CO2 sorption in ionic liquids (ILs) has been subject of attraction for scientists owing to potential use of ILs in CO2 capture and separation processes for industrial application. The current study addresses investigation of gas-solid equilibrium of four novel ILs based on phosphonium (tetraphenyl phosphonium PhP, benzyltriphenyl phosphonium bnPhP, (methoxymethyl)triphenyl phosphonium OMeMePhP, (cyanomethyl) triphenyl phosphonium CNMePhP) as cation sources with 2-acrylamido-2-methylpropanesulfonate AMPS anion. The current study affords insight outcomes of CO2 sorption at pressures up to 4 MPa and 298.5 K. Among the studied quaternary phosphonium based salts, bnPhPAMPS displayed the highest capability in terms of CO2 sorption by showing 0.7735 mole fraction at 1 MPa and 298.15 K. The molar volumes of respective ILs were estimated from experimentally determined densities for studying molar volume-CO2 sorption relationship. The DFT studies showed that higher interactional energies, negative Gibb's free energy and lower �EHOMO lead to higher CO2 sorption. The present study provides an in depth study to understand the structure-activity relationship among molar volumes and CO2 sorption of investigated ILs and explores the ways for enhanced CO2 sorption. © 2020 Elsevier B.V.