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Chapter 11: Problem 70

Describe how a cholesteric liquid crystal phase differs from a nematic phase.

Short Answer

Expert verified
A cholesteric liquid crystal phase, also known as chiral nematic phase, is characterized by chiral molecules that cause a helical molecular orientation. It consists of layers with well-defined spacing, but no positional order within each layer. In contrast, the nematic phase consists of rod-like molecules aligned along a common direction called the director, with positional disorder but orientational order. The key differences between these phases stem from the presence of chiral molecules and their resulting molecular arrangements, leading to distinct properties such as selective reflection of light in cholesteric phase, not seen in the nematic phase.

Step by step solution

01

Cholesteric Phase Properties

The cholesteric liquid crystal phase, or chiral nematic phase, is a special type of nematic phase. It is characterized by the presence of chiral molecules, which causes the molecular orientation to twist in a helical pattern. The pitch of this helix, or the distance over which the director (orientation of the molecules) rotates by 360 degrees, is called the cholesteric pitch. In a cholesteric phase, the molecules are arranged into layers with a well-defined layer spacing, but the positions of the individual molecules are not ordered within each layer.
02

Nematic Phase Properties

The nematic phase is a type of liquid crystal phase in which the rod-like molecules are aligned along a common direction, called the director. In this phase, the molecules possess positional disorder, meaning they are not arranged into a regular lattice structure. However, they maintain a degree of orientational order, as their long axes are preferentially aligned along the director. The nematic phase typically exhibits birefringence, which means that the refractive index depends on the polarization of the light relative to the director.
03

Comparison of Cholesteric and Nematic Phases

The main differences between the cholesteric and nematic phases lie in the presence of chiral molecules and the resulting molecular arrangement. In the cholesteric phase, the helical structure arises due to the presence of chiral molecules. In contrast, the nematic phase consists of rod-like molecules aligned in parallel without any helical twisting. This difference in molecular arrangement leads to distinct properties, such as the cholesteric phase's selective reflection of light, which is not observed in the nematic phase.

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Most popular questions from this chapter

If \(42.0 \mathrm{~kJ}\) of heat is added to a 32.0 -g sample of liquid methane under 1 atm of pressure at a temperature of \(-170{ }^{\circ} \mathrm{C},\) what are the final state and temperature of the methane once the system equilibrates? Assume no heat is lost to the surroundings. The normal boiling point of methane is \(-161.5^{\circ} \mathrm{C}\). The specific heats of liquid and gaseous methane are 3.48 and \(2.22 \mathrm{~J} / \mathrm{g}-\mathrm{K},\) respectively. [Section 11.4\(]\)

Arrange substances \(\mathrm{CCl}_{4},\) Si, and Ar in order of increasing boiling point.

Acetone, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO},\) is widely used as an industrial solvent. (a) Draw the Lewis structure for the acetone molecule and predict the geometry around each carbon atom. (b) Is the acetone molecule polar or nonpolar? (c) What kinds of intermolecular attractive forces exist between acetone molecules? (d) 1-Propanol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH},\) has a molecular weight that is very similar to that of acetone, yet acetone boils at \(56.5^{\circ} \mathrm{C}\) and 1-propanol boils at \(97.2^{\circ} \mathrm{C}\). Explain the difference.

Explain why any substance's heat of fusion is generally lower than its heat of vaporization.

List the three states of matter in order of (a) increasing molecular disorder and (b) increasing intermolecular attractions. (c) Which state of matter is most easily compressed?
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