1.2 Nanoscale Actors and Their Properties

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as with straight, saturated fatty acids, the higher the Tg. The less ordered the phase is,

as with bent, unsaturated fatty acids, the lower the Tg. A layer of fatty acids can also be

formed at the surface of water and then transferred onto a solid surface. These fatty acid

single layers are called self-assembled monolayers (SAM). They are also the reason why

amphiphiles are called surfactants (from “surface active”, or moving to the surface).

The most common lipids have their hydrophilic head attached to a hydrophilic head

group, thus overall still ending up with an amphiphilic molecule. Here, we will specifi-

cally mention phospholipids, or phosphoglycerides (Figure 1.6), since they are the main

lipids that make up cell membranes and the most commonly used lipids in nanotechnol-

ogy. The hydrophilic head group is phosphoglycerol, the hydrophobic tail contains two

fatty acid tails, either saturated or unsaturated, with somewhat different lengths.

Do to their amphiphilicity, lipids form liquid crystals in water (Figure 1.7). Depend-

ing on the shape of the lipid a bilayer can form, which is the basis of all lipid bilayer

cell membranes. But differently-shaped liquids can also form single-layer or double-

layer spheres, called micelles or liposomes, respectively. All of them have been used in

nanotechnology. The mechanical properties of lipid membranes can be manipulated by

changing the composition of the bilayers. A membrane containing more saturated lipids

will have a higher Tg, and thus will be stiffer, than a membrane with more unsaturated

lipids. The large, stiff cholesterol molecules further stiffen membranes, as do proteins,

especially transmembrane proteins, which are proteins that are long enough to cross

the membrane completely.

Figure 1.7: Common liquid crystal structures for phospholipids in water.