Air Partition Coefficients Research Articles

Compilation, Evaluation, and Selection of Physical-Chemical Property Data for α-, β-, and γ-Hexachlorocyclohexane

A complete set of temperature-dependent physical-chemical property data (aqueous solubility (SW), octanol−water partition coefficient (KOW), vapor pressure (P), Henry's law constant (H), octanol−air partition coefficient (KOA), and octanol solubility (SO)) for α-, β-, and γ-hexachlorocyclohexane (HCH) is derived by evaluating, averaging, and regressing all experimentally obtained values reported in the literature. Properties for each isomer are adjusted slightly to ensure thermodynamic consistency. That adjustment is sized according to, and is smaller than, the uncertainty apparent from the range of reported experimental values. The data analysis confirms that an unsually high KOA and an unusually low H reported for β-HCH are in agreement with the measured SW and P values when these are converted to the liquid state. Linear solvation energy relationships fail to predict the observed differences in the partitioning behaviors of the three isomers.

Approaching gas–particle partitioning equilibrium of atmospheric PCDD/Fs with increasing distance from an incinerator: measurements and observations on modeling

Abstract A comprehensive sampling campaign was carried out around a municipal solid waste incinerator (MSWI) in central Taiwan in order to evaluate the impact of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) emissions on the ambient air and describe their spatial variation of gas–particle partitioning. Stack flue gas samples from the MSWI were also analyzed to investigate the relationship between the source and the impacted sites. Results showed that the PCDD/F emissions from the MSWI were a major source for PCDD/Fs in the ambient air. Moreover, PCDD/Fs in the ambient air tended to be distributed between the gaseous and particulate phases based on molecular weight; the higher chlorinated congener occupied a higher particulate fraction. There was an increase in the proportion of particle-bound PCDD/Fs with increasing distance from the MSWI. This observation supports the assumption that PCDD/Fs emitted were initially more associated with gaseous phase and the gaseous PCDD/Fs would gradually migrate to particles via either adsorption or absorption process. The results obtained were also applied and compared with the subcooled liquid vapor pressure (PL0)-based model and with the octanol–air partition coefficients (Koa)-based model. These two models were found to be excellent descriptors for the gas–particle partitioning of PCDD/Fs. Regressions for log partition coefficient (Kp) versus log PL0 and log Kp versus log Koa both demonstrated that PCDD/Fs were approaching equilibrium partitioning with increasing distance from the MSWI in the current study.

Predicting blood:air partition coefficients using theoretical molecular descriptors

Three regression methods, namely ridge regression (RR), partial least squares (PLS), and principal components regression (PCR), were used to develop models for the prediction of rat blood:air partition coefficient for increasingly diverse data sets. Initially, modeling was performed for a set of 13 chlorocarbons. To this set, 10 additional hydrophobic compounds were added, including aromatic and non-aromatic hydrocarbons. A set of 16 hydrophilic compounds was also modeled separately. Finally, all 39 compounds were combined into one data set for which comprehensive models were developed. A large set of diverse, theoretical molecular descriptors was calculated for use in the current study. The topostructural (TS), topochemical (TC), and geometrical or 3-dimensional (3D) indices were used hierarchically in model development. In addition, single-class models were developed using the TS, TC, and 3D descriptors. In most cases, RR outperformed PLS and PCR, and the models developed using TC indices were superior to those developed using other classes of descriptors.

Molecular polarizability as a single-parameter predictor of vapour pressures and octanol–air partitioning coefficients of non-polar compounds: a priori approach and results

Abstract We present a one-parameter quantitative structure–property relationship (QSPR) which relates the vapour pressures, PL, and octanol–air partition coefficients, KOA, of a series of chlorinated organic compounds to their molecular polarizabilities. Polarizabilities, dipole moments and quadrupole moments were calculated for the complete set of chlorobenzenes, the mono-, di-, and tri-substituted chloronaphthalenes, the mono-, di-, tri-, and tetra-substituted chlorobiphenyls, and the chloroalkanes with one, two and three carbons using Density Functional Theory methods at the B3LYP/6-311G(d, p) level. Only the molecular polarizability was found to be significantly correlated with log P L and log K OA . The regression coefficients for plots of experimental values of log P L and log K OA as a function of molecular polarizability are approximately 0.98, indicating that non-specific intermolecular forces are dominant in governing the gas–condensed phase transition in these molecules. The predictive ability of the single-parameter QSPR is suggested by near-identical values of the residual sum of squares and PRESS parameters. We suggest that the QSPRs derived here could be used more generally.

