Entropic Control of Ethylene and Acetylene Uptake in Silicon-Functionalized Ionic Liquids? An Experimental and Molecular Dynamics Study

Distinguishing and rationalizing the physical absorption of small hydrocarbon gases is inherently challenging due to their nonpolar character. Similar interaction energetics obscure the molecular origin of solubility trends. Herein we combine isochoric saturation measurements and polarizable (CL&Pol) molecular dynamics simulations to examine ethylene absorption in seven silicon-functionalized imidazolium ionic liquids and extend the analysis to acetylene. Ethylene uptake increases with cation flexibility and free volume, with siloxane-functionalized cations and [NTf2]− salts giving the highest solubility. Thermodynamic analysis shows that differences in ethylene solubility are driven mainly by entropy, consistent with cavity formation in nonpolar regions as the dominant uptake mechanism. Simulations reproduce these trends and reveal that acetylene, unlike ethylene, also benefits from specific solute-anion interactions, especially with more basic anions. These results provide clear design rules for reversible C2-hydrocarbon sorbents: increase cation flexibility to enhance ethylene absorption, and tune anion basicity to amplify acetylene uptake selectively.