Supramolecular systems gelator

Hydrogels formed by PEO block copolymers have previously been proposed as sustained release matrix [85,86], The a-CD-PEO hydrogel delivery system differs in that the gelation relies on the formation of a polymer inclusion complex induced by the PEO-threaded CDs. The properties of the supramolecular hydrogel can be fine-tuned with the composition, molecular weight and chemical structure of the polymer or copolymers. 

This system has been investigated in detail, and the tunability of this type of two-component gelation system has been exemplified, as explained below. The acid-base interaction between the two components plays a primary role in the formation of the gel—if the acid is protected as an ester, no gel formation takes place. It is proposed that this interaction (Fig. 20) generates the gelator complex, which is the species that hierarchically self-assembles to form fibrous gel-phase aggregates. Interestingly, it was reported that macroscopic gelation still occurred when the acid-base interaction was swapped for an interaction between a crown ether and a protonated amine (Fig. 21) [71]. This indicates that different supramolecular interactions between the components can easily be employed. 

The addition of the second component therefore enhanced the gelation process for dendron A, but hindered the gelation process for dendron B. In siunmary, the gel-phase materials properties of the two dendrons responded in opposite ways to the presence of a diamine additive. This is a remarkable example in which dendritic generation and the use of a second (supramolec-ular) component can both control materials properties. This effect can be considered as orthogonal dendritic and supramolecular control. It was argued that, in the future, these systems may be exploited to develop switchable and highly controllable gel-phase materials. 

Many anion-driven supramolecular gels have been fabricated based on ion pairing or the analog state such as a mixture of acids and bases [8d]. There are a few examples of gelator molecules that are electronically neutral anion-responsive molecules, whose anion complexes also behave as gelators. This section mainly introduces several examples of anion-driven supramolecular gel systems as ion pairs, including ionic liquids and related materials, and cationic metal complexes accompanied by counteranions. 

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