n-alkanoate + n-alkane mixtures: folding of hydrocarbon chains of n-alkanoates

Abstract

The mixtures CH$_3$(CH$_2$)$_{u-1}$COO(CH$_2$)$_{v-1}$CH$_3$ ($u=5-13$, $v=1,2$; $u=1,2,3$, $v=3,4$; $u=1,2,4$, $v=5$) + n-alkane have been investigated using experimental data (viscosity and excess molar functions: enthalpy, $H_{\text{m}}^{\text{E}}$, volume, $V_{\text{m}}^{\text{E}}$, isobaric heat capacity, and isochoric internal energy, $U_{V\text{m}}^{\text{E}}$) and models (Flory, Grunberg-Nissan, Bloomfield-Dewan). They are characterized by weak orientational effects. Large structural effects are found in some systems, like those containing pentane. Some considerations from standard enthalpies of vaporization, cohesive energy densities and $V_{\text{m}}^{\text{E}}$ of heptane mixtures reveal the existence of structural changes in longer n-alkanoates, which lead to stronger interactions between them. The observed decrease of $H_{\text{m}}^{\text{E}}$ for systems with a given n-alkane seems to be more related to the steric hindrance of the COO group than to interactional effects. The $U_{V\text{m}}^{\text{E}}(n)$ function ($n=$ number of C atoms in the n-alkane) shows a minimum for systems with esters with ($u\geq4$, $v=1$); ($u\geq7$, $v=2$), or ($u\geq 1$, $v=4,5$). A similar dependence was found for n-alkane mixtures involving cyclic molecules (cyclohexane, benzene). This result suggests that certain n-alkanoates, in an alkane medium, can form quasi-cyclic structures. Viscosities are well described by means of free volume effects only. For systems with butyl ethanoate or methyl decanoate, the variation of $\Delta \eta (n)$ (deviation of dynamic viscosity) is consistent with that of $U_{V\text{m}}^{\text{E}}(n)$, which supports the existence of cyclic structures in these esters. The Flory model provides poor results on $H_{\text{m}}^{\text{E}}$ for systems with large structural effects. Results improve when the model is applied to $U_{V\text{m}}^{\text{E}}(n)$ data.