Thermodynamics of Mixtures Containing Amines. X. Systems with Cyclic Amines or Morpholine

Cyclic amine or morpholine + organic solvent mixtures have been investigated in terms of DISQUAC and of the Kirkwood-Buff formalism. The amines considered are cyclohexylamine; (c-CH2)(u)NH (u = 2-7) and (c-CH2)(u)N-CH3 (u = 4,5). The organic solvents are alkanes and methanol or ethanol. The DISQUAC interaction parameters are reported. The model describes correctly a whole set of thermodynamic properties: vapor-liquid equilibria (VLE); excess Gibbs energies, G(m)(E); excess enthalpies, H-m(E); excess heat capacities at constant pressure, C-P,m(E); and partial excess enthalpies at infinite dilution H-1(E,infinity), as well as the main features of the Kirkwood-Buff integrals. In (c-CH2)(u)NH + C6H12 mixtures, amine-amine interactions become weaker with increased u values. This is supported by the corresponding decrease of H-m(E) and H-1(E,infinity) and of the effective dipole moment. Interactions between amine molecules are also weakened when passing from a primary cyclic amine to an isomeric tertiary cyclic amine. The existence of an aromatic ring in the amine, as in aniline or pyridine, leads to stronger amine-amine interactions. Dipolar interactions between morpholine molecules are stronger than those between piperidine molecules, and reveal the existence of proximity effects between the two groups of morpholine. In systems with alkanes, interactions between amine molecules are preferred to those between unlike molecules. In piperidine + methanol and pyrrolidine + ethanol systems, the mixture structure is close to random mixing.