Design principles for supercritical CO2 viscosifiers

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 (scCO(2)). 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 scCO(2). 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 scCO(2) should have different geometric packing parameters (P-C(lim)) 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 rhyd. Using high pressure SANS (HP-SANS), and changing solvent from water to scCO(2) showed that all the M-F7H4 variants stabilise water-in-CO2 (w/c) microemulsion droplets on addition of water. In scCO(2) 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 scCO(2) 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 (eta(mic)/eta(CO2)).