Experimental Excess Enthalpies Research Articles

Modeling the temperature of maximum density of aqueous tert-butanol solutions

Short-chain alcohols at high dilution are among the very few solutes that enhance the anomalous behavior of water, in particular the value of the temperature of maximum density. This peculiar feature, first discovered experimentally in the early twenties, has remained elusive to a full explanation in terms of atomistic models. In this paper, we first introduce a two-site model of tert-butanol in which the interactions involving hydrogen bonding are represented by a Stillinger–Weber potential, following the ideas established by Molinero and Moore (2009) for water. Our model parameters are fit so as to semi-quantitatively reproduce the experimental densities and vaporization enthalpies of previously proposed united atom and all atom OPLS models. Water is represented using the aforementioned potential model introduced by Molinero and Moore, with cross interaction parameters between water and tert-butanol optimized to yield a reasonable description of the experimental excess enthalpies and volumes over the whole composition range of the mixture. We will see that our simple model is able to reproduce the presence of a maximum in the change of the temperature of maximum density for very low alcohol mole fractions, followed by a considerable decrease until the density anomaly itself disappears. We have correlated this behavior with changes in the local structure of water and compared it with the results of all-atom simulations of water/tert-butanol mixtures.

GE model independent application of the Gibbs-Helmholtz validation method to P-x data

Gibbs-Helmholtz (G-H) data validation for P-T-x/z (pressure-temperature-liquid mole fraction/overall mole fraction) binary VLE data sets, involves comparing calculated excess enthalpies (HE) from model derivation to experimental excess enthalpy data obtained independently from calorimetry. The usual methods have generally been applied to data at low pressures (usually 1–3 bars) and are dependent on the model selected for the activity coefficient (GE model), however measurements of this nature, where the vapour phase is not determined analytically, can also be conducted at moderate pressures (up to 15 bars) and can be modelled in a similar manner. It's proposed in this work that the G-H validation procedure can also be applied in these ranges if a modification is made for the description of the vapour phase non-idealities through a cubic equation of state. Furthermore, the dependence of the procedure on the activity coefficient model selected in the correlation of the VLE data is investigated. Additionally, a GE model independent method for Gibbs-Helmholtz validation of binary VLE systems at moderate pressures is proposed and tested, that involves the integration of the coexistence equation, using a cubic equation of state to describe vapour-phase non-idealities. It was found that the GE model independent validation procedure generally provides a superior prediction of the temperature trends and magnitude of HE than the model dependent method, and thus is a more reliable procedure for comparison to calorimetrically measured HE data.

Excess enthalpy of monoethanolamine + ionic liquid mixtures: how good are COSMO-RS predictions?

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 implementation of two different molecular models to define the structure of the IL: the IL as separate cation and anion [C+A] and the IL as a bonded single specie [CA]. Quantum-chemical calculations were performed to gain additional insight into the intermolecular interactions between the components of the mixture. For MEA + [hmim][NTf2] both the [C+A] and [CA] models predict endothermic behavior, but the [CA] model is in better agreement with the experimental results. For MEA + [OHemim][NTf2] the [C+A] model provides the best match to the experimental exothermic results. However, what is really surprising is that two different conformations of the cation-anion pair with nearly identical energies in the [CA] model result in completely different (exothermic vs endothermic) predictions of the excess enthalpy. Nonetheless, the results do show that the influence of the structure of the IL on the thermodynamic behavior of the mixture (endothermic vs exothermic) can be attributed to hydrogen bonding between the cation and the MEA molecule. However, this study highlights the importance of carefully selecting the molecular model and conformation in order to obtain even qualitatively correct predictions with COSMO-RS. The fact that even very slightly different conformations of the IL can drastically change the thermodynamic estimations using COSMO-RS is of significant concern. Overall, we believe the present work provides a better understanding of the behavior of mixtures involving amines and ILs, which is an important aspect to consider when evaluating the use of such solvent mixtures in CO2 capture technologies.

