Subject: Log P (o/w) of octanol

From: Butch Carreira <[email-masked]>
Date: Thu, 6 Apr 1995 19:26:32 -0400

Dear Kamal,

> Date: Thu, 06 Apr 1995 13:55:59 +0100
> To: [email-masked]
> From: [email-masked] (Kamal Azzaoui)
> Subject: CCL:Log P (o/w) of octanol !
> Sender: Computational Chemistry List <[email-masked]>
> Errors-To: [email-masked]
> Precedence: bulk
> 
> Dear netters
> 
> We are working in the field of quantitative structure-chromatographic
> retention relationships (QSSR) and ,
> among various physico-chemical descriptors, we used the calculated Log P
> (o/w)( With Ghose and Crippen=92s parameters).
> 
> One of our products is Octanol.
> 
> It is very easy to calculate the Log P of Octanol ( we found 2.54), this
> value is a by evidence a descriptor of the structure , but we have a
>philosophic problem:
> 
> What is the physical meaning of Log P(o/w) of Octanol?
> Subsidiary question: What is the Log P of water?
 
The answer is this:
 
Log P(o/w) for a solute is the difference in the log activities of 
that solute in water and octanol solvents. The log activity of octanol
in octanol is 0.0 (~0.04 or so if you take into account that it is really
wetted octanol). However, the log activity for octanol in water is about
4.0. If you express the log P using a moles/l reference rather than a mole
fraction reference (the usual) then the difference in the log of the 
molecularites of the solvents must be taken into account, for o/w this 
is ~0.82. So the log P would be 4.0-0.8 or around 3.1-3.2.

The same scheme will hold for the log P(o/w) of water.

Hope that helps.

Butch Carreira
====================================================================
Butch Carreira                             [email-masked]
Department of Chemistry                    (706) 542-2050 or 2051
The University of Georgia                  (706) 542-9454 FAX
Athens, GA  30602