Sublimation Pressure Data for Organic Compounds.

Abstract

c ONSIDERING T H E valuable information whichmay be derived either directly or indirectly from sublimation data, it is rather surprising that there is so little quantitative information available in the literature on the sublimation process. Apparently, no compilation of such data has been attempted. This summary of most of the available data, although not exhaustive, is the first serious attempt a t a tabulation for organic compounds.The variation of sublimation pressure with temperature may be represented to a first approximation by where p is the pressure of the saturated vapor in millimeters of mercury a t the absolute temperature T(T = to C. + 273.18). The constants A and B are related to the heat of sublimation and the entropy of sublimination (assuming these to be constant) in the following way:is the gas constant, 1.986 cal. per gram mole. The values of A and B are quoted to an accuracy a t least sufficient to reproduce sublimation pressures over the quoted temperature ranges to within the accuracy of the original observations. The AH and A S values derived from A and B are less certain if only because of the limitations in the relations used. Table I contains all the compounds for which full details are available. In cases where the authors have given an equation but have not deduced values for the heat and entropy of sublimation such values have been calculated and appear in bold face. The constants A and B are validfor the temperature ranges quoted for each compound. I t is obviously difficult to assess the quality of many of the results quoted, but an assessment has been attempted and the data considered most acceptable are footnoted. In arriving a t these conclusions the following facts were taken into account: In recent years there has been a considerable advance in the quality of the equipment available for the measurement of sublimation pressures.The adverse effect of impurities on the accuracy of the measurements was not fully appreciated by a considerable number of authors.In cases where doubts exist as to the validity of the results published, the data has been ignored.Failure by certain authors to ensure complete saturation in the air-flow method would invalidate their results.The effusion method is less sensitive to impurities than methods involving the use of a gauge.The fact that comparatively few measurements which have been described survive a critical appraisal, results in the paucity of reliable data. Table I1 contains a list of compounds for which limited sublimation pressure data are available. In the references given no heats or entropy values have been calculated for these compounds. ACKNOWLEDGMENT

New experimental sublimation energy measurements for some relevant astrophysical ices

“…These experiments (Armstrong et al 1955;Jones 1960;Stephenson & Malanowski 1987) use an experimental apparatus based in an evacuated vessel surrounded by a bath of liquid air in a Dewar flask whose temperature is given by a platinum resistance thermometer. The vacuum in the vessel is produced by a vacuum pump, and the final pressure is measured with a differential oil-manometer ranging pressures from 10 −1 to 10 3 mbar.…”

Estimation of solid vapor pressures of pure compounds at different temperatures using a multilayer network with particle swarm algorithm

“…Uncertainties are not given for any of the experimental data, which makes it impossible to interpret the deviations between the models and the experimental data. This is especially troubling given the history of unreliable experimental data that have been collected for the sublimation curve [5]. It would be much more useful and productive to select only the best experimental data sets (with well established uncertainties), and to use these data sets to develop and test models [1].…”

“…However, although small, the P S is needed in applications such as extraction of solids, in particular substances of biological origin, using high pressure gases [4]. Considering the valuable information which may be derived either directly or indirectly from sublimation data, it is rather surprising that there is so little quantitative information available in the literature on the sublimation process [5]. In addition, experimental techniques cannot in many cases be used to accurately obtain the P S [6][7][8].…”

“…An equation commonly used to relate the P S with the sublimation enthalpy ( H S ) is the Clausius-Clapeyron equation [5]:…”

The electrostatic term in lattice-energy calculations: C2H2, CO2and C2N2

“…But an additional possibility is that the empirical atomatom potentials we have employed may already include, in disguised form, a non-negligible fraction of the electrostatic contribution to the lattice energy. The measured sublimation energy AH r at temperature Tis adjusted to 0 K by the approximate expression: AH ° = AH~ + 2RT (Rae & Mason, 1968 References: (a) Jones (1960), (b) Pople (1954), (c) Perry & Bardwell (1925). A corresponding series of calculations has been performed for CO 2.…”