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 (HM) per mole of alcohol. For these systems, excluding the lower alcohols, the available i.r., HM, 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 HM 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 HM 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.