It often happens that a substance possessing a smectic liquid crystalline phase just above the melting point passes into a nematic liquid crystalline phase at a higher temperature. Account for this type of behavior.

Liquid crystals are a state of matter which have properties between those of conventional liquids and crystalline solids. They can flow like a liquid, but also have a structured arrangement of molecules similar to a crystalline solid. There are different types of liquid crystalline phases, and in this exercise, we are focusing on smectic and nematic liquid crystalline phases.

A smectic liquid crystalline phase is characterized by an organized structure of the molecules, where the molecules are arranged in layers. Within each layer, the molecules are parallel to each other and have a specific direction, called a director. The layers can slide past each other, which gives the smectic phase its fluid-like property. The more ordered structure of the smectic phase is present at a lower temperature range.

The nematic phase is another type of liquid crystalline phase. In this phase, the molecular ordering is less defined compared to the smectic phase. The molecules are not arranged in layers in the nematic phase, which means there is no layered structure. Instead, the molecules in the nematic phase have a common direction, which makes it an anisotropic liquid. The less organized nematic phase occurs at a higher temperature range than the smectic phase.

As the temperature increases, the kinetic energy of the molecules also increases. This increase in temperature leads to the reduction in the ordering of molecules. When the temperature is just above the melting point, the molecules can form a more ordered structure, forming the smectic phase with its layered structure.

As the temperature increases further, the kinetic energy of the molecules becomes high enough to disrupt the ordered layered structure of the smectic phase. The molecules become less ordered and lose their layered structure and instead align themselves in a common direction. This results in the transition from the smectic phase to the nematic phase at a higher temperature. In conclusion, this type of behavior of a substance passing from a smectic liquid crystalline phase just above the melting point to a nematic liquid crystalline phase at a higher temperature can be accounted for by the increasing kinetic energy of the molecules as the temperature increases. This increased kinetic energy disrupts the ordered structure found in the smectic phase, causing a transition to the less ordered nematic phase.