Question

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 in terms of the ideas developed in Chapter 11 relating molecular energies to temperature.

Short answer

The transition from a smectic to nematic liquid crystalline phase at higher temperatures occurs due to the increased molecular energy associated with a higher temperature. As temperature increases, molecular kinetic energy and motion increase, causing the molecular arrangement to lose its orderliness. This leads to the substance shifting from the more ordered smectic phase to the less ordered nematic phase.

Step-by-step solution

Understanding Smectic and Nematic Liquid Crystal Phases

Smectic and nematic phases are two types of liquid crystal phases that occur because of molecular arrangements. In a smectic liquid crystal phase, molecules are arranged in layers, while in a nematic liquid crystal phase, molecules are aligned parallel to each other but don't form distinct layers. The main difference between these two phases is the degree of order in their molecular structure.

Molecular Energies and Temperature

According to Chapter 11, molecular energies are related to temperature. As the temperature of a substance increases, the kinetic energy of its molecules also increases. This increase in molecular energy leads to an increased motion of the molecules, and as a result, the orderliness of the molecular arrangement decreases.

Transition from Smectic to Nematic Phase

The substance initially has a smectic phase at a temperature just above the melting point. In this phase, the molecules are organized in layers and have a relatively low molecular energy. As the temperature increases, the kinetic energy of the molecules increases, which leads to more molecular motion and a decrease in the orderliness of the molecular arrangement.

Explain the Behavior in Terms of Molecular Energies and Temperature

The transition from the smectic to the nematic phase occurs due to the increase in molecular energy associated with the increase in temperature. The higher temperature results in more kinetic energy and increased molecular motion, which causes the molecular arrangement to lose its orderliness. Since the smectic phase has a more ordered molecular arrangement compared to the nematic phase, the substance tends to shift to the less ordered nematic phase as the temperature increases. In conclusion, the behavior of a substance passing from a smectic liquid crystalline phase into a nematic liquid crystalline phase at higher temperatures can be accounted for by considering the relationship between molecular energies and temperature. As the temperature increases, molecular energies increase, which results in more molecular motion and a decrease in the orderliness of the molecular arrangement, ultimately leading to the transition from the smectic to the nematic phase.

Key concepts

Smectic Liquid Crystal Phase

Imagine a deck of cards neatly stacked together, where each card represents a molecule; this is what the structure in a smectic liquid crystal phase resembles. The molecules are arranged in layers, maintaining a uniform distance from each other, which results in a highly ordered structure. But unlike a solid, these layers can slide over each other, which gives the smectic phase its unique liquid-like properties. The alignment offers the material distinctive optical and electrical characteristics, which are utilized in various applications, such as digital displays and sensors.

In terms of molecular interactions, the orderly smectic structure is stabilized by intermolecular forces like Van der Waals forces or dipole-dipole interactions. With layers being the defining characteristic of the smectic phase, it can be envisaged as a middle ground between the rigid structure of a solid crystal and the random orientation of a normal liquid.

Nematic Liquid Crystal Phase

Transitioning from the organized layers of the smectic phase, the nematic liquid crystal phase is like a more relaxed gathering of the molecules with a 'go with the flow' attitude. In this phase, the molecules are aligned parallel to a common direction known as the director, but they do not form distinct layers. Their end-to-end positioning offers flexibility, yet a certain degree of order is maintained. This unique arrangement makes nematic liquid crystals important in the technology behind LCD monitors where their directional properties help to control light passage.

The nematic phase offers less structural order than the smectic phase, providing different mechanical and optical properties. The molecules have more freedom to move around while still maintaining overall directional order, which allows for the nematic liquid crystal's responsiveness to external fields, such as electric or magnetic fields, a feature that's widely exploited in electronic devices.

Molecular Energies and Temperature

The behavior of molecules within liquid crystal phases is largely governed by temperature because it directly influences molecular energies. To understand this, let's delve into the concept of kinetic energy, which is the type of energy associated with an object's movement. At higher temperatures, molecules receive thermal energy, translating into greater kinetic energy and more vigorous motion.

When applying heat to a substance, you are essentially pumping in energy, prompting molecules to vibrate and move more rapidly. The additional energy can disrupt orderly arrangements as it overcomes the forces holding the molecules in place. As the temperature climbs, this vibrational and rotational motion becomes intense enough to weaken the structured smectic layers, potentially leading to a phase transition, which ultimately showcases the intrinsic relationship between molecular energies and temperature in determining the physical state of a substance.

Transition Between Liquid Crystal Phases

The transformation from the smectic to the nematic phase is not simply a matter of molecules deciding to shift their positions. It's an energy-driven transition requiring sufficient thermal input to agitate the molecular arrangement. As the temperature rises, the smectic phase, with its well-established layers, loses stability because the molecules gain kinetic energy, giving them the push needed to break free from their ordered layout.

This loss of order leads to the less organized nematic phase, which retains directional alignment but without the constraint of layers. The shift is significant in the practical use of liquid crystals, as different phases exhibit divergent electro-optic properties. It's a dance of thermodynamics, where heat becomes the music that directs the molecules from one phase to another, illustrating a fascinating interplay between temperature and material properties.