Problem 80 - Chemistry Central Science

Question

Suppose that a liquid crystalline material such as cholesteryl benzoate is warmed to well above its liquid crystalline range and then cooled. On cooling, the sample unexpectedly remains clear until it reaches a temperature just below the melting point, at which time it solidifies. What explanation can you give for this behavior?

Step-by-Step Solution

Answer

The unexpected behavior of cholesteryl benzoate during cooling can be explained by supercooling. When heated above its liquid crystalline range, cholesteryl benzoate becomes an isotropic liquid, losing its ordered structure. As it cools, it remains clear and does not return to its liquid crystalline phase due to the supercooling phenomenon. It only solidifies upon reaching a temperature just below the melting point, as necessary conditions or a nucleation site for the formation of a crystal structure finally emerge.

Step 1: Understanding Liquid Crystalline Materials

Liquid crystalline materials are substances that have properties between those of conventional liquids and crystalline solids. They have an ordered structure in one or two dimensions and can flow like a liquid. In the case of cholesteryl benzoate, it usually forms a liquid crystalline phase within a specific temperature range.

Step 2: Heating the Material

When the cholesteryl benzoate is heated, it goes well above its usual liquid crystalline range. In this case, the cholesteryl benzoate becomes an isotropic liquid, which means it loses its ordered structure and behaves like a conventional liquid. This is why, when heated, it becomes clear.

Step 3: Cooling the Material

As the material is cooled, one would expect it to return to its liquid crystalline phase. However, in this scenario, the cholesteryl benzoate remains clear and does not display the characteristic properties of a liquid crystalline material.

Step 4: Understanding the Unexpected Behavior

The unexpected behavior of cholesteryl benzoate can be explained by a phenomenon called supercooling. Supercooling occurs when a substance remains in a liquid state, even though it is below the temperature at which it should solidify. This can happen when the cooling process does not provide the necessary conditions or a nucleation site for the formation of a crystal structure.

Step 5: Solidification at Melting Point

Once the temperature of the cholesteryl benzoate drops just below the melting point, it solidifies. This could be explained by the fact that at this temperature, the required conditions for crystallization are finally met, or the material comes into contact with an external factor that serves as a nucleation site, forcing it to solidify rapidly. In conclusion, the behavior of cholesteryl benzoate when heated and cooled can be explained by the properties of liquid crystalline materials and the phenomenon of supercooling. The material loses its ordered structure when heated, remaining clear during cooling, and only solidifies once it reaches a temperature just below the melting point due to supercooling.