Hey, bit of a late answer but it may still be helpful to other readers:

If Ethanol precipitation of oleic acid capped SPIONs does not work you did not use the proper Ethanol/1-Octadecene ratio. Chemically speaking you add a polar, protic solvent (EtOH) which is miscible with your aprotic one (1-ODC), which will turn your mixture more protic as you add more Ethanol. At some point (I estimate a three- or fourfold excess of EtOH over 1-ODC) your mixture will be too polar for the Oleic-acid coated SPIONs to stay in suspension and you can either collect them with a magnet or centrifuge them. You can redisperse the particles in chloroform or similar solvents. That worked fine for me at least. Some people add mixtures of Alcohol/chloroform to do it all in one step, but then you have to get the ratio right to get decent phase separation.

I have the experience that Heptadecane evaporates quite fast at 150°C in air flow. Maybe just heating to 150+°C in vacuo will take care of this problem. But the vacuum oven is probably messed up quite a bit afterwards.

Use supercritical fluid technology for drying.

Such equipment is not available at the moment. I had a go with vacuum evaporation which didnt seem to work either..I'm gonna try again with a better vacuum.

Perhaps you really need of better vacuum.

which temperature did you try?

i went all the way up to 280. at that temperature the crystals should begin to grow (normal boiling point of solvent is about 320 degrees. what i am thinking is that since the water in the uk at this time can barely exceed 10-12 degrees the condenser must have been freezing the solvent which has a freezing temperature of 14-16 degrees (indeed i found some at the upper most part of the condenser (that's why nothing was coming out). pfff 1-octadecene is such a pain. if someone tried a different solvent please speak up!

Octadecene has a double bond, so maybe at 280°C you polymerize the material. The evaporation will not work.

I know that there are several solvents for polyolefins and you basically have a wax, so a low molecular polyolefin. Probably xylene will dissolve it, but that of course is a rather nasty guy, which however can be removed in vacuo.

Research papers have used 1-octadecene at reflux for growing nanoparticles (b.p 320) so i dont think polymerisation took place..Also, in the case of polymerisation i would expect to see a change in the state of the resulting material i.e. not being pretty much the same viscous liquid (could become a waxy thing or similar) 

What size are your nanoparticles? Can you try separating them through filtration or magnetically?

they are (on TEM) about 17nm. I thought about size exclusion chromatography (or a different solvent) but i was asking in case someone tried a method with less effort and less time consuming than columns. what type of filtration are you talking about? cause i dont think the routine filtration methods can separate stuff on such a small scale. Magnetically, no! I havent tried that. have you tried something similar yourself? 

I haven't come across filters that small. Several groups have used magnetic NPs to capture analytes and use an external magnetic field for separation, but I haven't tried this, myself. See Nam, J.-M.; Thaxton, C. S.; Mirkin, C. A. Nanoparticle-Based Bio-Bar Codes for the Ultrasensitive Detection of Proteins. Science. 2003, 301, 1884-1886. (DOI: 10.1126/science.1088755) for an example.

Why did you used 1-octadecene in your process? Anyway solvents like hexane, heptane  should be able to extract it if not polymerized. In the latter case you will have to change your process.

Try acetone or chlorine-substituted methanes instead of alcohol, particles will be easier to separate and dry. If they are magnetic,use strong neodymium magnet instead of a centrifuge.

the high temperature decomposition method requires high temperatures which are directly related to the size of your nanoparticles. 1-octadecene allows for these temperatures to be reached and the starting material is soluble in it. (plus, many groups have used it before and they separated their particles with ethanol washing and centrifuging (yea right)). we use a strong magnet to decant stuff like washing etc, but this is not possible with octadecene, is really dense. Hexane didnt work either, and the particles are insoluble in acetone. We did really try but we have given up on the octadecene method, we got dioctyl ether (b.p 286) to give it a go maybe we can achieve big particles by let refluxing for longer. At the moment, we are trying various methods (some said, they managed to control size by varying the iron:acid-coating ratio) and see what comes out of it. But yea, we'd definitely given up on the octadecene method. STAY AWAY (you have been warned)

I found that chloroform will mix with the mother liquor, so add this then crash out your particles and collect them magnetically.

In what role this Octadecene in your reaction?.

you can achieve higher temperatures (315C) for the annealing/size focusing stage. higher temp=higher rate of Ostwald ripening=bigger particles (for an equivalent time, cf benzyl ether, bp 298C)

also it's non-coordinating, which has certain advantages

Hey, bit of a late answer but it may still be helpful to other readers:

If Ethanol precipitation of oleic acid capped SPIONs does not work you did not use the proper Ethanol/1-Octadecene ratio. Chemically speaking you add a polar, protic solvent (EtOH) which is miscible with your aprotic one (1-ODC), which will turn your mixture more protic as you add more Ethanol. At some point (I estimate a three- or fourfold excess of EtOH over 1-ODC) your mixture will be too polar for the Oleic-acid coated SPIONs to stay in suspension and you can either collect them with a magnet or centrifuge them. You can redisperse the particles in chloroform or similar solvents. That worked fine for me at least. Some people add mixtures of Alcohol/chloroform to do it all in one step, but then you have to get the ratio right to get decent phase separation.

Dear all, it's been great hearing from all of you and learning so many things through this question. I want to ask one thing. If one uses THF, will the octadecene + oleic acid part get dissolved in THF leading to the precipitation of only the inorganic nanoparticles?

You will only be able to dissolve chemicals not bound to the particles. However, do have some doubts whether particles capped with oleic acid are stably dispersed in THF. So you will have to evaluate this experimentally.

Did you get any success? I am facing the same problem. I am not able to precipitate my nanoparticles. 

I add acetone into the solution and then centrifuged to precipitate the nanoparticles. But i also have difficulties to dry the nanoparticles.

This is a very late answer, but I separate them magnetically and it is very fast and easy. I have made SPIONs by heating oleic acid, iron(III) oxide, and octadecene to 320°C as many papers have reported. Using solvent precipitations like ethanol, acetone, chloroform, etc. does work but it doesn't clean the excess oleic acid off of the particles very well. Just do magnetic separation. I pack a 5mL syringe to 3mL with steel wool at 0.9g/mL packing density, then dissolve my crude product in hexane (2:1 hexane to crude product by volume), then I flow the mixture through the syringe, which is between two powerful permanent magnets. Flush with hexane, then remove the syringe from the magnets and get a retentate in hexane. The retentate is stable in hexane, and you can evaporate and resuspend in THF and many other organic solvents.