Showing papers in "Journal of Solution Chemistry in 1998"

Volumetric properties of some α-amino acids in aqueous guanidine hydrochloride at 5, 15, 25, and 35°C

Abstract: The infinite-dilution apparent molar volumesV 2φ o for glycine, DL-alanine, DL-α-amino-n-butyric acid, DL-valine, DL-leucine, and L-serine in 6 mol-kg−1 aqueous guanidine hydrochloride were determined at 5, 15, 25, and 35°C from precise density measurements. Using these data, the standard volumes of transfer, Δt V°, from water to 6m> aqueous guanidine hydrochloride solution were calculated. A linear relationship was found between V 2φ o and temperature. Both V 2φ o and Δt V° vary linearly with increasing number of carbon atoms in the alkyl chain of the amino acids. The results show that the apparent molar volumes at infinite dilution for (NH 3 + ,COO-) groups increase with increasing temperature and those for CH2 and the other alkyl chains are almost constant. These results also shows that guanidine hydrochloride has stronger interactions with amino acids than urea. These phenomena are discussed in terms of the cosphere overlap model.

Thermodynamic properties of binary mixtures of butanediols with water

Abstract: Ultrasonic velocities and densities at five different temperatures over the entire composition range for aqueous solutions of 1,2, 1,3, 1,4, and 2,3 butanediols were measured. Excess volumes and adiabatic compressibilities were calculated using the experimental data. Apparent and partial molar volumes and compressibilities were analyzed to evaluate the departure from ideal solution behavior. From the analysis of the results, the interactions of isomeric butanediols with water are discussed in terms of the placements of hydroxyl groups in the isomeric butanediol molecules.

Apparent Molar Volume, Heat Capacity, and Conductance of Lithium Bis (trifluoromethylsulfone)imide in Glymes and Other Aprotic Solvents

Abstract: Lithium bis(trifluoromethylsulfone)imide (LiTFSI) is a promising electrolyte for high-energy lithium batteries due to its high solubility in most solvents and electrochemical stability. To characterize this electrolyte in solution, its conductance and apparent molar volume and heat capacity were measured over a wide range of concentration in glymes, tetraethylsulfamide (TESA), acetonitrile, γ-butyrolactone, and propylene carbonate at 25°C and were compared with those of LiClO4 in the same solvents. The glymes or n(ethylene glycol) dimethyl ethers (nEGDME), which have the chemical structure CH3−O−(CH2−CH2−O) n −CH3 for n = 1 to 4, are particularly interesting since they are electrochemically stable, have a good redox window, and are analogs of the polyethylene oxides used in polymer-electrolyte batteries. TESA is a good plasticizer for polymer-electrolyte batteries. Whenever required, the following properties of the pure solvents were measured: compressibilities, expansibilities, temperature and pressure dependences of the dielectric constant, acceptor number, and donor number. These data were used in particular to calculate the limiting Debye-Huckel parameters for volumes and heat capacities. The infinite dilution properties of LiTFSI are quite similar to those of other lithium salts. At low concentrations, LiTFSI is strongly associated in the glymes and moderately associated in TESA. At intermediate concentrations, the thermodynamic data suggests that a stable solvate of LiTFSI in EGDME exists in the solution state. At high concentrations, the thermodynamic properties of the two lithium salts approach those of the molten salts. These salts have a reasonably high specific conductivity in most of the solvents. This suggests that the conductance of ions at high concentration in solvents of low dielectric constant is due to a charge transfer process rather than to the migration of free ions.

A New Understanding of the Relationship Between Solubility and Particle Size

Abstract: Most discussions of the relationships between crystal solubility and particle size have hitherto been concerned with vapor condensation and have led to the prediction that the vapor pressure increases with curvature. Here, thermodynamic arguments are presented to show that such relationships, describing crystal solubility as a function of particle size, originally put forward by Ostwald and later corrected by Freundlich, may be unjustified for determining interfacial tension at solid–liquid interfaces. The Kelvin or Gibbs–Thomson equations are valid for liquid–vapor systems, but not for solid–liquid interfaces. Recent experimental observations have demonstrated that interfacial tension data obtained by the solubility–size approach are unreasonable. This leads to the conclusion that “Ostwald ripening” may not be due to a higher solubility of smaller crystals, but rather to a net negative interfacial tension between solid and solution.

