Question

The smectic liquid crystalline phase can be said to be more highly ordered than the nematic phase. In what sense is this true?

Short answer

The smectic liquid crystalline phase is more highly ordered than the nematic phase because, in the smectic phase, the rod-like molecules exhibit both parallel orientation and well-defined layered arrangement. In contrast, the nematic phase only displays parallel orientation of rod-like molecules without any layered arrangement, making it less ordered compared to the smectic phase.

Step-by-step solution

Introduction to Liquid Crystal Phases

Liquid crystals are an intermediate state of matter between the solid and liquid phase. They possess properties of both solid and liquid phases. The two main liquid crystal phases we will discuss are the smectic phase and the nematic phase. Both of these phases have a degree of molecular ordering, but the smectic phase is considered to be more highly ordered than the nematic phase.

Nematic Phase Ordering

In the nematic phase, the long axes of the rod-like molecules are oriented parallel to one another while maintaining a degree of rotational freedom around their long axis; however, they are not arranged in layers. Due to this orientation, the nematic phase is considered as less ordered than the smectic phase.

Smectic Phase Ordering

The smectic phase is characterized by a more organized molecular arrangement compared to the nematic phase. In the smectic phase, the rod-like molecules are not only oriented parallel to each other (similar to the nematic phase) but they also form well-defined layers. This layered arrangement adds an extra level of ordering in the smectic phase, making it more highly ordered compared to the nematic phase.

Conclusion

In summary, the smectic liquid crystalline phase is considered to be more highly ordered than the nematic phase because, in the smectic phase, the rod-like molecules are organized both in terms of their orientation and their arrangement in layers. On the other hand, the nematic phase exhibits only parallel orientation of rod-like molecules without any layered arrangement. This extra level of ordering in the smectic phase makes it more highly ordered compared to the nematic phase.