lindane extraction from aqueous solution using hexane

Question

I am studying the degradation of lindane using iron nanoparticles. I have prepared lindane solution with an acetone and water mixture (1:9 v/v). I tried to extract lindane after reaction with nanoparticles using hexane as a solvent. a viscous third layer has formed between the two layers. I have no idea why this happened. Can anyone please explain why this has happened?

Answer 1

According to this publication, lindane has the following properties:

Practically insoluble in water
Specific gravity = 1.85
Melting point = 113 $\mathrm{^oC}$
Solubility in acetone > 200 g/L
Solubility in n-heptane* = 10-14 g/L
* Used as surrogate for hexane solubility for this argument.

Notes: lindane is by far the most soluble in acetone, though only a small amount could have been used in the 1:9 acetone:water solvent. If we assume that the reaction mixture were extracted with an equal volume of hexane (conservative I think, probably used more hexane), then the solution would be about 1:10:10 acetone:water:hexane. Given the properties of lindane, I'd expect it to make for a gooey, separate, small layer between the water and hexane.

Prediction:

Bottom layer - Water, undissolved iron, dissolved iron chlorides.
Middle layer - Acetone, lindane, partially reacted lindane, some iron chlorides.
Top layer - hexane, Small amounts of lindane and organic byproducts.

Answer 2

What has happened is that the dreaded "third phase" problem has hit you.

With a very low concentration of lindane in water it will be possible to extract the lindane with hexane. But when the lindane concentration is higher then the organic phase is unable to support the lindane concentration.

I also suspect that the lower phase might be too concentrated in terms of lindane. I think you need to consider the question of "what am I trying to do". If you are trying to study the degradation of traces of lindane then I would suggest you consider working at lower concentrations of lindane which would redcue the loading on both the organic and aqueous phases.

You might need to work using a GCMS or ECD-GC to measure the lindane and the other organics, this would bring down the amount you need to work with.

If you are considering the degradation of large concentrations of lindane then I would suggest that you change to toluene. It is likely that this will be more able to extract lindane than hexane. If you are wanting to study the system with GC then I would suggest that you do the first extraction of the the toluene spiked with an internal standard.

Then transfer the organic phase (200 microlitre) into a new vial containing hexane (1.8 ml) and wash it with water (2 ml) to remove acetone and other hydrophilic substances. You can repeat this process several times before drying the extract with Na2SO4. Then inject into the GC. This method is biased in favour of the lipophilic compounds.

You might well want to try a few method development experiments in which you start with a known mixture of organics such as phenol, lindane, chlorocyclohexane, 1,2-dihydroxycyclohexane and other things which are similar to the things you are considering. Then you can get the recovery yeild of these things in the solvent extraction workup.

I think it is very important to choose solvent extraction conditions that do not give anything other than two clear layers. The reason is that you might lose one or more substances while leaving the other substances in the organic layer. This will cause your experiments to give a misleading impression of what is in the reaction you are sampling.

You might also want to consider the following method. Take a sample out and dilute it with acetone. Next add to it triethyl orthoformate and some tosic acid. Allow it to stand for some time (you will have to work out how long through experiments). Next add triethyl amine (excess) and evapourate it down to dryness. Now redissolve in ethyl acetate and then shake the organic phase with water to remove Et3NH OTs and the other very polar impurities. Now dry with Na2SO4 and filter. Now dilute with acetone for GC.

This is a method which works rather well for sugars and other things with 1,2-diols. It converts the 1,2-diols into acetone acetals which travel a lot better through a GC column. The action of the trethyl orthoformate will dehydrate the mixture and drive the reaction to make it complete.

If you want to make the products even more non polar for GC, the I would suggest that you do the following. After drying the ethyl acetate extract then evapourate it down and then redissolve in acetone. Add silver(I) oxide to this and then some methyl iodide. Allow it to stir in a sealed flask in the dark overnight. The next day add some methanol and stir for a while to destroy the excess methyl iodide. Now filter it and get the sample ready for GC. Now any remaining alcohols which did not react with the acetone will have been converted into methyl ethers.

I have used the acetone acetal and silver oxide / methyl iodide methods to allow me to do GC on sugars and these methods do work very nicely.