Liquid_Crystals - Prof. Shakhashiri www.scifun.org...

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Prof. Shakhashiri www.scifun.org Chemistry 103-1 LIQUID CRYSTALS To those who know that substances can exist in three states, solid, liquid, and gas, the term “liquid crystal” may be puzzling. How can a liquid be crystalline? However, “liquid crystal” is an accurate description of both the observed state transitions of many substances and the arrangement of molecules in some states of these substances. Many substances can exist in more than one state. For example, water can exist as a solid (ice), liquid, or gas (water vapor). The state of water depends on its temperature. Below 0 E C, water is a solid. As the temperature rises above 0 E C, ice melts to liquid water. When the temperature rises above 100 E C, liquid water vaporizes completely. Some substances can exist in states other than solid, liquid, and vapor. For example, cholesterol myristate (a derivative of cholesterol) is a crystalline solid below 71 E C. When the solid is warmed to 71 E C, it turns into a cloudy liquid. When the cloudy liquid is heated to 86 E C, it becomes a clear liquid. Cholesterol myristate changes from the solid state to an intermediate state (cloudy liquid) at 71 E C, and from the intermediate state to the liquid state at 86 E C. Because the intermediate state exits between the crystalline solid state and the liquid state, it has been called the liquid crystal state. Figure 1 . Arrangement of mole- cules in a solid crystal. Figure 2 . Arrangement of mole- cules in a liquid. Figure 3 . Arrangement of mole- cules in a liquid crystal. “Liquid crystal” also accurately describes the arrangement of molecules in this state. In the crystalline solid state, as represented in Figure 1, the arrangement of molecules is regular, with a regularly repeating pattern in all directions. (Molecules of substances with a liquid crystal state are generally oblong and rigid, that is, rod-shaped.) The
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This note was uploaded on 11/18/2010 for the course GEOLOGY 98975676 taught by Professor Donald during the Spring '09 term at Akademia Ekonomiczna w Krakowie.

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Liquid_Crystals - Prof. Shakhashiri www.scifun.org...

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