I made iron oxide nanoparticles via co-precipitation method and obtained a brown colour slurry with neutral pH and I did ex situ coating of oleic acid on nanoparticles and washed it with ethanol and water but i don't know if oleic acid is coated on my nanoparticles or not?

Procedure:

Nanoparticles slurry(100ml)

Oleic acid(1.6ml)

Ethanol(6.6ml)

Oleic acid:ethanol=1:4

Were magnetically stirred at 80C for 1 hour and washed with ethanol at 6000rpm for15 min and second washing was done with water at same rpm for 5 min but i didn't get two separate layers with brown colour layer on top of water. So i don't know if my particles are coated or not. All i got was brown pellet. Also i need to know the solvent which i should use for washing and also for suspending nanoparticles for my application