Interactions of Alkanolamines with Water: Excess Enthalpies and Hydrogen Bonding.

We report a transferable force field to describe the interactions of alkanolamines based on the N-C-C-O backbone with water, derived from a comparison with experimental excess enthalpies. This force field is tested on 2-aminoethan-1-ol (MEA), 2-amino-2-methylpropan-1-ol, 2-aminobutan-1-ol (ABU), and 1-aminopropan-2-ol. These alkanolamines are derivatives of MEA obtained by substitution with methyl and ethyl groups on the carbon atoms of the N-C-C-O backbone. A specific cross interaction site corresponding to the hydrogen bond between the hydroxyl group of the alkanolamine and the oxygen atom of water was introduced in order to reproduce quantitatively experimental excess enthalpies. The transferability of this specific site was assessed by predictions on alkanolamines that were not included in the parametrization data set. New data on enthalpy of mixing for ABU with water are reported, since they were not available in the literature. From the molecular simulations, several microscopic quantities of the alkanolamine-water mixtures were analyzed in order to improve our understanding of these systems. The structure of the solvation shells at varying compositions, statistics of hydrogen bonds, conformations, and energy decompositions served as bases for an interpretation of the molecular reasons underlying the behavior of the excess enthalpy. The prominent result is that water-water interactions play a major role in differentiating alkanolamine-water mixtures, which is a manifestation of the hydrophobic effect. Both the structural and energetic effects observed at the molecular level point to phenomena that have strong composition dependence, in particular, the interplay between the intramolecular hydrogen bond in the alkanolamine and the intermolecular hydrogen bonds with water.

Thermophysical properties of {R-fenchone + ethanol} at several temperatures and pressures

In this paper, experimental excess enthalpy and density values for the binary mixture {R-fenchone+ethanol} are reported at four temperatures (283.15, 298.15, 313.15 and 328.15) K and atmospheric pressure (virtually 0.1MPa). Densities have been also measured at the same temperatures and pressures ranging from 20MPa to 40MPa. In all cases the whole composition range has been considered. Excess molar volumes, isothermal compressibilities and isobaric thermal expansions have been calculated from the volumetric data at the four working temperatures. Excess enthalpies have been predicted using the UNIFAC (Dortmund) method and the Quantum Continuum Method COSMO-RS: UNIFAC shows the best performance. Four equations of state (Peng–Robinson, Peng–Robinson–Stryjek–Vera with volume translation, SAFT, PC-SAFT) have been tested as predictive models of the P-ρ-T behavior. Stryjek–Vera EOS lead to the best results with an average in the percentage absolute average deviation of 0.6% and a maximum percentage absolute average deviation of 0.9%.

Overview of the Excess Enthalpies of the Binary Mixtures Composed of Molecular Solvents and Ionic Liquids and Their Modeling Using COSMO-RS

In this work, an overview of the excess enthalpies of binary mixture containing ionic liquid and molecular solvent is reported, taking into account the various structural features of both the ionic liquid and molecular solvent. Subsequently, the capability of the conductor-like screening model for real system (COSMO-RS) to predict the excess enthalpies of the binary system of ionic liquid and molecular solvent was further evaluated. In general, a good agreement between experimental excess enthalpies and COSMO-RS prediction was obtained. In addition, the COSMO-RS was able to depict and screen the type of molecular interaction that governs the excess enthalpies of ionic liquid and molecular solvent. Thus, COSMO-RS proved very useful as an a priori method that could be used as a tool in the selection or design of a suitable ionic liquid candidate for a certain task and application before extensive experimental measurement.

