Question

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

Short answer

The smectic liquid crystalline phase can be considered more highly ordered than the nematic phase because, unlike the nematic phase, which only exhibits orientational order of molecules, the smectic phase exhibits both orientational and positional order. In the smectic phase, molecules are arranged in well-defined layers, with each layer maintaining a specific distance from its neighbors. This higher degree of molecular organization makes the smectic phase more ordered compared to the nematic phase.

Step-by-step solution

Introduction to Liquid Crystals

Liquid crystals are a unique state of matter that have properties between those of conventional liquids and those of solid crystals. They can flow as a liquid, but their molecules have a well-defined orientation like in solid crystals. There are two important types of liquid crystal phases: nematic and smectic.

Nematic Liquid Crystalline Phase

In the nematic phase, the molecules have no positional order, which means they can flow and move around easily. However, they do have orientational order. The molecules have a preferred direction in which they align, called the director. The director is not fixed in space, so its orientation can change as the liquid crystal flows or is subjected to external forces.

Smectic Liquid Crystalline Phase

The smectic phase is a more ordered state compared to the nematic phase. In addition to orientational order, smectic phases also have positional order. The molecules are organized into layers, with each layer maintaining a specific distance from its neighbors. These layers can slide past each other, allowing the material to flow, but the molecules within each layer maintain their relative positions.

Comparison between Nematic and Smectic Phases

When comparing smectic and nematic phases, smectic phases exhibit both orientational and positional order, while nematic phases only have orientational order. This implies that the smectic phase is more highly ordered than the nematic in terms of molecular organization.

Conclusion

Smectic liquid crystalline phases can be considered more highly ordered than nematic phases because, in addition to orientational order of molecules (like in nematic phases), smectic phases also exhibit positional order, with molecules arranged in well-defined layers. This difference in molecular organization is what makes the smectic phase more highly ordered in comparison to the nematic phase.