Thermodynamics of mixtures with strong negative deviations from Raoult's law. XVIII: Excess molar enthalpies for the (1-alkanol + cyclohexylamine) systems at 298.15 K and modelling

Abstract

Excess molar enthalpies, $H_{\text{m}}^{\text{E}}$, have been measured using a Tian-Calvet microcalorimeter for the systems CH$_3$(CH$_2$)$_{u-1}$OH ($u$ = 1,2,3,4,7,10) + cyclohexylamine at 298.15 K and 0.1 MPa. The $H_{\text{m}}^{\text{E}}$ values are large and negative, indicating the existence of strong interactions between unlike molecules, which is consistent with the also large and negative excess molar volumes, $V_{\text{m}}^{\text{E}}$, of these solutions, previously measured by us. The contribution from the equation of state term to $H_{\text{m}}^{\text{E}}$ has been evaluated for the 1-alkanol + cyclohexylamine, or + 1-hexylamine, or + aniline mixtures, and the corresponding excess molar internal energies at constant volume, $U_{\text{m,}V}^{\text{E}}$, have been determined. It is shown that such contribution is particularly important for the methanol + aniline system, in such a way that the excess functions $H_{\text{m}}^{\text{E}}$ and $U_{\text{m,}V}^{\text{E}}$ have different sign at $x_1$ = 0.5. The DISQUAC and ERAS models have been applied to the cyclohexylamine systems, and the interaction parameters have been reported. DISQUAC improves ERAS results on $H_{\text{m}}^{\text{E}}$. The latter model describes correctly the $V_{\text{m}}^{\text{E}}$ curves. The variation of of CH$_3$(CH$_2$)$_{u-1}$OH + cyclohexylamine, or + 1-hexylamine, or + aniline mixtures with $u$ along a homologous series with a given amine, or with the amine in mixtures with a given 1-alkanol is discussed in terms of the different interactional contributions to $H_{\text{m}}^{\text{E}}$.