A.H. Roux

Limiting Partial Molar Excess Enthalpies by Flow Calorimetry: Some Organic Solvents in Water

A Picker flow microcalorimeter was employed in conjunction with asymmetric syringe-type pumps to measure heats of mixing of highly dilute aqueous solutions of organic solvents. These data were used in turn to determine limiting partial molar excess enthalpies of the examined solvents in water. The measurements were carried out at 298.15 K for 29 common, oxygen and/or nitrogen containing solvents exhibiting complete miscibility with water. Except for only one compound, formamide, the limiting partial molar excess enthalpies are exothermic indicating that the process of dissolution is energetically favored. Comparison to literature data (in most cases to solution enthalpies at infinite dilution measured by batch calorimetry) proved the technique applied to be sufficiently accurate.

Limiting Partial Molar Excess Heat Capacities and Volumes of Selected Organic Compounds in Water at 25°C

Well-known Picker flow microcalorimeters for the differential measurements of volumetric heat capacities have been employed in conjunction with vibrating tube densimeters to determine the molar heat capacity, volume, and the apparent properties in dilute aqueous solutions for 17 organic solutes of moderate hydrophobicity. The dependence on concentration of the apparent properties allowed the limiting partial molar quantities at infinite dilution to be extrapolated and the limiting partial molar excess quantities to be evaluated. Comparison with available literature data shows good agreement. The application of group contribution rules to the limiting partial properties has been tested using the original method and parameters proposed by Cabani et al. The predicted values of the partial molar volumes are in fair agreement with the present data except for some less common solutes. With partial molar heat capacities, the agreement is less satisfactory. To improve the performance of the method, missing parameters for some types of monofunctional and bifunctional molecules have been evaluated.

Molar heat capacities and volumes of transfer of cytosine, thymine, caffeine and 1,3-diethylthymine to aqueous solutions of glycyl-glycine and L-α-alanyl-L-α-alanine at 25°C

Densities and specific heat capacities of ternary aqueous systems containing dipeptides (glycyl-glycine or L-α-alanyl-L-α-alanine) and nucleic acid bases (cytosine or thymine) or their alkyl derivatives (1,3-diethylthymine or caffeine) were determined at 25°C by flow calorimetry and flow densimetry. The partial molar volumes and heat capacities of transfer at infinite dilution of the different nucleic acid bases from water to water+dipeptide solutions were obtained therefrom. Except for the case of the transfer of cytosine to aqueous glycyl-glycine solutions where a small positive dependence of the transfer quantities was observed with the dipeptide concentration, the values of the heat capacities of transfer were in general low, positive or negative, depending on the compensation of hydrophobic-hydrophilic interactions between the dipeptide and the base. The volumes of transfer of most of the bases are very small, within the limit of the experimental error.

Excess molar heat capacities and enthalpies for 1-alkanol + n-alkane binary mixtures. New measurements and recommended data

Abstract Excess molar heat capacities at constant atmsopheric pressure C p E have been determined at 298.15 K as a function of the alcohol mole fraction x, for the following binary mixtures : ethanol + n-hexadecane, 1-butanol + n-decane, and 1-hexanol + n-hexane. The instrument used was a Picker flow calorimeter. The densities p for these mixtures at the same pressure and temperature were measured with a vibrating-tube densimeter ; the corresponding molar excess volumes V E were calculated therefrom. For the same mixtures as well as for the two other mixtures methanol + n-heptane and ethanol + n-heptane a complete literature search has yielded a selection of both the excess molar heat capacities and enthalpies H E . Several selected sets of data have been critically analyzed in terms of quality of measurements and precision. The best sets of data have been combined together and, also whenever possible, with our present experimental data to produce recommended data sets of the two calorimetric properties for the five binary mixtures, then considered as key-systems. In the course of this work particular attention was paid to keep data points in the whole concentration range especially in the dilute regions. Finally for each above mixture the recommended values of C E p and H E have been fitted accordingly with the same type of smoothing equation.

Apparent molar heat capacities and volumes of some alkylated derivatives of uracil and adenine in aqueous solution at 25°C

Densities and apparent molar heat capacities of some alkylated derivatives of uracil and adenine: 1-methyluracil, 1,3-dimethyluracil, 1,3-diethylthymine, 5,6-trimethylene-1,3-dimethyluracil, 5,6-tetramethylene-1,3-dimethyluracil, 5,6-pentamethylene-1,3-dimethyluracil, 2,9-dimethyladenine, 2-ethyl-9-methyladenine, 2-propyl-9-methyladenine, 8-ethyl-9-methyladenine, 6,8,9-trimethyladenine and 8-ethyl-6,9-dimethyladenine were determined using flow calorimetry and flow densimetry at 25°C. It was found that the partial molar volumes and heat capacities correlate linearly with the number of substituted methylene groups-CH2-as well as to the number of hydrogen atoms, nH, belonging to the skeleton of the molecule. In the case of alkylated uracils a difference was observed in the values at infinite dilution V2o and Cp2o, depending on the substitution of alkyl and cyclooligomethylene groups.

