Excess Free Enthalpy Research Articles

Study of molecular interactions between 2-methyl 1-propanol and m-substituted aniline at various temperatures (thermodynamic and acoustic properties)

ABSTRACT Excess molar volumes (V E), excess acoustic impedance (Z E), excess isentropic compressibility (κ s E), deviation in viscosity (Δη), excess Gibbs energy of activation of viscous flow (G *E), excess free length (L f E), excess enthalpy (H E) and excess speed of sound (U E) for binary mixtures of 2-methyl-1-propanol with meta-substituted aniline (3-chloroaniline, 3-methoxyaniline and 3-methylaniline) at selected compositions were determined from the measured values of densities (ρ), viscosities (η) and speeds of sound (u) of pure components and their mixtures in range of 303.15–313.15 K. The results are analysed in terms of interactions arising due to the formation of complex hydrogen bond electron in the binary mixtures. Different theoretical models were applied to check the applicability to the present values of speed of sound and viscosity. Finally, the Prigogine–Flory–Patterson theory is applied to identify the most predominant molecular interaction.

Development of Abraham model correlations for short-chain glycol-grafted imidazolium and pyridinium ionic liquids from inverse gas-chromatographic measurements

Infinite dilution activity coefficients and gas-to-liquid partition coefficients are herein reported for more than 45 organic solutes of varying polarity and hydrogen-bonding character dissolved within the ether-grafted ionic liquids 1-ethyl-3-(2-methoxyethyl)imidazolium bis(trifluoromethylsulfonyl)imide and N-(2-methoxyethyl)pyridinium bis(trifluoromethylsulfonyl)imide. Experimental values were determined in 10 K intervals from 323.15 to 373.15 K using the method of inverse gas chromatography. Measured infinite dilution activity coefficients were then used to determine the partial molar excess Gibbs free energies, enthalpies, and entropies associated with the dissolution of these model solutes into these two short-chain glycol-grafted ionic liquids. Finally, based on the measured infinite dilution activity coefficient data, Abraham model correlations have been established for solute transfer into these ionic liquids, including the calculation of cation-specific equation coefficients. The derived Abraham model correlations were found to back-calculate the observed partition coefficients to within 0.09 to 0.13 log units.

Characterization of the solubilizing ability of short-chained glycol-grafted ammonium and phosphonium ionic liquids

Abstract Infinite dilution activity coefficients and gas-to-liquid partition coefficients are reported for >40 organic solvents dissolved in (2-methoxyethyl) triethylphosphonium bis(trifluoromethylsulfonyl)imide, (2-methoxyethyl)tributylphosphonium bis(trifluoromethylsulfonyl)imide, and (2-methoxyethyl)triethylammonium bis(trifluoromethylsulfonyl)imide in the temperature range from 323.15 K to 373.15 K. Experimental values were determined using the method of inverse gas chromatography. Measured infinite dilution activity coefficients are used to calculate the partial molar excess Gibbs free energy, enthalpy and entropy associated with dissolution of the organic solutes into the three ionic liquid solvents. Abraham model correlations for solute transfer into each ionic liquid solvent have also been determined from the measured infinite dilution activity coefficient data.

Activity coefficients at infinite dilution of organic solutes, using novel N-(2′, 3′-epoxypropyl)-N-methyl-2-oxopyrrolidinium chloride ionic liquid by GLC

