5. CO₂-Responsive Wormlike Micelles

Despite its extensive use, pH as a trigger has several drawbacks including the need of stoichiometric quantities of acid and base, the accumulation of by-products, and costly disposal. In this context, the weak acid gas, CO₂, represents an interesting alternative because it is inexpensive, relatively benign, and can be easily removed, thus is free of contamination. In addition, CO₂ can be completely removed by streaming an inert gas into the solution and/or mild heating, which allows the CO₂-reactive functional groups to resort to their initial forms. Presented in this chapter are four classes of CO₂-responsive wormlike micellar systems, three of which are based on amines, either located on the hydrotrope molecules or on the skeleton of long-chain surfactants. Carboxylate-based anionic wormlike micelles are also sensitive to CO₂, but the pre-formed wormlike micelles are ruptured by the presence of CO₂ and thus the macroscopic viscosity drops. The “CO₂/air” switchable system based on a C22-tailed tertiary amine is particularly attractive for practical applications because it provides a cost-effective, environment-friendly, and low-energy way to regulate the viscoelasticity in aqueous media simply by bubbling CO₂ or air at ambient temperature, without the need of heat or the use of an inert gas such as N₂ or Ar.