Introduction to Liquid Crystals


      Surfactants (soap molecules) can form a variety of liquid crystalline phases in water. This ordering is due to hydrophobic/hydrophilic competitions. The hydrophilic (ionic) portion of the surfactant is more stable when solubilized by water, whereas the hydrophobic (alkane) portion of the surfactant is more stable when surrounded by other organic chains. Hence, the materials self-assemble in the liquid phase, giving it a higher than isotropic order. The structure of a typical surfactant (sodium laurate) is shown on the right. Below is a typical phase diagram for a surfactant, as a function of concentration (in water) and temperature. Amphiphiles have rich phase diagrams, with many novel morphologies. Schematics of some phases are shown below.

Soap phase diagram


Soap molecule


      In dilute solution, the surfactants do not form any particular structure. As the concentration is increased, however, the amphiphiles condense into well defined structures.

Dilute surfactant phase
Surfactant micelles


      The most readily formed structure is micelles, where the surfactants hide the hydrophobic tails inside a sphere, leaving only the water-soluble ionic heads exposed to solution.


      At higher concentration, surfactants can also form elongated columns that pack into hexagonal arrays. The columns have hydrophobic cores and hydrophilic surfaces. The columns are separated from one another by water.

Surfactant hexagonal columnar phase
Surfactant lamellar phase


      At extremely high concentration (neat soap), the surfactants crystallize into a lamellar structure, with elongated sheets separated by thin water layers. The structure is very reminiscent of the lipid bilayers seen in biological systems.




      The phospholipid/water mixture found in cell membranes is actually a liquid crystalline phase (the structure of dipalmitoylphosphatidylcholine is shown to the right). Phospholipids spontaneously form vesicles in water, encapsulating a small water droplet in a spherical shell of phospholipid molecules. Both the inner and outer wall of the shell are composed of hydrophilic heads, whereas the inside of the vesicle shell is the alkane tails. The image below is a slice through a spherical vesicle.


Phospholipid vesicle



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