This document discusses organic solvent-water partitioning, specifically: 1. It defines liquid-liquid partitioning as the equilibrium concentration of a solute between an organic solvent phase and water phase (Kilw= Cil/Ciw). 2. It explains that solute-solvent properties like molecular size, polarity, and inter- and intra-molecular interactions influence partitioning between phases. 3. It describes using the octanol-water partition constant (Kow) to measure and relate a compound's hydrophobicity, and methods for finding partition coefficients experimentally or through prediction models.

1. Organic Solvent-Water Partitioning
ENVR 725
March 5, 2013
Ariel Atkinson
1

2. Outline
• Why we care
• How we define l-w partitioning
• What influences partitioning
• Octanol-Water Partition Constant
• How to find partition coefficients
• Use of partition constants
22

3. Practically—
Why do we care?
• Liquid-Liquid Extractions
• Contaminant Fate
• Describe a compound- Measure of
Hydrophobicity/Hydrophilicity
• Relate partitioning to other things
• Remediation?
33

4. Definition
We could define partitioning between
and organic phase (solvent) and the
water phase at equilibrium as:
Kilw= Cil/Ciw
44

5. What physical-chemical factors determine or
influence partitioning between phases?
• Solvent-Solute Properties
– Molecular size
– Polarity
– Inter- and Intra- molecular interactions
• vdW
• H-bonding
• Co-solvation
– Activity
• Concentration?
• Solvent Matrices?
• Temperature?
5

6. 66
hexane
benzene
octanol
Methanol
Ex Solvents
Trichloroethylene
Tetrachlorobiphenyl
Dibromoacetic acid
Acrolein
Equilin
Water
Water Pollutants of Concern

7. Solute-Solvent Properties
Apolar, weakly polar: partition favorably from water to
organic solvent and their Kilw don’t vary too much
amongst organic solvents, since vdW dominate.
Partitioning from water to organic solvent can be enhanced
if there are complimentary i:l interactions
Example: water-trichloromethane for i=toluene
Trichloromethane = e- accepting/H donor α=0.15 β=0.02
Toluene = H accepting/e- donating α=0 β=0.14
Bipolar solutes don’t partition well into apolar
solvents from water!!
77

8. Activity
From the thermodynamics of Chapter 3, at equilibrium:
γilxiil = γiw xiw
going to molar concentrations CilVl= Xil
γilCil Vl = γiw Ciw Vw
88

9. The partition coefficient expresses the
difference between the nonideality of the
compound in the organic solvent and in the
aqueous phase
Activity
99
Solving for Cil/Ciw and defining a partitioning
coefficient Kilw
Kilw = Cil/Ciw gives
ln Kilw = lnγiw - lnγil + ln(Vw/Vl)
=-
∆𝐺 𝑒
𝑙𝑤
𝑅𝑇
+ constant

10. Co-solvation
Water is somewhat soluble in the octanol phase, such
that 1 out of every 4 molecules in the octanol phase will
be water.
In the aqueous phase- the activity coefficient for octanol
in water is:
γiw = 3.7x103
1/γiw = xiw = 2.7x10-4
So- only 27 molecules of octanol out of 100,000
will be soluble in the water phase.
If you can PRE-SATURATE!!!
1010

11. Concentration
Assume dilute solution-the activity coefficient of
compound i in water is independent of its
amount in the aqueous phase; ie. even at
saturation the probability of two solute
molecules “seeing” one another is small…This
is especially true for compounds with low
solubilities and large γw
sat
The octanol molecules/solvent in the water
phase do not affect the γw
sat of compound I
-This may not actually be the case for some
for some very hydrophobic molecules
1111

12. Solvent Matrices- Effect of Salt
• Multiple Solute systems
• Addition of salts to water containing organic solutes
increases γiw
• Ionic substances are not compatible with apolar
organic solvents, so we don’t expect salts to
dissolve in large amounts
• We expect that the influence of salt on γil to be
minimal, it is really the impact of salt on γiw that
matters:
1212

