvet type calorimeter connected with a conventional Sieverts-type apparatus was employed. Such installation permitted us to obtain simultaneously the P-X isotherms (P—equilibrium hydrogen pressure, X = H/CaSi) and variation of the partial molar enthalpies of the reaction of hydrogen desorption from CaSiHX with the hydrogen concentration in the metallic matrix. It was ascertained that in the CaSi-H2 system there was one region where values of the partial molar enthalpy of the reaction of hydrogen desorption from the CaSiHX hydride remained constant. This means that formation of one hydride phase in the CaSi-H2 system took place. The enthalpy and entropy values for the reaction of hydrogen desorption from the CaSiHX in the plateau range are ΔHdes = 53.8 ± 1.2 kJ mol−1 H2 and ΔSdes = 94.2 ± 2.7 J mol−1 H2 K−1 (ΔHdes and ΔSdes—the differential molar enthalpy and entropy desorption, respectively).

Heat effects due to mixing (dilution) the mixed acid solutions—Application of neural networks to approximate and generalize experimental data

The feed-forward neural networks have been used to approximate the specific molar enthalpy and the specific molar heat capacity of the mixed acid solutions. The nets have been trained with experimental data taken from the literature, so the values of the specific molar enthalpy and the specific molar heat capacity at the reference temperature T=0°C could be successively estimated for any composition of the mixed acid. Two principal methods have been considered and tested. In the first method two independent neural nets have been employed: the net NN-H, which approximates separately the specific molar enthalpy and the net NN-C, to approximate the specific molar heat capacity, respectively. In the second method only one net is employed (the net NN-HC), which simultaneously approximates both the specific molar enthalpy and the specific molar heat capacity. Then following both mentioned methods, the trained neural nets have been used to model the heat effects due to dilution of mixed acid solutions, carried out at various conditions – i.e. at any temperature and composition. Using these nets, both, the integral and the differential enthalpy balance can be carried out, so the smart and accurate method to model the mixed acid dilution has been elaborated. The proposed methods and their prediction accuracy have been successfully verified with our own experimental data carried out in the RC1 reaction calorimeter.

Influence of Structural Heterogeneity on Diffusion of CH4 and CO2 in Silicon Carbide-Derived Nanoporous Carbon.

We investigate the influence of structural heterogeneity on the transport properties of simple gases in a Hybrid Reverse Monte Carlo (HRMC) constructed model of silicon carbide-derived carbon (SiC-DC). The energy landscape of the system is determined based on free energy analysis of the atomistic model. The overall energy barriers of the system for different gases are computed along with important properties, such as Henry constant and differential enthalpy of adsorption at infinite dilution, and indicate hydrophobicity of the SiC-DC structure and its affinity for CO2 and CH4 adsorption. We also study the effect of molecular geometry, pore structure and energy heterogeneity considering different hopping scenarios for diffusion of CO2 and CH4 through ultramicropores using the Nudged Elastic Band (NEB) method. It is shown that the energy barrier of a hopping molecule is very sensitive to the shape of the pore entry. We provide evidence for the influence of structural heterogeneity on self-diffusivity of methane and carbon dioxide using molecular dynamics simulation, based on a maximum in the variation of self-diffusivity with loading. A comparison of the MD simulation results with self-diffusivities from quasi-elastic neutron scattering (QENS) measurements and, with macroscopic uptake-based low-density transport coefficients, reveals the existence of internal barriers not captured in MD simulation and QENS experiments. Nevertheless, the simulation and macroscopic uptake-based diffusion coefficients agree within a factor of 2–3, indicating that our HRMC model structure captures most of the important energy barriers affecting the transport of CH4 in the nanostructure of SiC-DC.

On the real performance of cation exchange resins in wastewater treatment under conditions of cation competition: the case of heavy metal pollution

Sorption performance of cation-exchange resins Amberlite® IRN77 and Amberlite™ IRN9652 toward Cs(I) and Sr(II) has been tested in single-component aqueous solutions and simulated waste effluents containing other monovalent (Effluent 1) or divalent (Effluent 2) metal cations, as well as nitrate, borate, or carbonate anions. The individual sorption isotherms of each main component were measured by the solution depletion method. The differential molar enthalpy changes accompanying the ion-exchange between Cs+ or Sr2+ ions and protons at the resin surface from single-component nitrate solutions were measured by isothermal titration calorimetry and they showed a higher specificity of the two resins toward cesium. Compared to the retention limits of both resins under such idealized conditions, an important depression in the maximum adsorption capacity toward each main component was observed in multication systems. The overall effect of ion exchange process appeared to be an unpredictable outcome of the individual sorption capacities of the two resins toward various cations as a function of the cation charge, size, and concentration. The cesium retention capacity of the resins was diminished to about 25% of the "ideal" value in Effluent 1 and 50% in Effluent 2; a further decrease to about 15% was observed upon concomitant strontium addition. The uptake of strontium by the resins was found to be less sensitive to the addition of other metal components: the greatest decrease in the amount adsorbed was 60% of the ideal value in the two effluents for Amberlite® IRN77 and 75% for Amberlite™ IRN9652. It was therefore demonstrated that any performance tests carried out under idealized conditions should be exploited with much caution to predict the real performance of cation exchange resins under conditions of cation competition.