Volumetric Properties of N, N-Dimethylformamide with 1,2-Alkanediols at 20°C

Abstract: Dilatometric measurements of excess molar volumes and excess partial molar volumes have been made for binary mixtures of N, N-dimethylformamide with 1,2-ethanediol, 1,2-propanediol, 1,2-butanediol, 1,2- pentanediol, and 1,2-hexanediol at 20°C over the entire composition range. The results are explained in terms of dissociation of the self-associated 1,2-alkanediol molecules and the formation of aggregates between unlike molecules through C = O ... 3H-O hydrogen bonding. Further, the effects of difference in chain lengths and steric factors on molecular interactions are also examined. From the experimental results, excess molar volumes were calculated and correlated by a Redlich–Kister type function in terms of mole fractions.

Thermodynamic Data for Complex Formation Between Cucurbituril and Alkali and Alkaline Earth Cations in Aqueous Formic Acid Solution

Abstract: Complex formation, between alkali and alkaline earth cations and the macrocyclic ligands cucurbituril and 18-crown-6, was studied by calorimetric titration in aqueous formic acid solution (50 vol.%). The solubility of cucurbituril strongly depends on the concentration of formic acid. At low and very high formic acid concentrations, the solubility of cucurbituril is low. For both ligands the formation of proton complexes influence the complexation reactions with cations. Thus, the complex formation in most cases is favored by entropic contributions. Only in the case of the Ba2+ ion stability constants and reaction enthalpies could be measured at lower formic acid concentrations. Using a highly sensitive calorimeter, the stability constants and reaction enthalpies for the reaction of Na+ and Ba2+ with cucurbituril in aqueous solution could be determined.

Comprehensive Investigation of Yttrium and Rare Earth Element Complexation by Carbonate Ions Using ICP–Mass Spectrometry

Abstract: Carbonate stability constants for yttrium and all rare earth elements have been determined at 25°C and 0.70 molal ionic strength by solvent exchange and inductively coupled plasma–mass spectrometry (ICP–MS). Measured stability constants for the formation of $${\text{MCO}}_3^ +$$ and $${\text{M}}\left( {{\text{CO}}_{\text{3}} } \right)_2^--$$ from M3+ are in good agreement with previous direct measurements, which involved the use of radio-chemical techniques and trivalent ions of Y, Ce, Eu, Gd, Tb, and Yb. Direct ICP–MS measurements of $${\text{MCO}}_3^ +$$ and $${\text{M}}\left( {{\text{CO}}_{\text{3}} } \right)_2^--$$ formation constants are also in general agreement with modeled stability constants for the metals La, Pr, Nd, Sm, Dy, Ho, Er, Tm, and Lu, based on linear-free energy relationship (LFER). The experimental procedures developed in this work can be used for assessing the complexation behavior of other geochemically important ligands such as phosphate, sulfate, and fluoride.

Solubility and phase behavior of Cr(III) oxides in alkaline media at elevated temperatures

Abstract: A platinum-lined, flowing autoclave facility is used to investigate the solubility behavior of Cr2O3 and FeCr2O4 in alkaline sodium phosphate, sodium hydroxide, and ammonium hydroxide solutions between 21 and 288°C. Baseline Cr(III) ion solubilities were found to be on the order of 0.1 nmolal, which were enhanced by the formation of anionic hydroxo and phosphato complexes. At temperatures below 51°C, the activity of Cr(III) ions in aqueous solution is controlled by a Cr(OH)3·3H2O solid phase rather than Cr2O3; above 51°C the saturating solid phase is γ-CrOOH. Measured chromium solubilities were interpreted via a Cr(III) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria were obtained from least-squares analyses of the data. The existence of four new Cr(III) ion complexes is reported: Cr(OH)3(H2PO4)−, Cr(OH)3(HPO4)2−, Cr(OH)3(PO4)3−, and Cr(OH)4(HPO4)-(H2PO4)4−. The last species is the dominant Cr(III) ion complex in concentrated, alkaline phosphate solutions at elevated temperatures.

