Question

For a given substance, the liquid crystalline phase tends to be more viscous than the liquid phase. Why?

Short answer

The higher viscosity of the liquid crystalline phase compared to the liquid phase in a given substance can be attributed to the orientational and partial positional order of molecules in the liquid crystalline phase. This order results in restricted motion, stronger molecular interactions, and consequently, higher resistance to flow compared to the more random and free movement of molecules in the liquid phase.

Step-by-step solution

Understanding the phases

In a given substance, there are mainly three phases: solid, liquid, and liquid crystalline state. The liquid phase is known to have fluidity while the solid phase has a well-organized structure where molecules have a fixed position. The liquid crystalline phase is an intermediate phase between the solid and liquid, in which the substance has properties of both solid and liquid phases.

Understanding liquid phase

In the liquid phase, the molecules are in constant motion and have a short-term order. They move freely and have no long-range order. The interactions between the molecules are weaker, which is why substances in the liquid phase have less viscosity in comparison to their solid or liquid crystalline states.

Understanding liquid crystalline phase

The liquid crystalline phase comes into existence when the substance has a partial ordering of its molecules between the solid phase's well-organized structure and the liquid phase's fluidity. In this phase, the molecules have both long-range orientational order and short-range positional order. They follow a certain order or direction but are not as rigid as in the solid phase. This phase displays properties of both solid and liquid phases.

Comparing the viscosity of liquid and liquid crystalline phases

Viscosity depends on the interactions between individual molecules, their mobility, and the degree of order between them. In the liquid crystalline phase, due to orientational and partial positional order, the molecules have restricted motions compared to the liquid phase, where they move freely and randomly. This restricted motion and the intermolecular forces in the liquid crystalline phase result in higher resistance to flow and more viscous behavior compared to the liquid phase. In conclusion, the reason behind the higher viscosity of the liquid crystalline phase as compared to the liquid phase in a given substance is due to the orientational and partial positional order of molecules, which results in restricted motion, stronger molecular interactions, and therefore higher resistance to flow.