Resolving the puzzle of ferrofluid dispersants

Oleic acid and stearic acid are similar surfactants which, however, lead re spectively to stability and to precipitation of ferrofluid suspensions: to understand this, the forces between layers of oleic-like surfactants and be tween layers of stearic-like surfactants across a hexadecane (HD) medium we re measured using a surface force balance (SFB). Separate measurements reve al that only the oleic layers are solvated by HD, while the SFB results rev eal that for both surfactants a marked net attraction is present between th e surfaces. Simple considerations based on these observations explain why, despite this attraction, ferrofluid dispersions are stabilized by oleic but not by stearic surfactants.