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3.11: Questions

  • Page ID
    35367
  • Quick questions

    You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

    Which of the following molecules is likely to form a liquid crystalline phase?

    a

    alt

    b

    alt

    c

    alt

    d

    alt

    Answer

    A, C, D

    Deeper questions

    The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

    The bright colours found on some insect wings are due to the existence of a thin membrane containing a chiral nematic liquid crystal on their surfaces. Keeping in mind that the light will be reflected by their wings rather than transmitted through them, how do these colours occur?

    Hint: The pitch of a typical chiral nematic is a few hundred nm.

    Answer

    The pitch of the liquid crystal in the insect wings must be equal to the wavelength of a particular colour of visible light. Therefore that wavelength is reflected by identical layers of molecules every half wavelength (corresponding to every half pitch � the director is either parallel or antiparallel to the original vector). The result is that constructive interference takes place between reflected beams of that particular wavelength, and so a bright colour is seen.

    In the introduction to this TLP lyotropic liquid crystals were mentioned. Unlike thermotropic species, their properties and mesophases are mainly affected by their concentration in solution, as well as other solutes & solvents present.

    Molecules that form lyotropic liquid crystals can usually be thought of as a long-chain molecule with a polar head attached to a non-polar hydrocarbon chain.

    What kind of structures do you think these liquid crystals would form when mixed with water? How would this differ if they were mixed with a solvent such as hexane?

    Hint: Think about the miscibility of different types of liquids.

    Answer

    The polar heads are kept in contact with the polar water, whilst shielding the non-polar tails. Similarly, when the liquid crystal is mixed with hexane (a non-polar molecule) we get the following miscelle structure:

    Which can be thought of as an inverse of the first, now with the non-polar tails in contact with the non-polar solvent and the polar heads shielded away.

    Note that these two structures are only the most basic that will form � varying factors such as concentration of the solute, temperature of the mixture and even the relative polar strength of the head region can result in more complex structures.

    The disclinations shown below all result in a schlieren brush visible under polarised light microscopy. For each disclination, select the brush that would be seen [Yes for Brush A or No for Brush B] (assume the two polarisers are aligned vertically and horizontally)):

    Brush A Brush B
    schlieren schlieren
    Yes No a

    alt

    Yes No b

    alt

    Yes No c

    alt

    Yes No d

    alt

    Answer

    A no

    B yes

    C yes

    D no

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