Excess Enthalpy and Excess Volume for Pyridine + Methyldiethanolamine and Pyridine + Ethanolamine Mixtures

Experimental excess enthalpy, excess volume, and density data for the binary liquid mixtures pyridine + n-methyldiethanolamine (MDEA) and pyridine + monoethanolamine (MEA) are reported at atmospheric pressure and (15, 25, and 80) °C. Experimental data are not currently available for these mixtures, and the reported values are particularly important for the ongoing development of group-contribution based thermodynamic models linked to the impact of specific molecular features on molecular interactions in mixtures and improving the parametrization of equation of state models. The experimental methods and procedures and data processing methods were validated using the heptane + toluene binary mixture, where it was possible to compare experimental and computational results obtained with literature values. Mixture liquid density, excess volume, and excess enthalpy data are also compared with values computed using the Peng–Robinson equation of state.

Oxygenated additives in bio-fuels: excess enthalpies of mixtures di-butyl ether + 1-propanol + heptane or + cyclohexane at 298.15 and 313.15 K

The increasing worldwide use of bio-fuels constitutes one of the measures considered to reduce greenhouse gas emissions. Bio-fuels also have an important part to play in promoting the security of energy supply, and promoting technological development and innovation. Di-butyl ether (DBE) is used as blending agent in reformulated gasoline and has been included in recent international regulations on the promotion of the use of energy from renewable sources for transport [1]. Recently 1-propanol has been proposed too as an alternative to conventional gasoline and diesel fuels [2]. Interest in propanols as a second-generation biofuels has increased because they have many advantages over other potential alternative fuel candidates such as ethanol, as they can be produced from biomass by microbial fermentation of biomass (cellulose). Ether + alcohol + alkane mixtures are of interest as model mixtures for gasoline in which the alcohol and the ether act as non-polluting, high octane number blending agents. From this point of view the study of the ternary mixtures dibutyl ether +1-propanol and heptane, or cyclohexane, are very interesting. This work is a new contribution to previous works on ether + alcohol + alkane mixtures of our group [3-7]. Experimental excess enthalpies of the ternary systems DBE + 1-propanol + heptane or + cyclohexane and the corresponding binary systems at 298.15 K and 313.15 K are reported in this work. Excess enthalpies have been measured with a quasi-isothermal flow calorimeter. The experimental data have been fitted using the RedlichKister polynomial equation for binary and ternary systems. The values of the standard deviation indicate the agreement between the experimental results and the fitted ones. Excess enthalpies have been determined with an estimated relative uncertainty of + 0.01•H E J•mol-1 at 298.15 and 313.15 K. We acknowledge support for this research to the Ministerio de Ciencia e Innovacion, Spain, Project ENE2009-14644-C02-02 and to the Agencia Espanola de Cooperacion Internacional para el Desarrollo AECID, project AP/041072/11. This work is part of the Doctoral Thesis of F.E.M. Alaoui. References

Excess Molar Enthalpies of Ternary and Binary Mixtures Containing 2-Methoxy-2-Methylpropane, Ethanol, and Nonane

Forming part of the scientific project entitled “Study on Physical Properties of Mixtures Hydrocarbon + Alcohol + Ether Like Alternative Fuels”, the present article reports experimental data of excess molar enthalpies for the ternary system {x1 2-methoxy-2-methylpropane (MTBE) + x2 ethanol + (1 – x1 – x2) nonane} and the involved binary mixture {x ethanol + (1 – x) nonane} at the temperature of 298.15 K and atmospheric pressure, over the whole composition range. No experimental excess enthalpy values were found in the currently available literature for the ternary mixture under study. Values of excess molar enthalpies were measured using a Calvet microcalorimeter. The results were fitted by means of different variable degree polynomials. Smooth representations of the results are presented and used to construct constant excess molar enthalpy contours on Roozeboom diagrams. The excess molar enthalpies for the binary and ternary system are positive over the whole range of composition. The binary mixture {x ethanol + (1 – x) nonane} is asymmetric, with its maximum displaced toward a high mole fraction of nonane. The ternary contribution is also positive, and the representation is asymmetric.

