Internal Equilibria and Partial Vapor Pressures of Mixtures of Primary Normal Alcohols with Normal Paraffin Hydrocarbons

Abstract

1. Heats of mixing for mixtures of primary n-alcohols and n-hydrocarbons have been determined down to practically infinite alcohol dilution over a temperature range from 10°C to 45°C. It has been found that (1) the heat of mixing is always negative and that the heat absorption, per mol of alcohol, increases with increasing dilution of the alcohols to a limit of 5800 cal. per mol; that the curves of the molar heat absorption against molar alcohol concentration are identical for all investigated systems—the first four primary n-alcohols in n-hexane and n-heptane. Furthermore, reports in the literature that n-hydrocarbons may be mixed with each other without any heat effect were confirmed. 2. The present experiments and evidence gathered from the literature indicate that (1) primary n-alcohols, in the pure liquid state, are completely associated to double molecules and that when mixed with a hydrocarbon they dissociate into single molecules; (2) that the association takes place between the two hydroxyl groups of two single molecules on account of their electrostatic dipole forces; (3) that the observed heat absorption is entirely due to the dissociation of double molecules, i.e., breaking up of the (electrostatic) hydroxyl bond and that there is no thermal effect of the aliphatic groups in the alcohol molecules upon the hydrocarbon solvent; and (4) that the equilibria between single and double molecules are identical at equal molar concentrations for all primary n-alcohols in all n-hydrocarbons. 3. This theory leads to an equation for the partial vapor pressures of n-alcohols in n-hydrocarbons: log p=log f−(λ−Q)/RT+B−b,where p=the partial vapor pressure at the mol fraction f, λ=the heat of vaporization of pure alcohol, Q=heat of mixing at mol fraction f and temperature T, and b=a universal function of Q. B is determined by the ordinary (simplified) vapor pressure equation log P=−λ/RT+Band is constant over the whole temperature range investigated. λ and B depend upon the nature of the alcohol, while Q and b depend upon molar concentration and temperature. 4. It is shown that after b has been determined once as a function of Q in a specific case, one can calculate partial vapor pressure curves of other systems. 5. An equation has been developed which permits the calculation of the degree of dissociation and therefore Q, for any temperature and concentration.