chapt7 web - Organic solvent-water partitioning the solvent...

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Organic solvent-water partitioning the solvent - water partitioning constant Near the end of the 19 th century people interested in pharmaceuticals discovered that organic drugs accumulated in organisms in way that was proportional to their partitioning in octanol. More recently environmental chemist have found similar correlations with partitioning to solid humus and other naturally occurring organic phases. by analogy to the dimentionless Henry’s law constant, K iaw = C ia /C iw we could define partitioning between and organic phase (solvent) and the water phase as: K isw = C is / C iw Water is somewhat soluble in the octanol phase, such that 1 out of every 4 1

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molecules in the organic phase or octanol phase will be water. In the aqueous phase the octanol activity coef. is γ iw = 3.7x10 3 1/ γ iw = x iw = 2.7x10 -4 only 27 molecules out of 100,000 will be soluble in the water phase. first , we must be concerned about the effects of intermolecular interactions of solute molecules with solvent molecules second, we must be aware that the high amount of water in the octanol phase can influence the molar volume of the octanol phase pure n-octanol o V _ = 0.16L/mol (at 25 o C) Water saturated octanol 2
V mix =(0.79)(0.16) + (0.21)(0.018) = 0.12 L mol- 1 What about the molar vol. of water saturated with octanol? What if we used hexane? What happens when we add a high γ water . organic to an octanol-water system? 3

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From the thermodynamics discussion, at equilibrium γ is x is = γ i w x iw going to molar concentrations C i = X i / V mix γ i s C i s V s = γ i w C i w V w solving for C i s /C i w and defining a partitioning coefficient K i sw K i sw = C i s /C i w gives s is w iw iw is isw V V C C K γ γ = = ln K isw = ln γ iw - ln γ is + ln s w V V If our solvent is octanol An octanol-water partitioning coef. is defined; when is it hexane, a hexane-water K sw is defined K i sw = C i s /C i w 4
Comparing partitioning in Octanol-water and hexane water; octanol is an amphiphilic solvent, e.g., it has both a non polar and polar group. K

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This note was uploaded on 08/28/2011 for the course ESE 544 taught by Professor Lee during the Fall '11 term at Purdue University-West Lafayette.

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chapt7 web - Organic solvent-water partitioning the solvent...

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