The activity coefficients at infinite dilution (γ13∞) for 31 solutes, from the following classes of compounds: alkanes, cycloalkanes, alkenes, cycloalkenes, aromatic hydrocarbons, alcohols, ketones, acetonitrile, thiophene and water were determined using an ionic liquid (IL) modified column utilizing gas liquid chromatography. In this work, N-(2′, 3′-epoxypropyl)-N-methyl-2-oxopyrrolidinium chloride IL was used to determine the activity coefficients of organic solutes in the range of temperatures from T = 323.15 K–343.15 K, using 10 K intervals, under atmospheric pressure. From these experimental γ13∞ data, the following thermodynamic properties at infinite dilution, including partial molar excess Gibbs free energies (ΔGiE,∞), enthalpies ΔHiE,∞and entropy TrefΔSiE,∞ were computed. Furthermore, selectivity (Sij∞) and capacity (Kj∞) were also calculated from experimental value of γ13∞ for separation of mixtures such as (heptane/benzene, heptane/thiophene, hexane/benzene, water/butanol and hexane/ethanol) at T = 323.15 K–343.15 K. Generally, the results of this work indicate that the investigated IL has good potential for separating aliphatic solutes from aromatic ones. Specifically, the chosen IL reduces the retention time of most of the solutes used, compared to other reported ILs. Furthermore, the findings of this study suggest that this IL [N(2′,3′-epoxypropyl)-N-methyl-2-oxopyrrolidinium chloride] may be useful for future applications such as separation and purification in petrochemical industries.

Thermodynamic analysis of EMISE–Water as a working pair for absorption refrigeration system

In this paper, thermodynamic analysis of novel ionic liquid 1 Ethyl-3-methylimidazolium ethyl sulphate (EMISE) as absorbent and water as green refrigerant for absorption refrigeration system (ARS) is performed. Thermodynamics excess properties like excess Gibbs free energy (GE), excess enthalpy (hE) and equilibrium Dühring’s plot (P-T-x1) of EMISE-H2O binary mixture are assessed using non-random two liquid (NRTL) activity coefficient model for composition x1 of 0.45–1. The performance of single effect ARS with SHE are mathematically modeled and simulated by applying first and second laws (exergy analysis) of thermodynamics. Simulated results of ARS are compared with other widely used working fluids and revealed higher COP of 0.66 for EMISE-H2O in comparison to NH3-H2O but lower than LiBr-H2O. In addition, the deviation in the COP and ECOP with the generator temperatures are compared for the evaporation temperature of 5, 10, 15 °C. Result showed noticeable difference between the optimum generator temperature based on COP and ECOP. This indicates the exergy analysis is used for evaluation of ARS and selection of heat source supply temperature obtained from waste heat and renewable solar energy.

Thermodynamics and activity coefficients at infinite dilution for organic solutes in the ionic liquid 1-butyl-1-methylpyrrolidinium dicyanamide

The activity coefficients at infinite dilution, γ13∞, for 46 solutes, including alkanes, cycloalkanes, alkenes, alkynes, aromatic hydrocarbons, alcohols, thiophene, ethers, ketones, esters, aldehyde, acetonitrile, pyridine and 1-nitropropane in the ionic liquid (IL) 1-butyl-1-methylpyrrolidynium dicyanamide, [BMPYR][DCA] were determined by gas-liquid chromatography at five temperatures in range of (313.15–373.15) K at pressure, p = 101 kPa. The thermodynamic functions at infinite dilution as partial molar excess Gibbs free energy, ΔG1E,∞, enthalpy ΔH1E,∞, and entropy term TrefΔS1E,∞ and the gas-liquid partition coefficients, KL at infinite dilution were calculated from the experimental γ13∞ values obtained over the temperature range. The values of selectivity and capacity for few separation problems as n-heptane/benzene, n-heptane/thiophene, n-heptane/1-heptyne, and ethylbenzene/styrene at T = 323.15 K were calculated from γ13∞ and compared to literature values for [DCA]-based ionic liquids and [BMPYR]-based ILs. In comparison with the former measured [BMPYR][TCM], or [BMPYR][SCN] ILs, the [DCA]-based IL presents high selectivities for the separation of aromatic hydrocarbons from aliphatic hydrocarbons, and especially for the extraction of thiophene, or pyridine from n-heptane with slightly lower capacity than those for [BMPYR][TCM], or [BMPYR][SCN]. New data shows that [BMPYR][DCA] IL may be proposed as an alternative solvent for the separation of aliphatic from aromatic hydrocarbons.

