Solubility (II)
© E. De Miguel – Linköping 2009
Solubility (II)
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Solubility
Non-polar solvents
Laboratory: n-hexane, n-octanol
Biota lipids.
Soil (sediment, suspended solids) organic
matter.
© E. De Miguel – Linköping 2009
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1
Non-polar solvents
Biota lipids:
Cell membranes are composed primarily of a thin bi-layer of phospholipids
(penetrated by bands of protein).
Crosby (1998)
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Non-polar solvents
One end of the molecule
is soluble in water
(hydrophylic head).
The other (two) end(s) is
a long-chain
hydrocarbon, fatty acid
(hydrophobic tails)
Bretscher (1985)
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2
Non-polar solvents
Lipids: High-molecular weight, non-polar to weakly
polar esters (fats).
CH2OOCR
CHOOCR’
CH2OOCR’’
Lipophylic (“fat loving”) compounds: Substances that
are soluble in lipid.
Can penetrate cell membranes
Are likely to have biological effects
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Partition coefficients
Solvent partitioning:
When an aqueous solution of an organic compound is
brought into contact with an organic solvent the solute
molecules will escape into the new solvent until equilibrium
is reached: Partitioning (distribution).
The ratio of the compound’s concentrations in the two
phases at equilibrium is constant under constant
environmental conditions: Partition coefficient.
K=
© E. De Miguel – Linköping 2009
Cphase1
Cphase2
=
Csolvent
C water
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3
Partition coefficients
Octanol-water partition coefficient:
1-Octanol is a good surrogate for biota lipids (it is “fatlike”) in many of its physical chemical properties.
K ow =
Coctanol
C water
KOW values can be measured in a shake-flask system by
adding the compound to the system and measuring the
concentrations after shaking to reach equilibrium.
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Solubility (II)
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Solubility (II)
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Partition coefficients
Connel (1997)
© E. De Miguel – Linköping 2009
4
Partition coefficients
Octanol-water partition coefficient:
KOW measures the lipophylicity of an organic compound.
log KOW<0
non-lipophylic
0< log KOW<2
2< log KOW<6.5
weakly lipoph.
lipophylic
log KOW>6.5
super-lipoph.
KOW is dependent on:
Polarity of the substance
Molecular weight of the substance
Typical lipophylic compounds: chlorohydrocarbons, hydrocarbons,
polychlorodibenzodioxins, polychlorodibenzofurans and related
compounds.
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Solubility (II)
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Organic acid and base speciation (1/2)
Acid dissociation constant:
AH + H2O ↔ A- + H3O+
H+ + H2O ↔ H3O+; K=1
AH ↔ A- + H+;
Ka =
[A ]×[H ]
−
[AH]
+
[A ] = pH - pK
[A ] + pH
−
[AH]
pKa = −logKa = −log
−
log
%AHdissoc. = 100 ×
[AH]
a
[A ]
100
100
[A ] + [AH] = 1 + [AH] = 1 + 10
[A ]
−
pK a −pH
−
−
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5
Organic acid and base speciation (2/2)
Base dissociation constant:
B + H2O ↔ BH+ + OH- ;
Conjugate acid: BH+ ↔ B + H+;
BH+
B + H 2O ↔
+
BH+ ↔ B + H+
H2O ↔ H+ + OH-
OH-
%Bdissoc. = 100 ×
pKb
pKa= − logKa = −log
[B][H+ ] = −log [B]
[BH ]
[BH ] + pH
[BH ] = pK − pH
log
+
+
+
pKb
pKa
pKw
[B]
a
[BH ] = 14 − pK
+
pKw=14=pKa + pKb
log
[BH ]
100
100
[BH ] + [B] = 1 + [B] = 1 + 10
[BH ]
[B]
+
+
pH-pK a
+
© E. De Miguel – Linköping 2009
=
b
− pH
100
1 + 10pH+pKb −14
Solubility (II)
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Solubility
Summary:
Ionic and polar substances dissolve completely or to a large extent in
water because water molecules form stronger attachments to the
solute than occur between the solute molecules and ions themselves.
Non-polar substances do not dissolve readily in water since solute and
solvent are not attracted and stabilisation of solute does not occur.
Non-polar and weakly polar substances will dissolve in non-polar
solvents like lipids. Substances that tend to become concentrated in
fatty tissues are called lipophylic.
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6
Solubility
Summary:
Covalent molecules usually have a mixture of polar and non-polar
sections and will partition between a polar phase such as water and a
non-polar phase such as lipids or octanol.
The tendency to leave an aqueous phase and enter a non-polar phase
can be measured by the KOW of a substance.
The less polar and the larger a molecule, the larger its KOW value.
© E. De Miguel – Linköping 2009
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Considering only their molecular structure, sort the following
compounds according to their water solubility.
NAME
Carbon
Tetrachloride
SIZE
SYMMETRY
(non-polar)
SW(mg L-1)
800
Chloroform
8000
Hexachlorobenzene
0.006
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Solubility (II)
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7
Considering only their molecular structure, sort the following
compounds according to their lipophilicity.
NAME
© E. De Miguel – Linköping 2009
SIZE
POLARITY
Kow
DDT
999909
Ethanol
0.49
Phenol
29
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