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Intermolecular Chemistry and Lyotropic Structures |  |
Applications and Importance of Lyotropic Liquid Crystals |  |
The previous section described some of the structures that amphiphilic molecules can form. Thermodynamic ideas help explain why those structures exist. This section will explore the contributions of temperature, energy, and entropy to amphiphilic structure formation.
Although lyotropic liquid crystals are characterized by the fact that concentration is the determining factor in their phase transitions, temperature also plays an important role. The amphiphilic molecules have temperature boundaries which depend on the specific kind of molecule. (Please refer to the phase diagram in the previous section for an illustration.) When the temperature is above this boundary, micelles and liquid crystals can form. Below it, the solution remains clear, although some small crystals may be suspended in the solvent.
One theory for the role of temperature points out that the forming of micelles requires the hydrocarbon chains to bend and fold in all directions. This bending and folding behavior is similar to what happens when hydrocarbons melt. One possibility, then, is that the temperature boundary is related to the melting point for the hydrocarbon tails.
In order to understand the following discussion, it will be necessary to recall some equations and terms relating to energy and entropy. We will begin, then, by going over those points. The equation dealing with the thermodynamic properties of chemical reactions is: DF = DH - TDS. In words, this reads, "The change in free energy is equal to the change in enthalpy minus the temperature in Kelvins times the change in entropy."
Almost any chemistry textbook will cite the familiar rule that "like dissolves like." This rule can be further understood by looking at the thermodynamic aspects of trying to dissolve hydrocarbons in water. In most cases, dissolving hydrocarbons in water would lead to a reduction in enthalpy. This is good for the reaction - it leads to an energy reduction since DH would be negative. The reaction, however, does not occur. Hydrocarbons do not dissolve in water. The reason? Entropy. In most cases, dissolving hydrocarbons in water would reduce the entropy of the system. One theory suggests that when the water molecules rearrange themselves around the large hydrocarbon molecules to restore their hydrogen bonds, the process increases order in that area. The more hydrocarbons dissolve, the more the order would increase.
These concepts relate to lyotropic liquid crystals by the fact that, for the amphiphilic molecules, the dominant form of entropy changes with concentration. At low concentrations, entropy is increased most by allowing the amphiphilic molecules to mix thoroughly with the water. As the concentration becomes higher, though, the order created by allowing organized structures becomes less important than the order created by forcing water molecules to rearrange themselves around dissolve hydrocarbons. At that point, structures begin to form in which the hydrocarbon tails are kept away from the water, preventing local order from increasing.
The following movie shows the formation of micelles as concentration increases.
The strong role of entropy makes lyotropic liquid crystals different from most other substances. In many substances, order exists at low temperatures when the low enthalpy is enough to reduce the free energy. Disorder arises at higher temperatures when high entropy is needed to reduce the free energy. In lyotropic liquid crystals, though, structure exists at high concentrations when the order created by dissolving hydrocarbons would be larger than the disorder of having them randomly distributed through the water. At low concentrations, entropy plays its usual role of encouraging complete solvation and structures do not form.
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Intermolecular Chemistry and Lyotropic Structures | |
Applications and Importance of Lyotropic Liquid Crystals | |