Thickening CO2 with Direct Thickeners, CO2-in-Oil Emulsions, or Nanoparticle Dispersions: Literature Review and Experimental Validation

High molecular weight associating random copolymers, such as poly(fluoroacrylate-styrene) (polyFAST), are direct CO₂ thickeners that can dissolve readily in CO₂ and significantly increase viscosity. Although polyfluoroacrylate (PFA) is more CO₂-soluble than polyFAST, PFA induces less thickening because it is non-associating. High molecular weight poly(dimethylsiloxane) (PDMS) and poly(vinyl acetate) (PVAc) can increase CO₂ viscosity if toluene cosolvent is introduced. Bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) can thicken CO₂ modestly if ethanol cosolvent is added. Little CO₂ thickening has been reported for low molecular weight polymers, including poly(vinyl ethyl ether) (PVEE) and poly-1-decene (P1D). Associating oligomers of poly(propylene oxide) (PPO) with pendant aromatic groups showed modest CO₂ viscosity enhancement. Cross-linked phosphate esters can increase the viscosity of CO₂ if a very substantial concentration of a light alkane cosolvent is added. Indirect thickening of CO₂ involves the generation of high apparent viscosity, completely waterless CO₂-in-oil (C/O) emulsions that can be attained when CO₂ is mixed with mineral oil containing a silicone-alkyl copolymeric surfactant; similar behavior was observed with a proprietary blend of oil and surfactants. Only one nanoparticle-based thickening of CO₂ involving the dispersal of dilute concentrations of nanoparticles that are surface-functionalized with highly CO₂-philic groups was reported (however, no explanation was provided to explain how the high molecular weight polymer used in this study could be CO₂-soluble). New experimental results were obtained for some readily available or easily synthesized CO₂-thickening candidates. PolyFAST yielded very substantial viscosity increases. PFA, PDMS-toluene, and PVAC-toluene could appreciably thicken CO₂. Very little detectable viscosity enhancement was achieved with low molecular weight PVEE or P1D. CO₂ was thickened slightly with AOT-ethanol. Regarding future studies, novel, nonfluorous associating CO₂-philic oligomers, and highly CO₂-philic phosphate esters that remain dissolved in CO₂ after cross-linking are the most obvious candidates for CO₂ direct thickeners.