Molecular Structure of MTBE

MTBE In Water

Due to the physiochemical properties of MTBE it is difficult to deal with and spreads quickly. It is highly soluble in water (50 g/L) compared to other organic compounds, therefore moves easily with both surface and ground water. It does not sorb to soil well due to it's low organic-carbon partition coefficient (Koc). Also, MTBE does not readily volatilize from water or gasoline due to it's low Henry's Law constant (Squillace, 1998). However, some MTBE will volatilize from water very slowly depending on the temperature and aeration of the water. The half-life of MTBE in water varies dramatically from 2 years in a still aquifer (Yu, 2006) to a few days in a rapidly moving stream.

MTBE In Plants

Plants can readily uptake MTBE into their shoots and leaves. The uptake and transpiration of organic compounds in plants depends on the octanol-water partition coefficient (log Kow). This is a measure of a chemical's affinity for water versus lipid or fats. The optimum range for plant uptake is 0.5-3.0 (Yu, 2006). MTBE has been shown to simply move up the transpiration path, with water, in several species of plants (Arnold, 2007) and volatilize from foliage.

MTBE is relatively recalcitrant to degredation in plants. The most common break-down product is tert-butyl alcohol (TBA). The initial step of hydrolysis to alcohol is difficult with MTBE because it is a hindered ether. Also, degredation of the tert-butyl group is difficult, requiring enzymes that plants do not typically produce (Davis, 2004). Therefore, almost all of the MTBE that is taken up by a plant is transpired in the same form in which it was taken up.

Physical State

Colorless liquid

Molecular formula

C5H12O

Molecular weigt

88.15

Melting point

-109°C

Boiling point

55.2°C

Water solubility

50 g/L @ 25°C

Density

0.74 g/mL @ 25°C

Koc

1.11

log Kow

1.24

Vapor pressure

245mmHg @ 25°C

Henry’s Law constant

0.02 @ 25°C

Odor threshold

15-40 ppb

Taste threshold

40-140 ppb

Volume in gasoline

Up to 15%

Water solubility in gasoline

5100 ppb

Table of Physical Properties of MTBE (Davis, 2004)