Thermodynamic properties of alcohol+alkane mixtures. II. Contributions to the excess energies other than those due to hydrogen bonding.

Abstract

Measured N.M.R. chemical shifts of the OH proton are reported for a number of n-alcohol + n-alkane systems at 30°C. When plotted as a function of OH group concentration these data show a remarkable similarity to the results obtained for the enthalpy of mixing (H^M) per mole of alcohol. For these systems, excluding the lower alcohols, the available i.r., H^M, and N.M.R. data correlate separately and each type of data is used to deduce the fraction of free OH as a function of concentration. The three deduced results are contradictory and it is postulated that the results from i.r. are correct whereas those from the H^M and also those from the N.M.R. data are invalid at high alcohol concentrations due respectively to a dipole-dipole enthalpy contribution and a related reaction field chemical shift. This also explains the similarity found between the two sets of data. At higher dilutions the results from i.r. and from H^M data are in very good agreement on the assumption of an effective hydrogen bond enthalpy of 25 kJ/mol while the infinite dilution results require a decreased value for lower multimers which is in accord with other evidence. The deduced dipole-dipole contribution to the enthalpy appears reasonable from other estimates and makes a large contribution to the enthalpies of mixing. A brief note on alcohol + alcohol systems is included.