A Comprehensive and Critical Compilation, Evaluation, and Selection of Physical–Chemical Property Data for Selected Polychlorinated Biphenyls

Accurate physical–chemical properties (aqueous solubility SW, octanol–water partition coefficient KOW, vapor pressure P, Henry’s law constant H, octanol–air partition coefficient KOA, octanol solubility SO) are of fundamental importance for modeling the transport and fate of organic pollutants in the environment. Energies of phase transfer are used to describe the temperature dependence of these properties. When trying to quantify the behavior of contaminant mixtures such as the polychlorinated biphenyls, consistent physical–chemical properties are required for each individual congener. A complete set of temperature dependent property data for sixteen polychlorinated biphenyls (PCB-3, 8, 15, 28, 29, 31, 52, 61, 101, 105, 118, 138, 153, 155, 180, 194) was derived, based on all experimentally obtained values reported for these congeners in the literature. Log mean values derived from the experimental data were adjusted to yield an internally consistent set of data for each congener. These adjusted data also show a greater degree of interhomologue consistency, which can be illustrated with the help of simple quantitative structure-property relationships that use molar mass and the number of chlorine substitutions in ortho-positions as descriptors. The extent of the required adjustment gives an indication of the uncertainty of the averaged measured values and is typically lower than might be expected from the range of the reported measured values.

Quantitative structure-property relationships for octanol–air partition coefficients of polychlorinated naphthalenes, chlorobenzenes and p, p′-DDT

The octanol–air partition coefficient ( K OA) is a key descriptor of chemicals partitioning between the atmosphere and environmental organic phases. Quantitative structure–property relationships (QSPR) are necessary to model and predict K OA from molecular structures. Based on 12 quantum chemical descriptors computed by the PM3 Hamiltonian, using partial least squares (PLS) analysis, a QSPR model for logarithms of K OA to base 10 (log K OA) for polychlorinated naphthalenes (PCNs), chlorobenzenes and p, p′-DDT was obtained. The cross-validated Q 2 cum value of the model is 0.973, indicating a good predictive ability of the model. The main factors governing log K OA of the PCNs, chlorobenzenes, and p, p′-DDT are, in order of decreasing importance, molecular size and molecular ability of donating/accepting electrons to participate in intermolecular interactions. The intermolecular dispersive interactions play a leading role in governing log K OA. The more chlorines in PCN and chlorobenzene molecules, the greater the log K OA values. Increasing E LUMO (the energy of the lowest unoccupied molecular orbital) of the molecules leads to decreasing log K OA values, implying possible intermolecular interactions between the molecules under study and octanol molecules.

Quantitative relationships between molecular structures, environmental temperatures and octanol–air partition coefficients of polychlorinated biphenyls

A quantitative model that incorporates information on both environmental temperatures ( T) and molecular structures, for logarithm of octanol–air partition coefficient to base 10 (log K OA) of polychlorinated biphenyls (PCBs) was developed. Partial least squares (PLS) analysis together with 16 theoretical molecular structural descriptors was used to develop the Quantitative relationships between structures, environmental temperatures and properties (QRSETP) model. The cross-validated Q cum 2 value for the optimal QRSETP model is 0.976, indicating a good predictive ability for log K OA of PCBs at different environmental temperatures. T, E LUMO (the energy of the lowest unoccupied molecular orbital), molecular size or average molecular polarizability ( α), and the net atomic charges on chlorine, hydrogen and carbon atoms of PCB molecules, are major factors governing log K OA. The lower the E LUMO, the greater the intermolecular interactions between octanol and PCB molecules, and thus the greater the log K OA values. Because of intermolecular dispersive forces, the more chlorine atoms in PCB molecules, the greater the molecular size or α, the greater the log K OA. The largest negative net atomic charge on a carbon atom ( q C −) and molecular size or average molecular polarizability ( α) are major factors governing temperature dependence of log K OA. PCB molecules with low q C − values and more chlorines (big size or α) tend to have strong temperature dependence, due to intermolecular electrostatic interactions and dispersive forces, respectively.

