(274b) Self-Degrading Molecular Organogels: Self-Assembled Gels Programmed to Spontaneously Liquefy after a Set Time

Gels are used in many applications in the oilfield, in medicine, and in consumer products. For example, during oil recovery, organogels are pumped underground into fractures within the oil-bearing rock. The gel must then act as a solid plug to block the flow of liquid (oil or water). However, after a period of time (days to weeks), the gel must liquefy (i.e., transform to a thin sol), thus removing the plug and enabling oil to be extracted through the permeable fractures. To degrade gels after a given time, many ‘degrading agents’ as well as external stimuli (e.g., heat) have been examined. Here, we explore a concept that avoids both external agents and stimuli: we demonstrate self-degrading organogels based on the self-assembly of molecular organogelators. The gels are (a) extremely strong and robust at time t = 0 and (b) degrade spontaneously into a sol after a set time t = t_degr that can be minutes, hours, or days. This combination of properties is achieved by combining two readily available molecules — the organogelator (1,3:2,4)-dibenzylidene sorbitol (DBS) and an acid (e.g., hydrochloric acid, HCl) — in an organic solvent. DBS self-assembles into nanoscale fibrils, which connect to form a 3-D network, thereby gelling the solvent. The DBS concentration sets the gel strength, and just 1% of DBS is enough to make a robust gel (having an elastic modulus G' > 10 kPa) that can be picked up and handled as a free-standing solid. At the same time, the acid type and concentration set the degradation kinetics and thereby the value of t_degr at a given temperature. For example, a 2% DBS gel in polyethylene glycol containing 20% of 0.3 M HCl degrades after 10 days at 30°C and after 2 h at 70°C. We use a combination of NMR and mass spectrometry to delineate the chemical mechanism for degradation. The acid is shown to induce a slow hydrolysis of the acetal groups, thereby converting DBS into small molecules (benzaldehyde and sorbitol) that cannot self-assemble into fibrils. Self-degrading DBS gels with a pre-programmed ‘degradation clock’ can be made with both polar and non-polar organic solvents and thus have wide-ranging applicability. The concept could be a game-changer for oil recovery as it promises to make the process safer, more efficient, and sustainable (due to reduced consumption of drilling fluids).