Using CO₂ to Modulate Self-Assembly—Viscosity Increase or Decrease in Surfactant Fluids Induced by CO₂

The self-assembly of surfactants in water can be tuned by a number of stimuli, including temperature, pH, and light. In particular, self-assembly into wormlike micelles (WLMs) has been shown to be sensitive to the above stimuli. WLMs are long, polymer-like chains that entange into transient networks. Thus, when WLMs are formed, the solution becomes highly viscoelastic. Conversely, if WLMs are converted into spherical structures (micelles or vesicles), the viscoelasticity is lost and the viscosity of the solution will drop to that of water.<br/><br/>Here, we focus on modulating the self-assembly of WLMs using carbon dioxide (CO₂). CO₂ is of concern due to its role in the atmosphere as a greenhouse gas (thus impacting global warming and climate change). Approaches for CO₂ sequestration often introduce gaseous CO₂ into a solution containing an amine such as diethanolamine (DEA), but such amines are a rather toxic molecules. We are interested in absorbing CO₂ in a fluid that is completely safe and biocompatible. Moreover, could CO₂ absorption transform the viscosity of the fluid?<br/><br/>In this regard, we have devised a fluid containing an anionic fatty acid and a nontoxic amine-bearing molecule. Both the constituents of our fluid are biocompatible and in fact, they are found in the body. We will show that the viscosity of the fluid can be switched from low to high (or high to low) by contact with CO₂. The low-viscosity state corresponds to spherical micelles (or vesicles) whereas the high-viscosity state corresponds to WLMs. By bubbling CO₂ into a fluid of given composition, we can rapidly (within seconds) drive a transition from one type of self-assembly to another. We are interested in exploiting such rapid transitions, not only for CO₂ sequestration but also for certain biomedical applications.