Compressibility of aqueous solutions of nonelectrolytes: an equilibrium model.

In this paper the parabolic-shaped dependences of compressibility kappa(S) of a nonelectrolyte in water solvent on concentration are explained and described semiquantitatively in terms of hydrate formation equilibrium reaction. It was demonstrated that only two fitted parameters, the hydration number n(h) and the formation constant of the hydrate K, are sufficient to reproduce satisfactorily the experimental data, in particular the characteristic minimums of kappa(S) versus composition observed at low contents of solutes.

Dimerization Constants from Acoustic Measurements: Solutions of Benzene, Cyclohexylamine and Aniline in Cyclohexane.

A model assuming that the formation of dimers determines the acoustic properties of liquid mixtures, in the inert solvent cyclohexane, was applied to describe the observed dependences of sound speed on composition. The dimerization constants were estimated. The results allow one to propose that the solutes tend to form associates larger than dimers in concentrated solutions, while in dilute systems solute-solvent interactions play an important role.

Ultrasonic and densimetric titration applied for acid-base reactions.

Classical acoustic acid-base titration was monitored using sound speed and density measurements. Plots of these parameters, as well as of the adiabatic compressibility coefficient calculated from them, exhibit changes with the volume of added titrant. Compressibility changes can be explained and quantitatively predicted theoretically in terms of Pasynski theory of non-compressible hydrates combined with that of the additivity of the hydration numbers with the amount and type of ions and molecules present in solution. It also seems that this development could be applied in chemical engineering for monitoring the course of chemical processes, since the applied experimental methods can be carried out almost independently on the medium under test (harmful, aggressive, etc.).

Peculiarity of aqueous solutions of 2,2,2-trifluoroethanol.

Aqueous solutions of 2,2,2-trifluoroethanol appear to show a structural transition at alcohol mole fraction equal to x(TFE) = 0.05, which can be concluded from a discontinuity of the speed of sound. At the same concentration, a discontinuity was observed in the parameters of the long-living component of the positron annihilation spectrum. Moreover, the partial molar volumes of components show transition-like behavior in the range of low solute contents, which is significantly different from nonsubstituted ethanol. The peculiarities of the low concentration system correlate with minor infrared spectra changes assigned to a mode composed of the CH(2) bending and CF(3) stretching internal vibrations being sensitive to polarity of the hydration shell surrounding the solute. The majority of the spectral changes arise from a gradual shift of the equilibrium between trans ↔ gauche isomers when the composition of the solution is changing. A possible explanation for the peculiar behavior of the system is a thermodynamic equilibrium between hydrated monomers and dimers at that respective mole number.

Hydration of amino acids from ultrasonic measurements.

In this paper the results of compressibility of aqueous solutions of amino acids in water and in aqueous HCl and NaOH solutions at 25 °C are presented. The effect of the charged protonated amino groups and deprotonated carboxylic groups on the hydration number was tested. The idea of additivity of the hydration number with the constituents of the solute molecule was successfully applied and discussed.

Determination of the conditional dimerization constants from the sound velocity data in binary liquid mixtures containing an associating solute and a nonassociating solvent.

A new model of calculating the self-association constants in mixtures of two liquids, an associating solute and a nonassociating solvent, is proposed and analyzed. The model assumes additivity of the time of transmission of the acoustic signal with the volume fractions of the components of the mixture. The model was tested for different systems, showing that the results are reliable and close to expectations. The presented attempt seems to be an interesting alternative in interpreting the experimental results of sound velocity measurements in liquid mixtures.