Density, speed of sound, refractive index and relative permittivity of methanol, propan-1-ol or pentan-1-ol + aniline liquid mixtures. Application of the Kirkwood-Fr\"ohlich model

TL;DR

This study investigates the physical properties of methanol, propan-1-ol, or pentan-1-ol mixed with aniline at various conditions, applying the Kirkwood-Fr"ohlich model to understand molecular interactions and structural effects.

Contribution

It provides comprehensive experimental data and analysis of excess thermodynamic and dielectric properties of these mixtures, applying the Kirkwood-Fr"ohlich model for the first time in this context.

Findings

Excess molar volume is negative, indicating structural effects.

Breaking of dipolar interactions dominates energy contributions.

Similar dipole orientation in methanol + aniline and + pyridine mixtures.

Abstract

Densities and speeds of sound at a temperature $T$ = 298.15 K, relative permittivities at 1 MHz and refractive indices at the sodium D-line at $T$ = (293.15 K to 303.15 K), all at a pressure p = 0.1 MPa, are reported for liquid mixtures methanol, propan-1-ol or pentan-1-ol + aniline. Excess molar volume ($V_{\text{m}}^{\text{E}}$), excess isentropic compressibility, excess speed of sound, excess refractive index, excess relative permittivity ($\varepsilon_{\text{r}}^{\text{E}}$) and its temperature derivative are calculated and fitted to Redlich-Kister polynomials. The agreement among the reported data and other literature sources is analysed by comparing $V_{\text{m}}^{\text{E}}$, $n_{\text{D}}^{\text{E}}$, $\varepsilon_{\text{r}}^{\text{E}}$ and the deviation of the speed of sound from mole-fraction linearity. The positive excess molar internal energies at constant volume ($U_{\text{m,}V}^{\text{E}}$) show the dominance of the breaking of interactions between like molecules in the energy balance on mixing, particularly the breaking of strong dipolar interactions between aniline molecules. This contribution is also dominant for the $\varepsilon_{\text{r}}^{\text{E}}$ values, as they are negative and decrease with the length of the alkan-1-ol chain. Calculations on the concentration-concentration structure factor are consistent with these statements, revealing homocoordination in the studied systems. The $V_{\text{m}}^{\text{E}}$ are negative, which together with the positive $U_{\text{m,}V}^{\text{E}}$ indicate the existence of important structural effects in the studied mixtures. The application of the Kirkwood-Fr\"ohlich model shows that the average relative orientation of neighbouring dipoles is similar in the mixtures methanol + aniline or + pyridine, in spite of the different character of the predominant interactions in the latter mixture (heterocoordination).