Molecular interactions in mixtures of carboxylic acids with amines. 1. Melting curves and viscosities

Abstract

In a previous paper, various properties of the mixture acetic acid + triethylamine were investigated and explained in terms of a very strong attractive interaction between a 1:1 complex acid + amine and the cyclic dimer of acetic acid, leading to a 3:1 aggregate of extreme thermodynamic stability. This gives rise to phase separation at negative values of the excess Gibbs energy. In the present series of articles, an attempt is made to present a unified treatment of systems of carboxylic acid + amine, with emphasis on triethylamine, but with inclusion of some recent results involving the tri-n-butylamine as well as secondary and primary n-butylamine. In this paper we present experimental results on solid-liquid equilibria of formic, propionic, and trimethylacetic (pivalic) acids + triethylamine and viscosities of formic, trifluoroacetic, propionic, and trimethylacetic acids + triethylamine, propionic and trimethylacetic acids + tri-n-butylamine, propionic acid + di-n-butylamine, and propionic acid + n-butylamine.

TGA–FTIR study of the vapors released by triethylamine–acetic acid mixtures

“…This was also theoretically predicted by ab initio calculations for the trimethylamine-formic acid and propylamine-propionic acid systems [22,30]. The ionic nature of the complexes was confirmed by both mid and near infrared spectroscopy [20,21,[24][25], FT-Raman [17], 1 H-NMR and 13 C-NMR [15,31], dielectric spectroscopy, conductivity measurements [18,23], melting and boiling point measurements, ultrasound absorption and relaxation spectroscopy [12,24,[32][33], and combinations of different measurement techniques including density, optical refractive index, and shear viscosity [23].…”

“…The interactions between amines (A) and carboxylic acids (C) have been comprehensively studied [12][13][14][15][16][17][18]. Amines and carboxylic acid mixtures interact strongly via hydrogen bonding [12,15].…”

“…Amines and carboxylic acid mixtures interact strongly via hydrogen bonding [12,15]. In 1:1 mixtures they form A 1 C 1 molecular complexes in the vapor phase [19][20][21][22][23][24][25][26][27][28][29][30] or ionic complexes in the liquid phase via Brønsted-Lowry acid-base reactions [17].…”

“…When the tertiary amine is replaced with a secondary or a primary amine the situation changes dramatically. The 1:1 complex becomes increasingly important and in the case of primary amines there is a tendency for these to form large clusters [15].…”

Brønsted acids in ionic liquids: how acidity depends on the liquid structure

“…10 Similarly, ionic liquids formed by proton transfer between an acid and a base, with the acid in excess, have been shown to form complex anions, for example [F(HF) x ] -, 11 [tfa(Htfa)] -, 6 [HSO 4 (H 2 SO 4 ) x ] -, 5 and a wide range of carboxylate-carboxylic acid clusters. 1,12,13 Poorly ionic mixtures of 1-methylpyrrolidine and acetic acid were shown to contain polymeric [OAc(HOAc) x ] -anions. 13 Such clusters are not specific to ionic liquids environment; they were observed in aqueous solutions, 14 non-polar solvents, 15 and are often found in crystal structures.…”