Interactions of Ionic Liquids and Water

Experimental excess enthalpies of ionic liquid and water mixtures in combination with calculated CHELPG atomic charges were used to investigate the interactions between the species in solution. The excess enthalpies of ionic liquids in water were obtained by calorimetry, using a Setaram C80 calorimeter, including temperatures from (313.15 to 348.15) K and the entire range of composition. The ionic liquids investigated all contain the 1-ethyl-3-methylimidazolium cation except one, which has an added hydroxyl group on the cation (1-(2-hydroxyethyl)-3-methylimidazolium cation). The anions investigated are ethylsulfate, methylsulfate, hydrogensulfate, trifluoromethanesulfonate, methanesulfonate, and trifluoroacetate, and these will demonstrate the effect of systematically varying the substituents on the anion. The CHELPG atomic charges on the cations and anions were calculated using the Gaussian 03 program. The CHELPG atomic charges are consistent with the observed trends in excess enthalpy and provide insight into cation/water, anion/water, and cation/anion interactions.

Study on the effect of increasing the chain length of the alkanol in excess molar enthalpies of mixtures containing 2-methoxy-2-methylpropane, 1-alkanol, decane

Excess molar enthalpies for the ternary system { x 1 2-methoxy-2-methylpropane + x 2 ethanol + (1 − x 1 − x 2) decane} and the involved binary mixture { x ethanol + (1 − x) decane} have been measured at the temperature of 298.15 K and atmospheric pressure, over the whole composition range. No experimental excess enthalpy values were found in the currently available literature for the ternary mixture under study. The results were fitted by means of different variable-degree polynomials. Smooth representations of the results are presented and used to construct constant excess molar enthalpy contours on Roozeboom diagrams. The excess molar enthalpies for the binary and ternary system are positive over the whole range of composition. The binary mixture { x ethanol + (1 − x) decane} is asymmetric, with its maximum displace toward a high mole fraction of decane. The ternary contribution is also positive, and the representation is asymmetric.

Extended UNIQUAC model for thermodynamic modeling of CO 2 absorption in aqueous alkanolamine solutions

The extended UNIQUAC model [K. Thomsen, P. Rasmussen, Chem. Eng. Sci. 54 (1999) 1787–1802] was applied to the thermodynamic representation of carbon dioxide absorption in aqueous monoethanolamine (MEA), methyldiethanolamine (MDEA) and varied strength mixtures of the two alkanolamines (MEA–MDEA). For these systems, altogether 13 interaction model parameters are adjusted. Out of these parameters, 11 are temperature dependent. All the essential parameters of the model are simultaneously regressed to a collective set of data on the single MEA and MDEA systems. Different types of data are used for modeling and they cover a very wide range of conditions. Vapor–liquid equilibrium (VLE) data for the aqueous alkanolamine systems containing CO 2 in the pressure range of 3–13,000 kPa and temperatures of 25–200 °C are used. The model is also regressed with the VLE and freezing point depression data of the binary aqueous alkanolamine systems (MEA–water and MDEA–water). The two just mentioned types of data cover the full concentration range of alkanolamines from extremely dilute to almost pure. The experimental freezing point depression data down to the temperature of −20 °C are used. Experimental excess enthalpy ( H E ) data of the binary MEA–water and MDEA–water systems at 25, 40, 65 and 69 °C are used as well. In order to enhance the calculation of the infinite dilution activity coefficients of MEA and MDEA, the pure alkanolamines vapor pressure data in a relevant temperature range (up to almost 230 °C) are included in the parameter estimation process. The previously unavailable standard state properties of the alkanolamine ions appearing in this work, i.e. MEA protonate, MEA carbamate and MDEA protonate are determined. The concentration of the species in both MEA and MDEA solutions containing CO 2 are predicted by the model and in the case of MEA compared to NMR spectroscopic data. Using only one set of parameters for correlation of different thermodynamic properties, the model has represented the experimental data with good precision.

