Problem 65 - Chemistry: The Central Science

Question

In terms of the arrangement and freedom of motion of the molecules, how are the nematic liquid crystalline phase and an ordinary liquid phase similar? How are they different?

Step-by-Step Solution

Answer

The nematic liquid crystalline phase and ordinary liquid phase both lack long-range positional order and their molecules are in constant motion, moving freely without constraints. However, nematic liquid crystals exhibit long-range orientational order, with molecules aligning along a preferred direction (director), while ordinary liquid phase has no orientational order. The freedom of motion in nematic liquid crystals is anisotropic, having easy movement parallel to the director but restricted movement perpendicular to it; in contrast, molecules in the ordinary liquid phase exhibit isotropic freedom, moving and rotating uniformly in all directions.

Step 1: Nematic Liquid Crystalline Phase

Nematic liquid crystals are characterized by a phase that has long-range orientational order but lacks positional order. The molecules in nematic liquid crystals are rod-like and tend to align along a preferred direction, called the director. These molecules can freely move parallel to the director but have restricted movement perpendicular to it. This leads to an intermediate phase between a well-ordered solid crystal phase and a disordered liquid phase.

Step 2: Ordinary Liquid Phase

In ordinary liquid phase, the molecules have no long-range positional or orientational order. The molecules exhibit random positions and orientations. These molecules have more freedom to move and rotate in all directions when compared to nematic liquid crystals.

Step 3: Similarities

Nematic liquid crystalline phase and ordinary liquid phase have the following similarities in terms of the arrangement and freedom of motion of the molecules: 1. Both phases lack long-range positional order which means that the molecules do not have any regular arrangement in terms of their positions. 2. The molecules in both phases are in constant motion and can move freely in the absence of any constraints.

Step 4: Differences

Nematic liquid crystalline phase and ordinary liquid phase have the following differences in terms of the arrangement and freedom of motion of the molecules: 1. In nematic liquid crystals, the molecules have a long-range orientational order, whereas in ordinary liquid phase, there is no orientational order. 2. The freedom of motion in nematic liquid crystals is anisotropic (direction dependent), meaning the molecules can easily move parallel to the director but have restricted movement perpendicular to it. In ordinary liquid phase, molecules have isotropic (uniform) freedom to move and rotate in all directions.