Organic Solvent-Water Partitioning

ENVR 725

March 5, 2013

Ariel Atkinson1

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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

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Practically—Why do we care?

• Liquid-Liquid Extractions• Contaminant Fate• Describe a compound- Measure of

Hydrophobicity/Hydrophilicity• Relate partitioning to other things• Remediation?

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Definition

We could define partitioning between and organic phase (solvent) and the water phase at equilibrium as:

Kilw= Cil/Ciw

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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?

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hexane

benzene

octanol

Methanol

Ex Solvents

Trichloroethylene

Tetrachlorobiphenyl

Dibromoacetic acid

Acrolein

Equilin

Water

Water Pollutants of Concern

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Solute-Solvent PropertiesApolar, 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=0.02Toluene = H accepting/e- donating α=0 β=0.14 Bipolar solutes don’t partition well into apolar solvents from water!!

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Activity

From the thermodynamics of Chapter 3, at equilibrium:

γilxiil = γiw xiw

 going to molar concentrations CilVl= Xil γilCil Vl = γiw Ciw Vw

 

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The partition coefficient expresses the difference between the nonideality of the compound in the organic solvent and in the aqueous phase

Activity

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Solving for Cil/Ciw and defining a partitioning coefficient Kilw

 Kilw = Cil/Ciw gives ln Kilw = lnγiw - lnγil + ln(Vw/Vl)

=-

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Co-solvationWater 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 100,000will be soluble in the water phase.

If you can PRE-SATURATE!!!10

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ConcentrationAssume 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

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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:

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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 = HilE

- HiwE

HiwE

is pretty small for most organic solutes (± 0 kJ mole-1)

HilE

is ~ ± 10 kJ mole-1

For most organic solutes, the l-w partitioning is only weakly temperature dependent! 13

Effect of Temperature on Organic Solvent-Water Partitioning

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Effect of Temperature on Organic Solvent-Water Partitioning

ΔlwHi = HilE

- HiwE

Exceptions to general rule of weak temperature dependence:

Small bipolar compounds in apolar solvents:-these compounds tend to have negative Hiw

E

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The n-Octanol-Water Partition Constantn-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

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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

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Proposed structural model for natural organic matterBenzene in octanol:

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How to measure partitioning coefficients

 

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• Measure directly:• Slow stir• Shake-Flask

• Predict:• HPLC method• Correlation with other partition

coefficients• Fragmentation methods

Experiment

DBPs

DBPs

DBPs

-co-saturated solvents-pH 7.0-temperature 25 C-headspace free

-Measure concentration in water and octanol

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Kiow from Chromatographic Data

 

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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. 21

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Kiow from Chromatographic Data

Since the time that a partitioning compound spends in the mobile phase will depend on the partitioning coefficient (K iow 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

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Kiow from Chromatographic Data

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Generally non-polar Kiow and Ki hex-w correlate well

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Correlation with other Partition Coefficients

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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

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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:

Now we can use the multiparameter LFERs for air-solvent and air-water previously developed.. (Chp. 6)

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Fragmentation MethodsMost Used modeling software-EPISUITE

KOWWIN fragment coefficient (fi)-CH2- 0.4911-CH 0.3614-Cl 0.3102C 0.9723-Br 0.3997-I 0.8146-C#N -0.9218-OH -1.4086-O- -1.2566

correction factor (ci)monohalo acetamides 0.3263polyhalo acetamides 0.6365

log(Kow)=Sum(fi*ni)+Sum(ci*ni)+0.229

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Fragmentation Methods-Example

Trichloroacetonitrile

ni*fi3*-Cl 3*0.31021* C 0.97231* C#N -0.9218  0.229 SUM 1.21

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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