Question

Describe what is occurring at the molecular level as a substance passes from the solid to the nematic liquid crystalline to the isotropic (normal) liquid phase upon heating.

Short answer

Upon heating a substance, it transitions through different phases involving changes in molecular mobility and long-range order. In the solid phase, molecules are closely packed in a highly ordered crystal lattice. As the temperature increases, molecules gain kinetic energy and transition to the nematic liquid crystalline phase, characterized by long-range order in a parallel arrangement along one axis. Further heating leads to the isotropic liquid phase, where molecules are disordered, and there's no long-range order, exhibiting properties similar to conventional liquids.

Step-by-step solution

Solid Phase

In the solid phase, the molecules are very closely packed together. They're held in a regular, repeating, and highly ordered structure called a crystal lattice. The molecules vibrate about their fixed positions but don't have the freedom to change positions or move around.

Transition from Solid to Nematic Liquid Crystalline Phase

Upon heating the solid, the molecules gain kinetic energy, and their vibrations become more vigorous. At a certain temperature called the melting point, the energy becomes sufficient for the molecules to overcome the intermolecular forces that hold them in their fixed positions. However, they still maintain some degree of order, which leads to the formation of the nematic liquid crystalline phase. In the nematic liquid crystalline phase, the molecules have more mobility and can move past one another. However, there's still a long-range order, meaning that the molecules are oriented in a parallel arrangement along one axis called the director. This anisotropic arrangement gives them unique optical and flow properties.

Transition from Nematic Liquid Crystalline Phase to Isotropic Liquid Phase

As the heating continues and the temperature increases, the kinetic energy of the molecules increases further, leading to the disruption of the remaining long-range order present in the nematic liquid crystalline phase. Eventually, the sample reaches another transition temperature called the isotropic or clearing point, where the liquid loses all preferential molecular orientation. In the isotropic liquid phase, the molecules are disordered and have the highest freedom to move around. There is no long-range order, and the liquid obtains properties similar to those of a conventional liquid. This lack of order is responsible for the isotropic nature of the phase, which means that the properties are the same, irrespective of the direction of observation. To summarize, the transitions between different phases upon heating involve the gradual increase of molecular mobility and loss of long-range order: from highly ordered and restricted movement in the solid phase to the parallel arrangement in the nematic liquid crystalline phase and finally to the disordered isotropic liquid phase where molecules have the highest degree of freedom.