Viscosity, Surface Tension, and Refractive Index Measurements of Mixtures of Isomeric Butanediols with Water

Abstract: Solution properties of aqueous mixtures of isomeric butanediols have been investigated employing viscosity, surface tension, and index of refraction measurements as functions of temperature. The deviation of viscosity, surface tension, and molar refraction from ideal solution behavior is evaluated from the experimental data. The deviation from ideality is discussed in terms of molecular interactions between the components. Surface activity of the diols is evident from the surface tension measurements. It is found that the degree of hydrophobicity of the diols varies in the order 1,2 > 2,3 > 1,3 > 1,4. The strength of interaction of diols with the water varies in the order 2,3 > 1,4 ≃ 1,3 > 1,2.

Different Views on the Stability of Protein Conformations and Hydrophobic Effects

Abstract: Apparently contradictory statements about the thermodynamics of aqueous protein solutions and of hydrophobic effect are quoted and discussed Some credibility is found in the divergent points of view and it is pointed out that they focus attention on different aspects of the complicated conditions in aqueous solutions, some of which are more important than others for the stability of protein conformations

Measurement and Correlation of Viscosities, Densities, and Water Activities for the System Poly(propylene glycol) + MgSO4 + H2O at 25°C

Abstract: Viscosities, densities, and water activities for binary and ternary systems of poly(propylene glycol) 425 + H2O and poly(propylene glycol) 425 + MgSO4 + H2O have been measured at 25°C. From density and viscosity measurements, excess volumes and excess viscosities of the binary poly(propylene glycol) 425 + H2O, over the entire composition range, were obtained and correlated by means of a Redlich–Kister type equation. Viscosity, density. and water activity data for ternary system of poly(propylene glycol) 425 + MgSO4 + H2O were correlated by using a semiempirical equation.

Speed of Sound, Isentropic Compressibilities, and Excess Molar Volumes of Binary Mixtures Containing p-Dioxane

Abstract: The speed of sound u in and densities ρ of eight binary mixtures of p-dioxane (p-C4H8O2) with methylcyclohexane (c-C6H11CH3), 1-chlorohexane (C6H13C1), 1-bromohexane (C6H13Br), p-xylene [C6H4(CH3)2], propylbenzene (C6H5C3H7), methyl acetate CH3COOCH3), butyl acetate (CH3COOC4H9), and amyl acetate (CH3COOC5H11) were measured over the whole composition range at 30°C. Isentropic compressibilities (K S), Rao's molar sound functions (R), excess molar volumes (V E), excess isentropic compressibilities (K S E ) together with relative change in volume ΔV/φ1φ2 values, have been obtained for all measured mole fractions. The excess partial molar volume (V1-V 1 0 ) of p-dioxane in different solvents have also been estimated. The experimental results have been analyzed in terms of the Prigogine–Flory–Patterson theory of solutions.

Osmotic and Activity Coefficients of Nonaqueous Electrolyte Solutions. 1. Lithium Perchlorate in the Protic Solvents Methanol, Ethanol, and 2-Propanol

Abstract: Vapor pressure lowering by the addition of LiClO4 to the protic solvents methanol (0.04–5.1 m), ethanol (0.03–1.5 m), and 2-propanol (0.05–1.5 m) was measured at 25°C with high precision. The experimental data of the corresponding osmotic coefficients are compared to those obtained by the use of Pitzer equations and chemical model calculations. Mean activity coefficients are derived from the osmotic coefficients.

