United States
Environmental Protection
Agency
National Exposure
Research Laboratory
Athens, GA 30605-2700
Research and Development
EPA/600/S-96/006 September 1996
&EPA
ENVIRONM
RESEARCH
NTAL
RIEF
Octanol/Water Partition Coefficients for Eight Phthalate Esters
J. Jackson Ellington and Terry L Floyd1
Abstract
Octanol/water partition coefficients (Kow) for eight dialkyl phtha-
late esters were measured by the "slow-stir" method. The Kows
and water solubilities for the esters were also computed by a
computational expert system (SPARC). The largest difference
between measured and computed values was for bis(2-
ethylhexyl) phthalate; the difference was 0.27 log units at a log
Kow of >7. Values for diethyl-, butylbenzyl-, and diamyl phtha-
late differed by only 0.04 log units.
Background
The Clean Water Act, the Marine Protection, Research, and
Sanctuaries Act, the Endangered Species Act, and the Com-
prehensive Environmental Response, Compensation, and Li-
ability Act (Superfund) require EPA to address sediment quality
issues. A water quality-based approach is currently used to
implement pollution control and prevention strategies for toxics
in the water column. A similar approach is under development
for sediments. Development of Sediment Quality Criteria (SQC)
is an on-going component of the current Office of Research
and Development (ORD) sediment research program. The
method employed for deriving SQC uses equilibrium partition-
ing to estimate the amount of bioavailable contaminants) in
sediment porewater for comparison with the water quality crite-
ria for the contaminant(s). This approach is referred to as the
equilibrium partitioning (EqP) model of bioavailability. The EqP
method uses .partition coefficients for relating the freely dis-
solved and total concentrations of chemicals in the water col-
umn, and pore water concentrations to total sediment concen-
trations of chemicals in sediments. The EqP method requires
'Ecosystems Research Division, National Exposure Research Laboratory, U.S.
Environmental Protection Agency, Athens, GA 30605-2700.
sediment (K^), particulate (K^) and dissolved (K^) organic
carbon partition coefficients, and the octanol/water partition
coefficient (Kow). Correlations have been developed for log K^.,
log K , and log Kdoc with log Kow that allow their calculation
from aknown value of log Kow. Uncertainties in SQC values for
a chemical are closely related to the uncertainties in the log Kow
of the chemical.
A proposal by the Phthalate Ester Panel (PEP) of the Chemical
Manufacturers Association offered an opportunity for EPA to
test the EqP method on dialkyl phthalate esters with log Kow s
ranging from 1.5 to 8.5. The Environmental Research Task
Group (ERTG) of the Panel drafted a preproposal for a sedi-
ment research program designed to evaluate the environmen-
tal safety of sediment-associated phthalate esters. The
preproposal was submitted to the Sediment Quality Program in
the Health and Ecological Criteria Division of the Office of
Water of the U.S. Environmental Protection Agency (EPA) for
review. After EPA review of the proposal, a joint meeting was
held between ERTG and EPA, and consensus was reached on
a research plan that would address joint research needs. The
ERTG needed to assess the possible effects of phthalate
esters on benthic organisms, and therefore required data to
characterize potential phthalate ester exposures from contami-
nated sediments. One EPA objective for the research was to
test the EqP method for predicting the toxicity of sediment-
associated phthalate esters to benthic organisms. EPA agreed
to determine the octanol/water partition coefficients (Kow ) of
the phthalate esters by the "slow-stir" method (1-3). These log
Kw values and water-only toxicity data generated in prelimi-
nary experiments will be used to calculate sediment spiking
concentrations for use in experiments designed to determine
the potential for adverse acute effects at environmentally rel-
evant concentrations. Data generated in these spiked sediment
toxicity tests will be used to evaluate or "compare" the extrapo-
Printedon Recycled Paper
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lation of the EqP model across sediments and organisms. A
second EPA objective was to compare the laboratory mea-
sured "slow-stir" log K^, to values computed by SPARC (4,5).
Procedure
A detailed description of the method is reported elsewhere (6).
Briefly, at the beginning of an experiment, water (0.5-9L) is
brought into the equilibration vessel together with a Teflon-
coated magnetic stirring bar, and a minimum vortex is initiated
through the stirring action. When the water temperature has
equilibrated to 25°C, the stirring is stopped and 10 to 25 ml of
octanol are carefully layered onto the surface. Stirring is re-
sumed. After 24 hours, stirring is stopped and octanol (30-100
ml) containing one of the esters to be measured is carefully
added in a manner that avoids mixing the two phases. The
concentration of the ester in the octanol is calculated to yield a
final octanol layer concentration of 2 to 4 parts per thousand.
Samples of octanol and water are analyzed daily until the ratio
of the concentration of the chemical in the octanol divided by
the concentration of the chemical in the water is the same for
three successive days.
The water sample for each analysis is removed through a
stopcock at the base of the equilibration vessel. The ester is
extracted from the water sample by either methylene chloride
or Empore™ extraction disks (acetonitrile extraction solvent).
The extract is usually concentrated to a final volume of 0.5 to 1
ml by blowdown with a gentle stream of nitrogen. The concen-
tration factors usually range from 10 to 8,000. Analysis of the
octanol layer is performed by removing 1 ml and successively
diluting it with solvent until gas or liquid chromatographic analy-
sis of equal volume (1ul gas chromatograph and 10 ul liquid
chromatograph) injections of both the final octanol dilution and
the water extract concentrate yield the same (within a factor of
two) detector response. The K^ is then calculated:
vow
(Detector response, octanol) x Dilution factor
(Detector response, water) •+• Concentration factor
where M0 and Mw are solvent molecularities of octanol and
water, respectively. Activity coefficients for either solvent or
solute are computed by solvation models that are built from
structural constituents requiring no data besides the structures.
