Development of Organogelators Based on Supramolecular Chemistry

When a saturated solution of sodium chloride in hot water is left at room temperature, sodium chloride of the quantity which corresponds to the difference in solubility is deposited as crystal. This process is called recrystallization, and the crystallization of sodium chloride occurs smoothly. In contrast, there is a case in which the supersaturated solution is so stable that crystallization does not occur. In general, when a saturated solution of low molecular weight compounds is cooled, it crystallizes unless a supersaturated solution is formed. However, we rarely observe the phenomenon in which the entire system is solidified like jelly. This phenomenon is called gelation and gels are defined as soft, solid, or solid-like materials consisting of two or more components, one of which is a liquid present in substantial quantity. 

The thermally-reversible gel returns to the solution because the droving forces are noncovalent bonds such as hydrogen bonding, hydrophobic interaction, n-n interaction and electrostatic interaction. These noncovalent bonds are broken down by heating, so the gel reverts to the sol. Since the thermally-irreversible gel forms a network structure by firm chemical covalent bonds, the gel formed never changes to the sol. This is why the thermally-irreversible gel is called physical gel and the thermally-irreversible gel is referred to as chemical gel. Moreover, the gelation ability is found in not only biopolymers but also low molecular weight compounds. 

This section deals with the development of organogelators that are low molecular weight compounds capable of gelating liquid by the simple operation of cooling after heating. The features of organogelators can be summarized as follows  

The disposal of crude oil spills and used cooking oil are serious problems of environmental pollution. Organogelators that solidify waste oils upon addition in small amounts will play a role in environmental protection. Moreover, it is not acceptable to discard liquids such as spent organic solvents in industrial waste. If waste organic solvents can be rapidly solidified by adding organogelators, utilization as fuel becomes a possibility. 

---