Showing papers on "Enthalpy published in 2014"

Chemically Tunable Ionic Liquids with Aprotic Heterocyclic Anion (AHA) for CO2 Capture

Abstract: Ionic liquids (ILs) with aprotic heterocyclic anions, or AHAs, can bind CO2 with reaction enthalpies that are suitable for gas separations and without suffering large viscosity increases. In the present work, we have synthesized ILs bearing an alkyl-phosphonium cation with indazolide, imidazolide, pyrrolide, pyrazolide and triazolide-based anions that span a wide range of predicted reaction enthalpies with CO2. Each AHA-based IL was characterized by NMR spectroscopy and their physical properties (viscosity, glass transition, and thermal decomposition temperature) determined. In addition, the influence of substituent groups on the reaction enthalpy was investigated by measuring the CO2 solubility in each IL at pressures between 0 and 1 bar at 22 °C using a volumetric method. The isotherm-derived enthalpies range between −37 and −54 kJ mol–1 of CO2, and these values are in good agreement with computed enthalpies of gas-phase IL-CO2 reaction products from molecular electronic structure calculations. The AHA ...

Ceria–Zirconia Solid Solutions (Ce1–xZrxO2−δ, x ≤ 0.2) for Solar Thermochemical Water Splitting: A Thermodynamic Study

Abstract: The redox behavior of ceria–zirconia solid solutions (or Zr-substituted ceria, ZSC) with a Zr content of up to 20 mol % is studied by thermogravimetry (TG) between 600 °C and 1490 °C under controlled atmospheres. Thermodynamic properties, specifically standard oxidation enthalpy, ΔHoxd⊖, and entropy, ΔSoxd⊖, are derived from TG data. The raw TG results show that the extent of reduction is significantly increased (compared with undoped ceria), even at a low Zr substitution level of 5 mol %. Concomitantly, the magnitude of the thermodynamic functions dramatically decreases as a function of Zr content, particularly at low values of oxygen non-stoichiometry, δ (<3 mol %). Thermochemical fuel production from Zr-substituted ceria generally increases with increasing Zr content under both two-temperature and isothermal cycling conditions. In the case of two-temperature cycling, the benefit is accompanied by a penalty in the (computed) steam-to-hydrogen conversion ratio, whereas it is accompanied by a gain in this...

Effect of Water on the Density, Viscosity, and CO2 Solubility in Choline Chloride/Urea

Abstract: To study the effect of water on the properties of choline chloride (ChCl)/urea mixtures (1:2 on a molar basis), the density and viscosity of ChCl/urea (1:2) with water were measured at temperatures from 298.15 K to 333.15 K at atmospheric pressure, the CO2 solubility in ChCl/urea (1:2) with water was determined at 308.2 K, 318.2 K, and 328.2 K and at pressures up to 4.5 MPa. The results show that the addition of water significantly decreases the viscosity of ChCl/urea (1:2), whereas the effects on their density and CO2 solubility are much weaker. The CO2 solubility in ChCl/urea (1:2) with water was represented with the Nonrandom-Two-Liquid Redlich–Kwong (NRTL-RK) model. The excess molar volume and excess molar activation energy were further determined. The CO2 absorption enthalpy was calculated and dominated by the CO2 dissolution enthalpy, and the magnitude of the CO2 dissolution enthalpy decreases with the increase of water content.

Adsorption and thermodynamic study of Cephalosporins antibiotics from aqueous solution onto MgO nanoparticles

Abstract: The adsorption characteristics of Cephalosporins antibiotics from aqueous solution were investigated using magnesium oxide (MgO) nanoparticles. Liquid phase adsorption experiments were conducted. Batch adsorption studies are carried out by observing the effect of experimental parameters, namely amount of adsorbents, pH, temperature and contact time. The drug removal data were fitted on Langmuir adsorption equations. The pseudo second-order kinetic model was indicated with the activation energy of 7.6682 and 5.3745 kJ/mol for Cephalexin and Cefixime, respectively. The values of the enthalpy (ΔH#) and entropy of activation (ΔS#) were 5.0851 kJ/mol and −286.67 J/mol K for Cephalexin; 3.1952 kJ/mol and −303.70 J/mol K for Cefixime, respectively. The free energy of activation (ΔG#) at 30 °C was 97.9 kJ/mol. Thermodynamic analysis showed negative values of ΔG° indicating adsorption was favourable and spontaneous, negative values of ΔH° indicating exothermic physical adsorption of Cephalosporins using MgO nanoparticles.

