This work reveals new principles for designing new self-assembling additives as viscosity enhancers for supercritical fluid CO₂. Employing the approaches outlined in this work a maximum increase in CO₂ viscosity of 100% has been achieved, the highest ever reported for surfactant CO₂ viscosifiers. A series of semi-fluorinated F–H hybrid surfactants were synthesised based on the pentadecafluoro-5-dodecyl (F7H4) sulfate anion, but featuring different metallic counterions: Li-F7H4, K-F7H4, Na-F7H4 and Rb-F7H4 (denoted M-F7H4). These M-F7H4 variants were designed to reveal the effects of hydrated cation radius (r_hyd) on packing self-assembly in supercritical CO₂ (scCO₂). The CO₂-philic M-F7H4 surfactants were investigated at the air–water interface by surface tensiometry, and in the bulk by small-angle neutron scattering (SANS), in both aqueous solutions and scCO₂. Surface tensiometry of aqueous solutions showed that interfacial packing density and limiting head group area at the critical micelle concentration (A_cmc), depends implicitly on the identity of M⁺, suggesting that micelles in water or scCO₂ should have different geometric packing parameters (P^lim_C) as a function of M⁺. SANS measurements on M-F7H4 aqueous micellar solutions confirmed sphere → ellipsoid → vesicle transitions as a function of M⁺ with decreasing r_hyd. Using high pressure SANS (HP-SANS), and changing solvent from water to scCO₂ showed that all the M-F7H4 variants stabilise water-in-CO₂ (w/c) microemulsion droplets on addition of water. In scCO₂ Li-F7H4 forms prolate ellipsoidal droplets, Na-F7H4 forms long cylindrical droplets, K-F7H4 forms ellipsoidal droplets, whereas Rb-F7H4 stabilises spherical microemulsion droplets. The ability of M-F7H4 additives to enhance the viscosity of scCO₂ was shown by high pressure falling cylinder viscometry. The HP-SANS and viscometry experiments were shown to be quantitatively consistent, in that aggregate aspect ratios (X) determined in structural studies account for the enhanced relative viscosities (η_mic/η_CO2).