A fully consistent experimental and molecular simulation study of methane adsorption on activated carbon

The adsorption of pure methane in activated carbon Ecosorb was studied by combining grand canonical ensemble Monte Carlo molecular simulations and an experimental approach based on a gravimetric device. Experimental and calculated adsorption isotherms of methane were determined in supercritical conditions at 303.15 and 353.15 K and pressures up to 10 MPa. The comparison between both experimental and estimated data proves the consistency of the methodology used in this work, starting from the characterization of the porous media in terms of pore size distribution, the determination of the experimental adsorption isotherms, and the final estimation of computational results through estimated isotherms determination. Moreover, additional differential enthalpy of adsorption calculations were compared with experimental values obtained by means of a manometric/calorimetric technique. The good agreement shows the strength and the originality of this paper by combining experimental and computational homemade results allowing a complete characterization of the activated carbon substrate and its methane storage capacity.

Dissolution properties of ammonium dinitramide in N-methyl pyrrolidone

The dissolution properties of ammonium dinitramide (ADN) in N-methyl pyrrolidone (NMP) were studied with a RD496-2000 Calvet microcalorimeter at four different temperatures under atmospheric pressure. The heat effects were determined for ADN in NMP. The molar enthalpies and the differential molar enthalpies for ADN in NMP were also obtained at the same time. The corresponding kinetic equations that describe the four dissolution processes are discussed.

Moisture sorption isotherms and thermodynamic properties of Jack pine and palm wood: Comparative study

Currently, the impact of the controlled climate on moisture sorption isotherms of Jack pine and palm wood still remains unclear. The aim of this paper was to determine and compare the moisture sorption isotherms and thermodynamic properties of Jack pine and palm wood. The equilibrium moisture content of wood samples was measured within the range of 5 to 90% relative humidity at 50°C, 60°C, 70°C and 80°C. For both wood varieties, the equilibrium moisture content decreased with increasing temperature, however the effect of temperature is very remarkable for palm wood. The isosteric heat of desorption, the enthalpy and the entropy decreased with the increase of the moisture content. A good straight line correlated differential enthalpy and entropy, according to the enthalpy–entropy compensation theory. The spreading pressure decreased with increasing temperature at a given water activity, and increased with increasing water activity at a constant temperature. Predictive correlations of the two wood properties, based on experimental methods and physical laws, were established and compared.

Effect of Hemicellulose as a Coating Material on Water Sorption Thermodynamics of the Microencapsulated Fish Oil and Artificial Neural Network (ANN) Modeling of Isotherms

In this study, effect of hemicellulose as a coating material on sorption behavior of the microencapsulated fish oil is presented. For this purpose, water sorption isotherms of the fish oil microcapsules prepared using gum arabic (GA) and gum arabic-hemicellulose (GA-HC) were determined at 10, 25, and 35 °C using the gravimetric technique. The equilibrium moisture content (EMC) data were fitted to several empirical mathematical models, namely Oswin, BET, GAB, and Freundlich equations and the novel artificial neural network (ANN) model. The GA-HC microcapsules had higher EMC values than GA, due to the sugar content of hemicellulose. The EMC of the samples were more accurately predicted by the ANN model than by the empirical models at the entire range of a w. Heat of sorption of fish oil microcapsules was then predicted using the data obtained from the ANN model and fitted to the power model (R 2 = 0.985–0.978). A linear relation existed between the differential enthalpy and the differential entropy for microcapsules, thus satisfying the enthalpy-entropy compensation theory.

Thermodynamic and Modeling Study of Thin n-Heptane Films Adsorbed on Magnesium Oxide (100) Surfaces

Thermodynamic properties of n-heptane adsorbed on the MgO(100) surface were investigated using high-resolution, volumetric adsorption isotherms in the temperature range of 205–275 K. Two distinct molecular layers were observed in all isotherms. The heat, differential enthalpy, differential entropy, and isosteric heat of adsorption were determined. Using the temperature dependence of the two-dimensional compressibility, two phase transitions were observed at 246.0 ± 2.0 K and 249.3 ± 1.2 K for the first and second layers, respectively. The average area per n-heptane molecule adsorbed on MgO was estimated to be 99 ± 10 A2, suggesting that the carbon backbone is preferentially oriented parallel to the surface. The COMPASS force field was used to calculate the minimum energy configurations of an n-heptane molecule on the MgO(100) surface. The calculations support experimental evidence of the carbon backbone oriented parallel to the surface and additionally suggest a preferential alignment of the molecule alon...

