What makes oil and soap slippery?

Discussion in 'Chemistry' started by Magical Realist, Jan 15, 2014.

  1. Magical Realist Valued Senior Member

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    Is there something different in the structure of the molecules that allows for less friction?
     
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  3. exchemist Valued Senior Member

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    I would imagine it is because they are surfactants which adsorb to the surface, forming a viscous layer that keeps the surfaces apart more effectively than water does. Lubrication at low relative rates of sliding generally requires high viscosity lubricants, so that the lubricant is not squeezed out of the contact zone.
     
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  5. Magical Realist Valued Senior Member

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    But what makes that layer more viscous? Maple syrup forms a thick layer that keeps surfaces apart but actually makes them more sticky. Why is soap or oil different from maple syrup viscosity-wise?
     
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  7. exchemist Valued Senior Member

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    No I bet it lubricates them (i.e. keeps the surfaces from contacting one another) fine. But the viscosity of that stuff is so high that you feel the resistance. Thing is with soap and water you have a viscous layer next to the surface, but then water in between, which is low viscosity. So you sort of get the best of both worlds. Or that would be my best guess, anyway.
     
  8. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    Try this for an answer.
    Oil is naturally slippery because the molecules are non-polar and slide over each other easily.
    Non-polar means they don't have positive and negative ends.
    Water is not slippery because it is polar. It joins loosely to make chains and clumps, so there is friction.
    What soap does it to attract the water to its surface so that they are all facing the same way, so they don't form chains.
    With less friction, it becomes more like oil.
     
  9. exchemist Valued Senior Member

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    Nice try, but try telling that to the people who make aviation turbine lubricants from organic esters, for example. And of course water IS slippery, when you fall on a patch of ice.

    Polarity determines propensity to adsorb to polar surfaces, but not lubricating ability.
     
  10. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    Here's the definition from wiki of an oil.

    An oil is any neutral, nonpolar chemical substance, that is a viscous liquid at ambient temperatures, and is both hydrophobic (immiscible with water, literally "water fearing") and lipophilic (miscible with other oils, literally "fat loving"). Oils have a high carbon and hydrogen content and are usually flammable and slippery.

    http://en.wikipedia.org/wiki/Oil

    There may be some non-polar liquids which are not slippery, but they usually are.
    Viscosity might be important, but I don't know why.
     
  11. exchemist Valued Senior Member

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    Yes but this is the definition of an oil, which is by no means synonymous with a lubricant, q.v. :http://en.wikipedia.org/wiki/Lubricant


    What a lubricant does is keep sliding surfaces apart, usually by forming a pressurised wedge of fluid in the contact zone, due to the sliding of one surface relative to the other. This is termed hydrodynamic lubrication. Viscosity is important, as the ability of the sliding surface to "drag" enough lubricant into the contact zone to pressurise it sufficiently to keep the surfaces apart will depend on the sliding speed and the viscous drag exerted. At high enough sliding speeds, and/or with little pressure in the contact, even water can be a lubricant. But for most machine applications, something of higher viscosity is needed. The subjective perception of "slipperiness" merely reflects whether or not a fluid can keep apart the contact zone formed by your thumb and forefinger when you rub them together.

    Whether a lubricant is polar or not is rather beside the point, as far as basic hydrodynamic lubrication goes. However it can be important for regimes in which the hydrodynamic film cannot be sustained, e.g. with extreme pressures or with sliding motions that are irregular (e.g. with "finger" type cam followers in IC engines, the complex, combined rolling and sliding between gear teeth, etc). Polar agents that bind to the surface and form a harder film that can be sheared, while protected the two metal surfaces, may be used for such applications. These are known as antiwear or EP agents. And detergents and dispersants, which are polar of course, are added to engine oils to suspend the contaminants introduced by combustion.
     
    Last edited: Jan 16, 2014
  12. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    The question was about what makes oil and soap slippery.
    The lubricant graphite, for example, is not a non-polar liquid, I accept that.
    My explanation for soap making water slippery was that it arranges the polar molecules so that they move over each other
    rather than attracting each other.
    What's your explanation at a chemical level?
     
  13. exchemist Valued Senior Member

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    Eh? I was not thinking one was needed, as if soap makes a viscous film on your fingers that should be enough. But, thinking about your idea of some process akin to what makes graphite lubricate, I do recall that the "smectic" phase of liquid crystals is named after soap. I've therefore been trying to look up on the web articles about the crystal structure of soap. There are a couple of tantalising ones but both require subscription to access the full content. You may be right that the molecules are aligned with glide planes parallel to the direction of sliding and this explains it.
     
  14. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    Not everything viscous is slippery.
    Glue for example.
     
  15. exchemist Valued Senior Member

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    Well it is, actually, until it starts to set, at which point of course it ceases to behave like a real fluid. When I help my son with his Airfix kits, the glue can be very slippery, at just the wrong moments.
     
  16. Olinguito Registered Member

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    Wet paint can be slippery too.
     
  17. exchemist Valued Senior Member

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    Another viscous fluid.

    Not sure about dogshit though - maybe that needs an explanation more like Capt. Kremmen's…...
     
  18. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    So slippery stuff is slippery because it's got slippery stuff in it.
     

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