Conductance of selected alkali metal salts in aqueous binary mixtures of 2-methoxyethanol at 25°C

Abstract: Precise conductance measurements have been performed for lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, sodium perchlorate, and sodium tetraphenylborate in 2-methoxyethanol–water mixtures at four different mole fractions, i.e., 0.056, 0.136, 0.262, and 0.486 of 2-methoxyethanol (69.73 ≥ D ≥26.55) at 25°C in the concentration range 0.0004–0.0642 mol-dm−3. The limiting molar conductivity Λ°, the association constant KA, and the association distance R for the solvent mixtures have been evaluated from the conductance concentration data using the 1978 Fuoss conductance equation. The single-ion conductances have been estimated using the “reference electrolyte” tetrabutylam-monium tetraphynylborate(Bu4NBPh4). The analysis of the data indicates that for most salts ion association is appreciable in the solvent mixtures with a mole fraction of the cosolvent of 0.262 or higher. The results have been interpreted in terms of ion-solvent interactions and structural changes in the mixed solvent media.

Hydrogen Bonding of Carboxylic Acids in Aqueous Solutions—UV Spectroscopy, Viscosity, and Molecular Simulation of Acetic Acid

Abstract: The UV spectra of aqueous acetic acid solutions up to 2M were investigated. At these wavelengths, the carboxylic acids exhibit an absorption peak, attributed to the C=O group, which shifts when hydrogen bonds are formed.. The measured spectra were best fitted to several bands, either of Gaussian or Lorentzian shape, which can be explained as several types of structural units formed by hydrogen bonds established between acetic acid and water molecules and between acetic acid molecules themselves. Molecular dynamics simulation of these mixtures was also performed, confirming the occurrence of several types of hydrogen bonds and showing the presence of dimers at higher concentrations. The viscosity and density of these solutions were also measured at different concentrations and temperatures. These results give a more complete picture of the hydrogen bond network of the system.

Viscosities and Densities for the 1-Propanol + n-Heptane System at Several Temperatures

Abstract: Viscosities and densities have been measured for 1-propanol + n-heptane at 20, 25, 30, and 35°C and atmospheric pressure. Kinematic viscosities were determined using a capillary viscosimeter; densities were measured using vibrating-tube densimetry. The viscosity deviations were evaluated. Viscosity results were fitted to the equations of Grunberg–Nissan, McAllister, Auslander, and Teja. The experimental excess molar volumes were compared with the results obtained with the Nitta–Chao model.

Complex Permittivity Spectra of Binary Pyridine–Amide Mixtures Using Time–Domain Reflectometry

Abstract: Frequency spectra of the complex permittivity for pyridine–amide binary mixtures have been determined over the frequency range 10 MHz to 10 GHz, at 5, 15, 25, and 40°C, using the time–domain reflectometry method, for 11 compositions of each pyridine–amide system, e.g., formamide, N-methylformamide, and N,N-dimethylformamide. The relaxation in these systems can be described by a single relaxation time using the Debye model. The static dielectric constant, relaxation time, the corresponding excess dielectric properties, Kirkwood correlation factor, and molar activation energy of the mixtures have been determined. The excess permittivity is found to be positive in the amide-rich region and negative in the pyridine-rich region. The excess inverse relaxation time is negative, except in the pyridine-rich region. The static dielectric constants for the mixtures have been fitted with the modified Bruggeman model. The temperature-dependent relaxation times show the expected Arrhenius behavior.

Modeling of Thermodynamic Properties of Amino Acids and Peptides Using Additivity and HKF Theory

