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Will anybody tell me what is the relationship between log D of a
molecule with its surface active property?
The background of this query is that I am working with a zwitterionic
molecule, which shows higher surface activity (ability to lower the
surface tension) at acidic pH as compared to neutral pH. When I
predicted its log D by PALLAS, the log D increases as the pH increases
(negative value at pH<6 and 0.2 at pH 7). The trend was entirely
opposite when I practically conducted the log D determination. It
showed a decrease in log D as the pH increased and also no log D value
comes out to be negative value.(A surprising aspect is that this
molecule has very good solubility at acidic pH as compared to neutral
1. Why there is an entirely opposite behaviour of practical vs.
2. What may be the reason(s) for this typical behaviour of this
3. Will there be any correlation of log D with its surface active
4. Will there be hydrogen bonding between octanol and the moecule
playa role in the partitioning? (This molecule has around 20
one donor and acceptor groups). If so, will log D be a good
absorption of this particular molecule?
Dear friends, I want to get valuable suggestions about this aspect from
you all. If you need any more details, I will be happy to send them.
Thanks in advance.
T. THANGA MARIAPPAN,
DEPT. OF PHARMACEUTICAL ANALYSIS,
NATIONAL INSTITUTE OF PHARMACEUTICAL EDUCATION AND RESEARCH (NIPER),
PHASE-10, SECTOR-67, MOHALI-160 062,
Thanga,Back to the Top
The molecule could be a hydrotrope if it has both hydrophobic (ring)
and hydrophilic moieties. In which case, solubility measurements may be
demonstrating self-associating behavior rather than actual molecular
solubility. Current scientific experimental results are ambiguous as to
whether hydrotropes influence surface tension, and if so, how. I would
expect that both the balance of the strength of the hydrophobic and
hydrophilic forces, and their physical position on the molecule, would
For a quick and simple test of hydrotropic behavior, consider local
advancing and receding hydration front microscopy. Use crossed
polarization microscopy to view the results of exposing the powder to a
water drop on the microscope slide. As water drop first advances and
then recedes, the powder grains of the drug substance are hydrated to
various degrees and then allowed to dry. If it is a hydrotrope, then
the result will remind you of a liquid crystal, for hydrotropes are
lyotropic Liquid Crystals, in which the formation is concentration and
solvent dependent, (not thermotropic Liquid Crystals, in which the
formation is temperature dependent, and which are further characterized
as Calamitic nematic, or Calamitic smectic or Discotic).
I will give a presentation on this subject at the Barnett
"Polymorphism" conference, in Philadelphia, PA, on June 24-25, 2004.
Here are some references from it:
1) Srinivas, V.; Rodley, G. A.; Ravikumar, K.; Robinson, W. T.;
Turnbull, M. M.; Balasubramanian, D. Langmuir 1997, 13, 3235. *
2) Buurma, Niklass Jan, Doctoral thesis "Water, a Unique Medium
for Organic Reactions", February 2003, Universtiy of Groningen, p. 101.
See also Chapter 1, "Water, the Unique Reaction Medium" and Chapter 4,
"Hydrolysis in Aqueous Solutions Containing Hydrotropes", [an On Line
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