Pharmacokinetics of ethylene in man by on-line laser photoacoustic detection

The pharmacokinetics of ethylene are determined using laser-based photoacoustic detection and a closed chamber setup. Concentration–time data are analyzed using a two-compartment and a physiologically based pharmacokinetic (PBPK) model. Endogenous production was 92 ± 13 pmol/h/kg for the two-compartment model and 75 ± 10 pmol/h/kg for the PBPK model. These values agree with previous work at our department but are significantly lower than published values based on gas chromatography. The blood:air partition coefficient in the PBPK model was determined by curve fitting, because simulations based on published values did not agree well with data. Curve fitting gave a value of 0.092 ± 0.029. The real-time nature and high sensitivity of photoacoustic detection make it a useful addition to gas chromatography in closed chamber studies.

Gas/particle concentrations and partitioning of PCBs in the atmosphere of Korea

Abstract Air samples were taken in a rural area of Kyonggi-do of South Korea to investigate gas/particle concentration and partitioning. Gas/particle phase concentrations of polychlorinated biphenyls (PCBs) were measured over 48 h periods from September 2001 to July 2002. The average contribution (%) of gas to particle was about 90%, which suggests that in the atmosphere PCBs predominantly existed in the gas phase. The correlation coefficients (r) between total PCBs and temperature (°C) showed negative correlation for particle phase and positive correlation for gas phase. The contribution (%) of PCB congeners to total PCBs in gas phase increased with decreasing chlorine substitution. The particle contribution (%) of PCB congeners increased for the less volatile congeners, i.e., those with the higher chlorine number. The coefficient of determination (R2) between log (Cparticle/Cgas) and log KOA was 0.86, which implies that heavier PCB congeners exist more in particle phase than gas phase compared to lighter PCBs. Partitioning of PCBs between gas and particle phases correlated well with the sub-cooled liquid vapor pressure (PoL) and the octanol–air partition coefficient (KOA) for all samples, but the coefficient of determination (R2) differed for each season.

Relationship between Olive Oil:Air, Saline:Air, and Rat Brain:Air Partition Coefficients of Organic Solvents In Vitro

Partition coefficients of 28 volatile organic solvents (13 alkylbenzenes, 10 chlorinated hydrocarbons, and 5 ketones) in olive oil, saline, and rat brain tissue homogenates were measured by equilibration in a closed vial and subsequent gas-chromatographic analysis of headspace air. The values of oil and saline partition coefficients correlate well with previously reported data. Brain partition coefficients were fit to a bilinear equation of the form P brain:air = f o P oil:air + f s P saline:air + c. The regression coefficients accurately predicted previously reported rat brain partition coefficients of 19 solvents with distinct physicochemical properties within a factor of 2.5. The combined data set of presently determined and previously reported brain partition coefficients (n = 46) yields tissue-specific regression coefficients for solvent partitioning in rat brain of 0.028 for f O' , 0.845 for f S' , and 0.90 for the intercept, with coefficients of variation amounting to 11%, 4%, and 463%, respectively. The generalized empirical relationship predicts the brain partition coefficients within a factor of 2.5 accurately for 95% of the compounds. The ratios of rat brain concentrations calculated from predicted and measured P brain:air and P blood:air values were within a factor of 4 for 95% of the compounds. It was concluded that the enlargement of the empirical data set leads to more reliable predictions of rat brain partition coefficients, particularly for the lipophilic volatile organic compounds.

Is vapor pressure or the octanol–air partition coefficient a better descriptor of the partitioning between gas phase and organic matter?

