Heats of mixing. V. Systems of n-alcohols with n-hexane

Abstract

Heats of mixing have been measured at 25°, 35°, and 45° for mixtures of ethanol, propan-1-ol, butan-1-ol, hexan-1-ol, and octan-1-ol with n-hexane and for methanol with n-hexane at 45°. These heats of mixing were found to increase with an increase in temperature and to decrease with an increase in the size of the alcohol molecule. They showed anomalies for the systems containing methanol and ethanol. An explanation of these anomalies has been given. The contribution of the hydroxyl groups to the enthalpy of any mixture of a n-alcohol with a n-alkane is defined as the enthalpy change on replacing the alcohol by its homomorphic alkane. The experimental results given here are consistent with the postulate that the contribution of the hydroxyl groups (per mole of alcohol) to the enthalpy depends only on the ratio of the number of hydroxyl groups to that of hydrocarbon units in the mixture. This postulate, if generally true, can be used for the prediction of the heats of mixing of any n-alcohol with a n-alkane and of the enthalpy difference between any n-alcohol and its liquid homomorphic alkane. Other data support the conclusion that the postulate holds for all concentrations of alcohol, but show that the analogous postulate applied to free energies holds only for infinitely dilute solutions of alcohols. The interaction of the hydroxyl groups with the hydrocarbon units contribute -7.8 and -4.5 kJ/mole of alcohol to the enthalpies and free energies respectively of these infinitely dilute solutions. The required values of the heats of mixing of the lower alkanes with n-hexane were estimated from the enthalpies of the alkanes using the principle of congruence; these were found to be appreciable and negative. They are in sign agreement with values extrapolated, from measured data at a higher temperature, using a principle of corresponding states for chain molecules.