12.1 Liquid Crystals

We have seen that when a solid is heated to its melting point, the added thermal energy overcomes the intermolecular attractions that provide molecular order to the solid. (For more information, see Section 11.1) The liquid that forms is characterized by random molecular orientations and considerable molecular motion. Some substances, however, exhibit more complex behavior as their solids are heated.

In 1888 Frederick Reinitzer, an Austrian botanist, discovered that an organic compound he was studying, cholesteryl benzoate, has interesting and unusual properties. When heated, the substance melts at 145°C to form a milky liquid, and at 179°C the milky liquid suddenly becomes clear. When the substance is cooled, the reverse processes occur: The clear liquid turns milky at 179°C (Figure 12.2), and the milky liquid solidifies at 145°C. Reinitzer's work represents the first systematic report of what we now call a liquid crystal.

Instead of passing directly from the solid to the liquid phase when heated, some substances, such as cholesteryl benzoate, pass through an intermediate, liquid-crystalline phase that has some of the structure of solids and some of the freedom of motion possessed by liquids. Because of the partial ordering, liquid crystals may be very viscous and possess properties intermediate between those of the solid and liquid phases. The region in which they exhibit these properties is marked by sharp transition temperatures, as in Reinitzer's example.

From the time of their discovery in 1888 until about 30 years ago, liquid crystals were largely a laboratory curiosity. They are now widely used as pressure and temperature sensors and in the displays of electrical devices such as digital watches, calculators, and laptop computers (Figure 12.3). These uses of liquid crystals result from the fact that the weak intermolecular forces that hold the molecules together in a liquid crystal are easily affected by changes in temperature, pressure, and electromagnetic fields.

Types of Liquid-Crystalline Phases

Substances that form liquid crystals are often composed of long, rodlike molecules. In the normal liquid phase, these molecules are oriented in random directions [Figure 12.4(a)]. Liquid-crystalline phases, by contrast, exhibit some ordering of the molecules. Depending on the nature of the ordering, liquid crystals can be divided into three categories: nematic, smectic, and cholesteric.

FIGURE 12.4 Ordering in liquid-crystalline phases, as compared with a normal (nonliquid-crystalline) liquid.

In the nematic liquid-crystalline phase, the molecules are aligned along their long axes, but there is no ordering with respect to the ends of the molecules [Figure 12.4(b)]. The arrangement of the molecules is like that of a handful of pencils whose ends are not aligned.

In the smectic liquid-crystalline phases the molecules exhibit additional ordering beyond that of the nematic phase. The smectic phases resemble a handful of pencils whose ends are more nearly aligned. There are different kinds of smectic phases, designated by the letters A, B, C, and so forth. In the smectic A phase the molecules are arranged in layers, with their long axes perpendicular to the layers [Figure 12.4(c)]. Other smectic phases display different types of alignments. For example, in the smectic C phase the molecules are aligned with their long axes tilted relative to the layers in which the molecules are stacked [Figure 12.4(d)].

Examples of molecules that exhibit nematic and smectic phases are shown in Figure 12.5. Note that these molecules are fairly long in relation to their thicknesses. The CN and NN double bonds and the benzene rings add stiffness. The flat benzene rings help the molecules stack against one another. In addition, many of the molecules contain polar groups; these give rise to dipole-dipole interactions that promote matching alignments of the molecules. (For more information, see Section 11.2) Thus, the molecules order themselves quite naturally along their long axes. They can, however, rotate around their axes and slide parallel to one another. In smectic phases the intermolecular forces between the molecules (such as London dispersion forces, dipole-dipole attractions, and hydrogen bonding) limit the ability of the molecules to slide past one another.

FIGURE 12.5 Structures and liquid-crystal temperature intervals of some typical liquid-crystalline materials.

Figure 12.6 shows the ordering of the cholesteric liquid-crystalline phase. The molecules are aligned along their long axes as in nematic liquid crystals, but they are arranged in layers with the molecules in each plane twisted slightly in relation to the molecules in the planes above and below. These liquid crystals derive their name from the fact that many derivatives of cholesterol adopt this structure. One such molecule is cholesteryl octanoate, whose molecular and three-dimensional structures are shown in Figure 12.7. Even though this is a rather complex molecule, it can be described approximately as a flat rod with a flexible tail. In the liquid crystal the molecules sit side by side in layers. The tail, however, causes one layer to be twisted relative to the next. The slight twist in their orientation from layer to layer tends to make these liquid crystals colored. Changes in temperature and pressure change the order and hence the color (Figure 12.8). These liquid crystals have been used to monitor temperature changes in situations where conventional methods are not feasible. For example, they can detect hot spots in microelectronic circuits, which may signal flaws. They can also be fashioned into thermometers for measuring the skin temperature of infants.

FIGURE 12.6 (a) Ordering in a cholesteric liquid crystal. The molecules in successive layers are oriented at a characteristic angle with respect to those in adjacent layers, to avoid unfavorable interactions. The result is a screwlike axis, as shown in (b).

FIGURE 12.7 (a) Molecular structure of cholesteryl octanoate. Note that the rings in this molecule are not benzene rings. Each corner of the rings has a carbon atom and as many hydrogen atoms or other bonds as is needed to satisfy the carbon valency of four. (b) A three-dimensional molecular model of cholesteryl octanoate.

Sample Exercise 12.1

Which of the following substances is most likely to exhibit liquid-crystalline behavior?

SOLUTION Molecule (i) is not likely to be liquid crystalline because it does not have a long axial structure. Molecule (iii) is ionic; the generally high melting points of ionic materials (Section 8.2) and the absence of a characteristic long axis make it unlikely that this substance will exhibit liquid-crystalline behavior. Molecule (ii) possesses the characteristic long axis and the kinds of structural features that are often seen in liquid crystals (Figure 12.5).

Practice Exercise

Suggest a reason why the following molecule, decane, does not exhibit liquid-crystalline behavior:

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

Answer: Because rotation can occur about carbon-carbon single bonds, molecules whose backbone consists of CC single bonds are too flexible; the molecules tend to coil in random ways and thus are not rodlike.