Enthalpies Of Dilution Research Articles

Enthalpies of dilution of some aqueous transition metal sulfate solutions at 25°C

Enthalpies of dilution of aqueous solutions of the transition metal sulfates CoSO4, MnSO4, and NiSO4, were measured from 1.5 to 0.2 mol-kg−1 at 25°C. The apparent molal enthalpy equations of Pitzer were fit to the resulting data and the parameters for these equations presented.

Calorimetric, Viscosimetric, and Light Scattering Studies of the Aggregation of, and the Solubilization of Water by, Triton X-35 inn-Heptane

The behavior of a nonionic surfactant TX-35 in solution inn-heptane in the presence and absence of added water has been examined using the microcalorimetric, viscosimetric, and quasielastic light scattering experimental methods. In this paper, we were interested in the aggregation process of the poly(oxyethylene) glycol alkylphenyl ether inn-heptane and in the solubilization of water in the reverse micelle of the surfactant (micellar solubilization). The analysis of the differential molar enthalpies of dilution of TX-35 in driedn-heptane has shown the occurrence of a gradual exothermic aggregation process on a very wide range of concentration which takes place at particular concentration so-called “operational CMC”. This operational CMC value has been confirmed by viscosities measurements. The differential molar enthalpies of hydration of TX-35 were also measured and found to be exothermic. The maximal hydration ratio (w0) was found to be equal to 3.2 mol H2O per mole of TX-35 before the point of phase separation. The measurements of the variation of the amount of water contained in TX-35 solutions at different concentrations inn-heptane also show the occurence of a gradual aggregation process and confirm the value of the maximal hydration ratio already determined by microcalorimetry. In the absence of added water, from the quasielastic light scattering experiments, a mean diameter of the aggregates close to 45 Å has been determine, while in the presence of water, a mean diameter of 61 Å was detected and remained unchanged with increasing the hydration ratio indicating that the size of the aggregate is more influenced by the presence of water than by the amount. In the presence of water, it is relevant to discuss aggregates of lamellar or filament shape.

Chiral recognition in solution. Interactions of α-amino acids in concentrated aqueous solutions of urea or ethanol

Enthalpies of dilution of the L and D forms of glutamine, citrulline, and phenylalanine in concentrated aqueous solutions of urea or ethanol were measured calorimetrically at 298 K. Glycine, urea, formamide, and phenol were also studied under the same experimental conditions, to get information about the behaviour of the zwitterion and of the functional group in the side chain of the cited amino acids when the concentration of the cosolvent changes. The derived pairwise enthalpic interaction coefficients for the three amino acids were rationalized according to the preferential configuration model. Indications are that, in concentrated urea, the coefficients for citrulline and glutamine are determined mainly by the interactions between the cosolvent and the hydrophilic groups in the molecule of the amino acids. For phenylalanine, coefficients are less positive than in water, because the presence of urea, which solvates preferentially the zwitterions, attenuates hydrophobic interactions between the benzene rings. In ethanol, coefficients for the three amino acid become negative or more negative than in water, because in this medium hydrophilic interactions are enhanced. Chiral recognition, namely the difference in the values of homo- and heterochiral interaction coefficients, was detected only for phenylalanine in urea. Hence, the nature of the cosolvent, influencing differently hydrophilic and hydrophobic interactions, can lead to the detection of chiral recognition also for those systems that, as phenylalanine, do not present this effect in pure water.Key words: α-amino acids, excess functions, molecular interactions, preferential configuration.

Proximity effects and cyclization in oxaalkanes+CCl 4 mixtures DISQUAC characterization of the Cl–O interactions. Comparison with Dortmund UNIFAC results

Thermodynamic properties, vapor–liquid equilibria (VLE), molar excess Gibbs energies ( G E), molar excess enthalpies ( H E) and natural logarithms of activity coefficients at infinite dilution (ln γ i ∞) or partial molar excess enthalpies at infinite dilution ( H i E,∞) of mixtures of oxaalkanes, linear or cyclic monoethers, diethers or acetals, and CCl 4 are studied in the framework of DISQUAC. The oxygen/CCl 4 contacts are characterized by dispersive (DIS) and quasichemical (QUAC) interaction parameters, which are reported. Contacts of the type (polar group)/CCl 4 are usually characterized by DIS parameters only. The effects of proximity and cyclization on the interchange coefficients are examined. In comparison with systems of oxaalkanes and n-alkanes, some differences exist; e.g., linear monoethers and diethers+CCl 4 mixtures are represented by different interaction parameters due to proximity effects of oxygen atoms (i.e., –O–C–C–O–) in diethers. In solutions with cyclic molecules, ring strain seems to be now more important. DISQUAC results are compared with those obtained using the Dortmund version of UNIFAC. From this comparison, it is concluded that it is necessary to distinguish at least between monoethers, diethers and acetals when treating mixtures with oxaalkanes and that each cyclic molecule should be characterized by its own set of interaction parameters.

