Chapter 7 Organic LiquidWater Partitioning

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Chapter 7Organic Liquid-Water Partitioning

• Klw and Kow

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Equilibrium partitioning between water and any
organic liquid
large?
prime refers to mole fraction basis better to
use molar concentration note how this
equilibrium constant is related to those
discussed previously
small?
phase change costs cancel
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Effect of salinity on waterorganic liquid
partitioning
Salinity will increase tendency to partition into
the organic phase by decreasing the solubility
(increasing the activity coefficient) of the
solute in water. Assuming that salts are largely
insoluble in the organic phase. Account for
salinity effects via Setschenow constant
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Effect of Temperature on waterorganic
partitioning
As per previous discussions, assume enthalpy
change of the partitioning process is constant
over the relevant range of T
water
HEiw
DlwH
solvent
Total enthalpy change different between excess
enthalpy of solubilization in water and solvent
HEil
Pure liquid
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Temperature dependence of Klw

• Typically HEiw and HEil are similar in magnitude,
  so the temperature dependence of Klw is small
  (negligible)
• not true when there is great dissimilarity
  between solute and solvent, i.e. PCBs, PAHs in
  water, ethanol in nonpolar solvent
• As usual, correct for temperature (when
  necessary) via

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Comparing different organic solvent-water systems

• Recall that partitioning will be driven by
• vdW forces
• polarity/polarizability
• H bonding
• The better the match between chemical properties
  of solute and solvent, the higher the equilibrium
  constant
• for example, better extraction solvent

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LFERs for relating different organic
solvent-water systems

• IF the two solvents are similar, then simple
  linear FER can be used for a series of similar
  compounds

For example, hexadecane and octanol
Works well for apolar and weakly polar
solutes. Does not work for very polar compounds,
such as phenols
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fig 7.1
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Predicting Kilw
molar volume describes vdW forces
refractive index describes polarity
additional polarizability term
H-bonding
cavity term
This is a generic equation for estimating the
partition of a compound between water and any
solvent. It is similar to the equation we used to
estimate VP, solubility, HLC, etc.
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Now that weve seen this approach four times
these equilibrium constants are related therefore
we can subtract their estimation equations to
yield the coefficients we need
table 7.2
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Octanol-water partition coefficient

• Importance
• Huge database of Kow values available
• Method of quantifying the hydrophobic character
  of a compound.
• Can be used to estimate aq. solubility
• Can be used to predict partitioning of a compound
  into other nonpolar organic phases
• other solvents
• natural organic material (NOM)
• biota (like fish, cells, lipids, etc.)
• Why octanol?
• Has both hydrophobic and hydrophilic character
  ("ampiphilic")
• Therefore a broad range of compounds will have
  measurable Kow values

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An experiment mix pure water and pure octanol,
let come to equilibrium At equilibriumwater
contains 8 octanol for every 100,000
wateroctanol contains 1 water for every 4
octanol Add organic compound At equilibrium,
fo fw ?o?o ?w ?w ?oCoVo ?wCwVw
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IF ? ?w is large ? ?o is close to 1 then Kow is
largely driven by aqueous activity
coefficient Assume Cwsat 1/(Vw?w)
?o doesnt change much
fig 7.2
Note subcooled liquid solubility used
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• Because Kow is primarily determined by aqueous
  solubility.
• corrections of Kow for T, salt, are made by
  adjusting the aq. solubility term
• T effect on aq. solubility (and Kow) is small
• Phase change costs cancel

? LFERs can be used to predict Kow from Csat
(and vice versa)
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From aq. solubility relation
Recall assumptions ?w , ?o are independent of
concentration ?w is not influenced by octanol in
water Concentrations are sufficiently low not to
affect Vo-w (water saturated octanol) Result
for nonpolar, hydrophobic chemical with Cwsat gt
10-6 mol/L, ?o 1 to 10 Apparently, ?o
increases with molecular size Big PCBs, PAHs,
dioxins, etc. Increasingly incompatible with
water-saturated octanol. Thus, LFERs
developed Note subcooled liquid solubility
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table 7.3
Difference between b and b is related to molar
volume of water
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Methods of measuring Kow Shake flask method
(compounds with log Kow lt 5) Solute is
equilibrated between the two phases by shaking
Generator column method
Chromatographic data Relate Kow of known
compounds to capacity factor, k', on a
reverse-phase C18 HPLC column Measure k' for
known compounds, develop relationship between Kow
and k'
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Kow from fragment constants structure-property
relationships structure-property relationships
used to predict many things ? specific
structural units increase or decrease and
compound's Kow by about the same amount. Hansch
and Leo Kow estimation method
where f are factors for structural units, and F
are geometric factors which account for affects
such as branching, flexing, polyhalogenation,
etc. Note factors for fragments attached to
aliphatic carbons (f) are not the same as those
attached to aromatic carbons (f?) Example Cl
f 0.06, f? 0.94
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• billions of maddening factors virtually ensure
  human error!
• can be avoided by
• ClogP
• starting with a structurally similar compound
• Factors make sense
• -H 0.23 (positive, increases Kow, more
  hydrophobic)
• -OH -1.64 (negative, decreases Kow, less
  hydrophobic)
• based on size, polarizability, hydrogen bonding
  contribution
• Factors are not same as for KH estimation

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Hansch and Leo correction factors (from old
edition of text)
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More on fragments

• Different constants for fragments attached to
  double bonds and aromatic rings reflects greater
  electron delocalization.
• NO2 f -1.16 f? -0.03
• difference of a full log unit or more in f
• For vinylic carbons, Hansch and Leo recommend
• for vinylic halogens f (f? f)/2
• for vinylic polar groups (with N or O) f (2/3
  f? 1/3 f)/2

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Meylan and Howard (1995)

• similar to Hansch and Leo, perhaps (?) simpler

where n frequency of each type of fragment f
factors for each type of fragment c
correction factors (235!)
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tables 7.4 and 7.5
New edition of text gives only Meylan and Howard
model
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Predicting Kow via computational chemistry use
a variety of computational models ab initio
Hartree-Fock, MP, DFT semi-empirical to
extract important parameters surface area
(cavity term) surface electrostatic potential
(dipolarity) hydrogen bonding term then
construct a multivariable least-squares fit to
known Kow data
Where VI van der Waals volume ? d ?
polarizability term ?, ? hydrogen bond donor
and acceptor terms
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Dissolution of organic compounds in water from
organic liquid mixtures

• LNAPLs (gasoline, heating oil)
• DNAPLs (chlorinated solvents)
• PCBs, hydraulic oils

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Issues to ignore

• cosolvent effect?
• examples, gasohol, MTBE
• salts?
• Assuming these effects are negligible

solubility of liquid
define organic mixture-water partition
coefficient
in many cases gimix 1
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Problem 7.3

• You want to extract 1-naphthol out of 20 mL
  aqueous samples, achieving 99 extraction
  efficiency, so that you can analyze the compound
  by GC.

• which solvent, and how much, should you use?
• what if you extract twice?
• what if you add 3.56 g NaCl to the 20 mL
  sample?