Gas saturation and CO2 enhancement potential of coalbed methane reservoirs as a function of depth

The influence of moisture, temperature, coal rank, and differential enthalpy on the methane (CH4) and carbon dioxide (CO2) sorption capacity of coals of different rank has been investigated by using high-pressure sorption isotherms at 303, 318, and 333 K (CH4) and 318, 333, and 348 K (CO2), respectively. The variation of sorption capacity was studied as a function of burial depth of coal seams using the corresponding Langmuir parameters in combination with a geothermal gradient of 0.03 K/m and a normal hydrostatic pressure gradient. Taking the gas content corresponding to 100% gas saturation at maximum burial depth as a reference value, the theoretical CH4 saturation after the uplift of the coal seam was computed as a function of depth. According to these calculations, the change in sorption capacity caused by changing pressure, temperature conditions during uplift will lead consistently to high saturation values. Therefore, the commonly observed undersaturation of coal seams is most likely related to dismigration (losses into adjacent formations and atmosphere). Finally, we attempt to identify sweet spots for CO2-enhanced coalbed methane (CO2-ECBM) production. The CO2-ECBM is expected to become less effective with increasing depth because the CO2-to-CH4 sorption capacity ratio decreases with increasing temperature and pressure. Furthermore, CO2-ECBM efficiency will decrease with increasing maturity because of the highest sorption capacity ratio and affinity difference between CO2 and CH4 for low mature coals.

Moisture Sorption Characteristics of Pineapple Processing Waste: Shell and Central Cylinder

AbstractThe moisture adsorption isotherms of pineapple processing waste – shell and central cylinder – were determined at 40, 50, 60 and 70C using a gravimetric method. The sorption data were fitted to six well‐known sorption isotherm models. The Guggenheim–Anderson–de Boer model was found to be the most satisfactory to represent the equilibrium moisture content data for the pineapple shells and central cylinders. The thermodynamic properties – differential enthalpy and entropy – were estimated by the Clausius–Clapeyron method. The differential enthalpy of the pineapple shells and central cylinders ranged between 11.486 to 1.671 kJ/mol and 23.655 to −0.945 kJ/mol, respectively. The differential entropy had minimum values at 0.04 and 0.005 kgwater/kgdry solids for the pineapple shells and central cylinders, respectively. The pineapple shells presented two lines of compensation, one controlled by entropy and the other controlled by enthalpy and the pineapple central cylinders showed only one zone controlled by enthalpy mechanisms.Practical ApplicationsThis work presents information about the moisture sorption characteristics of pineapple processing waste. This kind of information is important for designing the unit operations necessary in the waste process use. In addition, knowledge of the moisture sorption characteristics is useful with respect to selection of packaging materials and storage stability. In general, this work gives relationship between water activity, entropy, enthalpy and moisture content of pineapple processing waste.

Thermophysical properties of MOF-5 powders

We present a comprehensive assessment of the thermophysical properties of an industrial, pilot-scale version of the prototype adsorbent, metal–organic framework 5 (MOF-5). These properties are essential ingredients in the design and modeling of MOF-5-based hydrogen adsorption systems, and may serve as a useful starting point for the development of other MOF-based systems for applications in catalysis, gas separations, and adsorption of other gasses or fluids. Characterized properties include: packing density, surface area, pore volume, particle size distribution, thermal conductivity, heat capacity, stability against hydrolysis, differential enthalpy of H2 adsorption, and Dubinin–Astakhov isotherm parameters. Hydrogen adsorption/desorption isotherms were measured at six temperatures spanning the range 77–295K, and at pressures of 0–100bar.

To determine the half-life for gemcitabine hydrochloride using microcalorimetry

The enthalpies of dissolution of gemcitabine hydrochloride in 0.9 % normal saline (medical) and citric acid solution were measured using a microcalorimeter at 309.65 K under atmospheric pressure. The differential enthalpy \( \left( {\Updelta_{\text{dif}} H_{\text{m}}^{{{\theta}}} } \right) \) and molar enthalpy \( \left( {\Updelta_{\text{sol}} H_{\text{m}}^{{{\theta}}} } \right) \) of dissolution were determined, respectively. The corresponding kinetic equation described the dissolution were elucidated to be da/dt = 10−3.84(1 − a)0.92 and da/dt = 10−3.80(1 − a)1.21. Besides, the half-life, \( \Updelta_{\text{sol}} H_{\text{m}}^{{{\theta}}} ,\;\Updelta_{\text{sol}} G_{\text{m}}^{{{\theta}}} \) and \( \Updelta_{\text{sol}} S_{\text{m}}^{{{\theta}}} \) of the dissolution were also obtained. Obviously, it will provide a simple and reliable method for the clinical application of gemcitabine hydrochloride.