CO ₂ ‐Responsive Alkyl Amidine Surfactants: Hydrophobic Chain Length Dependent Behaviors and Applications

We synthesized a homologous series of alkyl amidines with chain lengths from C 4 to C 18 and systematically investigated their physicochemical properties, CO 2 responsiveness and applications. Key findings demonstrate a dual dependence of properties on the alkyl chain length and CO 2 ‐induced protonation. Both unprotonated and protonated surfactants show an inverted volcano‐shaped relationship between surface tension and chain length, with optimal surface activity at intermediate lengths (C 8 –C 14 ), whereas oil–water interfacial tension shows minimal dependence on either chain length or CO 2 ‐induced protonation. Interestingly, these alkyl amidines exhibit distinct chain‐length‐dependent and CO 2 ‐switchable emulsification‐demulsification behaviors. The unprotonated short‐chain surfactants (C 4 –C 10 ) form O/W emulsions but cause demulsification upon protonation, whereas the unprotonated long‐chain surfactants (C 12 –C 18 ) cannot emulsify but their protonated forms can stabilize O/W emulsions. The resultant emulsions stabilized by unprotonated short chains show a negatively charged interface due to hydrogen bond‐induced water deprotonation by the amidine group, thereby generating the interfacial OH − ions. In contrast, emulsions stabilized by protonated long‐chain amidines possess a positively charged interface via CO 2 ‐induced cationization. Importantly, the unique chain length dependence and CO 2 responsiveness were successfully leveraged for diverse applications, including clay stabilization, crude oil washing, and CO 2 ‐enhanced foam fire suppression.