CO 2 -responsive gels.

CO 2 -responsive materials undergo a change in chemical or physical properties in response to the introduction or removal of CO 2 . The use of CO 2 as a stimulus is advantageous as it is abundant, benign, inexpensive, and it does not accumulate in a system. Many CO 2 -responsive materials have already been explored including polymers, latexes, surfactants, and catalysts. As a sub-set of CO 2 -responsive polymers, the study of CO 2 -responsive gels (insoluble, cross-linked polymers) is a unique discipline due to the unique set of changes in the gels brought about by CO 2 such as swelling or a transformed morphology. In the past 15 years, CO 2 -responsive gels and self-assembled gels have been investigated for a variety of emerging potential applications, reported in 90 peer-reviewed publications. The two most widely exploited properties include the control of flow (fluids) via CO 2 -triggered aggregation and their capacity for reversible CO 2 absorption-desorption, leading to applications in Enhanced Oil Recovery (EOR) and CO 2 sequestration, respectively. In this paper, we review the preparation, properties, and applications of these CO 2 -responsive gels, broadly classified by particle size as nanogels, microgels, aerogels, and macrogels. We have included a section on CO 2 -induced self-assembled gels (including poly(ionic liquid) gels).