Vapour pressures, heats of sublimation and degrees of association of some azo compounds in the vapour phase

Abstract

The vapour pressures of a number of aromatic azo compounds, similar in structure to disperse dyes, have been measured over a range of temperatures by two independent methods. Correlation of the results allows the calculation of the mean degree of association in the vapour over the observed temperature range. Most compounds appear to be dimeric in the vapour although a degree of association of 4·7 was found for 3′-nitro-4-N-dimethylaminoazobenzene. Heats of sublimation in relation to structure are discussed. Consistently higher vapour pressures of calibrating substances, found when all-glass effusion vessels were used, are attributed to a contribution to the total effusion flux by migration over the surface of the cell orifice.