Saturated Fatty Acids

I cannot find a good explanation about why saturated fatty acids are considered "lighter weight" than unsaturated fatty acids.  I understand that naturally hydrogenated (saturated) fatty acids have a 'cis-configuration' (varying on the same sides of the carbon chain), and artificially hydrogenated fats have a trans-configuration (opposite sides of the carbon chain) making lay tighter together... but how does that change the molecular weight as compared to unsaturated fatty acids with empty double bonds.  Anyone able to articulate this phenomenon?

Comments

  • Bill_TogeBill_Toge Member, Professional Chemist
    on a fundamental level, it's due to the shape of the molecules

    saturated and trans-unsaturated fatty acids adopt a linear configuration, which means they occupy a relatively low volume of space and can move freely

    cis-unsaturated fatty acids, by their nature, can only adopt a cuneiform (wedge-shaped) configuration, because the double bond is completely inflexible, so they occupy a much larger volume of space than linear fatty acids and cannot move as freely

    as a result, they have higher melting and boiling points than the linear fatty acids with the same carbon chain length, and are usually liquids at room temperature; the linear fatty acids with the same carbon chain length are usually solids, which gives them a different skin feel
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • Thanks, that's very interesting.

    So when they refer to something as being "lighter" weight, that means it just "feels" different, either more flexible or stiffer... right?  It's not truly "heavier" or "lighter" weight by the technical/scientific definition.

    And when you say something takes up more "volume of space", or "less volume of space", does that mean there's actually unidentified matter in the unoccupied spaces, or some other chemical element?  I hope I'm asking the question in the right way.


  • Bill_TogeBill_Toge Member, Professional Chemist
    yes, 'light' and 'heavy' are generally used to refer to the physical sensations rather than the actual weight

    the best explanation of molecular volume is an illustration:

    space-filling model of stearic acid (unsaturated C18 acid)
    space-filling model of oleic acid (monounsaturated C18 acid)

    (the black parts represent carbon atoms, the white parts represent hydrogen atoms, and the red parts represent oxygen atoms)

    because it has two rotational axes rather than one, oleic acid takes up more space than stearic acid
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • OKay that makes a lot of sense.  That visual was better than some of other rudimentary ones I've seen.  So that must be why the polyunsaturated feel so much heavier.  Instead of just a "kink" or "bend" in the carbon chain length, the double bonds make more of a curved "hook", which would make them even more inflexible.  Thanks.
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