Properties of n-alcohol + n-alkane mixtures

Abstract

The total polar contributions (A_P) to three properties [infrared absorbance, mixing enthalpies (H^M) and excess free energies (G^E)] of alcohol + alkane (alp) systems are separated into a direct hydrogenbond contribution ( A_B) from the formation of isolated imers and a dipole-dipole contribution (A_D) resulting from dipolar correlation between these transient imers.

Dilute concentration range data giving the A_B contributions to these properties were found dependent only on OH group concentration (c) and are used to show the serious inadequacies of previous theories. A new proposed association model having only two parameters, that are fixed for all systems, does give good results for the AB contributions and further is quite compatible with the effect of temperature change and with the n.m.r. chemical shift (ε) and apparent mean square dipole moment (p²) data that are also studied. Thus association theory has been made quantitative for the A_B contributions to three properties of a/p systems and the approach given for deriving models appears capable of wider application.

The model was used to extrapolate the A_B contributions into the concentrated alcohol range to thus give the A_D contributions by difference. The latter are then shown to be the origin of the distinctive behaviour shown by lower alcohols in their pure and binary mixture properties either with alkanes or with other alcohols where for the latter the principle of congruence is shown to be completely misleading.

Two contributions (Ag and AD) explain the different c dependence shown by the i.r., H^M and the δ data for a/p systems and, qualitatively, the H^M data for alcohol+alcohol systems while the existence of a significant dipole term is strongly supported by the remarkable similarities found between the p²(c) data and the derived dipole-dipole contribution to the entropy of a/p systems.

A method is given for predicting latent heats and partial molar enthalpies of higher alcohols from the H^M data for one a/p system and a refined estimate is made of the enthalpy of formation of a hydrogen bond. Polar structure and non-linear dielectric effects are also discussed.