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Chapter 7
Organic Liquid-Water Partitioning
Klw and Kow
Air
KH = PoL/Csatw
Kow = Csato/Csatw
Koa = Csato/PoL
A gas is a gas is a gas
T, P
Koa
KH
Po L
Octanol
Water
Fresh, salt, ground, pore
T, salinity
Csatw
Kow
Pure Phase
(l) or (s)
Ideal behavior
NOM, biological lipids,
other solvents
Csato
Equilibrium partitioning between water and
any organic liquid:
x il
 iw large?
'
K ilw 

prime refers to
x iw
 il
mole fraction basis
small?
better to use molar
concentration
note how this
equilibrium
constant is related
to those discussed
previously
K ilw
Cil Vw  iw


Ciw Vl  il
K ilw
K iaw

K ial
phase change costs cancel
Effect of salinity on water:organic
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:
K ilw, salt  K ilw 10
 K is [ salt ]tot
Effect of Temperature on water:organic
partitioning
As per previous discussions, assume enthalpy change of the
partitioning process is constant over the relevant range of T
 lw H i
ln K ilw  
 cst
RT
lwHi  H  H
E
il
E
iw
Total enthalpy change = different
between excess enthalpy of
solubilization in water and solvent
water
lwH
HEiw
solvent
HEil
Pure liquid
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:
 lw H i
ln K ilw  
 cst
RT
Comparing different organic solventwater 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
LFERs for relating different organic solventwater systems
IF the two solvents are similar, then simple linear FER can be
used for a series of similar compounds:
log Ki1w  a  log Ki 2w  b
For example, hexadecane and octanol:
log Kihw  1.21 log Kiow  0.43
Works well for apolar and weakly polar solutes.
Does not work for very polar compounds, such as phenols
fig 7.1
Predicting Kilw
molar volume
describes vdW
forces
refractive index
describes polarity
2



n
2/3
Di  1
  p( i )
ln K ilw  s Vix   2
 nDi  2 

additional
 a( i )  b(  i )  v(Vix )  cst
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.
Now that we’ve seen this approach four times…
Kiaw
Kilw 
Kial
table 7.2
these equilibrium constants are
related
therefore we can subtract their
estimation equations to yield the
coefficients we need
Octanol-water partition coefficient:
Kow 
Co (mol / Lo )
C w (mol / Lw )
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
An experiment
mix pure water and pure octanol, let come to equilibrium
At equilibrium:
water contains 8 octanol for every 100,000 water
octanol contains 1 water for every 4 octanol
Add organic compound
At equilibrium,
Kow 
fo = fw
 o o =  w w
oCoVo = wCwVw
Co (mol / Lo )  w Vw


C w (mol / Lw )  o Vo
e
Vw
Gow
ln Kow  ln  w  ln  o  ln  
 constant
Vo
RT
IF
 w is large
 o is close to 1
then Kow is largely driven by aqueous activity coefficient
Assume
Cwsat
= 1/(Vww)
K ow 
1 1 1
 
sat
Cw  o Vo
o doesn’t
change much
fig 7.2
Note subcooled
liquid solubility
used
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
 Cwsat 
 Kow ,salt 
s
log  sat   K salt t  log 

C
K
 ow 
 w ,salt 
 LFERs can be used to predict Kow from Csat (and vice versa)
From aq. solubility relation:
log Kow   log Cwsat  log  o  logVo
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 > 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:
log Kow  a logCwsat ( L)  b'
Note: subcooled liquid solubility
table 7.3
log Kow  a log iw  b
log Kow  a logCwsat ( L)  b'
Difference between b and
b’ is related to molar
volume of water
Methods of measuring Kow
Shake flask method (compounds with log Kow < 5)
Solute is equilibrated between the two phases by shaking:
octanol
water
Generator column method
octanolsaturated
water
glass beads coated with
compound of interest
solid
adsorbent
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'
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:
log Kow   f i   Fj
i
j
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
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:
-Cl
-OH
Car-NO
KH
-0.30
-3.21
-1.83
Kow
+0.06
-1.64
-0.03
Hansch and Leo
correction
factors (from old
edition of text)
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
Meylan and Howard (1995)
similar to Hansch and Leo, perhaps (?) simpler
log Kow   nk  f k   n j  c j  0.23
k
j
where:
n = frequency of each type of fragment
f = factors for each type of fragment
c = correction factors (235!)
New edition of text gives only
Meylan and Howard model
tables 7.4 and 7.5
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
log Pow  mVI  x( *  d )  bm  am  C
Where VI = van der Waals volume
* + d  = polarizability term
,  = hydrogen bond donor and acceptor terms
Dissolution of organic compounds in water
from organic liquid mixtures
• LNAPLs (gasoline, heating oil)
• DNAPLs (chlorinated solvents)
• PCBs, hydraulic oils
xiw  imix

ximix  iw
V mix  imix
Ciw  Cimix 

V w  iw
V mix 
M mix
 mix
Issues to ignore
• cosolvent effect?
– examples, gasohol, MTBE
• salts?
Assuming these effects are negligible:
Ciw  Cimix V mix   imix  C (L)
sat
iw
solubility of liquid
define organic mixture-water partition coefficient:


1
Cimix
sat
K imixw 
 V mix   imix  Ciw ( L)
Ciw
in many cases imix = 1
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.
Solvent
n-hexane
benzene
chloroform
ethyl acetate
n-octanol
logKlw
0.52
1.89
1.82
2.6
2.9
• 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?