Normally PEG is best option if you are interested to use NPs in physiological environment, and water will be solvent. But for polymerization you should use some organic solvent and oleic acid.
The choice between oleic acid or PEG is definitely dependent on your next application. Water may be the best solvent, but to have easier way of drying you better choose ethanol. By "next application" I wanted to draw your attention to the nature of the two coats. Oleic acid is acidic and the PEG is neutral to slight alkaline... Good luck...
You better get a hydrocarbon liquid magnetic: magnetite nanoparticles + oleic acid + kerosene. Try instead of kerosene, styrene or divinylbenzene. It will be interesting and original way to solve your problem.
Dear Anis, your solvent choice for PEG because Magnetite won't be dissolve/soluble in any solvent just try to concentrate to dissolve PEG, I think organic polar solvent is good for this, moreover I agree in solvent choice with Sir Yuri Mirgorod.
For me its would be alkyl phosphate or phosphonate dissolve into a light alkane. An important point is to asses the necessary amount for coating your iron oxide. For this purpose, you have to know the specific surface area of your solid or to establish the isotherm of adsorption. Have a look also in the handbook of Fillers
Normally PEG is best option if you are interested to use NPs in physiological environment, and water will be solvent. But for polymerization you should use some organic solvent and oleic acid.