Effects of hydrophobic and hydrophilic hydration on the compressibility, volume and viscosity of mixtures of water with β-picoline

The speed of ultrasound in and the density of mixtures of water with beta-picoline were measured in the whole concentration range and within the temperature limits 293 divided by 298 K. The viscosities of those mixtures were measured at 293 K. The molar compressibility, molar volume and the corresponding excess and partial functions as well as the coefficients of the adiabatic compressibility and the excess activation energy of viscous flow were calculated. The results were discussed in terms of hydrophobic and hydrophilic molecular interactions. The hydrophobic hydration becomes evident in the thermodynamic properties of dilute aqueous solutions in that most probably labile semiclathrate structures, resembling cubic II solid clathrates (symmetry group Fd3m) but with guest molecules hydrogen-bonded to the water host lattice, arise. With increasing concentration of the organic component the shortage of water leads to a gradual decay of the water polyhedra around the picoline molecules and hydrophilic hydration becomes predominant.