A novel paradigm for the design of surfactants for water/CO2 (W/C) microemulsions is presented. The paradigm focuses on the fractional free volume (FFV) available to CO2 at the interface. The FFV is an unambiguous geometric parameter that is calculated directly from surfactant tail geometry and surface coverage. We present an analysis of recent experimental studies indicating that low FFV is a necessary, although not sufficient, condition for W/C microemulsion formation and that both microemulsion and macroemulsion stability correlate qualitatively with FFV. This correlation is understood by noting that a decrease in FFV tends to favor the factors that stabilize W/C microemulsions, namely, decreased interfacial tension, reduced overlap between tails (weakening attractive interdroplet interactions), and increased interfacial curvature. These factors are more challenging to achieve in CO2) than in alkane solvents, implying that low FFV is especially important for W/C microemulsions.