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Laundry detergent and grease question

  1. May 22, 2007 #1
    When detergent acts on grease (their claim it cuts the grease) what this really means is that the detergent surrounds the grease making it water-soluable correct? thats what i got so far, but im not sure how that works. can someone explain it to me on a molecular level?
  2. jcsd
  3. May 22, 2007 #2


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    Gold Member

    I am no expert but my limited understanding is that there are two components - a detergent and an emulsifier:

    Grease - being organic fat and oil based - is water-repellent, so the polar configuration of water can't tear it apart (which is what it usually does to most things). Fats and oils are some of the few things that are immune to water's "nigh-universal solvent" property.

    But the detergent molecule has a shape with two different ends - one end has an affinity for oils and attaches itself to the grease, the other end has an affinity for water and allows water to attach to it.

    The grease now literally has a handle that the water can grab and pull. The blob of grease is torn into smaller micro droplets, which are further surrounded by the oil-loving detergent molecules. The micro droplets are prevented from coalescing back into a large blob of grease while at the same time the water is free to transport the droplets away.

    Using hot water gives the water molecules more "kick", making them more efficient at ripping apart the fats and oil molecules.
  4. May 23, 2007 #3
    detergent molecules have two ends: one is hydrophobic (usually alkyl or benzyl) and the other is hydrophilic (usually carboxlyic acid or sulfate, something that is charged).

    the detergent molecules surround the hydrophobic solute (grease, protein, etc.) by sticking with their hydrophobic end, and the hydrophilic ends maintain a solvation shell around the new micelle that has encapsulated the solute.
  5. May 23, 2007 #4

    jim mcnamara

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    Staff: Mentor

    Dave's explanation is a good one. Surfactants like a detergent break down the surface tension of water, too. But the ionic "handles" are really the trick, as Dave explained. It is also why detergents in dishwater eventually stop working. The effectiveness of the detergent drops over time- detergent is deactivated and taken out of play by attaching itself to fats and oils.
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