Multi-purpose gel: From supramolecular envirogel to high-value applications

Dibenzylidene-D-sorbitol (DBS) has remained a well-known low molecular weight gelator of organic solvents for over 100 years. As such, it constitutes a very early example of supramolecular gels. It has found widespread applications such as personal care products and high-tech applications.
Despite the versatility of DBS as an organogelator and industrial feedstock, none of its existing derivatives could gel water. Herein, functionalization of the aromatic ‘wings’ with various functionalities and their gelation profiles were described. None of the synthesized derivatives but DBS-COOH and DBS-CONHNH2 could undergo gelation in water. DBS-COOH formed a stable hydrogel by pH-switching while the DBS-CONHNH2 formed stable hydrogels across a wide range of pH values by a heat-cool cycle. CD spectroscopy and SEM were used to show that these functionalities control the aggregation mode of the parent DBS.
DBS-COOH and DBS-CONHNH2 hydrogels demonstrated a significant uptake of dyes from model polluted water and hence we call them supramolecular envirogels. DBS-COOH demonstrated higher affinity towards methylene blue at ambient conditions but DBS-CONHNH2 demonstrated a pH-dependent selectivity towards various dyes. Lead, cadmium and mercury were significantly adsorbed by DBS-CONHNH2 .The maximum metal loading of 1:1 molar ratio was obtained at neutral ambient pH condition and 2:1 molar ratio at basic pH. The adsorption processes fit pseudo-second-order kinetic and the Langmuir isotherm models.
DBS-CONHNH2 were used to recover precious metals from a model mine tailing with greater selectivity towards precious metals than earth-abundant metals. The adsorbed precious metals were reduced in situ to nanoparticulate form. Interestingly, the hydrogel–NPs exhibited higher conductance and electrocatalytic activities than bare gels and/or carbon electrodes.
Solubility of the APIs in water was enhanced by forming complexes with the DBS-CONHNH2 while the mechanical stability and morphology of the native hydrogel was influenced by the presence of APIs. APIs were released under basic pH conditions in a controlled-manner.