Skip to content
CleaningHQSearch

Why oil resists water

Water molecules attract each other so strongly that they exclude oil rather than dissolving it. Nothing about scrubbing changes that, which is what a detergent is for.

In detail

Water molecules are polar and hydrogen-bond to one another with unusual strength. An oil molecule offers nothing to bond with, so water molecules prefer each other's company and the oil is squeezed out of the network rather than taken into it. That is immiscibility, and it is why hot water alone runs off a greasy plate, why a rinsed pan still feels slippery, and why the water on a greasy surface beads instead of spreading. Two consequences follow that decide the whole approach. First, no amount of agitation makes water dissolve oil — mechanical action can break oil into droplets, but without something to stabilise them the droplets recombine the moment the agitation stops. Second, the fix is not a stronger version of water but a different molecule: a surfactant, with a water-loving head and an oil-loving tail, which wraps each droplet so that water can carry it. That is why washing-up liquid works at a concentration that seems far too low to matter, and why a rinse is part of the job rather than the end of it — the oil is suspended, not destroyed, and an evaporating under-rinsed film puts it straight back.

Why

Sources for this page

Sources

  1. Kirk-Othmer Encyclopedia of Chemical Technology

    John Wiley & Sons · Reference work

    Relied on for: That surfactants are amphiphilic molecules which reduce interfacial tension and emulsify oils that water alone cannot take up. (background and mechanism only)

    Verification: the work exists and is described correctly. Nobody has yet read the specific passage.

  2. Spot Solver: spot and spill removal procedures

    The Carpet and Rug Institute (CRI) · Trade or professional body

    Relied on for: That greasy and oily deposits are routed to a solvent branch rather than to an aqueous one. (states this directly)

    Verification: a person has read the passage relied on.

    Read on — Sprint 5-10: fifteen entries, stain by stain. Sprint 30: enumerated at 296 from the publisher's own sitemap, 134 more read plus 'Unknown'. Sprint 38: 56 more, chosen against stains the catalogue lacked. 192 of 296 read. Supports the claim as stated.

Last reviewed by a person on .

Share this page