logD is a log of partition of a chemical compound between the lipid and aqueous phases. The most popular lipid phase is octanol. LogP is equivalent to logD for non-ionisable compounds and represents the partition of the neutral form for ionizable compounds (and, hence, is a virtual, unmeasurable, property). LogD/logP are used as meaurable, though imperfect, proxies of an illusive and unmeasurable concept of hydrophobicity. They are particularly useful in drug discovery and development.
logD is a log of partition of a chemical compound between the lipid and aqueous phases. The most popular lipid phase is octanol. LogP is equivalent to logD for non-ionisable compounds and represents the partition of the neutral form for ionizable compounds (and, hence, is a virtual, unmeasurable, property). LogD/logP are used as meaurable, though imperfect, proxies of an illusive and unmeasurable concept of hydrophobicity. They are particularly useful in drug discovery and development.
In short, during HPLC method development, the analyst is setting up a concentration partition of the molecule(s) of interest between the mobile phase(s) and the stationary phase or column. While logD is helpful information, it is not required.
logD refers the the water:octanol partition coefficient at a specific pH, normally pH 7.4. For ionizable compounds (acids and bases), logD would be altered by pH because the distribution of charged and uncharged forms would change, and the uncharged form is more hydrophobic.
Can there be a huge difference between log P and log D? say for Log P is 5.5 and log D 0.10
Yes, there can be a large difference if the compound contains ionizable groups.
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