(a)     State one other example of a lyotropic liquid crystal and describe the difference between lyotropic and thermotropic liquid crystals.

(b)     Name a thermotropic liquid crystal.

(c)     Explain the liquid-crystal behaviour of the thermotropic liquid crystal named in part (b), on the molecular level.

(a)     soap / kevlar / fatty acids / lipid bilayer / cellulose / silk proteins / DNA;

lyotropic liquid crystals

solutions that show the liquid-crystal state at certain concentrations;

thermotropic liquid crystals

(pure substances that) show liquid-crystal behaviour over temperature ranges (between the solid and liquid states);

(b)     biphenyl nitriles/cyanobiphenyls;

(c)     nitrile groups make molecule polar;

intermolecular forces are strong enough to align in a common direction;

biphenyl groups make molecules more rigid/rod-shaped;

(long) alkane chain ensures that molecules cannot pack together closely (to maintain liquid-crystal state);

This question was very poorly completed by most candidates. Candidates were usually able to name another example of a lyotropic liquid crystal and define thermotropic and lyotropic liquid crystals. However, candidates struggled to give an example of a thermotropic liquid crystal and often confused this with providing an example of the use of thermotropic liquid crystals.

In part (c), even when a thermotropic liquid crystal was mentioned in the previous question, candidates struggled to score in this part because they described the general functioning of liquid crystals which was asked in a previous paper. This was another example where candidates either did not read the question correctly or did not know and just wrote what they knew about.