Ângela F.S. Santos

Chemical Thermodynamics of Ultrasound Speed in Solutions and Liquid Mixtures

A comprehensive formalism is developed to treat thermodynamically speed of ultrasound data for solutions and liquid mixtures. For solutions, the apparent speed of ultrasound of a solute is introduced and proposed to take the place of empirically defined quantities. The partial speed of ultrasound of a solute is defined and related to the partial molar volume and partial molar isentropic compression. For liquid mixtures, the concept of speed of sound before mixing pure liquids is presented and used to define the change in speed of ultrasound upon ideal mixing, which is predicted to be generally a negative quantity. A new thermodynamic equation is derived linking the values for excess speed of ultrasound, excess molar volume and excess molar isentropic compression of a mixture, and its applications are discussed. Ideal and excess apparent speeds of ultrasound, as well as ideal and excess partial speeds of ultrasound, are defined for substances making up a liquid mixture. Accurate speeds of ultrasound in 31 mixtures of water with the amphiphile 2-(ethylamino)ethanol at 293.15 K are reported. These data are used to demonstrate the ability of the apparent speed of ultrasound to describe the impact of solutes on sonic properties of solutions and the advantages of analysing thermodynamic properties of binary liquid mixtures in terms of the dependence on composition of Balankinas ratios between excess and ideal values. It is concluded that the new thermodynamic functions defined for speeds of ultrasound in solutions and liquid mixtures give, at the least, equivalent information on molecular aspects to the usual functions related to the isentropic compressibility, without needing density data for this purpose.

Dipole moments of isomeric alkoxyalcohols in cyclohexane. Comparison of Hedestrand and Fröhlich procedures with a new formula

Limiting dipole moments of four isomeric alkoxyalcohols dissolved in cyclohexane at 298.15 K were determined from measurements of the relative permittivity of at least 17 dilute solutions up to solute mole fraction of 0.03. In addition, 5 to 7 data points were obtained up to a mole fraction of 0.1. A stepwise dilution device ensured dielectric measurements to be performed in highly dilute solutions with accurately determined concentrations. Densities of these solutions and refractive indices of the pure liquids were independently measured. Limiting dipole moments were calculated using Hedestrands equation and an improved method of implementing Fröhlichs equation, which circumvents extrapolation difficulties referred to in the literature. A new formula, based on the one-liquid approach for extending the Onsager-Kirkwood-Fröhlich equation to liquid mixtures, is introduced and shown to yield a reliable and robust procedure for estimating dipole moments of polar molecules dissolved in non-polar solvents. Limiting dipole moment values for 2-tert-butoxyethanol (2.31 D), 1-propoxypropan-2-ol (2.14 D), 2-butoxyethanol (2.14 D) and 2-isobutoxyethanol (2.08 D) are recommended. The relative order of these values appears to determine the order of hydrophilicity of these four alkoxyalcohols as suggested by their recently reported limiting partial molar volumes and isentropic compressions in aqueous solutions.