Glass transition dynamics and boiling temperatures of molecular liquids and their isomers

The relation between a dynamic and a thermodynamic temperature, glass transition T-g and boiling point T-b, is investigated for various glass-forming liquids, with emphasis on monohydroxy alcohols. As is well known, T-b and T-g are positively correlated across a large variety of liquids. However, we found that the same quantities show a negative correlation within an isomeric series, i.e., T-b decreases with increasing T-g for different isomers of the same chemical formula. For the alcohol series, CnH2n+1OH with 3 <= n <= 10, a master curve of the negative T-g - T-b correlation is obtained if the temperatures are normalized to the respective values of the n-alkanols. This T-g - T-b dependence of isomeric liquids is linked to entropic effects and responsible for much of the scatter of the correlation observed for a large number of molecular organic glass-formers with 45 < T-g < 250 K. Dielectric relaxation is measured for three groups of isomers: (a) 3-methoxyl-1-butanol and 2-iso-propoxyethanol, (b) 1,4-, 1,2-, and 2,4-pentanediol, and (c) di-n- and di-iso-butyl phthalate. Two key parameters of the dynamics, fragility m and stretching exponent beta, are found to be indistinguishable within isomers of moderately different T(g)s. Larger fragility differences are readily expected with pronounced structural change, but no systematic trend is observed within an isomer series. The results provide a useful tool for assessing T-g, m, and beta for marginal glass formers on the basis of their isomers.