The effect of halogen hetero-atoms on the vapor pressures and thermodynamics of polycyclic aromatic compounds measured via the Knudsen effusion technique

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Abstract

Knowledge of vapor pressures of high molar mass organics is essential to predicting their behavior in combustion systems as well as their fate and transport within the environment. This study involved polycyclic aromatic compounds (PACs) containing halogen hetero-atoms, including bromine and chlorine. The vapor pressures of eight PACs, ranging in molar mass from (212 to 336) g · mol−1, were measured using the isothermal Knudsen effusion technique over the temperature range of (296 to 408) K. These compounds included those with few or no data available in the literature, namely: 1,4-dibromonaphthalene, 5-bromoacenaphthene, 9-bromoanthracene, 1,5-dibromoanthracene, 9,10-dibromoanthracene, 2-chloroanthracene, 9,10-dichloroanthracene, and 1-bromopyrene. Enthalpies of sublimation of these compounds were determined via application of the Clausius–Clapeyron equation. An analysis is presented on the effects of the addition of halogen hetero-atoms to pure polycyclic aromatic hydrocarbons using these data as well as available literature data. As expected, the addition of halogens onto these PACs increases their enthalpies of sublimation and decreases their vapor pressures as compared to the parent compounds.

Introduction

A variety of applications requires vapor pressure data for predicting many phenomena; e.g. the pharmaceutical industry uses vapor pressures in developing inhalation delivery systems and for estimating the potential hazards associated with the inhalation of toxic vapors [1]. The vapor pressures of polycyclic aromatic compounds (PACs) are important in flame modeling [2] and in determining thermal remediation conditions for sites contaminated with coal tars and other fossil fuels, such as former manufactured gas plants [3]. There exist relatively few vapor pressure and thermodynamic data concerning PACs.

This study focused on those PACs containing chlorine and bromine atoms, persistent organic pollutants (POPs) included in many toxic risk assessments [4]. Polycyclic aromatic hydrocarbons (PAHs) are found at processing sites for fossil fuels. The PAHs may undergo a variety of halogenation mechanisms to yield PACs. For example, Hu et al. [5] show that the presence of bromine ions significantly increases the reaction rate of the chlorination of pyrene, while simultaneously producing brominated pyrenes, including 1-bromopyrene. The promotion of soot formation in flames by chlorine and chlorinated hydrocarbons is linked to PAH formation, as PAHs are believed to be soot precursors [6]. In the presence of bromine and chlorine, it may be possible to promote the formation of halogenated and non-halogenated PAHs [5].

A further step in this research was to analyze available literature data on halogenated PACs in conjunction with the newly obtained data set to investigate whether trends exist with the substitution of halogens onto these environmentally critical aromatic compounds. An extensive literature exists on group contribution methods for predicting phase equilibrium for organic compounds, yet many of these models fall short when polycyclic aromatics are considered [7]. Thus, we aim towards a future ability to describe, systematically, the thermodynamic effect of halogen substitution on the vapor pressures of PACs, for which the presently reported data would be very valuable.

To perform this work, we use the Knudsen effusion technique, which enables the measurement of vapor pressures of high molar mass, semi-volatile organic compounds indirectly via the molecular effusion of a vapor through an orifice under a high vacuum at low to moderate temperatures. This technique eliminates the need for the high temperatures required to measure low volatility compound vapor pressures directly, as high temperatures result in the thermal degradation of these compounds.

The Knudsen effusion method is used to determine vapor pressures by measuring the molecular leak rate from an effusion cell through a small orifice, without disturbing the thermal and chemical equilibrium within the cell. Application of the Knudsen theory stipulates the conditions of thermal and chemical equilibrium within the sample cell. The rate of molecular effusion through the pinhole leak (measured as the mass loss of sample from the cell) equals the rate at which molecules would strike an area of wall equal to the area of the hole, if the hole were not present; the long mean free path of the vapor molecules as compared to the radius of the orifice justifies this assumption. The use of high vacuum (<10−7 Torr) guarantees this required long mean free path, enabling measurements of vapor pressures as low as 10−6 Torr, at experimental temperatures sufficiently low enough to prevent thermal degradation of the sample. The vapor pressures measured are actually sublimation vapor pressures because of the low temperatures and solid state of the polycyclic aromatic compounds examined. By assuming a constant enthalpy of sublimation, ΔsubH, over the temperature ranges employed, the Clausius–Clapeyron equation models the vapor pressure data as a function of temperature:lnP=-ΔsubH/RT+ΔsubS/R,where P is the saturation vapor pressure, T is the absolute temperature, ΔsubS is the entropy of sublimation, and R is the universal gas constant. This integrated form of the Clausius–Clapeyron equation results in a fairly linear vapor pressure curve of ln P versus 1/T, well representing data in the pressure region of (10−6 to 10−3) Torr.

Section snippets

The Knudsen effusion technique

The present implementation of the Knudsen effusion method is described in previous publications [3], [8]. Measurements were made under isothermal conditions, using an Omega type K thermocouple, calibrated to ±0.1 K, located directly above the effusion cell opening. The cell was suspended on one arm of a Cahn 2000 microbalance with a sensitivity of 0.5 μg and hangs inside a black copper capsule within the glass-enclosed thermo-gravimetric apparatus (TGA). The balance is interfaced with a National

Results

The experimental technique was validated by comparing vapor pressures obtained using the present technique applied to fluorene, anthracene, and pyrene, and comparing these to available literature values, spanning a temperature range of 298 K to 381 K [8].

Table 1 presents data obtained using isothermal steps. These data were examined using the Knudsen effusion equation (equation (1)) to obtain the enthalpies and entropies of sublimation of each compound, displayed in table 2, which also details

Discussion

Overall, the successive addition of halogens to polycyclic aromatic compounds always decreases the vapor pressure as compared to the unsubstituted compound. As expected, the more halogens substituted, the lower the vapor pressure. Single chlorine substitution had a significantly smaller effect on vapor pressure than did bromine substitution at the same position for naphthalene, even though the enthalpy of sublimation was not much influenced by the chlorine heteroatom. The vapor pressure at T = 298

Acknowledgements

The project described was supported by Grant Number 5 P42 ES013660 from the National Institute of Environmental Health Sciences (NIEHS), NIH and the contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIEHS, NIH.

The authors appreciate the help of Daniel Lim at Brown University in measuring the melting temperatures of the halogenated PAHs and Professor G. Diebold for the use of his Digimelt apparatus.

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