Answers to questions on Liquid crystals

1. (a) The cyanide group is polar (due to the difference in electronegativity between carbon and nitrogen). This ensures that the intermolecular forces are string enough to align in a particular direction.

(b) The pentyl group is a long alkyl chain, which ensures that the molecules cannot pack to closely together. This helps to maintain the liquid crystal state.

(c) The two disubstituted phenyl groups help to ensure that the molecules are rigid and therefore more rod-shaped.

2. (a) Soap solutions show lyotropic liquid crystal behaviour but they need to be reasonably concentrated in order for soap molecules to group together to form the micelles, which become ordered in the liquid crystal phase.

(b) Cholesteryl benzoate shows thermotropic crystal behaviour at 145.5 ^oC but turns into the normal liquid state at 178.5 ^oC and above.

(c) Because liquid crystals only work within a specified temperature range they are likely to malfunction at low temperatures.

(d) Cathode ray tubes require a relatively large electric current to function and this produces heat, LCDs use extremely small electric currents so do not give off nearly as much heat.

(e) The molecules in the liquid crystals sandwiched between two polarisers need to be lined up in the same direction (the nematic phase) so they can interact with the plane-polarised light.

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