Simultaneous determination of solubility, dissolution and dilution enthalpies of a substance from a single calorimetric experiment

The thermodynamics of dissolution in water of a set of substances has been studied calorimetrically. The examined substances were: potassium chloride, (glycyl-glycyl)diketopiperazine, (alanyl-alanyl)diketopiperazine, (leucyl-glycyl)diketopiperazine. They were chosen on the basis of their solubilities, going from a highly soluble electrolyte to the sparingly soluble diketopiperazines. It is shown that, using a commercially available calorimeter, it is possible to perform in a single calorimetric experiment the simultaneous determination of all thermodynamic parameters characterizing dissolution of a substance in a given solvent, i.e. solubility, dissolution enthalpy and dilution enthalpy. The solubility values in water obtained through the proposed method are in good agreement with those reported in the literature and obtained by other techniques.

Thermodynamic Properties of Aqueous Magnesium Chloride Solutions From 250 to 600 K and to 100 MPa

A new general model that describes the thermodynamic properties of MgCl2(aq) has been developed from a global fit to experimental results, including isopiestic molalities, vapor pressure measurements, freezing-point depressions, enthalpies of dilution, heat capacities, and densities, for this system. The model is based on a recent ion-interaction treatment with extended higher-order virial terms, and on experimental results from 240 to 627 K at pressures to 100 MPa and molalities to 25 mol⋅kg−1.

Determination of the entropy change for electrode reaction and dilute enthalpy of some ions by thermo-eletrochemical technology

Based on thermo-electrochemical equation for an electrode reaction, the entropy change of it can be obtained by the thermo - electrochemical technology (TECT). The entropy changes of Fe (CN) 6 −3 /Fe (CN) 6 −4 , H+/H2(P0), Cu+2/Cu and Zn+2/Zn electrode reaction systems and the dilute enthalpies of the H+, Cu2+ and Zn2+ ions under the ion concentrations studied have been determined by a specially designed thermo-electrochemical equipment. The enthalpy change and entropy change for the five systems at unlimitedly diluted concentrations agree well with the literature.

The System NaCl-CaCl 2-H 2O: Part 3. Heats of Dilution and Mixing at 373 to 573 K and 21.5 MPa Using a New High-Temperature, Flow-Through Calorimeter

New heat of mixing and heat of dilution data are presented for the system {NaCl+CaCl 2}(aq) at 373, 423, 473, 523, and 573 K and 21.5 MPa. At each temperature heat of mixing measurements were made over an ionic strength range between 0.5 and 4.0 mol · kg −1 H 2O and a constant ionic strength fraction of 0.5. Heats of dilution of CaCl 2(aq) were measured at each temperature over the ionic strength range of 0.3–18 mol/kg H 2O. All measurements were made using heats of dilution of NaCl(aq) to calibrate the device. CaCl 2 dilution enthalpies compare favorably with the available published data to 523 K. Mixing enthalpies at these temperatures have not been previously measured. While dilution enthalpies for the aqueous salt binaries in this system are exothermic, mixing enthalpies are endothermic. At fixed ionic strength and pressure, mixing enthalpies in this system have a roughly quadratic dependence on temperature with a broad minimum between 373 and 473 K. The data have been fit within the Pitzer ion-interaction formalism, incorporating published ion-interaction parameters for the aqueous salt binaries. In order to reproduce the data within their accuracy, the model must include the mixing parameters Φ NaCa L and Ψ NaCaCl L.

