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4.18.2  What is LogD? 

The hydrophobicity (also known as the lipophilicity) is a very important physicochemical property 

used in different areas of chemistry, medicine, and pharmacology.  It is a very important indicator 

of transport and permeation through membranes, interaction with biological receptors and 

enzymes, toxicity, and biological potency.  In environmental sciences the hydrophobicity is often 

used to predict solubility (S), the bioconcentration factor (BCF), and the organic carbon 

adsorption coefficient (Koc). 

The octanol water partition coefficient, logP, is a measure of compound hydrophobicity, which in 

many cases correlates well with the above chemical and biological properties.  However, the logP 

can be accurately calculated only for uncharged substances.  If the chemical compound contains 

one or more ionogenic groups (i.e., functional groups which can easily form ions), it may exist as 

a mixture of different ionic forms.  The composition of this mixture depends strongly on pH. 

In such cases the apparent partition coefficient D for dissociative systems (or logD) gives a more 

appropriate description than logP of the complex partitioning equilibria.   

The following scheme presents an example of the complex equilibrium which exists for a solution 

of alanine in a water octanol mixture:

O

H

CH

3

O

NH

2

K

K

1(org)

3(org)

P

2

O

H

CH

3

K

K

1

O

NH

2

3

O

CH

P

O

CH

O

H

CH

O

H

CH

3

3

3

P

3

1

H O

4

2

+

+

O

NH

O

NH

O

NH

O

NH

2

2

3

3

O

CH

3

K

K

2

4

+

O

NH

3

P

3

K

K

2(org)

4(org)

O

CH

3

Organic phase

+

O

NH

3

K

i

 the ionization constant of the i th microspecies 

K

i(org)

 the ionization constant of the i th microspecies in octanol 

P

i

 the partition coefficient of the i th microspecies 

So, if such complex equilibria exist, then the partitioning of chemicals between water and organic 

phase must be a function of:  

(i)  The extent of ionization, and  

(ii) The partition constants.  

of the numerous different microspecies shown in the diagram above. 

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