Water solubilities are computed in a similar manner.
A goal for SPARC is to compute values that are as accurate as
values obtained experimentally for a fraction of the cost re-
quired to measure them. Because SPARC does not depend on
laboratory measurements conducted on compounds with struc-
tures closely related to that of the solute of interest, it does not
have, for instance, the inherent problems of phase separation
encountered in measuring highly hydrophobic compounds (log
Kow > 5). For these compounds, SPARC's computed value
should, therefore, be more reliable than a measured one.
However, at this time, no SPARC version number has been
assigned for the physical property calculator. Data computed
after future refinement in the calculator may, therefore, be
slightly different.
Results and Discussion
Table 1 contains the list of phthalate esters studied and their
log Kow s measured by the "slow-stir" method and computed by
SPARC. Because the water solubility of a chemical is an
important physico-chemical parameter, the SPARC computed
water solubilities are also listed. Three equilibration vessels
were used in the "slow-stir" measurements of each phthalate
ester. For the esters with n>3 in Table 1, multiple samples
were withdrawn from each flask. For example, each dimethyl
phthalate vessel was sampled 4 times to yield a total of 12
determinations. For bis(2-ethylhexyl) phthalate (DEHP) and
dioctyl phthalate (OOP) the contents of a single vessel were
sampled on days 5, 6 and 8. The measured values and
calculated values are in close agreement. The greatest differ-
ence between the SPARC-computed value and the mean of
the "slow-stir" values is 0.27 log units for DEHP. Log Kow values
reported in the literature for DEHP range from 5.11 to 9.61.
Computed and measured values for diethyl-, butylbenzyl-, and
diamyl phthalate differ by only 0.04 log units.
Computed Data
SPARC (SPARC Performs Automated Reasoning in Chemis-
try) is a computational expert system that predicts chemical
reactivity. The system has the capability of crossing chemical
class boundaries to cover all organic chemicals and using
algorithms based on fundamental chemical structure theory to
estimate parameters. SPARC quantifies reactivity by classify-
ing molecular structures and selecting appropriate "mechanis-
tic" models. It uses an approach that combines principles of
quantitative structure-activity relationships, linear free energy
theory (LFET), and perturbed molecular orbital (PMO) or quan-
tum theory to describe quantum effects such as delocalization
energies or polarizabilities of K electrons. SPARC computes
the log of the octanol/water partition coefficient from activity
coefficients in the octanol (yj and water (y~w) phases:
log K = log -$ + log
MO
M
Acknowledgments
The authors thank Dr. Samuel W. Karickoff and Dr. J. MacArthur
Long of the U.S. Environmental Protection Agency, Athens,
Georgia, for providing the SPARC-calculated Kows and water
solubilities.
References
1. De Bruijn, J., F. Busser, W. Seiner, and J'. Hermens. 1989.
Determination of octanol/water partition coefficients for
hydrophobic organic chemicals with the "slow-stirring
method, Environ. Toxicol, and Chem., 8:499-512.
2. Brooke, D.N., A.J. Dobbs, and N. Williams. 1986.
OctanoNwater partition coefficients (P): Measurement,
estimation, and interpretation, particularly for chemicals
with P > 10s. Ecotoxicol. Environ. Safety 77:251-260.
3. Marple, L, B. Berridge, and L. Throop. 1986. Measurement
of the water-octanol partition coefficient of 2,3,7,8-
tetrachlorodibenzo-p-dioxin, Environ. Sci. Technol. 20:397-
399.
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4. Karickhoff, S.W., V.K. McDaniet, C. Melton, A.N. Vellino,
D.E. Nute, and L.A. Carreira. 1991. Predicting chemical
reactivity by computer. Environ. Toxicol. Chem., 10
5. Hilal, S.H., L.A. Carreira, and S.W. Karickoff. 1989.
Estimation of chemical reactivity parameters and physical
properties of organic molecules using SPARC. IN:
Quantitative Treatments of Solute/Solvent Interactions,
Theoretical and Computational Chemistry, Vol. 1, Politzer,
P. and Murrary, J.S., (Eds.), Elsevier Science.-Amsterdam,
pp. 291-353.
Ellington, J.J., and T.L. Floyd. 1996. Measuring octanol/
water partition coefficients by the slow-stirring method,
U.S. Environmental Protection Agency, Athens, Georgia,
in press.
Table 1. Log Kox Values for Eight Phthalate Esters
Chemical Name
Dimethyl phthalate
Diethyl phthalate
Dibutyl phthalate
Butylbenzyl phthalate
Diamyl phthalate
Dihexyl phthalate
Bis(S-ethylhexyl) phthalate
Dioctyl phthalate
CAS Number
131-11-3
84-66-2
84-74-2
85-68-7
131-18-0
84-75-3
117-81-7
117-84-0
Log Km
"Slow-Stir"
1.60 ± 0.04 (n=12)
2.42 ± 0.04 (n=9)
4.50 ± 0.03 (n=9)
4.73 ± 0.06 (n=6)
5.62 ± 0.04 (n=6)
6.82 ± 0.10 (n=5)
7.27 ± 0.04 (n=3)
8.10 + 0.11 (n=3)
V
SPARC
1.48
2.51
4.63
4.77
5.66
6.67
7.54
8.30
Water Solubility
(SPARC), mg/L
3.3E3
4.0E2
4.9EO
2.4EO
4.9E-1
4.9E-2
2.6E-3
4.6E-4
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