Application of the Flory Theory and of the Kirkwod-Buff Formalism to the Study of Orientational Effects in 1-Alkanol + Linear or Cyclic Monoether Mixtures

Orientational effects present in binary mixtures of 1-alkanols with linear or cyclic monoethers have been studied using the Flory model and the Kirkwood-Buff formalism. The alcohols considered are those from methanol to 1-decanol, and the ethers are the following: dimethylether (DME), diethylether (DEE), dipropylether (DPE), dibutylether (DBE), tetrahydrofuran (THF), and tetrahydropyran (THP). In terms of the Flory model, orientational effects have been investigated by analyzing the concentration dependence of the interaction parameter, X-12, and comparing the deviations obtained between experimental molar excess enthalpies, H-m(E), and calculated values with all X-12 parameter determined from H-m(E) measurements at equimolar composition. Due to the facts that structural effects are relevant for many of the considered mixtures and their excess molar Volumes, V-m(E), are negative, we have also determined the excess molar internal energies, U-V,m(E), and X-12 parameters from these data. From the X-12 variation with the alkanol size, we have shown than interactions between unlike molecules are nearly constant along a homologous series with a given ether. In the framework of the Kirkwood-Buff theory, the study has been developed through the Kirkwood-Buff integrals and related local mole fractions, obtained from vapor-liquid equilibria and V-m(E) data available in the literature. Ideal compressibilities for the mixtures were assumed. Although the Flory model does not describe the complex structural effects present in the investigated mixtures and fails when predicting V-m(E), both theories provide consistent results on the orientational effects in these solutions. In addition, the results are in agreement with those previously obtained from the ERAS model. Systems involving linear monoethers are mainly characterized by orientational effects related to the self-association of the alcohol, in such way that the mentioned effects decrease when the size of the alcohol is increased in solutions with a given ether. Orientational effects become more relevant when the chain length of the linear monoether mixed with a fixed 1-alkanol increases. It has been shown that dispersive interactions merely differ by size effects for solutions of 1-alkanol (from 1-propanol) and DPE, or DBE. Systems with cyclic monoethers are characterized by a strengthening of dipolar interactions and a weakening of association effects. As a result, orientational effects are weaker in this type of solution. It is remarkable that mixtures with the longer 1-alkanols (from 1-hexanol) show a behavior close to random mixing. Random mixing is also observed when the temperature increases, as in the case of 1-propanol or 1-butanol + diethylether systems at 321.15 K.