Abstract Both the sub-cooled liquid vapor pressure (PL) and the octanol–air partition coefficient (KOA) are used to describe the partitioning of non-polar organic compounds between the gas phase and a variety of natural organic substrates in soil, atmospheric particles and foliage. Whether the former is preferable over the latter depends on whether the interaction of the organic compound with the organic matter (OM) resembles more those in the pure liquid than those in liquid octanol. The activity coefficient in octanol (γOct) is a quantitative measure of the difference between these two interactions. An analysis of PL and KOA values for several sets of non-polar and semi-volatile organic compounds (chlorobenzenes, PCBs, PCNs, PCDD/Fs, PBDEs), and of the γOct values derived from these, reveals that γOct tends to range from 1 to 10 suggesting that PL and KOA are very highly correlated. Furthermore, the estimated standard deviation of γOct tends to be so large that PL and KOA are virtually indistinguishable within the measurement uncertainty. Whether γOct within a group of related compounds increases, decreases or stays the same with increasing molecular mass depends on the specific KOA and PL data set used in the calculation of γOct. This implies that with the current precision of KOA, PL and partition coefficients involving OM it is impossible to judge one parameter better than the other.

Quantitative structure–property relationships for physiologically based pharmacokinetic modeling of volatile organic chemicals in rats

The objective of present study was to develop quantitative structure–property relationships (QSPRs) for the chemical-specific input parameters of rat physiologically based pharmacokinetic (PBPK) models (i.e., blood:air partition coefficient ( P b), liver:air partition coefficient ( P l), muscle:air partition coefficient ( P m), fat:air partition coefficient ( P f), and hepatic clearance ( CL h)), for simulating the inhalation pharmacokinetics of volatile organic chemicals (VOCs). The literature data on P b, P l, P f, and P m for 46 low-molecular-weight VOCs as well as CL h for 25 such VOCs primarily metabolized by CYP2E1 (alkanes, haloalkanes, haloethylenes, and aromatic hydrocarbons) were analysed to develop QSPRs. The QSPRs developed in this study were essentially multilinear additive models, which imply that each fragment in the molecular structure has an additive and constant contribution to partition coefficients and hepatic clearance. Most of the values in the calibration set could be reproduced adequately with the QSPR approach, which involved the calculation of the sum of the frequency of occurrence of fragments (CH 3, CH 2, CH, C, CC, H, Cl, Br, F, benzene ring, and H in benzene ring structure) times the fragment-specific contributions determined in this study. The QSPRs for P b, P l, P m, P f, and CL h were then included within a PBPK model, which only required the specification of the frequency of occurrence of fragments in a molecule along with exposure concentration and duration as input for conducting pharmacokinetic simulations. This QSPR–PBPK model framework facilitated the prediction of the inhalation pharmacokinetics of four VOCs present in the calibration dataset (toluene, dichloromethane, trichloroethylene, and 1,1,1-trichloroethane) and four VOCs that were not part of the calibration set (1,2,4-trimethyl benzene, ethyl benzene, 1,3-dichloropropene, and 2,2-dichloro-1,1,1-trifluoroethane) but that could be described using the molecular fragments investigated in the present study. The QSPRs developed in this study should be potentially useful for providing a first-cut evaluation of the inhalation pharmacokinetics of VOCs prior to experimentation, as long as the number and nature of the fragments do not exceed the ones in the calibration dataset used in this study.

Air−Soil Exchange of Organochlorine Pesticides in Agricultural Soils. 2. Laboratory Measurements of the Soil−Air Partition Coefficient

This is the second of two papers that investigate soil−air exchange of organochlorine (OC) pesticides. In the present paper, a fugacity meter was used to measure soil−air partition coefficients (KSA) under controlled laboratory conditions for OC pesticides in three different soils as a function of temperature and soil organic carbon content. A strong temperature dependence of KSA was observed for all three soils. A linear relationship between KSA and the octanol−air partition coefficient (KOA) was observed for all soils. The enthalpy of soil−air exchange (ΔHSA) was calculated and shown to be slightly greater than the enthalpy of octanol−air exchange or the enthalpy of vaporization, possibly because of interactions between the pesticides and the organic matter in the soil. ΔHSA values were very similar for all soils, irrespective of organic matter content/type. An expression for predicting KSA values on the basis of KOA, fraction of organic carbon, and soil density (Karickhoff relationship) was tested against the chamber measurements and found to produce consistent values of KSA. A small offset was observed between field-derived values of QSA (soil−air quotient, an approximation of KSA) and chamber-derived values of KSA. This was attributed to several factors including possible incomplete equilibrium during the field measurements and fluctuation of meteorological parameters that govern soil−air exchange. Ultimately, the findings of this work and the previous field study of soil−air exchange indicate that the Karickhoff relationship is useful and valid from a modeling standpoint where it is necessary to compromise accuracy in exchange for simple relationships that utilize readily available input data.

