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Organic Solvent-Water Partitioning
ENVR 725
March 5, 2013
Ariel Atkinson
1
Outline
• Why we care
• How we define l-w partitioning
• What influences partitioning
• Octanol-Water Partition Constant
•...
Practically—
Why do we care?
• Liquid-Liquid Extractions
• Contaminant Fate
• Describe a compound- Measure of
Hydrophobici...
Definition
We could define partitioning between
and organic phase (solvent) and the
water phase at equilibrium as:
Kilw= C...
What physical-chemical factors determine or
influence partitioning between phases?
• Solvent-Solute Properties
– Molecular...
66
hexane
benzene
octanol
Methanol
Ex Solvents
Trichloroethylene
Tetrachlorobiphenyl
Dibromoacetic acid
Acrolein
Equilin
W...
Solute-Solvent Properties
Apolar, weakly polar: partition favorably from water to
organic solvent and their Kilw don’t var...
Activity
From the thermodynamics of Chapter 3, at equilibrium:
γilxiil = γiw xiw
going to molar concentrations CilVl= Xil
...
The partition coefficient expresses the
difference between the nonideality of the
compound in the organic solvent and in t...
Co-solvation
Water is somewhat soluble in the octanol phase, such
that 1 out of every 4 molecules in the octanol phase wil...
Concentration
Assume dilute solution-the activity coefficient of
compound i in water is independent of its
amount in the a...
Solvent Matrices- Effect of Salt
• Multiple Solute systems
• Addition of salts to water containing organic solutes
increas...
ΔlwHi -the enthalpy of transfer of i from water to organic solvent- is
the difference in excess enthalpies of the compound...
Effect of Temperature on Organic
Solvent-Water Partitioning
ΔlwHi = Hil
E - Hiw
E
Exceptions to general rule of weak tempe...
The n-Octanol-Water Partition
Constant
n-Octanol:
• Amphiphilic character: apolar part
and bipolar functional group
• Bipo...
The n-Octanol-Water Partition
Constant
• Most commonly measured
partition constant
• Used to define
hydrophobicity/hydroph...
Proposed structural model for
natural organic matterBenzene in octanol:
1717
How to measure
partitioning coefficients
1818
• Measure directly:
• Slow stir
• Shake-Flask
• Predict:
• HPLC method
• Cor...
Experiment
DBPs
DBPs
DBPs
-co-saturated solvents
-pH 7.0
-temperature 25 C
-headspace free
-Measure concentration in water...
Kiow from Chromatographic Data
2020
The column in many HPLC systems is reverse phase,
(column is non-polar and mobile phase is polar, often
ACN-water or MeOH-...
Kiow from Chromatographic Data
Since the time that a partitioning compound spends in
the mobile phase will depend on the p...
Kiow from Chromatographic Data
2323
Generally non-polar Kiow and Ki hex-w correlate well
2424
For apolar and weakly polar compounds
Kiow and Kilw are related:
Log Kihw = 1.21 log Kiow -0.43
For apolar and
weakly pola...
Not surprisingly, there are multiparamaters
LFERs for Kilw ..
If we assume that mutual saturation has little effect
on γiw...
27
Fragmentation Methods
Most Used modeling software-EPISUITE
KOWWIN fragment coefficient (fi)
-CH2- 0.4911
-CH 0.3614
-Cl...
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
2929
What to do with K?
• Use in models to predict behavior, understand
behavior, fate in environment
• Method development...
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Env chem guest lecture organic solvent water partitioning_030513

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Guest Lecture from 030513 in ENVR 725 on Organic Solvent-Water Partitioning

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Env chem guest lecture organic solvent water partitioning_030513

  1. 1. Organic Solvent-Water Partitioning ENVR 725 March 5, 2013 Ariel Atkinson 1
  2. 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. 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. 4. Definition We could define partitioning between and organic phase (solvent) and the water phase at equilibrium as: Kilw= Cil/Ciw 44
  5. 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. 6. 66 hexane benzene octanol Methanol Ex Solvents Trichloroethylene Tetrachlorobiphenyl Dibromoacetic acid Acrolein Equilin Water Water Pollutants of Concern
  7. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 17. Proposed structural model for natural organic matterBenzene in octanol: 1717
  18. 18. How to measure partitioning coefficients 1818 • Measure directly: • Slow stir • Shake-Flask • Predict: • HPLC method • Correlation with other partition coefficients • Fragmentation methods
  19. 19. Experiment DBPs DBPs DBPs -co-saturated solvents -pH 7.0 -temperature 25 C -headspace free -Measure concentration in water and octanol
  20. 20. Kiow from Chromatographic Data 2020
  21. 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. 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. 23. Kiow from Chromatographic Data 2323
  24. 24. Generally non-polar Kiow and Ki hex-w correlate well 2424
  25. 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. 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. 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. 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. 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

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