Ultrasound-Induced Gelation of Organic Liquids by L-Cysteine-Derived Amphiphile Containing Poly(ethylene glycol) Tail

Amphiphile containing L-cysteine covalently linked with poly(ethylene glycol) (PEG) chain (PEG(360)-Cys) was observed to produce transparent gel at room temperature in polar aprotic solvents not only by heating-cooling (HC) but also when subjected to ultrasound (US). It was observed that a suspension of PEG(360)-Cys when treated with US readily formed gel at much lower critical gelation concentration. US irradiation has been established to control the gel properties at the molecular level. The morphological change of the organogels produced by the HC and US methods was confirmed from scanning as well as transmission electron microscopy. The organogels produced by the two external stimuli (HC and US) were characterized in detail by FTIR spectroscopy, differential scanning calorimetry, and rheology to shed light on the molecular packing during gelation. It is important to note that the US-induced organogels showed almost 13-fold increase in gel strength compared to the organogels obtained by the HC method. Further, US-induced gels were found to be thermally more stable than the heat-set gels. All these studies clearly demonstrate that hydrogen-bonding interaction is the main driving force for both the gelation processes, but the mode of hydrogen bonding at the molecular level is different.