Air−Soil Exchange of Organochlorine Pesticides in Agricultural Soils. 1. Field Measurements Using a Novel in Situ Sampling Device

Initial results are presented for in situ measurements of soil−air partitioning for a range of organochlorine (OC) pesticides in two contaminated agricultural soils. A soil survey was conducted and used to identify high levels of several OC pesticides in two regions of southern Ontario that are known for their intensive agriculture, the Tobacco Belt and the Holland Marsh. Experiments were conducted at one field in each region by sampling air very close to the soil surface using a disc-shaped sampler. The equilibrium status of the sampled air was tested by comparing the chiral signature of the soil with the signature in air sampled by the device and ambient air. Although results showed that 104% of trans-chlordane (TC) and 96% of cis-chlordane (CC) in the air under the sampler originated from the soil, the propagated errors in these results (34% SD for TC and 26% SD for CC) are too large to provide conclusive evidence for equilibrium. Therefore, a soil−air quotient (QSA) is reported here instead of the soil−air partition coefficient (KSA). This value is an approximation of the “true” KSA. Results show a linear relationship between log QSA and log KOA and fit in with the relationship KSA = 0.411ρφOCKOA where ρ is the soil density (kg L-1). Using this relationship, fugacities were calculated in air and soil. Results of this calculation identify a strong disparity that favors soil-to-air transfer. This gradient is confirmed by measurements at different heights over one of the fields. Soil−air exchange is a key process in the overall fate of OC pesticides. The results from this study will improve our ability to model this process and account for differences between soils.

Quantitative predictive models for octanol–air partition coefficients of polybrominated diphenyl ethers at different temperatures

Quantitative predictive models for octanol–air partition coefficients of polybrominated diphenyl ethers at different environmental temperatures ( T) were developed. Partial least squares (PLS) regression was used for model development. A list of 18 theoretical molecular structural descriptors was screened by PLS analysis. The optimal model was selected from the one containing nine theoretical molecular descriptors and 1/ T as predictor variables. The cross-validated Q 2 cum value for the optimal model is 0.975, indicating a good predictive ability and stability of the model. Intermolecular dispersive interactions play a leading role in governing the magnitude of log K OA. The lower the E LUMO (the energy of the lowest unoccupied molecular orbital), the greater the intermolecular interactions between octanol and PCB molecules, and thus the greater the log K OA values.

Patterns of contamination among vascular plants exposed to local sources of polychlorinated biphenyls in the Canadian Arctic and Subarctic

Polychlorinated biphenyl (PCB) concentrations were examined in 1043 Arctic vascular plant specimens comprising 31 genera and the soils they grew in. The samples were collected at 61 abandoned military and Coast Guard sites across the Canadian Arctic and Subarctic and in nine remote background locations. Genus-specific differences in PCB uptake, partitioning of PCBs among different plant tissues, and congener-specific uptake of PCBs were examined. Mean PCB concentrations and plant vs. soil regression relationships were significantly different among genera. The highest concentrations were found in Poa and Luzula and the lowest mean concentration was found in Betula. Among the genera examined, PCB concentrations in the genus Luzula exhibited the greatest increase relative to increasing soil PCB concentrations. Bioaccumulation factors were not fixed within a single genus or species, but decreased with increasing soil concentrations, suggesting that at higher levels of exposure accumulation of PCBs may be kinetically limited by redistribution processes within the plant. The accumulation of specific congeners was related to the primary mode of exposure and the octanol–air partition coefficient ( K oa) of the congener. In plants exposed mainly to atmospheric PCBs, uptake increased with increasing K oa, as has been reported elsewhere. By contrast, there was a negative correlation between accumulation and K oa in plants that were mainly exposed through direct contact with contaminated soil. Only congeners 132/153 were found at concentrations greater than predicted from their K oa. The presence of these congeners in plants is proposed as the explanation for their predominance in terrestrial animal tissues.

