A Comparative Study of CO2-Responsive Worm-like Micelles Prepared by Macromolecules and Small Molecules

Hypothesis: Worm-like micelles triggered by carbon dioxide (CO₂), as an abundant, inert, and green stimulus have recently attracted much interest. These materials have many potential applications, including heat transfer, rheological control, personal protection and enhanced oil recovery (EOR). An ideal CO₂-responsive worm-like micelle reveals a reversible transition state (from sol to gel state and vice versa) in response to environmental changes. The most important feature of these systems during these transitions is that CO₂ does not accumulate in the system upon repeated cycles. Herein, we prepared two types of materials based on 3-(dimethylamino)-1-propylamine sodium dodecyl sulfate (DMAPA-SDS) as a small molecule, and poly(2-(dimethylamino)ethyl methacrylate-b-polymethyl mthacrylate)-SDS [(PDMAEMA-b-PMMA)-SDS] as a macromolecule to examine possible formation of CO₂-responsive worm-like micelles.
Methods: Amine groups in the structure of DMAPA and PDMAEMA-b-PMMA can be protonated and ionized to quaternary ammonium salts by CO₂ bubbling and interact with SDS to possibly form a worm-like micelle through non-covalent electrostatic attraction. The viscosity and structural features of aqueous solutions were evaluated before and after being exposed to CO₂ by rheometry and ¹H NMR, respectively. The rheometry results showed shear thinning and gel-like behaviors at high shear rates and frequencies, respectively.
Findings: The results showed that for a DMAPA-SDS small molecule an ideal reversible CO₂-responsive worm-like micelle was formed and a sol-to-gel transition was observed, whereas in using a macromolecule an irreversible agglomeration occurred. The absence of reversible sol-gel transitions and the presence of heavy agglomeration for the (PDMAEMA-b-PMMA)-SDS macromolecule was attributed to entanglements of its long polymer chains. Therefore, DMAPA-SDS as small molecule with its ideal CO₂-responsive worm-like micelle has potential in different useful applications, particularly in EOR.