Excess Enthalpies of Binary and Ternary Mixtures Containing Dibutyl Ether, Cyclohexane, and 1-Butanol at 298.15 K

Experimental excess enthalpies of the ternary system dibutyl ether (DBE) + cyclohexane + 1-butanol and the corresponding binary systems at 298.15 K are reported. A quasi-isothermal flow calorimeter has been built and tested to make the measurements. All the binary and the ternary systems show endothermic character. The experimental data have been fitted using a polynomial equation for the binary and ternary systems. The values of the standard deviation indicate good agreement between the experimental results and those calculated from the equation.

Excess Molar Enthalpies of Ternary and Binary Mixtures Containing 2-Methoxy-2-methylpropane, 1-Propanol, and Nonane

Excess molar enthalpies for the ternary system {x1 2-methoxy-2-methylpropane (MTBE) + x2 1-propanol + (1 − x1 − x2) nonane} and the involved binary mixture {x 1-propanol + (1 − x) nonane} have been measured at the temperature of 298.15 K and atmospheric pressure, over the whole composition range. No experimental excess enthalpy values were found in the currently avaliable literature for the ternary and binary mixtures under study in similar conditions. Values of excess molar enthalpies were measured using a Calvet microcalorimeter. The results were fitted by means of different variable degree polynomials. Smooth representations of the results are presented and used to construct constant excess molar enthalpy contours on Roozeboom diagrams. The group contribution model of the UNIFAC model (in the versions of Larsen et al. and Gmehling et al.) was used to estimate ternary excess enthalpy values. Several empirical expressions for estimating ternary properties from binary results were also tested.

Isothermal Vapor−Liquid Equilibrium of Binary Mixtures Containing 1-Chlorobutane, Ethanol, or Acetonitrile

Vapor−liquid equilibrium has been measured at 333.15 K for the following binary mixtures: acetonitrile (3) + 1-chlorobutane (1), ethanol (2) + 1-chlorobutane (1), and ethanol (2) + acetonitrile (3) at 333.15 K. Values of the maximum of GE ranging from 700 J·mol−1 to 1150 J·mol−1 were found for the three systems. Consistency with data already published at different temperatures has been demonstrated as well as with experimental excess enthalpy for the ethanol + acetonitrile mixture. The results have been used to test the ability of a lattice-fluid theory to fit the GE and to predict the HE and VE excess functions. Although reasonable fits were found for the systems containing ethanol, worse results were found for 1-chlorobutane + acetonitrile. In general, the theoretical predictions of HE and VE are rather poor when the binary energy parameter of the theory is fitted to GE.

Study of the effect of increasing the chain length of the alkane in excess molar enthalpies of mixtures containing methyl tert-butyl ether, 1-propanol, alkane

Excess molar enthalpies for the ternary system { x 1 methyl tert-butyl ether + x 2 1-propanol + (1 − x 1 − x 2) decane} and the involved binary mixture { x 1-propanol + (1 − x) decane} have been measured at the temperature of 298.15 K and atmospheric pressure, over the whole composition range. No experimental excess enthalpy values were found in the currently avaliable literature for the ternary mixture under study. Values of excess molar enthalpies were measured using a Calvet microcalorimeter. The results were fitted by means of different variable degree polynomials. Smooth representations of the results are presented and used to construct constant excess molar enthalpy contours on Roozeboom diagrams. The group contribution model of the UNIFAC model (in the versions of Larsen and Gmehling) was used to estimate ternary excess enthalpy values. Several empirical expressions for estimating ternary properties from binary results were also tested. A study of the effect of increasing the chain length of the n-alkane is made.

