ENVIRONMENTAL

RESEARCH BRIEF

Octanol/Water Partition Coefficients for Eight Phthalate Esters

Research and Development EPA/600/S-96/006 September 1996

J. Jackson Ellington and Terry L. Floyd1

United States National Exposure

Environmental Protection Research Laboratory

Agency Athens, GA 30605-2700

Abstract

Octanol/water partition coefficients (Kow) for eight dialkyl phthalate 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 phthalate 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 Comprehensive Environmental Response, Compensation, and Liability 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 partitioning to estimate the amount of bioavailable contaminant(s) in

sediment porewater for comparison with the water quality criteria 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 dissolved and total concentrations of chemicals in the water column, and pore water concentrations to total sediment concentrations of chemicals in sediments. The EqP method requires

sediment (Koc), particulate (Kpoc) and dissolved (Kdoc) organic

carbon partition coefficients, and the octanol/water partition

coefficient (Kow). Correlations have been developed for log Koc,

log Kpoc, and log Kdoc with log Kow that allow their calculation

from a known 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 sediment research program designed to evaluate the environmental 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 contaminated sediments. One EPA objective for the research was to

test the EqP method for predicting the toxicity of sedimentassociated 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

Kow values and water-only toxicity data generated in preliminary experiments will be used to calculate sediment spiking

concentrations for use in experiments designed to determine

the potential for adverse acute effects at environmentally relevant concentrations. Data generated in these spiked sediment

toxicity tests will be used to evaluate or “compare” the extrapo- 1

Ecosystems Research Division, National Exposure Research Laboratory, U.S.

Environmental Protection Agency, Athens, GA 30605-2700.
2

lation of the EqP model across sediments and organisms. A

second EPA objective was to compare the laboratory measured “slow-stir” log Kow 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 Tefloncoated magnetic stirring bar, and a minimum vortex is initiated

through the stirring action. When the water temperature has

equilibrated to 25o

C, the stirring is stopped and 10 to 25 ml of

octanol are carefully layered onto the surface. Stirring is resumed. 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 EmporeTM 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 concentration 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 analysis of equal volume (1μl gas chromatograph and 10 μl 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 Kow is then calculated:

Computed Data

SPARC (SPARC Performs Automated Reasoning in Chemistry) 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 classifying molecular structures and selecting appropriate “mechanistic” models. It uses an approach that combines principles of

quantitative structure-activity relationships, linear free energy

theory (LFET), and perturbed molecular orbital (PMO) or quantum theory to describe quantum effects such as delocalization

energies or polarizabilities of π electrons. SPARC computes

the log of the octanol/water partition coefficient from activity

coefficients in the octanol (γ ∞

o) and water (γ ∞w) phases:

log Kow = log 

γ

4

w

γ

4

o

+ log 

Mo

Mw

where Mo 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 required to measure them. Because SPARC does not depend on

laboratory measurements conducted on compounds with structures 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 (DOP) 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 difference 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.

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.

Octanol:water partition coefficients (P): Measurement,

estimation, and interpretation, particularly for chemicals

with P > 105

. Ecotoxicol. Environ. Safety 11:251-260.

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.

Kow = ÷

(Detector response, octanol) x Dilution factor

(Detector response, water) Concentration factor
3

4. Karickhoff, S.W., V.K. McDaniel, C. Melton, A.N. Vellino,

D.E. Nute, and L.A. Carreira. 1991. Predicting chemical

reactivity by computer. Environ. Toxicol. Chem., 10

(11):1405-1416.

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.

6. 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 Kow Values for Eight Phthalate Esters

Log Kow Water Solubility

Chemical Name CAS Number "Slow-Stir" SPARC (SPARC), mg/L

Dimethyl phthalate 131-11-3 1.60 ± 0.04 (n=12) 1.48 3.3E3

Diethyl phthalate 84-66-2 2.42 ± 0.04 (n=9) 2.51 4.0E2

Dibutyl phthalate 84-74-2 4.50 ± 0.03 (n=9) 4.63 4.9E0

Butylbenzyl phthalate 85-68-7 4.73 ± 0.06 (n=6) 4.77 2.4E0

Diamyl phthalate 131-18-0 5.62 ± 0.04 (n=6) 5.66 4.9E-1

Dihexyl phthalate 84-75-3 6.82 ± 0.10 (n=5) 6.67 4.9E-2

Bis(2-ethylhexyl) phthalate 117-81-7 7.27 ± 0.04 (n=3) 7.54 2.6E-3

Dioctyl phthalate 117-84-0 8.10 ± 0.11 (n=3) 8.30 4.6E-4
4

United States

Environmental Protection Agency

Center for Environmental Research Information

Cincinnati, OH 45268

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT NO. G-35

Official Business

Penalty for Private Use $300

EPA/600/S-96/006

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