The role of preferential interactions between similar domains in determining the behavior of aqueous solutions of aminoalkanols. A microcalorimetric study

Enthalpies of dilution of binary aqueous solutions containing 1 mol kg −1 NaOH as cosolvent and the homologous series of α,ω-aminoalkanols and 2-amino-1-alkanols as solutes have been determined by flow microcalorimetry at 298 K. Enthalpic self and cross interaction coefficients of the virial expansion of the excess enthalpies were evaluated. These coefficients are positive and increase at increasing alkyl chain length. The difference between the coefficients of positional isomers of aminoalkanols resembles that shown by the corresponding diols in water. The cross interaction coefficient relative to the ternary system containing hexane-1,6-diamine and hexane-1,6-diol is smaller than those relative to the respective binary solutions. The coefficients were interpreted using the “preferential configuration” model: it is inferred that interactions between like hydrophilic groups are preferred when compared to the like–unlike ones.

ChemInform Abstract: Enthalpy of Dilution in Aqueous Systems of Single Solutes Ammonia, Sodium Sulfate and Ammonium Sulfate: Experimental Results and Modeling.

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Acid-based properties of silicas modified by organic compounds as determined by inverse gas chromatography

The results of silica surface modification by selected modifiers were examined by inverse gas chromatography (IGC). Experiments were carried out at infinite dilution and Gibbs free energies and enthalpies of specific interactions were determined according to Papirer's and Dong-Donnet's approaches, KA and KD values, calculated from enthalpies of specific interactions, as well as their ratio Sc = KD/KA, were used for characterization of the examined surfaces. The influence of the modifier type and its amount upon the physicochemical parameters investigated is presented and discussed.

Application of the Bjerrum association model to electrolyte solutions. II. Enthalpies of dilution in water and nonaqueous solvents

The association theory based on the Bjerrum model, which had been developed for the treatment of apparent and partial molar volumes of electrolyte solutions, is extended to apparent molar relative enthalpies L2,φ. Experimental enthalpies of dilution for tetrabutylammonium bromide in acetonitrile, propylene carbonate, and γ-butyrolactone and for lithium perchlorate and sodium thiocyanate in acetonitrile were obtained and analyzed with this model. Literature data for various electrolytes in water, acetonitrile, and n-propanol were also reanalyzed. Through the Bjerrum equations, enthalpies of dilution can be extrapolated to infinite dilution and reliable L2,φ obtained for associated electrolytes. The model can be used to estimate the association constant KA of the electrolyte and these KA are compared with literature values (generally obtained from conductivity). Considering the difference in the concentration ranges investigated in L2,φ and Λ measurements, KA extracted from L2,φ generally fall within an expected range of deviation from the ones obtained by conductivity, provided that no specific interactions are present in solution.

Enthalpies of Solution and Dilution of Some L-α-Amino Acids in Water at 298.15 K

The enthalpies of solution and dilution of glycine, L-α-alanine, L-valine, L-α-leucine, L-α-serine and L-α-threonine in water were measured at 298.15 K. The results were applied to calculate the enthalpic coefficients of homogeneous interactions between zwitterions of L-α-amino acids in aqueous solutions.

Enthalpies of Dilution for Aqueous Solutions of 3:1 Electrolytes: Variation with Temperature

The enthalpies of dilution of aqueous solutions of [Co(en)3]Cl3 and [Co(tn)3Cl3 (where en=1,2-diaminoethane, and tn=1,3-diaminopropane) were measured at 278.15 K up to 1 mol kg-1, using a large isoperibolic calorimeter, with the 'long-jump' method. Relative apparent molar enthalpies, LΦ>, were extracted via an empirical equation relating LΦ> and molality. These new values were compared with previous results at 298.15 K on the same aqueous systems. Theoretical predictions were satisfied, but an unexpectedly broad divergence was found for such close temperatures. A tentative explanation is put forward.

Heats of Dilution, Heat Capacities and Apparent Molal Enthalpies of Li2B4O7-Li2SO4-LiCl-H2O Systems at 298.15 K

The heats of dilution and heat capacities of the Li2B4O7-Li2SO4-LiCl-H2O quaternary system and the Li2B4O7-Li2SO4-H2O and Li2B4O7-LiCl-H2O ternary systems were measured in the range of ionic strengths from 19 to 0.1 at 298.15 K. The data of the heat of dilution were extrapolated to infinite dilution by use of Debye-Huckel limiting law to obtain relative apparent molal enthalpies over the range of ionic strengths I from 19 to 0.0001.