A comprehensive comparison between thermodynamic perturbation theory and first-order mean spherical approximation: Based on discrete potentials with hard core

Using the TL (Tang and Lu, 1993) method, Ornstein-Zernike integral equation is solved perturbatively under the mean spherical approximation (MSA) for fluid with potential consisting of a hard sphere plus square-well plus square-shoulder (HS+SW+SS) to obtain first-order analytic expressions of radial distribution function (RDF), second-order direct correlation function, and semi-analytic expressions for common thermodynamic properties. A comprehensive comparison between the first-order MSA and high temperature series expansion (HTSE) to third-, fifth- and seventh-order is performed over a wide parameter range for both a HS+SW and the HS+SW+SS model fluids by using corresponding “exact” Monte Carlo results as a reference; although the HTSE is carried out up to seventh-order, and not to the first order as the first-order MSA the comparison is considered fair from a calculation complexity perspective. It is found that the performance of the first-order MSA is dramatically model-dependent: as target potentials go from the HS+SW to the HS+SW+SS, (i) there is a dramatic dropping of performance of the first-order MSA expressions in calculating the thermodynamic properties, especially both the excess internal energy and constant volume excess heat capacity of the HS+SW+SS model cannot be predicted even qualitatively correctly. (ii) One tendency is noticed that the first-order MSA gets more reliable with increasing temperatures in dealing with the pressure, excess Helmholtz free energy, excess enthalpy and excess chemical potential. (iii) Concerning the RDF, the first-order MSA is not as disappointing as it displays in the cases of thermodynamics. (iv) In the case of the HS+SW model, the first-order MSA solution is shown to be quantitatively correct in calculating the pressure and excess chemical potential even if the reduced temperatures are as low as 0.8. On the other hand, the seventh-order HTSE is less model-dependent; in most cases of the HS+SW and the HS+SW+SS models, the seventh-order HTSE improves the fifth- and third-order HTSE in both thermodynamic properties and RDF, and the improvements are very demonstrable in both the excess internal energy and constant volume excess heat capacity; for very limited cases, the seventh-order HTSE improves the fifth-order HTSE only within lower density domain and even shows a bit of inadaptation over higher density domain.

Wettability of Binary Solid-Supported Films of Zwitterionic/Anionic Phospholipids

The effect of different amounts of anionic phosphatidylglycerol in the zwitterionic phosphatidylcholine monolayers on their properties was quantitatively described in terms of the interactions between these two compounds as well as the changes of mixed monolayers' apparent surface free energy and its components. For all Langmuir films, surface pressure–area per molecule (π–A) isotherms were used to monitor the surface thermodynamics (i.e. the excess area and excess free enthalpy of mixing). This allowed to evaluate the kind and magnitude of interactions that have an influence on the phase behaviour and structural properties of the monolayers. Then, assuming that the molecular properties of Langmuir films are preserved upon transfer to a solid support, further characterizations were accomplished with advancing and receding contact angle measurements on the Langmuir–Blodgett films deposited at 35 mN/m on mica. These enabled the possibility to track changes in the wettability of the tightly packed or phase-separated phospholipid films and their surface free energy as a function of the monolayer composition. It is believed that these results throw a light on better understanding of the organizational structure of lipid films and their interactions with the surrounding environment. Moreover, the obtained results can be also helpful to identify the optimal lipid composition in the production of more effective therapeutic surfactants.

Influence of surfactant hydrophobic segment length on the thermodynamic parameters of binary mixed micelles of homologues polysorbates and Triton X-100

It is important to find binary surfactant systems having synergistic interactions (negative value of the interaction parameter) since it means that a binary surfactant mixture composition exists in these systems, having lower critical micelle concentration value than more hydrophobic mixture component. In this work, influence of hydrophobic segment length of homologues Tweens in mixed micelles with Triton X-100 on the interaction parameter value (βij) and on the excess Gibbs energy (GE) was studied. Also, influence of temperature on the stability of investigated mixed micelles has been investigated (298.15–328.15K). The critical micelle concentration values of pure surfactants and their binary mixtures were determined spectrofluorimetrically using pyrene as a probe molecule. It was found for every investigated temperature that increase in length of saturated aliphatic side chain of a polysorbate increases the stability of investigated mixed micelles (βij<0 and GE<0) compared to the ideal mixed micelles.Presence of the C9 double bond in oleic acid residue of Tween 80 reduces synergistic effect of this polysorbate compared to Tween 60 which contains hydrophobic segment with the same number of C atoms but without olefin bond.The excess free enthalpy below the 298.15K is determined with entropic contribution, but above 298.15K with enthalpic contribution.

