Physical Chemical Properties Affecting Environmental Distribution

Physical-chemical properties dictate the behavior of a chemical in a given environment. The most important physical-chemical properties of several phthalate esters are given in Table 1. Phthalates differ greatly in chemical properties due to their varying chain lengths. Hence, the distribution of phthalates over the various environmental compartments also differs. Molecular weights range from about 194 to 550 gmol-1, water solubility relates inversely to the molecular weight from about 4200 mgl-1 to <0.001 mgl~ (i.e., a variance of over 6 orders of magnitude), but especially molar volume has been shown to be an excellent indicator of water solubility. As with most organic hydrophobic chemicals, phthalate esters are less soluble in saltwater than in freshwater. Experimental determination of the water solubility of phthalates meets difficulties as the esters are bipolar in nature. They may

Table 1 Physical-chemical properties of several phthalate esters

Molecular

Water

weight

solubility

log

Name

Abbreviation

(gmol1)

(mgl1)

KOW

Dimethyl

DMP

194.19

4200

1.61

phthalate

Dibutyl

DBP

278.35

11.2

4.45

phthalate

Diethylhexyl

DEHP

390.56

0.003

7.50

phthalate

Diisononyl

DINP

425

<0.001

>8

phthalate

Diisodecyl

DIDP

447

<0.001

>9

phthalate

form micelles or other types of aggregates. Consequently, experimentally determined water solubilities for individual esters often span several orders of magnitude, and calculated water solubilities based on structure-activity relationships give generally lower estimates for water solubility than experimental results. The environmental distribution of phthalates is nevertheless driven by their partitioning to organic carbon. Octanol-water partition coefficients (Kow) increase (i.e., hydrophobicity increases) with increasing chain length. The reason for this increase is that solubilities in water decrease more with increasing chain length than the solubilities in octanol. The high Kow values of phthalates indicate that these substances are very hydrophobic and will adsorb strongly to organic matter and surfaces. In fact, a linear relationship would be expected between Kow and Koc (the organic carbon-water partition coefficient). However, a linear relationship fails for phthalates, with log Kow exceeding 6. Instead, fairly constant log Koc values of c. 6 are reported for most of the higher phthalates. This discrepancy may be caused by problems in measuring truly dissolved phthalate concentrations in water related to sorption to colloidal matter in the aqueous phase.

Octanol-air partition coefficients are appropriate to describe the partitioning between air and organic phases in soils, plants, and atmospheric aerosols. However, especially for phthalates with alkyl chains of more than six carbon atoms, limited vapor pressure data are available.

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