UNIFAC prediction of infinite dilution activity coefficients and enthalpies of mixing of chlorophenols in aqueous and 1-octane-ol solutions

Modelling of thermodynamic properties of chlorinated phenols in water and in 1-octane-ol is presented. Activity coefficients at infinite dilution and enthalpies of mixing were considered. The UNIFAC method as presented by Gmehling et al. [J. Gmehling, J. Li, M. Schiller, A modified UNIFAC model: 2. Present parameter matrix and results for different thermodynamic properties, Ind. Eng. Chem. Res. 32 (1993) 178–193] and Larsen et al. [B.L. Larsen, P. Rasmussen, A. Fredenslund, A modified UNIFAC group-contribution model for prediction of phase equilibria and heats of mixing, Ind. Eng. Chem. Res. 26 (1987) 2274–2286] was used. A method combining the group contribution approach with correlation based on physical descriptors yields results within experimental error.

Calorimetric investigation of water/lecithin/cyclohexane microemulsions

The enthalpies of dilution of water/lecithin/cyclohexane microemulsion-gels at variousR values (R=[water]/[lecithin]) and molar enthalpies of solution of water in lecithin/cyclohexane at a fixed lecithin concentration were determined calorimetrically at 25°C. Through a description of the process of dilution of water/lecithin/cyclohexane microemulsion-gels as one involving mainly the scission of lecithin reversed micelles in to smaller ones, the concentration dependence of the enthalpy was rationalized. Surprisingly, in order to account for the dilution enthalpies, it was not necessary to hypothesize a thermal effect arising from the breakage of the micellar network present in the micremulsion-gels. This result indicates that the building-up of the micellar network mainly arises from topological entanglements and steric hindrances among the lecithin reversed micelles. Attribution of the molar enthalpy of solution of water essentially to the thermal effect arising from the intermicellar aggregation process induced by the water addition allowed a rough estimation of the scission energy (the energy necessary to break an Avogadro's number of micelles) and of the micellar aggregation number.

The enthalpic stabilization on molecular inclusion of butanediol isomers into cyclodextrin cavities

The enthalpy of dilution of dilute aqueous solution of meso-2,3-butanediol, x<2×10 −3, and enthalpy of inclusion of meso-2,3-butanediol into α- and β-cyclodextrin cavities in dilute aqueous solutions have been determined by using a rocking twin-microcalorimeter of a heat-conduction type at 298.15 K under atmospheric pressure. The enthalpies of inclusion of meso-2,3-butanediol into α- and β-cyclodextrins are exothermic and smallest among three isomers, 1,3-, 1,4- and meso-2,3-butanediols. The correlation between behaviors and molecular shapes are discussed.

Enthalpy of dilution in aqueous systems of single solutes ammonia, sodium sulfate and ammonium sulfate: Experimental results and modeling

The enthalpy of dilution in the sodium sulfate-water, ammonium sulfate-water and ammonia-water systems was measured at temperatures from 313 to 373 K with a Calvet-type batch calorimeter using specially designed mixing cells. The mixing cells allow to pressurize the solutions. The formation of a gas phase during the experiments is thus avoided. Experimental results for the enthalpy of dilution are reported and compared to literature data and correlations.

Thermodynamics ofN,N,N-octylpentyldimethyl-ammonium chloride in water-urea mixtures

Specific conductivities, densities, heat capacities, and enthalpies of dilution at 25‡C were measured forN,N,N-octylpentyldimethylammonium chloride (OPAC) in water-urea mixtures at various urea concentrations mu as functions of the surfactant concentration ms. From conductivity data, the cmc and the degree of the counterion dissociation Β of the OPAC micelles were calculated. The cmc increases linearly with increasingm u while Βvs. mu is a smooth concave curve. From the experimental thermodynamic data, the apparentY Φ and partialY 2 molar properties (volumes, heat capacities, and relative enthalpies) are derived as functions of mu andm s . The effect of urea on the dependences of the different properties on ms are discussed. From data in the premicellar region the standard partial molar volumesV 2 0 and heat capacitiesC p2 0 were evaluated. It was observed thatV 2 0 increases linearly withm u whileC p2 0 decreases. The properties of OPAC in the dispersed and micellized forms at the cmc were obtained and, therefore, the thermodynamic functions of micellization were calculated on the basis of the pseudo-phase transition model.