Solubilities of Carbon Dioxide in Eutectic Mixtures of Choline Chloride and Dihydric Alcohols

Abstract: The solubilities of CO2 in eutectic mixtures containing choline chloride and dihydric alcohols (including 1,4-butanediol, 2,3-butanediol, and 1,2-propanediol) with a molar ratio of choline chloride to dihydric alcohol of 1:3 and 1:4 were measured at 293.15 K, 303.15 K, 313.15 K, and 323.15 K under pressures up to 600.0 kPa using an isochoric saturation method. Henry’s constant and the dissolution Gibbs free energy, enthalpy, and entropy changes of CO2 solvation were obtained by correlating the experimental data. The solubility of CO2 in the mixtures increased linearly with the increasing pressure or the decreasing temperature. The enthalpies of solution were negative at all conditions.

Preparation and characterization of activated carbon from pine cone by microwave-induced ZnCl 2 activation and its effects on the adsorption of methylene blue

Abstract: In this work, activated carbon prepared from pine cone (PCAC) with ZnCl2 as an activation agent under microwave radiation was investigated. The activation step was performed at the microwave input power of 400 W and radiation time of 5 min. The properties of activated carbon were characterized by N2 adsorption Brunauer–Emmett–Teller (BET), scanning electron microscopy and Fourier transform infrared spectroscopy. Results showed that the BET surface area, Langmuir surface area, and total pore volume of PCAC were 939, 1,486 m2/g and 0.172 cm3/g, respectively. Adsorption capacity was demonstrated by the iodine numbers. The adsorptive property of PCAC was tested using methylene blue dye. Equilibrium data was best fitted by the Langmuir isotherm model, showing a monolayer adsorption capacity of 60.97 mg/g. The pseudo-first- and pseudo-second-order kinetic models were examined to evaluate the kinetic data, and the rate constants were calculated. Adsorption of the dyes followed pseudo-first order kinetics. Thermodynamic parameters such as free energy, enthalpy and entropy of dye adsorption were obtained.

Adsorption of lead ions using a modified lignin hydrogel

Abstract: Adsorbent composites comprising of bentonite/sodium lignosulfonate graft-polymerized with acrylamide and maleic anhydride (BLPAMA) were synthesized, characterized, and used to remove Pb2+ from aqueous solution with a large adsorption capacity. The fundamental adsorption behavior of the as-prepared BLPAMA was studied as a function of adsorbent dosage, initial pH, contact time, temperature, concentration of Pb2+, and additive electrolyte. Results showed that the adsorption of Pb2+ on BLPAMA was temperature-independent but strongly pH dependent. The Pb2+ adsorption kinetics was well described by a pseudo-second order model and the adsorption isotherms closely adhered to the Langmuir model, indicating a monolayer chemical adsorption. Thermodynamic parameters such as Gibbs free energy, enthalpy, and entropy changes were also evaluated. Their values demonstrated that the adsorption of Pb2+ on BLPAMA was spontaneous and endothermic in nature. The competitive adsorption of Pb2+, Cu2+, Cd2+ and Zn2+ in their quaternary metal solution was investigated and the results showed that Pb2+ was preferably adsorbed on BLPAMA. TGA and SEM characterization proved that Pb2+ was adsorbed onto BLPAMA. FTIR and XPS analyses further revealed that the adsorption mechanism of Pb2+ on BLPAMA were mainly through chelation and ion exchange mechanism.

