Page 82 - Partition & Adsorption of Organic Contaminants in Environmental Systems

LIPID–WATER SYSTEMS 73

Determined K tw values, corresponding K ow values, solubilities in water (S w),
V
and molar volumes ( ) of 38 organic solutes at room temperature (20 to 25°C)
are listed in Table 5.5. The S w values for solid solutes are the values of their
supercooled liquids, calculated from solid solubilities, heats of fusion (DH fus),
and melting points (T m) according to Eqs. (3.9) and (3.25). For 1,2,3-
trichlorobenzene, 1,3,5-trichlorobenzene, 1,2,3,4-tetrachlorobenzene, 1,2,3,5-
tetrachlorobenzene, and pentachlorobenzene, which have low melting points
(T m < 370K), calculations were made with the assumption of DH fus = 56.5T m
(J/mol), along with the solid solubilities of 16.3, 10.6, 7.18, 3.23, and 0.385mg/L.
The molar volumes are those for solutes in the liquid state; densities of 1,2,3-
trichlorobenzene, 1,2,3,4-tetrachlorobenzene, pentachlorobenzene, hex-
achlorobenzene, and DDT at their melting points were determined and used
to calculate their V values. Liquid molar volumes of 1,3,5-trichlorobenzene
and 1,2,3,5-tetrachlorobenzene were assumed to be the same as those of 1,2,3-
trichlorobenzene and 1,2,3,4-tetrachlorobenzene. Liquid molar volumes of
PCBs were approximated by using the densities of liquid Arochlor PCB mix-
tures that have approximately the same chlorine numbers as the individual
PCBs.
We now show more explicitly the calculated g* t values by Raoult’s law and
their dependence on solute molecular size for the solutes in Table 5.5. For
small solutes with V t /V > 6, Eq. (5.14) leads to g* t = 0.27 to 0.42. This implied
serious negative deviation from Raoult’s law is not justified by the lack of spe-
cific solute–solvent interactions between these solutes and triolein, but rather,
is an artifact of the model calculation (Chiou and Manes, 1986). As expected,
the assumed molecular-size effect on g* t by Raoult’s law becomes progressively
reduced (i.e., the g* t increases toward 1) as the solute molecular size increases.
Although the resulting g* t values for large solutes, such as hexachlorobenzene
(HCB), DDT, and some PCBs, are greater than 1, they are not physically
rigorous because the observed solubility of DDT and others cannot be well
accounted for by Raoult’s law, as shown earlier.
With the noted limitation of Raoult’s law, Chiou (1985) treated the solute
partition coefficient in a triolein–water mixture by application of the Flory–
Huggins model [Eq. (3.13)], which gives
*
logK tw =- logS V - ( [ 1 - V V t ) + c ] . 2 303 - log( g w g* w) (5.15)
w
where V is the molar volume of the solute. Other terms remain as defined
earlier. The water content in triolein at 25°C is 0.11% by weight (or 5.6 ¥
-2
10 M), which is significantly less than that in octanol (2.3M). This gives
V * = 0.919L/mol, or logV * t =-0.037, on the assumption of volume additivity
t
for triolein and water. To simplify the analysis further, again the term
log (g w /g* w ) accounting for the solute solubility enhancement in water by dis-
solved triolein is assumed to be zero.
Since Eq. (5.15) accommodates effectively the measured logK tw values
for all the solutes, it is used as the basis for interpreting the solute behavior in