Association and vapor pressure isotope effect of variously deuterated methanols in n-hexane

Activity coefficients have been determined for solutions of CH3OH, CH3OD, CD3OH, and CD3OD in n-hexane between 35 and 750C from isothermal vapor pressure measurements. Using Wilson's equations for representing the activity coefficients, curves for the partial pressure ratios of the deuterioisomeric methanols do not show oscillating behavior as they do when using the Redlich--Kister type representation. In each case, Wilson's parameters, association constants, association energies, heats of vaporization, and excess Gibbs free energies suggest that the energy of the deuterium bond of the methanols is larger than that of their hydrogen bond. An association energy of about 5 kcal/mole is derived using Wilson's representation, while the value of 6 to 7 kcal/mole derived from the Redlich--Kister representation is too large. The ratios p(CD3OH)/p(CH3OH) and p(CD3OD)/p(CH3OD) are inverse (greater than 1) in the whole concentration range, whereas p(CH3OD)/p(CH3OH) and p(CD3OD)/p(CD3OH) changes from normal (less than 1) to inverse as the mole fraction goes to zero. Previous measurements on methylamines showed a similar behavior, except that the inversion point (p/p' = 1) occurs at a higher mole fraction. The pronounced difference is due to the weaker hydrogen bonding of the amino group compared to the hydroxyl group