A new family of urea-based low molecular-weight organogelators for environmental remediation: the influence of structure

Abstract

Gelation processes grant access to a wealth of soft materials with tailorable properties, in applications as diverse as environmental remediation, biomedicine and electronics. Several classes of self-assembling gelators have been studied and employ non-covalent bonds to direct assembly, but recently attention has come to focus on how the overall shape of the gelator molecule impacts its gelation. Here we study a new sub-family of low molecular weight organogelators and explore how steric rearrangement influences their gelation. The gels produced are characterised with X-ray diffraction and small-angle neutron scattering (SANS) to probe their ex situ and in situ gelation mechanisms. The best examples were then tested for environmental remediation applications, gelling petrol and oils in the presence of water and salts.

Fig. 1. Schematic of urea gelation, directed by 1D H-bonding to form long chains, which then self-assemble into the overall fibrillary network (SAFIN). The 3D shape of the LMWO strongly influences the subsequent SAFIN formation.

Scheme 1. Structure of the gelators used in this study along with a general synthetic scheme from isocyanate precursors.

Fig. 2. SEM images of xerogels from various solvents at 5% w/v. (a) 5 from PE, (b) 4 from cyclohexane, (c) 5 from cyclohexane and (d) 6 from cyclohexane. All scale bars 20 μm.

Fig. 3. SCXRD structures of gelators 4, 5 and 6, showing the number of independent molecules in the asymmetric unit cell of the crystallographic cell (4, 2, and 3, respectively) and the chain network formed by N–H···O hydrogen-bonding.

Fig. 4. SANS profiles for (a) the three different gelators at 5% w/v, and (b) gelator 5 at a range of concentrations, at 25 °C in d₁₂-cylcohexane.

Fig. 5. SANS profiles for (a) 5, 10% w/v, and (b) 6, 5% w/v, at a range of temperature, in d₁₂-cylcohexane.

Fig. 6. Images of gels of 5 and 6 in various polluting liquids formed on the surface of both DI and sea water: 5% pump oil gels of (a) 6 with water, (b) 5 with water and (c) 5 with sea water, each supporting 5 mL of water. (d) 120 g of water (coloured with CuSO₄) supported by a 5% gel of 6 and (e) sol–(f) gel phases of 5 in dirty engine oil.