Thickening CO2with 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.