Thermodynamics and activity coefficients at infinite dilution for organic solutes, water and diols in the ionic liquid choline bis(trifluoromethylsulfonyl)imide

The activity coefficients at infinite dilution, γ13∞, for 63 solutes, including alkanes, cycloalkanes, alkenes, alkynes, aromatic hydrocarbons, alcohols, water, thiophene, ethers, ketones, esters, aldehyde, acetonitrile, pyridine and 1-nitropropane and 6 diols in the ionic liquid (IL) choline bis(trifluoromethylsulfonyl)imide, [N1112OH][NTf2] were determined by (gas+liquid) chromatography at six temperatures in range of (318.15 to 368.15)K and at three temperatures for diols in the range of (388.15 to 418.15)K. The thermodynamic functions at infinite dilution as partial molar excess Gibbs free energy, ΔG1E,∞, enthalpy ΔH1E,∞, and entropy term TrefΔS1E,∞ were calculated from the experimental γ13∞ values obtained over the temperature range. The density of [N1112OH][NTf2] was measured within temperature range (313.15 to 353.15)K. The (gas+liquid) partition coefficient KL was calculated for all solutes. The values of selectivity and capacity for a few separation problems as hexane/benzene, cyclohexane/benzene, heptane/thiophene at T=328.15K were calculated from γ13∞ and compared to literature values for similar ionic liquids, N-methyl-2-pyrrolidinone (NMP), and sulfolane. In comparison with the former measured ammonium-based ILs and the morpholinium-based ILs, the [N1112OH][NTf2] shows average selectivity for the separation of aromatic hydrocarbons, or sulfur compound from aliphatic hydrocarbons, and very high selectivity for pyridine/heptane separation. New data show that [N1112OH][NTf2] IL may be proposed as an alternative solvent for the separation of sulfur or nitrogen compounds from alkanes.

Conditional Solvation Thermodynamics of Isoleucine in Model Peptides and the Limitations of the Group-Transfer Model

The hydration thermodynamics of the amino acid X relative to the reference G (glycine) or the hydration thermodynamics of a small-molecule analog of the side chain of X is often used to model the contribution of X to protein stability and solution thermodynamics. We consider the reasons for successes and limitations of this approach by calculating and comparing the conditional excess free energy, enthalpy, and entropy of hydration of the isoleucine side chain in zwitterionic isoleucine, in extended penta-peptides, and in helical deca-peptides. Butane in gauche conformation serves as a small-molecule analog for the isoleucine side chain. Parsing the hydrophobic and hydrophilic contributions to hydration for the side chain shows that both of these aspects of hydration are context-sensitive. Furthermore, analyzing the solute–solvent interaction contribution to the conditional excess enthalpy of the side chain shows that what is nominally considered a property of the side chain includes entirely nonobvious contributions of the background. The context-sensitivity of hydrophobic and hydrophilic hydration and the conflation of background contributions with energetics attributed to the side chain limit the ability of a single scaling factor, such as the fractional solvent exposure of the group in the protein, to map the component energetic contributions of the model-compound data to their value in the protein. But ignoring the origin of cancellations in the underlying components the group-transfer model may appear to provide a reasonable estimate of the free energy for a given error tolerance.