Abstract: Relative densities, \(\left( {\rho - \rho _o } \right)\), and heat capacity ratios, \([(c_p \rho /c_{p1}^o \rho _o ) - 1],\) of aqueous L-histidine, L-phenylalanine, L-tyrosine, L-tryptophan, and L-2,3-dihydroxyphenylalanine (L-dopa) have been measured at 15, 25, 40, and 55°C and 0.1 MPa. Apparent molar volumes, V2,Φ, apparent molar heat capacities, CP2,Φ, partial molar volumes at infinite dilution,\(V_2^o\), and partial molar heat capacities at infinite dilution, \(V_{p2}^o\), have been calculated from these measurements and compared to available literature values. The partial molar properties at infinite dilution for these systems have been added to those previously obtained for amino acids and peptides in water and the combined set used as input to a novel additivity analysis. The model we develop is based upon the equations of state of Helgeson, Kirkham, and Flowers (HKF) and has been constructed with additive parameters. The model may be used to predict thermodynamic properties of many aqueous biochemicals over an extended temperature range. Group contributions to the parameters in our model and effective Born coefficients are reported for 24 aqueous amino acid and peptide systems. Our results are compared to data previously published in the literature.

Excess, Partial, and Molar Volumes of n-Alkanes in Near-Critical and Supercritical Water

Abstract: Densities of solutions of n-pentane, n-hexane, n-heptane, and n-octane in near-critical and supercritical water were measured at pressures between 4 and 38 MPa and temperatures from 643.15 to 648.15 K over the entire composition range. The measurements were performed at three isotherms: 643.15, 647.05, and 648.15 K. A constant-volume piezometer was used to measure the PVTx data. The overall accuracy of the pressure, density, temperature, and mole fraction data are ±0.15%. ±0.5%, ±10mK and ±0.0002, respectively. From these results, excess and partial molar volumes were determined. The uncertainties of the derived results are given. Analysis of the results for dilute water + n-alkane mixtures showed that partial molar volume of n-alkane (solute) and excess molar volume of the mixture near the critical point of pure water (solvent) exhibit remarkable anomalies. The experimental values of molar volumes are compared with predicted values based upon scaling theory. Analysis of the results confirms the prediction of scaling theory that along the critical temperature and pressure of water the limiting partial molar volume of alkane as mole fraction x → 0 is proportional to x−γ/βδ, where γ/βδ ≍ 0.79. Our results contribute to understanding of supercritical solubility in near-critical fluids.

Partial Molar Volumes of Hydrophobic Compounds—Insight into the Solvation Shell? Part I

Abstract: A simple model for the interaction of the solute with the solvent is presented. The relation between the molecular volume, partial molar volume, and the volume of the solvation shell of the compound studied is expressed by the α parameter, defined as the relative density of the solvation shell. The values of α are calculated for many organic compounds in aqueous solutions. It is stated that in a given series of compounds, α depends strongly on the number of CH2 groups in the compound studied. The correlation of α and the polarity (defined as the ratio of the surface area of polar groups and atoms exposed to the total molecular surface area of the molecule studied) was found.

Kamlet-Taft Solvatochromic Parameters for 25 Glycol Ether Solvents and Glycol Ether Aqueous Solutions

Abstract: The Kamlet–Taft parameters for 25 glycol ethers and their aqueous solutions were measured. Values for the three Kamlet–Taft parameters: the hydrogen-bond donor ability, hydrogen-bond acceptor ability, and the dipolarity/polarizability, as well as the index of refraction, were determined for each pure glycol ether and each aqueous glycol solution. A correlation matrix between other known solvent parameters and the measured Kamlet–Taft values revealed only one correlation, suggesting that the three measured Kamlet–Taft parameters for the glycol ethers are independent solvent descriptors. Last, trends in the measured Kamlet–Taft values were related to functional group modifications to the basic glycol ether structure.

Complexation of Inorganic Anions and Small Organic Molecules with Alpha-Cyclodextrin in Water

Abstract: The complexation of esters, amides, carboxylic acids, and inorganic ions with alpha-cyclodextrin (α-CD) in aqueous medium was studied by calorimetry. Thermodynamic parameters for the formation of the host–guest complex have been determined. All these processes seem to be enthalpy driven. The entire ester molecule probably penetrates the α-CD cavity. The carboxylic acids hydrogen bond to the α-CD while the alkyl part of the acid enters the cavity. Amides undergo only a weak interaction with the host molecule α-CD. Larger anions interact more strongly with α-CD than do smaller anions; this may be, in part, due to the relative ease of desolvation of the larger anion. The cation seems to play some role in the complexation process with α-CD.

