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Chapter 11: Problem 70

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

Short Answer

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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

Look up and compare the normal boiling points and normal melting points of \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{H}_{2} \mathrm{~S}\). Based on these physical properties, which substance has stronger intermolecular forces? What kinds of intermolecular forces exist for each molecule?

For many years drinking water has been cooled in hot climates by evaporating it from the surfaces of canvas bags or porous clay pots. How many grams of water can be cooled from \(35^{\circ} \mathrm{C}\) to \(20^{\circ} \mathrm{C}\) by the evaporation of \(60 \mathrm{~g}\) of water? (The heat of vaporization of water in this temperature range is \(2.4 \mathrm{~kJ} / \mathrm{g}\). The specific heat of water is \(4.18 \mathrm{~J} / \mathrm{g}-\mathrm{K} .)\)

The boiling points, surface tensions, and viscosities of water and several alchohols are as follows: $$ \begin{array}{lrcc} & \begin{array}{l} \text { Boiling } \\ \text { Point }\left({ }^{\circ} \mathbf{C}\right) \end{array} & \begin{array}{l} \text { Surface } \\ \text { Tension }\left(\mathbf{J} / \mathbf{m}^{2}\right) \end{array} & \begin{array}{l} \text { Viscosity } \\ (\mathbf{k g} / \mathbf{m}-\mathbf{s}) \end{array} \\ \hline \text { Water, } \mathrm{H}_{2} \mathrm{O} & 100 & 7.3 \times 10^{-2} & 0.9 \times 10^{-3} \\ \text {Ethanol, } \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} & 78 & 2.3 \times 10^{-2} & 1.1 \times 10^{-3} \\ \text {Propanol, } \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} & 97 & 2.4 \times 10^{-2} & 2.2 \times 10^{-3} \\ n \text { -Butanol, } \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} & 117 & 2.6 \times 10^{-2} & 2.6 \times 10^{-3} \\\ \text {Ethylene glycol, } \mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH} & 197 & 4.8 \times 10^{-2} & 26 \times 10^{-3} \end{array} $$ (a) For ethanol, propanol, and \(n\) -butanol the boiling points, surface tensions, and viscosities all increase. What is the reason for this increase? (b) How do you explain the fact that propanol and ethylene glycol have similar molecular weights \((60\) versus \(62 \mathrm{amu}),\) yet the viscosity of ethylene glycol is more than 10 times larger than propanol? (c) How do you explain the fact that water has the highest surface tension but the lowest viscosity?

The fact that water on Earth can readily be found in all three states (solid, liquid, and gas) is in part a consequence of the fact that the triple point of water \(\left(T=0.01^{\circ} \mathrm{C}, P=0.006 \mathrm{~atm}\right)\) falls within a range of temperatures and pressures found on Earth. Saturn's largest moon Titan has a considerable amount of methane in its atmosphere. The conditions on the surface of Titan are estimated to be \(P=1.6\) atm and \(T=-178^{\circ} \mathrm{C}\). As seen from the phase diagram of methane (Figure 11.30 ), these conditions are not far from the triple point of methane, raising the tantalizing possibility that solid, liquid, and gaseous methane can be found on Titan. (a) What state would you expect to find methane in on the surface of Titan? (b) On moving upward through the atmosphere the pressure will decrease. If we assume that the temperature does not change, what phase change would you expect to see as we move away from the surface?

Referring to Figure 11.28 , describe all the phase changes that would occur in each of the following cases: (a) Water vapor originally at 0.005 atm and \(-0.5^{\circ} \mathrm{C}\) is slowly compressed at constant temperature until the final pressure is 20 atm. (b) Water originally at \(100.0^{\circ} \mathrm{C}\) and \(0.50 \mathrm{~atm}\) is cooled at constant pressure until the temperature is \(-10^{\circ} \mathrm{C}\).
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