Comparative Study of Associated Species in Vapors of Alcohols and Water by Measurement of Thermal Conductivity

摘要

Measurements of the thermal conductivities of methanol, trifluoroethanol, ethanol, and water vapors have been made as a function of pressure and temperature with a thick hot wire cell using a relative method in which the cell was calibrated with N sub 2 , Ar, and Kr. Comparisons of the results for the different vapors can readily be made since all the measurements were performed with the same apparatus. At any given temperature (330 to 400 exp 0 K), the thermal conductivities increased with pressure (7 to 160 kPa), indicating the presence of associated species in all the vapors. From the data, association constants were deduced as well as standard enthalpies and standard entropies for the formation of the associated vapor species. Dimeric molecules were detected in trifluoroethanol, ethanol, and water vapor; and a tetramer species was detected in methanol and ethanol. In quantum mechanical calculations, the cyclic hydrogen bonded tetrameric species is especially stable, supporting the conclusions that the pressure dependence of the thermal conductivities of methanol and ethanol is related to the presence of tetramer. Consistent with the quantum mechanical calculations, the measurements indicate that the trifluoroethanol dimer bond is strongest. Transport parameters deduced from the data are also reported. (ERA citation 03:004551)