The incorporation of ester functions in the side chains in 1-alkyl-3-methylimidazolium cations seems to increase the biodegradability of these ionic liquids. We study here how the presence of ester functional groups affects the liquid-state structure (namely, the microphase segregation between polar and nonpolar domains in these ionic liquids) and the way in which the solvation of gases can be understood in these solvents. We use molecular simulation to study the structure of the ionic liquids 3-methyl-1-(pentoxycarbonylmethyl)imidazolium octylsulfate, [C(1)COOC(5)C(1)im][C8SO4]; and 3-methyl-1-(pentoxycarbonylmethyl)imidazolium bis-(trifluoromethylsulfonyl)imide, [C(1)COOC(5)C(1)im][NTf2] in the liquid phase and to assess the molecular mechanisms of salvation of carbon dioxide and ethane. The presence of ester functions influences the relative size of the polar and nonpolar domains in the ionic liquids, but does not significantly affect the solvation of gases.