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 addition to possessing long-range orientational order, smectic phase molecules also exhibit positional order, where they are arranged in well-defined layers. This additional level of organization results in a higher degree of order in the smectic phase as compared to the nematic phase.

Step-by-step solution

1. Understanding Nematic and Smectic Liquid Crystal Phases

Nematic and smectic phases are two types of liquid crystal phases. A liquid crystal is a state of matter that has properties between those of a conventional liquid and those of a solid crystal. In both nematic and smectic phases, the molecules have long, rod-like shapes. In the nematic phase, the molecules have a preferred orientation, but their position with respect to each other is random. In the smectic phase, the molecules are also oriented, but they have an additional level of organization where they form layers.

2. Comparing Order and Molecular Arrangements

To understand the difference in order between the nematic and smectic phases, we can examine their molecular arrangements. In the nematic phase, the molecules are on average aligned with each other, exhibiting long-range orientational order. However, they don't have any positional order, meaning there is no specific arrangement of molecules relative to each other. In the smectic phase, the molecules also exhibit orientational order, but in addition to that, they have positional order, where the molecules are arranged in well-defined layers. Within these layers, the molecules are more closely packed than in the nematic phase, and their positions are more highly ordered.

3. Concluding the Sense of Higher Order

The smectic liquid crystalline phase is considered more highly ordered than the nematic phase because not only do the molecules have long-range orientational order like in the nematic phase, but they also possess positional order through the formation of distinct layers. This additional level of organization results in a higher degree of order in the smectic phase as compared to the nematic phase.