N-Alkanoate + N-Alkane Mixtures: Folding of Hydrocarbon Chains of N-Alkanoates

Abstract

The mixtures CH₃(CH₂)_u-1COO(CH₂)_v-1CH₃ (u = 5–13, v = 1,2; u = 1,2,3; v = 3,4; u = 1,2,4, v = 5) + n-alkane have been investigated on the basis of excess molar functions, enthalpy (\(H_{{\text{m}}}^{{\text{E}}}\)), volume (\(V_{{\text{m}}}^{{\text{E}}}\)), isobaric heat capacity (\(C_{{p{\text{m}}}}^{{\text{E}}}\)), and isochoric internal energy (\(U_{{V{\text{m}}}}^{{\text{E}}}\)), and viscosity data, and by means of different models (Flory, Grunberg-Nissan and Bloomfield-Dewan). Solutions are characterized by weak orientational effects. Large structural effects are encountered in a number of systems, such as those containing pentane. The variation with the ester size of the difference between the standard enthalpy of vaporization at 298.15 K of an ester and that of the homomorphic alkane along an homologous series formed by methyl or ethyl n-alkanoates reveals the existence of structural changes in longer n-alkanoates, which lead to stronger interactions between them. A similar result is obtained from values of cohesive energy density. The variation of \(V_{{\text{m}}}^{{\text{E}}}\) values of the corresponding heptane mixtures supports this statement. The observed decrease of \(H_{{\text{m}}}^{{\text{E}}}\) for systems with a given n-alkane (heptane, e.g.) seems to be more related to the COO group is more sterically hindered than to interactional effects. The \(U_{{V{\text{m}}}}^{{\text{E}}}\) (n) function (n is the number of C atoms in the n-alkane) shows a minimum for systems with esters characterized by (u \(\ge\) 4, v = 1); (u \(\ge\) 7, v = 2), or (u \(\ge\) 1, v = 4,5). A similar dependence of \(U_{{V{\text{m}}}}^{{\text{E}}}\) (n) was encountered 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. Viscosity data 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 the mentioned cyclic structures in these esters. The Flory model provides poor results on \(H_{{\text{m}}}^{{\text{E}}}\) for systems characterized by large structural effects. Results are improved when the model is applied to \(U_{{V{\text{m}}}}^{{\text{E}}}\) data.