Excess molar volumes and excess molar enthalpies of binary mixtures for 1,2-dichloropropane + 2-alkoxyethanol acetates at 298.15 K

The excess molar volumes V m E and excess molar enthalpies H m E over the whole range of composition have been measured for the binary mixtures formed by 1,2-dichloropropane (1,2-DCP) with three 2-alkoxyethanol acetates at 298.15 K and atmospheric pressure using a digital vibrating-tube densimeter and an isothermal calorimeter with flow-mixing cell, respectively. The 2-alkoxyethanol acetates are ethylene glycol monomethyl ether acetate (EGMEA), ethylene glycol monoethyl ether acetate (EGEEA), and ethylene glycol monobutyl ether acetate (EGBEA). The V m E of the mixture has been shown positive for EGMEA, ‘S-shaped’ for EGEEA, being negative at low and positive at high mole fraction of 1,2-DCP, and negative for EGBEA. All the H m E values for the above mixtures showed an exothermic effect (negative values) which increase with increase in carbon number of the 2-alkoxyethanol acetates, showing minimum values varying from −374 J mol −1 (EGMEA) to −428 J mol −1 (EGBEA) around 0.54–0.56 mol fraction of 1,2-DCP. The experimental results of H m E and V m E were fitted to Redlich–Kister equation to correlate the composition dependence of both excess properties. In this work, the experimental excess enthalpy data have been also correlated using thermodynamic models (Wilson, NRTL, and UNIQUAC) and have been qualitatively discussed.

Experimental and theoretically estimated excess molar enthalpies for tert-butyl methyl ether+ 1-pentanol+octane at 298.15 K

Forming part of the scientific project entitled ‘Study on physical properties of mixtures hydrocarbon+alcohol+ether like alternative fuels’, the present article reports experimental data of excess molar enthalpies for the ternary mixture {x1tert-butyl methyl ether (MTBE)+x21-pentanol+(1−x1−x2)octane} and the involved binary mixture {x1-pentanol+(1−x) octane} at the temperature of 298.15 K and atmospheric pressure. No experimental excess enthalpy values were found in the currently available literature for the ternary mixture under study. The group contribution model of the UNIFAC (in the versions of Larsen, and Gmehling) were used to estimate excess enthalpy values. Several empirical expressions for estimating ternary properties from binary results were also tested.

Excess enthalpy, density, and speed of sound determination for the ternary mixture (methyl tert-butyl ether + 1-butanol + n-hexane)

Density, ( ρ), and speed of sound, ( u), from T = 288.15 to T = 308.15 K, and excess molar enthalpies, ( h E) at T = 298.15 K, have been measured over the entire composition range for (methyl tert-butyl ether + 1-butanol + n-hexane). In addition, excess molar volumes, V E, and excess isentropic compressibility, κ s E , were calculated from experimental data. Finally, experimental excess enthalpies results are compared with the estimations obtained by applying the group-contribution models of UNIFAC (in the versions of Dang and Tassios, Larsen et al., Gmehling et al.), and DISQUAC.

Excess enthalpies of binary and ternary mixtures containing tert-amyl methyl ether (TAME), tert-amyl alcohol (TAOH) and hexane at 298.15 and 313.15 K

Experimental excess enthalpies of the ternary system tert-amyl methyl ether (TAME)+ tert-amyl alcohol (TAOH)+hexane and the corresponding binary subsystems at 313.15 K are reported. Also the systems TAME + TAOH and TAOH + hexane have been measured at 298.15 K. A quasi-isothermal flow calorimeter has been modified and tested to make the measurements. The experimental data have been fitted using a polynomial equation for binary and ternary systems. The values of the standard deviation indicate the agreement between the experimental results and the fitted ones. Positive experimental excess enthalpies were obtained for all compositions of the ternary mixture investigated, which are larger than for the binary subsystems. Temperature increase from 298.15 to 313.15 K enhances endothermic effect in the binary subsystems, TAME + TAOH and TAOH + hexane, studied here. Excess Gibbs energy data published previously and excess enthalpies have been used for calculating the excess entropy of the binary and ternary systems at 313.15 K.