No abstract

The 36 Ar- 40 Ar vapor pressure isotope effect in the liquid and solid phases has been experimentally determined at about 100 temperatures between 83 and 97 K using a high-accuracy double-differential manometric technique. Differences between the vapor pressure of a highly enriched sample of 36 Ar and that of natural argon were measured simultaneously with the absolute vapor pressure of natural argon. Absolute and differential vapor pressure measurements show an estimated accuracy of 0.15% and 0.001%, respectively, with respect to the absolute vapor pressure of argon. Data were obtained in both the solid and liquid regions. The vapor pressure of 36 Ar is always higher than that of 40 Ar by about 0.5%. Triple-point temperatures and pressures were also measured for both isotopes. All data compare favorably with previous results reported in the literature and are nicely interpreted within the framework of Bigeleisen's theory of isotope effects. Using a large temperature extrapolation of our data, liquid-vapor isotope fractionation factors were successfully inferred from the vapor pressure measurements; the estimated values agree with the experimental ones up to close to the critical point. The isotopic difference in the molar enthalpies of vaporization and sublimation was also calculated from the vapor pressure data. Additionally, molar volume and second virial coefficient isotope effects are estimated using several different theoretical approaches.

At the highest metrological level, natural gas standards are commonly prepared gravimetrically as PSMs (primary standard mixtures). This international key comparison is a repeat of CCQM-K1e-g. The mixtures concerned contain nitrogen, carbon dioxide and the alkanes up to butane. The only difference with CCQM-K1e-g is the addition of iso-butane to the list. The results usually agree within 1% (or better) with the key comparison reference value. For ethane, nitrogen and carbon dioxide, the agreement is within 0.5% (or better), and for methane within 0.1% (or better) of the KCRV.Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

No abstract

Densities and ultrasound speeds were determined in aqueous binary mixtures of 2-diethylaminoethanol over the whole composition range at intervals of 5 K in the temperature range between 283 and 303 K. Thermal expansibility effects on this amphiphile/water mixture are analysed in terms of excess molar isobaric expansions E E P;m for the mixture and of excess apparent molar isobaric expansions E E P;f;i for both chemical substances in the mixture. Different strategies are used and discussed for obtaining limiting (infinite dilution) excess partial molar isobaric expansions. Compressibility effects are described in terms of excess molar isentropic compressions K E S;m and excess partial molar isentropic compressions K E S;i . The latter properties are analytically calculated from the fit of experimental K E S;m data to a Redlich-Kister equation. A method based on this equation yields limiting excess partial molar isentropic compressions. Additionally, excess ultrasound speeds u E are also examined. All these excess properties are referred to a thermodynamically defined ideal liquid mixture. Interesting insights into the mixing process are gained from the visual impact of plots showing the composition and temperature dependence of different excess molar thermodynamic properties. Comparison of expansibility-and compressibility-related quantities shows that these two types of thermodynamic properties probe different aspects of intermolecular and packing effects on the process of mixing amphiphiles and water.y Electronic supplementary information (ESI) available: Tables S1-S4. See

At the highest metrological level, natural gas standards are commonly prepared gravimetrically as PSMs (Primary Standard Mixtures). This international key comparison is a repeat of CCQM-K1e-g. The mixtures concerned contain nitrogen, carbon dioxide and the alkanes up to butane. The only difference from CCQM-K1e-g is the addition of iso-butane to the list. The agreement of the results in this key comparison is very good. For all parameters, with a few exceptions, the results agree within 0.5% (or better) with the key comparison reference value. For methane, the results are generally within 0.1% (or better) of the KCRV.Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

No abstract

Positive excess molar Gibbs energies were detected in liquid mixtures of ( 36 Arϩ 40 Ar) using a high-accuracy double differential manometric technique. The vapor pressure differences between a mixture of ( 36 Arϩ 40 Ar) and 40 Ar and between 36 Ar and 40 Ar were measured simultaneously with the vapor pressure of 40 Ar. Four different mixtures were selected and 80 temperatures were chosen in the 85-96 K temperature range. The results show that the vapor pressure of an equimolar liquid mixture is greater than what would be expected if the solution were strictly ideal, by approximately 2.5 Pa in the 85-96 K temperature range. The calculated excess Gibbs energy for the equimolar mixture at 85 K is G 1/2 E ϭ(0.021Ϯ0.004) J mol Ϫ1 . For the excess molar enthalpy a value of H 1/2 E ϭ(0.12Ϯ0.04) J mol Ϫ1 was found. The corresponding deviations from Raoult's law are therefore extremely small ͑3 to 4 orders of magnitude smaller than those typically found in binary mixtures of nonisotopic constituents͒. The evidence of nonideal behavior in such a fundamental and simple system is relevant for the interpretation of the properties of liquid mixtures of more general importance. The pitfalls of the determination of activity coefficients through the comparison between vapor pressure isotope effect data and liquid-vapor isotope fractionation factors are discussed in some detail. The results were used to test three theoretical models, namely, the vdW-1 fluid theory, the 1cLJ perturbation theory, and the statistical theory of isotope effects in mixtures. While the first two proved to be inadequate, the isotope effect theory agrees reasonably well with experiment. The excess Gibbs energy of monatomic isotopic mixtures are related with the molar volume isotope effect and with the mean Laplacian of the potential energy in the liquid phase.