Apparent molar heat capacities and volumes of electrolytes and ions in acetonitrile-water mixtures

Apparent molar volumes Vφ and heat capacities Cp,φ of NaCl, KCl, KNO3, AgNO3, KI, NaBPh4 and Ph4PCl have been measured in acetonitrile (AN)-water mixtures up to xAN=0.25 by flow densitometry and flow microcalorimetry. Limited data have also been obtained for NaF, LiCl and KBr up to xAN=0.15. Single ion volumes and heat capacities of transfer were obtained using the assumption ΔtXφ(PH4P+) = ΔtXφ(BPh4-) where X=V or Cp and ΔtXφ is the change in Xφ for a species on transfer from H2O to AN-H2O mixtures. Volumes and heat capacities for simple salts show relatively little dependence on solvent composition. However, ΔtXφ for simple ions show more pronounced variations, exhibiting at least one extremum. These extrema are similar to but much less pronounced than those derived previously for ions in t-butanol-water mixtures. Surprisingly little correlation is found between the present data and other thermodynamic transfer functions. This is attributed to the predominance of ion-solvent over solvent-solvent interactions in AN-H2O solutions. ΔtVφ and ΔtCp,φ for the silver ion differ markedly from those of the alkali metal ions as a result of the well-known specific interaction between Ag+ and AN.

Apparent molar heat capacities and volumes of electrolytes and ions int-butanol-water mixtures

The apparent molar volumes Vϕ and heat capacities Cp,ϕ of NaCl, LiCl, NaF, KI, NaBPh4 and Ph4PCl have been determined in solutions of H2O containing up to 40 mass% t-butyl alcohol (TBA) by flow densitometry and flow microcalorimetry. Combination of these results with literature data allows calculation of Vϕ and Cp,ϕ for 16 ions in these mixtures using the assumption that ΔtXφ(Ph4P+) = ΔtXφ(BPh4−) where X=V or Cp and ΔtXφ is the change in Xϕ for a species on transfer from H2O to TBA-H2O mixtures. These are the first reported single ion values for Cp,ϕ in a mixed solvent. While whole electrolyte volumes and heat capacities show relatively smooth changes with solvent composition, ΔtXφ(ion) exhibit two well-developed extrema at around 10 and 25 mass% TBA. The shape of the ΔtXφ(ion) curves shows considerable uniformity among the alkali metal cations and the halide ions but the extrema become more pronounced with increasing size among the tetraalkylammonium ions. These extrema are analogous to those observed in aqueous organic mixtures of surfactants and are probably indicative of microphase transitions in these strongly interacting solvent mixtures.

Thermodynamics of aqueous mixtures of 2-(2-Hexyloxyethoxy)ethanol at 5°C

The densities and the ultrasonic speeds of the aqueous solutions of 2-(2-hexyloxyethoxy)ethanol (C6E2) were measured over the entire range of mole fractions at 5°C. Excess molar volumes VE were readily calculated from the densities. The densities, in combination with the ultrasonic speeds, furnish estimates of the molar (and excess molar) isentropic compressibilities KS and the deviations uD of the ultrasonic speeds from the values calculated for ideal mixtures. Radical changes in the mole fraction derivatives of the excess molar properties of the (C6E2 + water) system, in the vicinity of an amphiphile mole fraction of 0.003, indicate that C6E2 like C6E3 is capable of micelle formation. Our data have been compared with those reported earlier for (C4E2 +, C2E2 +, and C6E3 + water). We have employed both mass action and pseudophase approaches to data analysis, together with the four-segment model approach.

Thermobarometric and differential scanning calorimetric study of the polymorphism of some even and odd paraffins (C26, C27, C40, C60)

Abstract Thermal and thermobarometric analysis of three n-paraffins having an even number of carbon atoms (C26, C40 and C60) and of one paraffin having an odd number (C27) are reported. The two techniques used yield consistent results in agreement with high-pressure literature data. The thermobarometric technique allows one to detect low-enthalpy or weak transitions, especially in the case of the short paraffins exhibiting more complex phase diagrams. It appears that the stability domain with respect to pressure and temperature of the rotative phase, which is stable in the fusion vicinity for paraffins, decreases when the pressure and/or the chain length increases.

THERMOCHEMISTRY OF AQUEOUS SOLUTIONS OF ALKYLATED NUCLEIC ACID BASES. Part VII. Apparent molar heat capacities and volumes of aqueous solutions of alkylated derivatives of uracil at 298.15 K

Abstract Densities and heat capacities of some alkylated derivatives of uracil (1,3,6-tnmethyluracil, 1,3-dimethyl-6-ethyluracil, 1,3-dimethyl-6-propyluracil and 1,3-dimethyl-6-butyluracil) in dilute aqueous solutions are measured using flow calonmetry and flow densimetry at 298.15 K. Apparent molar volumes and heat capacities were derived and analyzed as a function of concentration.