Vapor pressures and sublimation enthalpies of seven heteroatomic aromatic hydrocarbons measured using the Knudsen effusion technique

Abstract

The vapor pressures of seven heteroatom-containing cyclic aromatic hydrocarbons, ranging in molecular weight from (168.19 to 208.21) grams⊕mol −1 were measured over the temperature range of (301 to 486) Kelvin using the isothermal Knudsen effusion technique. The compounds measured include: anthraquinone, 9-fluorenone, 9-fluorenone oxime, phenoxazine, phenoxathiin and 9H-pyrido [3,4-b]indole. These solid-state sublimation measurements provided values that are compared to vapor pressures of parent aromatic compounds (anthracene and fluorene) and to others with substituent groups in order to examine the effects of alcohol, ketone, pyridine, and pyrrole functionality on this property. The enthalpies and entropies of sublimation for each compound were determined from the Clausius-Clapeyron equation. Though there is no consistent trend in terms of the effects of substitutions on changes in the enthalpy or entropy of sublimation, we note that the prevalence of enthalpic or entropic driving forces on vapor pressure depend on molecule-specific factors and not merely molecular weight of the substituents.

Thermodynamic properties of 9-fluorenone: Mutual validation of experimental and computational results

“…All measured vapor and sublimation pressures are plotted in figure 4. The consistency in slope for ln(p/p°) against (T/K) À1 can be seen there for the literature sublimation studies, even though the pressures reported by Goldfarb and Suuberg [48] are low. A comparison of figures 3 and 4 also serves to demonstrate that a plot of ln(p/p°) against (T/K) À1 is a poor choice for the purpose of vapor pressure comparisons because the logarithmic scale masks large differences between the data sets.…”

“…The enthalpy of sublimation D g cr H was measured calorimetrically by Sabbah et al [43] measured sublimation pressures by Verevkin [45] fD g cr HðT ¼ 336 KÞ ¼ ð93:9 AE 1:6Þ kJ Á mol À1 g and Hansen and Eckert [47] fD g cr HðT ¼ 336 KÞ ¼ 92:2 kJ Á mol À1 with no reported uncertaintyg are in good accord with the present research. The value derived by Goldfarb and Suuberg [48] is slightly lower, but with a relatively large uncertainty estimate; fD g cr HðT ¼ 336 KÞ ¼ ð88:5 AE 3:7Þ kJ Ámol À1 g. The temperatures associated with the literature values is the midpoint of the experimental temperature range in each case. All measured vapor and sublimation pressures are plotted in figure 4.…”

“…We consider this agreement to be good because of the difficulties inherent in the transpiration method. In contrast, the sublimation pressures measured with Knudsen effusion by Goldfarb and Suuberg [48] are 50% to 60% low. Goldfarb and Suuberg did not report uncertainties for these values, but uncertainties near 10% should be achievable with the Knudsen method.…”

“…Sublimation pressures were measured by Hansen and Eckert [47] and Verevkin [45] with transpiration methods and recently by Goldfarb and Suuberg [48] with the Knudsen effusion technique. Sublimation pressures were estimated in the present research through combination of vapor pressures extrapolated to below T tp {parameters of table 8 with equation (3)}, the thermodynamic functions for the crystals (table 6), and thermodynamic functions for the super cooled liquid based, in part, on extrapolated liquid heat capacities (table 6 and plotted in figure 1).…”

Thermodynamics of Multicomponent PAH Mixtures and Development of Tarlike Behavior

“…The results presented here are consistent with those previously reported for real coal tars4, though the enthalpies are a bit lower for the present mixtures than for the coal tars. The coal tars have significant heteroatomic (oxygen and nitrogen) contents, particularly in the case of Wyodak and Pittsburgh coal tars and it is known that heteroatomic substituted PAHs can have higher enthalpies of sublimation compared with their parent PAHs 24. In addition, it should be noted that the present mixtures are also of somewhat lower molecular weight than the reported averages for the lowest molecular weight fractions of the coal tars, and this would also lead to the enthalpies of vaporization being higher for the latter materials.…”