Thermodynamics and activity coefficients at infinite dilution for organic solutes and water in the ionic liquid 1-butyl-1-methylmorpholinium tricyanomethanide

The activity coefficients at infinite dilution, γ13∞, for 61 solutes, including alkanes, cycloalkanes, alkenes, alkynes, aromatic hydrocarbons, alcohols, water, thiophene, ethers, ketones, esters, aldehyde, acetonitrile, pyridine and 1-nitropropane in the ionic liquid (IL) 1-butyl-1-methylmorpholinium tricyanomethanide, [BMMOR][TCM] were determined by gas–liquid chromatography at six temperatures within the range of (318.15 to 368.15)K. The thermodynamic functions at infinite dilution as partial molar excess Gibbs free energy ΔG1E,∞, enthalpy ΔH1E,∞, and entropy term TrefΔS1E,∞ were calculated from the experimental γ13∞ values obtained over the temperature range. The density of [BMMOR][TCM] was measured over the temperature range (288.15 to 368.15)K. The gas–liquid partition coefficient KL was calculated for all solutes. The values of selectivity and capacity for a few separation problems such as hexane/benzene, cyclohexane/benzene, heptane/thiophene at T=328.15K were calculated from γ13∞ and compared to literature values for similar ionic liquids, viz. N-methyl-2-pyrrolidinone (NMP), and sulfolane. In comparison with the previously measured values for [BMPYR][TCM], the morpholinium IL presents high selectivity for the separation of aromatic hydrocarbons from aliphatic hydrocarbons, and especially thiophene, or piridine from heptane with a slightly lower capacity. New data show that [BMMOR][TCM] IL may be proposed as an alternative solvent for the separation of sulphur or nitrogen compounds from alkanes.

Study of molecular interactions in binary mixtures of formamide with 2-methoxyethanol and 2-ethoxyethanol at varying temperatures

The thermophysical properties of binary mixtures of formamide with 2-methoxyethanol and 2-ethoxyethanol have been investigated in this article. Densities, refractive index, ultrasonic velocity and viscosity for the two binary mixtures viz. formamide with 2-methoxyethanol and 2-ethoxyethanol have been measured over the entire composition range at 293, 303 and 313 K and at atmospheric pressure. The excess molar volume, the molar refraction deviation, excess Gibb's free energy of activation for viscous flow, excess isentropic compressibility, deviation in viscosity, excess free volume and excess molar enthalpy have been computed using experimental data. These excess parameters have been correlated with Redlich–Kister polynomial equation. The results have been interpreted on the basis of strength of intermolecular interaction occurring in these mixtures. Densities, refractive index and ultrasonic velocity were correlated with second-order polynomial equation. The molar volume and excess partial molar volume at infinite dilution have also been calculated for both the mixtures.

Miscibility and phase separation in mixed erucylphosphocholine–DPPC monolayers

Binary Langmuir monolayers composed of 1,2-dipalmitoyl-sn-phosphatidylcholine (DPPC) and erucylphosphocholine (ErPC) – a new generation anti-cancer drug of phospholipids-like structure, have been studied with classical Langmuir technique complemented with Brewster angle microscopy (BAM) and Grazing Incidence X-ray Diffraction (GIXD). In the course of surface pressure (π)–area (A) isotherms for film containing mole fraction of ErPC (XErPC) equal to 0.1, 0.2 and 0.3, two collapses appear – the value of the first one is close to the collapse pressure observed for pure ErPC, while the second one occurs at the pressure similar to that for pure DPPC. Such a behavior could imply that the investigated system is immiscible – the expulsion of a component collapsing at a lower pressure (ErPC) from mixed monolayer occurs at the first transition while DPPC (collapsing at a higher pressure value) is expelled at the final collapse. To confirm this hypothesis further analysis has been performed. Namely, thermodynamic analysis based on the calculation of the excess free enthalpy of mixing (ΔGMexc) evidenced for mutual miscibility of DPPC and ErPC in the region of surface pressures below the first collapse, which has been additionally confirmed with BAM images. On the other hand, at high surface pressures, above the first collapse (50mN/m) GIXD experiments complemented with BAM images proved phase separation of the investigated lipids and 2D crystalline domains formation. The interpretation of the X-ray scattering results enabled us to propose a possible model of molecular packing, according to which only condensed domains of the 1:1 lipids ratio are periodically ordered; whereas at the other proportions the lipids are located randomly within the domains.

