Thermodynamic study of the ternary system NaCl–H2O–Et3N at 25 °C. Part 1.—Volumes

Abstract

The densities of ternary systems of triethylamine in aqueous sodium chloride solutions have been measured at 25 °C, for molalities in sodium chloride ranging from 0 to 0.3 mol kg^–1. Concentrations from 0 to ca. 0.35 mol kg^–1 in triethylamine were measured, for each fixed NaCl concentration.

Values of the apparent molar volumes, V^obs_ϕ, for the hypothetical species, Et₃N · H₂O, were first calculated, by applying a correction for the hydrolysis of triethylamine in the reference aqueous solutions. Limiting partial molar volumes, V^∞_Et3N, of the free amine were thus calculated, by linear extrapolation to infinite dilution. Volumes of transfer from water to aqueous sodium chloride solutions were obtained. The V^∞_Et3N values decrease with increasing NaCl concentration. The results have been interpreted using a hard-sphere model showing that the packing efficiency of the solvent molecules is the essential factor justifying the experimental results. The interference of the amine group in the water structure, by making the packing efficiency of the molecules decrease, seems to be of secondary importance. Comparison of the calculated volumes of transfer with the experimental ones suggests that an electrostatic interaction, between the sodium ion and the free electron pair of the amine group, may be operating in the most concentrated solution.