Solubility of Crystalline Calcium Isosaccharinate

Abstract: The solubility of calcium isosaccharinate Ca(ISA)2(c) was determined at 23°C as a function of pH (1–14) and calcium ion molality (0.03–0.52). The similarity of solubility from the over- and undersaturation directions for different equilibration periods indicated that equilibrium in these solutions was reached rapidly (< 7 days) and that these data can be used to develop thermodynamic equilibrium constants. The solubility data were interpreted using the Pitzer ion–interaction model. The logarithms of the thermodynamic equilibrium constants determined from these data were 1.30 for the dominant reaction at pH < 4.5 [Ca(ISA)2(c) + 2H+ ⇌ Ca2+ + 2HISA(aq)], and −2.22 for the dominant reaction at 4.5 [Ca(ISA)2(c)+ ⇌ Ca(ISA)2(aq)]. In addition, the logarithm of the dissociation constant of HISA [HISA(aq) ⇌ ISA- + H+] was calculated to be −4.46.

Thermodynamics of the Second Dissociation Constant and Standards for pH of 3-( N Morpholino) Propanesulfonic Acid (MOPS) Buffers from 5 to 55°C

Abstract: The second dissociation constant pK2 of 3-(N-morpholino)propanesulfonic acid (MOPS) has been determined at eight temperatures from 5 to 55°C by measurements of the emf of cells without liquid junction, utilizing hydrogen electrodes and silver–silver chloride electrodes. The pK2 has a value of 7.18 ± 0.001 at 25°C and 7.044 ± 0.002 at 37°C. The thermodynamic quantities ΔG°, ΔH°, ΔS°, and ΔC p o have been derived from the temperature coefficients of the pK 2. This buffer at ionic strength I = 0.16 mol-kg−1 close to that of blood serum, has been recommended as a useful secondary pH standard for measurements of physiological fluids. Five buffer solutions with the following compositions were prepared: (a) equimolal mixture of MOPS (0.05 mol-kg−1) + NaMOPS, (0.05 mol-kg−1); (b( MOPS (0.05 mol-kg−1) + NaMOPS (0.05 mol-kg−1) + NaCl (0.05 mol-kg−1); (c) MOPS (0.05 mol-kg−1) + NaMOPS (0.05 mol-kg−1); + NaCl (0.11mol-kg−1); (d) MOPS (0.08 mol-kg−1) + NaMOPS (0.08 mol-kg−1); and (e)MOPS (0.08 mol-kg−1) + NaMOPS (0.08 mol-kg−1) + NaCl (0.08 mol-kg−1).The pH values obtained by using the pH meter + glass electrode assembly are compared with those measured from a flow–junction calomel cell saturated with KCl (cell B), as well as those obtained from cell (A) without liquid junction at 25 and 37°C. The conventional values of the liquid junction potentials E j have been obtained at 25 and 37°C for the physiological phosphate reference solution as well as for the MOPS buffers (d) and (e) mentioned above.

Conductivity Studies on Aqueous Solutions of Stereoisomers of Tartaric Acids and Tartrates. Part III. Acidic Tartrates

Abstract: Conductivity measurements on aqueous solutions of sodium hydrogen tartrate and potassium hydrogen tartrate were performed in the temperature range 5–35°C. By including the dissociation equilibria, the experimental conductivities of the acidic salts can be well described using equivalent limiting conductivities of the bitartrate anion λ∞(HTar−) and the tartrate anion λ∞ (1/2 Tar2−) when applying the Quint and Viallard equations for unsymmetrical electrolytes.

Solvent effects on the electrochemical behavior of iron(III) Schiff base complex

Abstract: Electrochemical reduction of FeIIILCl where L is a Schiff base has been investigated in various aprotic solvents. From a plot of the half wave potential E1/2 for the reduction of these complexes vs. E1/2 for the oxidation of ferrocene, the solvent–solute interactions were studied: the E1/2 variation is found to be a function of Lewis-type acceptor–donor interactions. The diffusion coefficients D in the different solvents were also been determined by linear sweep voltammetry. The variation of D is discussed in terms of viscosity and dielectric constant.