Activity coefficient at infinite dilution measurements for organic solutes (polar and non-polar) in fatty compounds – Part II: C18 fatty acids

In this work, activity coefficients at infinite dilution (γ13∞) have been measured for 21 solutes (subscript 1) (alkanes, cycloalkanes, alkenes, aromatic compounds, alcohols, esters, ketones, and halogenated hydrocarbons), in four solvents (subscript 3), namely one saturated fatty acid and three unsaturated fatty acids: stearic (octadecanoic) acid – C18:0, oleic (cis-9-octadecenoic) acid – C18:1 9c, linoleic (cis,cis-9,12-octadecadienoic) acid – C18:2 9c12c, and linolenic (cis,cis,cis-9,12,15-octadecatrienoic) acid – C18:3 9c12c15c, by gas–liquid chromatography. The measurements were carried out at temperatures from (303.13 to 368.19)K and the partial molar excess Gibbs free energy (ΔG1E,∞), enthalpy (ΔH1E,∞), and entropy (ΔS1E,∞), at infinite dilution were calculated from experimental γ13∞ values obtained over the temperature range. The uncertainties in determination of γ13∞ and ΔH1E,∞ are 4% and 20%, respectively. The results for stearic acid obtained in this study have been compared to those available in the Dortmund Data Bank (DDB). The real behaviour of fatty systems could be better understood through the results obtained in this work.

Activity coefficients at infinite dilution and physicochemical properties for organic solutes and water in the ionic liquid 1-(2-methoxyethyl)-1-methylpiperidinium trifluorotris(perfluoroethyl)phosphate

This work is continuation of our systematic study of activity coefficients at infinite dilution, γ∞ of different organic solutes and water in the ionic liquids. New data of γ∞ were determined for 62 solutes, including alkanes, alkenes, alkynes, cycloalkanes, aromatic hydrocarbons, alcohols, thiophene, ethers, ketones, esters, 1-nitropropane, butanal, acetonitrile, and water in the ionic liquid 1-(2-methoxyethyl)-1-methylpiperidinium trifluorotris(perfluoroethyl)phosphate by inverse gas chromatography at the temperature range from (318.15 to 368.15)K. The basic thermodynamic functions, namely partial molar excess Gibbs free energies, ΔG1E,∞, enthalpies, ΔH1E,∞ and entropies, ΔS1E,∞ at infinite dilution were calculated from the experimental γ∞ values obtained over the temperature range. Additionally the gas–liquid partition coefficients, KL were determined. Results are compared to previously investigated ionic liquids with the same cations or anions. Values of the selectivity and capacity at infinite dilution for heptane/benzene, heptane/thiophene, and heptane/methanol extraction problems were calculated from experimental γ∞ values to verify the possibility of investigated ionic liquid as an entrainer in liquid–liquid extraction.

Activity coefficient at infinite dilution measurements for organic solutes (polar and non-polar) in fatty compounds: Saturated fatty acids

The activity coefficients at infinite dilution , γ13∞ (the subscript 1 and 3 correspond to solute and solvent, respectively), for 21 solutes, including alkane, cycloalkane, alkene, aromatic compounds, alcohol, ester, ketone, and halogenated hydrocarbons, in four solvents, that are the saturated fatty acids: capric (decanoic) acid, lauric (dodecanoic) acid, myristic (tetradecanoic) acid, and palmitic (hexadecanoic) acid, were determined by gas–liquid chromatography at temperatures from 314.10K to 358.33K. Comparison with previously published for selected solutes in palmitic (hexadecanoic) acid were also performed. The values of the partial molar excess Gibbs free energy, ΔG13∞, enthalpy, ΔH13∞, and entropy, ΔS13∞, at infinite dilution were calculated from experimental γ13∞ values obtained over the temperature range. Results obtained in this work allow a more accurate description of the real behavior of fatty systems.