Quantifying Crystallinity in High Molar Mass Poly(3-hexylthiophene)

Abstract: We demonstrate a method to determine absolute crystallinity in high molar mass poly(3-hexylthiophene) (P3HT), as used in commercially relevant organic photovoltaic devices, using enthalpy of fusion and melting temperature values derived from differential scanning calorimetry (DSC) and 13C CPMAS NMR. By studying P3HT with molar masses ranging from 3.6 to 49 kg/mol and using recent work on oligomeric 3-hexylthiophene, we demonstrate several critical items. First, that proper extrapolation to infinite chain lengths, i.e., crystal size, yields values for the equilibrium melting temperature Tm0 of 272 ± 6 °C and the enthalpy of fusion per crystalline repeat unit ΔHu of 49 ± 2 J/g for Form I crystals of P3HT. Second, that a simple correction for crystal size using Tm0 is critical for determining an accurate degree of crystallinity from enthalpy measurements because of finite crystal size effects. Furthermore, our results demonstrate that the ordered fraction of P3HT measured from 13C NMR is indistinguishable fr...

Insight into adsorption equilibrium, kinetics and thermodynamics of lead onto alluvial soil

Abstract: In the present study, adsorption of lead (II) ions from aqueous solution by alluvial soil of Bhagirathi River was investigated under batch mode. The influence of solution pH, sorbent dose, initial lead (II) concentration, contact time, stirring rate and temperature on the removal process were investigated. The lead adsorption was favored with maximum adsorption at pH 6.0. Sorption equilibrium time was observed in 60 min. The equilibrium adsorption data were analyzed by the Freundlich, Langmuir, Dubinin–Radushkevich and Temkin adsorption isotherm models. The kinetics of lead (II) ion was discussed by pseudo first-order, pseudo second-order, intra-particle diffusion, and surface mass transfer models. It was shown that the adsorption of lead ions could be described by the pseudo second-order kinetic model. The activation energy of the adsorption process (E a) was found to be −38.33 kJ mol−1 using the Arrhenius equation, indicating exothermic nature of lead adsorption onto alluvial soil. Thermodynamic parameters, such as Gibbs free energy (∆G 0), the enthalpy (∆H 0), and the entropy change of sorption (∆S 0) have also been evaluated and it has been found that the adsorption process was spontaneous, feasible, and exothermic in nature. The results indicated that alluvial soil of Bhagirathi River can be used as an effective and low cost adsorbent to remove lead ions from aqueous solutions.

Size effects and hydrogen storage properties of Mg nanoparticles synthesised by an electroless reduction method

Abstract: Magnesium nanoparticles have been synthesized by reducing di-n-butylmagnesium with lithium in the presence of naphthalene as an electron carrier. The reactants' ratio significantly influenced the particle size leading to some control of the nucleation and growth process with magnesium particles as small as 8 nm produced at high lithium/naphthalene ratios. Studies of hydrogen storage properties showed that these materials were capable of hydrogen uptake at low temperatures (<150 °C) with kinetics dependent to some extent on the particle size. More remarkably, thermodynamics were found to significantly deviate from that of bulk MgH2 with a significant decrease of both enthalpy (to 63.5 ± 1.8 kJ mol−1 H2) and entropy (to 118.4 ± 3.1 J K−1 mol−1 H2) for particle sizes below 25 nm. These effects are much larger than previously thought and demonstrate that even relatively large isolated nanoparticles can undergo significant thermodynamic alterations.

Influence of temperature and organic solvents (isopropanol and 1,4-dioxane) on the micellization of cationic gemini surfactant (14-4-14).

