Lyotropic Liquid Crystals
These liquid crystals form the basic fabric of all biomembranes in living
cells, and are found in all manner of consumer products, from shampoos and
fabric softeners to ice cream stabilisers and cake mixes. The molecules
involved, membrane lipids, detergents and surfactants, all share the common
feature that they are amphiphilic.
This typical
amphiphile, DOPC, has two major molecular elements: a polar headgroup and two
oily hydrocarbon chains.
Because the headgroup wants to be in contact with water, whereas the
hydrocarbon chains hate water, when we mix an amphiphile with water, structures
form spontaneously which hide the oily chains from the water, but allow the
headgroups to be hydrated. These are the lyotropic liquid crystalline phases.
Some have rather simple structures, such as the lamellar phase:
The
amphiphiles, represented as two-tailed tadpoles, form bilayers, which stack
periodically. The amphiphiles are free to diffuse around in each monolayer,
making this a two dimensional liquid and a one dimensional crystal.
Other phases have interfaces which bend, sometimes in a simple manner, as in
the inverse hexagonal (HII) phase:
Here the
interface has curved into cylinders with water on the inside. The cylinders pack
onto a two dimensional hexagonal lattice, so now we have a two dimensional
liquid and a two dimensional crystal.
Sometimes the interface may bend in an extremely complex way, as the gyroid
cubic phase:
In this
tortured structure, the interface has been bent into convoluted saddles which
are arranged on a cubic lattice. This is a two dimensional liquid and a three
dimensional crystal.
Nature has devised many other beautiful structures which allow the headgroups
to hydrate, yet hide the chains from water. We spend our time searching for new
structures, trying to understand the energetics of why they form, the geometrical route ,by which one structure grows from
another, and developing new instrumentation for probing
this fascinating and delicate phase of matter.