Activity coefficients at infinite dilution and physicochemical properties for organic solutes and water in the ionic liquid 1-(2-methoxyethyl)-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)-amide

The activity coefficients at infinite dilution, γ∞, gas–liquid partition coefficients, KL, the partial molar excess Gibbs free energy ΔG1E,∞, enthalpy ΔH1E,∞ and entropy ΔS1E,∞ at infinite dilution for 61 organic solutes and water in the ionic liquid 1-(2-methoxyethyl)-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)-amide were determined by inverse gas chromatography at the temperatures from (318.15 to 368.15)K. The values of the selectivity for selected compounds which form azeotropic mixtures were calculated from the γ∞ and compared to the literature values for other ionic liquids based on bis(trifluoromethylsulfonyl)-amide anion.

Measurements of activity coefficients at infinite dilution for organic solutes and water in the ionic liquid 1-ethyl-3-methylimidazolium methanesulfonate

The activity coefficients at infinite dilution, γ13∞, for 43 solutes, including alkanes, cycloalkanes, alkenes, alkynes, aromatic hydrocarbons, alcohols, water, thiophene, ethers, ketones, acetonitrile, pyridine, and 1-nitropropane in the ionic liquid 1-ethyl-3-methylimidazolium methanesulfonate [EMIM][MeSO3] were determined by gas–liquid chromatography at six temperatures within the range (308.15 to 358.15)K. The interaction of these 42 organic compounds and water with the ionic liquid were described with the partial molar excess Gibbs free energy, ΔG1E,∞, enthalpy ΔH1E,∞, and entropy TrefΔS1E,∞ at infinite dilution, calculated from the experimental γ13∞ values obtained over the temperature range. The gas–liquid partition coefficients, KL were calculated for all solutes. The values of the selectivity for few separation problems as heptane/toluene, cyclohexane/benzene, heptane/xylenes and heptane/thiophene were calculated from γ13∞ and compared to literature values for N-methyl-2-pyrrolidinone (NMP), sulfolane, and other ionic liquids based on [EMIM]+ cation. Comparison with the results published earlier for [EMIM][MeSO3] show much lower selectivities in the discussed separation problems. By contrast with the former measured ILs [EMIM][MeSO3] reveals the average selectivity for the separation of aromatics/aliphatics and low capacity for benzene. An interesting high value of selectivity was observed in the separation of thiophene/aliphatics. Very high values of selectivity may be expected in the separation of the azeotropic systems including water. The density of [EMIM][MeSO3] within temperature range from (298.15 to 358.15)K was measured and compared to the literature values.

Separating the Role of Protein Restraints and Local Metal-Site Interaction Chemistry in the Thermodynamics of a Zinc Finger Protein

We express the effective Hamiltonian of an ion-binding site in a protein as a combination of the Hamiltonian of the ion-bound site in vacuum and the restraints of the protein on the site. The protein restraints are described by the quadratic elastic network model. The Hamiltonian of the ion-bound site in vacuum is approximated as a generalized Hessian around the minimum energy configuration. The resultant of the two quadratic Hamiltonians is cast into a pure quadratic form. In the canonical ensemble, the quadratic nature of the resultant Hamiltonian allows us to express analytically the excess free energy, enthalpy, and entropy of ion binding to the protein. The analytical expressions allow us to separate the roles of the dynamic restraints imposed by the protein on the binding site and the temperature-independent chemical effects in metal-ligand coordination. For the consensus zinc-finger peptide, relative to the aqueous phase, the calculated free energy of exchanging Zn2+ with Fe2+, Co2+, Ni2+, and Cd2+ are in agreement with experiments. The predicted excess enthalpy of ion exchange between Zn2+ and Co2+ also agrees with the available experimental estimate. The free energy of applying the protein restraints reveals that relative to Zn2+, the Co2+, and Cd2+-site clusters are more destabilized by the protein restraints. This leads to an experimentally testable hypothesis that a tetrahedral metal binding site with minimal protein restraints will be less selective for Zn2+ over Co2+ and Cd2+ compared to a zinc finger peptide. No appreciable change is expected for Fe2+ and Ni2+. The framework presented here may prove useful in protein engineering to tune metal selectivity.