13. ΔlwHi -the enthalpy of transfer of i from water to organic solvent- is
the difference in excess enthalpies of the compound in the two
phases (depends on the nature of i and of the solvent):
ΔlwHi = Hil
E - Hiw
E
Hiw
E is pretty small for most organic solutes (± 0 kJ mole-1)
Hil
E is ~ ± 10 kJ mole-1
For most organic solutes, the l-w
partitioning is only weakly
temperature dependent! 1313
Effect of Temperature on Organic
Solvent-Water Partitioning

14. Effect of Temperature on Organic
Solvent-Water Partitioning
ΔlwHi = Hil
E - Hiw
E
Exceptions to general rule of weak temperature
dependence:
Small bipolar compounds in apolar solvents:
-these compounds tend to have negative Hiw
E
1414

15. The n-Octanol-Water Partition
Constant
n-Octanol:
• Amphiphilic character: apolar part
and bipolar functional group
• Bipolar functional group ensures
favorable interactions with mono
and bipolar compounds
• Can accommodate large range of
compounds
1515

16. The n-Octanol-Water Partition
Constant
• Most commonly measured
partition constant
• Used to define
hydrophobicity/hydrophilicity of
liquids
• Used as an indicator for
environmental fate
1616

17. Proposed structural model for
natural organic matterBenzene in octanol:
1717

18. How to measure
partitioning coefficients
1818
• Measure directly:
• Slow stir
• Shake-Flask
• Predict:
• HPLC method
• Correlation with other partition
coefficients
• Fragmentation methods

19. Experiment
DBPs
DBPs
DBPs
-co-saturated solvents
-pH 7.0
-temperature 25 C
-headspace free
-Measure concentration in water and octanol

20. Kiow from Chromatographic Data
2020

21. The column in many HPLC systems is reverse phase,
(column is non-polar and mobile phase is polar, often
ACN-water or MeOH-H2O)
Kiow from Chromatographic Data
Partitioning of a hydrophobic organic may be
viewed as sorbing into the non-polar C18
alkanes on the LC packing and then back into
the mobile polar phase. 2121

22. Kiow from Chromatographic Data
Since the time that a partitioning compound spends in
the mobile phase will depend on the partitioning
coefficient (Kiow or Kism)
log Kiow = a log t +b;
where t is the retention time of the non-polar compound
log Kiow = a log (t-to )/to +b
where to is the retention time of some non-retained
species
2222

23. Kiow from Chromatographic Data
2323

24. Generally non-polar Kiow and Ki hex-w correlate well
2424

25. For apolar and weakly polar compounds
Kiow and Kilw are related:
Log Kihw = 1.21 log Kiow -0.43
For apolar and
weakly polar
compounds, vdW
interactions dominate
The 1 parameter
LFER doesn’t hold
for polar compounds
2525

26. Not surprisingly, there are multiparamaters
LFERs for Kilw ..
If we assume that mutual saturation has little effect
on γiw or γil we can relate Kilw to air-solvent and air-
water constants:
𝐾𝑖𝑙𝑤=
𝐾 𝑖𝑎𝑤
𝐾 𝑖𝑎𝑙Now we can use the
multiparameter LFERs for air-
solvent and air-water previously
developed.. (Chp. 6)
2626

27. 27
Fragmentation Methods
Most Used modeling software-EPISUITE
KOWWIN fragment coefficient (fi)
-CH2- 0.4911
-CH 0.3614
-Cl 0.3102
C 0.9723
-Br 0.3997
-I 0.8146
-C#N -0.9218
-OH -1.4086
-O- -1.2566
correction factor (ci)
monohalo acetamides 0.3263
polyhalo acetamides 0.6365
log(Kow)=Sum(fi*ni)+Sum(ci*ni)+0.229
27

28. 28
Fragmentation Methods-Example
Trichloroacetonitrile
ni*fi
3*-Cl 3*0.3102
1* C 0.9723
1* C#N -0.9218
0.229
SUM 1.21
28

29. 2929
What to do with K?
• Use in models to predict behavior, understand
behavior, fate in environment
• Method development: LLE, chromatography
• Use to find other parameters: activity, Kiaw, Kial,
ΔlwH