Selecting internally consistent physicochemical properties of organic compounds

Methods are presented for selecting values of chemical properties of vapor pressure, water solubility, Henry's law constant, and octanol-water and octanol-air partition coefficients, which are subject to thermodynamic constraints, while taking advantage of all measurements. The aim of the mathematical procedures is to find the one set of internally consistent partitioning parameters that is minimally divergent from the experimental values. Information about the reliability or uncertainty of reported values can be accounted for by weighing factors. A similar approach is applied to the temperature dependence of these properties. The influence of partial miscibility of the octanol-water system is discussed and a correction is suggested for this effect. The selection method is applied to 50 mostly aromatic chemicals for which multiple measured partitioning data are available. The resulting sets of consistent property data are presented and discussed.

A Study of the Biological Partitioning Behavior of n-Alkanes and n-Alkanols in Causing Anesthetic Effects

n-Alkanes and n-alkanols are two groups of common volatile organic compounds (VOCs) having potential anesthetic effects on workers and building occupants. A partition model based on the octanol–air partition coefficient was developed in this investigation to describe the biological partitioning of n-alkanes and n-alkanols in causing general anesthesia. Data on anesthetic potency (minimum alveolar concentration, MAC) for the test groups in rats were found to fit the model. The slight difference between the n-alkanes and n-alkanols in testing the model could be largely eliminated by correcting for the potential partial pressure gradients of the long-chain alkanes across the blood–brain barrier. The corrected MAC data for the two test groups fit well onto one common activity–partition regression line. This suggests that n-alkanes and n-alkanols may share a common biophase or mechanistic pathway for anesthesia. The present findings may provide some useful insight into setting anesthesia-based health standards for VOC mixtures.

Quantitative structure-property relationships for octanol-air partition coefficients of polychlorinated biphenyls.

Based on nine quantum chemical descriptors computed by PM3 Hamiltonian, using partial least squares analysis, a significant quantitative structure-property relationship for the logarithm of octanol-air partition coefficients (logK(OA)) of polychlorinated biphenyls (PCBs) was obtained. The cross-validated Q2cum value of the model is 0.962, indicating a good predictive ability. The intermolecular dispersive interactions and thus the size of the PCB molecules play a key role in governing log K(OA). The greater the size of PCB molecules, the greater the logK(OA) values. Increasing ELUMO (the energy of the lowest unoccupied molecular orbital) values of the PCBs leads to decreasing logK(OA) values, indicating possible interactions between PCB and octanol molecules. Increasing Q(Cl)+, (the most positive net atomic charges on a chlorine atom) and Q(C)- (the largest negative net atomic charge on a carbon atom) values of PCBs results in decreasing lg K(OA) values, implying possible intermolecular electrostatic interactions between octanol and PCB molecules.

Determination of Octanol−Air Partition Coefficient (KOA) Values for Chlorobenzenes and Polychlorinated Naphthalenes from Gas Chromatographic Retention Times

Isothermal capacity factors k on three gas chromatographic columns of different polarity were determined for chlorobenzenes (CBzs) and polychlorinated naphthalenes (PCNs). All possible multiple linear regressions of the logarithm of these capacity factors with previously reported temperature-dependent octanol−air partition coefficient (KOA) values of selected CBzs and PCN congeners were performed to derive predictive relationships for the remaining congeners. A comparative evaluation of the regression coefficients and the standard error of the predicted KOA values revealed that regressions with the capacity factor on a single nonpolar column (DB-5ms) yielded the best results for both CBzs and PCNs. Using the best regression equations, KOA values for 6 CBzs and 27 PCNs were obtained for the temperature range from 10 °C to 50 °C. The internal energies of air−octanol phase transfer ΔOAU for these chemicals were calculated by correlating the interpolated log KOA values with reciprocal absolute temperature.