Abstract: The micellar properties of gemini surfactant, tetramethylene-1,4-bis(dimethyltetradecylammonium bromide) (14-4-14) in binary aqueous mixtures of isopropanol (IP) and 1,4-dioxane (DO) were investigated by tensiometric, conductometric and microcalorimetric methods in the temperature range of 298 to 323 K. The values of both critical micelle concentration (cmc) and degree of dissociation increase with increasing temperature and concentration of cosolvent. The energetics of micellization was determined from the temperature dependence of critical micelle concentration values. The standard Gibbs free energy of micellization (ΔG) was found to be negative and the negative value decreases with both temperature and concentration of cosolvent. The Gibbs free energy of micellization (ΔG) is mainly controlled by tail transfer free energy. The enthalpy of micellization obtained from direct calorimetry, Gibbs-Helmholtz equation and van't Hoff methods are presented and compared. Entropic contribution is found to be larger than the enthalpy and for all the systems, an enthalpy-entropy compensation phenomenon was obtained. Some interfacial parameters, e.g., Gibbs surface excess (Γmax), minimum area per surfactant molecule (Amin), surface pressure (Πcmc) were been calculated. The fluorimetric technique was used to understand the microenvironment of the solution under the influence of cosolvent. The micellar aggregation number of 14-4-14 in a binary mixed solvent was also determined from fluorimetry using pyrene as a probe. Two fluorophores, fluorescein and curcumin delivered the information of the peripheral region of the micellar interface and palisade region. The self-diffusion coefficients of the micellar media were evaluated using the cyclic voltammetry (CV) method. Such multi-technique investigation provides a new look into the role of solvation in micellization.

Excess Enthalpy of Monoethanolamine + Ionic Liquid Mixtures: How Good are COSMO-RS Predictions?

Abstract: Mixtures of ionic liquids (ILs) and molecular amines have been suggested for CO2 capture applications. The basic idea is to replace water, which volatilizes in the amine regeneration step and increases the parasitic energy load, with a nonvolatile ionic liquid solvent. To fully understand the thermodynamics of these systems, here experimental excess enthalpies for binary mixtures of monoethanolamine (MEA) and two ILs: 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [hmim][NTf2], and 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [OHemim][NTf2], were obtained by calorimetry, using a Setaram C80 calorimeter, over the whole range of compositions at 313.15 K. Since it is the temperature derivative of the Gibbs energy, enthalpy is a sensitive measure of intermolecular interactions. MEA + [hmim][NTf2] is endothermic and MEA + [OHemim][NTf2] is exothermic. The reliability of COSMO-RS to predict the excess enthalpy of the (MEA+IL) systems was tested based on the implement...

Solubility and Solution Thermodynamics of Some Sulfonamides in 1-Propanol + Water Mixtures

Abstract: The solubilities of sulfadiazine (SD), sulfamerazine (SMR) and sulfamethazine (SMT) in some 1-propanol + water co-solvent mixtures were measured at five temperatures from 293.15 to 313.15 K over the polarity range provided by the aqueous solvent mixtures. The mole fraction solubility of all these sulfonamides was maximal in the 0.80 mass fraction of 1-propanol solvent mixture (δ solv = 28.3 MPa1/2) and minimal in water (δ = 47.8 MPa1/2) at all temperatures studied. The apparent thermodynamic functions Gibbs energy, enthalpy, and entropy of solution were obtained from these solubility data by using the van’t Hoff and Gibbs equations. Apparent thermodynamic quantities of mixing were also calculated by using the ideal solubilities reported in the literature. Nonlinear enthalpy–entropy relationships were observed for these drugs in the plots of enthalpy versus Gibbs energy of mixing. The plot of ∆mix H° versus ∆mix G° shows different trends according to the slopes obtained when the mixture compositions change. Accordingly, the mechanism for the solution process of SD and SMT in water-rich mixtures is enthalpy driven, whereas it is entropy driven for SMR. In a different way, in 1-propanol-rich mixtures the mechanism is enthalpy driven for SD and SMR and entropy driven for SMT. Ultimately, in almost all of the intermediate compositions, the mechanism is enthalpy driven. Nevertheless, the molecular events involved in the solution processes remain unclear.