There is alot of good information out there about how best to store essential oils and essential oil-infused DIY products. Click here for doTerra's information on it.
I like to keep things simple and intuitive so here are my quick guides on the matter.
Glass is impervious to essential oils and is the best way to keep DIY products fresh. It is also easy to clean with soap and water. With plastics, there's a possibility that the oils will leech onto/dissolve the plastic. The higher quality plastic -HDPE- has stronger bonds and longer chain lengths that are not broken up by essential oils. PET is also resistant but be wary of the dilution ratio. To test if your plastic container is resistant, add a drop of the oil you want to use (different oils have different impact) to the outside of the container and make sure there is no change to the plastic after a few minutes.
To be safe, again, glass is best.
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