Why is octanol prioritized for partition coefficients?

[SemM]

Hi, it is well established that octanol is the preferred solvent for partition coefficient studies in environmental chemistry, however, some studies ui.e Noble A. Parition coefficients (n-octanol-water) for pesticides) J Chromat. 642 3-14, mention that estimating toxicity based on solubility in octanol and water is not optimal. Other alternatives are present, such as hexadecane, however, octanol is established as the main partition coefficient for toxicity studies. What are the reasons for this?

Thanks

 

[Bystander]

What are the reasons for this?

"This" is a procedural prescription; there is no "why" beyond that "that" was the only solvent in the laboratory when the procedure was developed.

 

[SemM]

"This" is a procedural prescription; there is no "why" beyond that "that" was the only solvent in the laboratory when the procedure was developed.

I see. The reason for this question, is that several new emerging pollutants are actually discerned better for toxicity and membrane crossing potential using other solvents, and particularly some toxic compounds, such as the perfluoro compounds are to part not even calculable at all using log Kow empirically (see report on PFOS in the EU commission) and therefore can require other methods and perhaps a new standard.

 

[zbikraw]

The discussion is about models, laboratory and mental procedures. The more general is a model, the less precise is in fitting to a given new situation. For biochemical considerations, more than century ago, there was precedure of measuring oil/water partition coefficients. Applying various plant and animal-derived oils gave various results. Laboratories begun to use single triglycerides, which was highly expensive. For some solutes, there was ordering of oil phase with triglyceride condensation (sometimes even crystallization) in unexpected places of laboratory stills. Less than century ago it was realized that pure triglycerides have melting points above "room temperature", and its liquid existence is mainly a result of difficulties in crystallization. These are times of beginning of interpretation of compounds physical properties using molecular structures and a start to mental incompatibility between chemists and medicine/hygiene/biology/food/ecology/toxicology workers. The former try to understand structure-properties ralations, the later need inexpensive liquid simulating hydrophobic behaviour of matter. During single generation it was practicised that n-octanol is quite a good such liquid. Work started from n-octane and isooctane, but even their mixtures were inappropriate as a models for laboratory hydrophobic phases. During shaking with aqueous solutions dispersions were formed and phase separations demand centrifuging or even more sophisticated treatment. Also saturation of hydrocarbon phase with solute from aqueous one was sluggish, because of almost complete non-mixing. Minute amount if interphase mixing/solution was needed and n-octanol gives it. This of course gaves some errors, but biologist do not need absolute values. Quite satisfactionary were no- dimensional and no-name numbers easy to obtain is standarized laboratory conditions.
Today we understand that n-octanol is a good structural analog of lipid phase because of its short range ordering of dipoles. On the border with aqueous phase there even is short duration surface activity which speeds interphase transport of solutes.
Of course, these numbers are hopeless as single indicators of molecular properties and useless for quantifying membrane properties, but in combination wirth other data (mainly chromatographic retention factors or times) are useful in classification of properties of small groups of structurally related substances. To achieve undestanding of molecular analogies in cellular transport of species, one must use vast number of results of experiments in partition between stationary and mobile phases of various structures. Many-dimensional analysis can give properties of membrane and solute as arrays of numbers. Such a complex work do not guarantee obtaining simple answer to simple questions, so the problem waits for solution. For many practical purposes partition with n-octanol must be satisfactionary.

zbikraw