Potentiometric investigation of the weak association of sodium and carbonate ions at 25°C

Abstract: The weak association between sodium and carbonate ions has been investigated at 25°C using high-precision sodium ion-selective electrode potentiometry in solutions of ionic strength ranging from 0.5 to 7.0 M in CsCl and in 1.0 M Me4NCl media. The protonation constants of CO 3 2- (aq) were also measured, using a H+-responsive glass electrode in 1.0 M Me4NCl and NaCl. The value of the ion-pair association constant calculated from the difference in the protonation constants in these two media was in excellent agreement with that obtained from the Na+ISE measurements. Evidence is also presented for the formation of extremely weak ion pairs between Na+ and HCO 3 - and between Cs+ and CO 3 2- .

Molecular interactions of macrocycles with dipeptides in aqueous solutions. Partial molar volumes and heat capacities of transfer of a chiral 18-crown-6 and of a calix[4]resorcinarene derivative from water to aqueous dipeptide solutions at 25°C

Abstract: Densities and specific heat capacities of ternary aqueous systems containing a dipeptide (alanyl-alanine, alanyl-glutamic acid, alanyl-serine or L-seryl-L-leucine) and a macrocycle (D-α-manno-naphtho-18-crown-6-ether or 2,8,14,20-tetrakis[-methyl (aminoformyl)]-4,6,10,12,16,18,22,24-octahydroxycalix[4]arene) were determined at 25°C by flow densimetry and flow calorimetry. The partial molar volume and heat capacity of transfer of a macrocycle from water to the dipeptide solution was determined as a function of the dipeptide concentration. Positive values for transfer volumes and transfer heat capacities are observed with all the solutions studied. With the crown ether, except for alanyl-glutamic acid where a 1:1 complex is clearly evidenced due to specific interactions of the side-chain functional group of the peptide with the crown ether, no stoichiometric complexes are confirmed and the partial molar quantities of transfer increase with the hydrophobic character of the dipeptide. Partial quantities of transfer are smaller with the calixarene than with the crown ether and stoichiometric complexes [calixarene]/[dipeptide] from 2:1 to 1:4 are evidenced, depending on the nature and the concentration of the dipeptide.

Molar Volume, Molar Refraction, and Isentropic Compressibility Changes of Mixing at 25°C for the System Ethanol + Methanol + Dibutyl Ether

Abstract: The densities, refractive indexes, and sound velocities for mixtures of ethanol + methanol + dibutyl ether at 25°C and atmospheric pressure, were determined and used to calculate molar volumes, molar refractions, and isentropic compressibilities. The excess molar volumes and the deviations of molar refractions and isentropic compressibilities from mole fraction and volume fraction averages, respectively, of these properties of the pure components were satisfactorily correlated with the composition data by means of the Redlich–Kister polynomial.

Molar Volumes and Heat Capacities of Electrolytes and Ions in Nonaqueous Solvents: 1. Formamide

Abstract: Apparent molar volumes and heat capacities of 27 electrolytes have been measured as a function of concentration in formamide at 25°C using a series-connected flow densimeter and Picker calorimeter system. These data were extrapolated to infinite dilution using the appropriate Debye–Huckel limiting slopes to give the corresponding standard partial molar quantities. Ionic volumes and heat capacities at infinite dilution were obtained by an appropriate assumption based on the reference electrolyte Ph4PBPh4 (TPTB). The ionic volumes, but not the heat capacities, agree reasonably well with previously published statistically based predictions. The values obtained are discussed in terms of simple models of electrolyte solution behavior and a number of interesting features are noted, including, possible dependencies of ionic volumes on solvent isothermal compressibility and of ionic heat capacities on solvent electron acceptor abilities.