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Structure and Properties of Lyotropic Liquid Crystals |  |
Thermodynamic Considerations |  |
The dual nature of amphiphilic substances leads to a struggle between the hydrophilic heads attempting to increase their contact with water and the hydrophobic tails trying to avoid it. This tension leads to a sort of compromise - an optimal surface area for the water-amphiphile interface specific to each type of molecule. The shapes in which the molecules arrange themselves depend partly on the optimal surface area, as well as partly on the fluid volume of the hydrocarbon chains and the maximum length at which they can still be considered fluid. Although many structures can fit the geometry, one is usually best from a thermodynamic perspective. Large structures create too much order, while small structures cause the surface area to be larger than optimal, so a medium-sized structure usually wins out.
This section will examine the relationships between characteristics of molecules and the structures they form in solution.
When the amphiphiles have a large optimal area but a small hydrocarbon chain volume, they often form spherical micelles in which the radius is no larger than the maximum fluid length of the hydrocarbon chain. Most often, molecules that form spherical micelles have a single hydrocarbon chain as that has a smaller volume. Also, heads carrying a net charge are more common since they have a higher affinity for water which results in a higher optimal interface area. If one or more of the factors are just a little bit off from that required for a spherical micelle, then structures form which are nearly but not perfectly spherical.
When the factors are too far off for even an imperfect sphere to form, the molecules might assemble into cylindrical micelles. Like those that make spherical micelles, molecules that form cylindrical micelles usually have single chains. Their optimal interface areas, however, are smaller. Amphiphiles that usually form spheres can be induced to form cylinders instead if a salt is added to the solution. The ions in the salt weaken the hydrophilic reaction of the charged head.
If the molecules have a higher chain volume and a lower interface area, they are likely to form bilayers. These molecules are likely to have two hydrocarbon chains. The extra chain makes them more hydrophobic and also raises the volume to length ratio. Amphiphilic bilayers are not as dynamic as micelles - molecules stay in the bilayer for a relatively long time rather than continually diffusing in and out as happens with micelles. If the head group is especially small or is anionic and in a salt solution, or if the chains are saturated and frozen, a planar bilayer may form. Planar bilayers have very little motion and do not form into vesicles.
Vesicles are bilayers that have folded into a three-dimensional spherical structure, sort of like a micelle with two layers of molecules. Vesicles form because they get rid of the edges of bilayers, protecting the hydrophobic chains from the water, but they still allow for relatively small layers. In order for a flat bilayer to be without edges, it would have to be infinite. Molecules that form vesicles usually have a fluid double chain and a large optimal area. Lipids found in biological membranes spontaneously form vesicles in solution. Please refer to the illustration to the left for a cross-section of a vesicle.
Under certain circumstances, as mentioned earlier, the amphiphiles can form inverse micelles, with the heads on the inside and tails on the outside. Molecules that form this structure usually have a small optimal interface area or a large chain volume to length ratio. Double chains, nonionic heads, and cis unsaturated chains are also common. When inverse micelles form, the solution changes from appearing as oil droplets in water to water droplets in oil.
The section above discussed some of the most common conditions for various geometries to form. Changing a few factors, however, can sometimes cause a structure to form where it wouldn't otherwise. This can be done by changing the head, changing the chain, or mixing different amphiphiles.
If a salt is added to the solution or if the pH is lowered, the hydrophilic interaction of the head is reduced and the optimal interface area is lowered. The additional ions in solution also reduce the repulsive interactions between head groups, reducing the radius of curvature. This makes the molecules more likely to form bilayers or inverse micelles. Using these methods to reduce the interface area often has the additional effect of straightening the hydrocarbon chains.
If the hydrocarbon chains are unsaturated or branched, their length is reduced. This increases the volume to length ratio and again makes bilayers and inverse micelles more likely.
When two different kinds of amphiphiles are mixed, the characteristics of the solution are similar to an average of the characteristics of solutions of the individual types, provided the two types can mix freely in solution. Furthermore, carefully adding more of one kind of molecule can cause the solution to form structures of different shapes or sizes than either molecule would form alone.
NOTE: This page was completed in its current form July 31, 2003.
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Structure and Properties of Lyotropic Liquid Crystals | |
Thermodynamic Considerations | |