Order and Disorder

Molecules never rest. The formally rigid all-trans chains in crystalline phospholipids have a measure of vibrational freedom that increases toward the methyl group tail of the chain (Figure 9). In liquid crystal phases the chains are disordered, any given segment rapidly changing its orientation. We expect the motion of a given segment to be more limited near the tightly packed head groups than at the tail ends of the chains but since the orientation of a given segment is time-dependent and unpredictable, we can only make statistical statements about motion. In this chapter we show that a variety of spectroscopic techniques allow us to make such statements since they provide quantitative measures of order, a term we shortly shall define. The theoretic framework that we use was invented to deal with the physical properties of liquid crystals.