Polysorbate 20 (Tween)
Polysorbate 20, or "Tween", is an emulsifying agent to help essential oils stay mixed with water.
In general, one uses as much Tween as they do essential oil in a blend -- but we've found we could use much less than that if we don't mind shaking the bottle before we use it. It has an odd aroma that dissipates after a while - perhaps a day later we don't notice it at all.
We've used it specifically for making insect repellent sprays, and one could do the same for Lavender linen sprays, room aromatherapy sprays and similar applications.